Cliff, some coolant loss will not leave any evidence such as puddles under car, clouds of steam, etc, but a working temperature gauge will indicate something is not normal. I've experienced such events myself. One turned out to be a bad head gasket. I'd have to add coolant once a month. I discovered the problem by accident while working on a running engine with the back of my hand near the cylinder head and exhaust manifold. The head gasket was leaking but the vapor leak was invisible to the eye. The car was a 67 GTO with less than 30k miles.
Another event was a Ford Econoline kept loosing coolant with no puddle evidence left behind. About once a month I would add coolant. No odd smells at the exhaust or extra clean spark plugs indicating a problem. Another accidental discovery found the problem. I was removing some air intake ducting and saw a fine crack on top of the radiator which is the plastic area. I tried a product called K-Seal that worked for months until I got around to having the radiator professionally repaired and flushed.
My advice would be go to Orges Cave and find out the proper radiator cap & T-Stat for your Fiero. Also any cracks in your coolant recovery bottle can cause a problem as well as some of the issues mentioned on this topic by others.
------------------ "Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut
Where from? How much? Any strange puddles under the car? Like I said in a previous post, I had low coolant levels and I saw problems similar to what you are describing. I was able to identify the leak because of a big puddle of coolant.
Does your oil look like a chocolate milk shake? Got any sweet smelling exhaust (if you have antifreeze, anyway.)? What do your plugs look like? Do you have any that are really, really clean?
I don't see it leaking anywhere. No puddles under my car. Oil looks normal, no weird smelling exhaust (or white smoke). Haven't checked the plugs (they haven't been removed for over 20 years, I'm afraid they will break off).
Cliff, some coolant loss will not leave any evidence such as puddles under car, clouds of steam, etc, but a working temperature gauge will indicate something is not normal. I've experienced such events myself. One turned out to be a bad head gasket. I'd have to add coolant once a month. I discovered the problem by accident while working on a running engine with the back of my hand near the cylinder head and exhaust manifold. The head gasket was leaking but the vapor leak was invisible to the eye. The car was a 67 GTO with less than 30k miles.
Well, I know one (maybe both) of the the exhaust manifolds has a leaking gasket.
My problem is that I don't have a garage to work on the car in. All work needs to be done at the side of the road, so I can't do any repairs that might take longer than an afternoon to finish. Also, anything left "open" will collect water and sand (I live a few hundred yards from the beach).
Many repair and part shops have a tester but not T-stat adapter to test Fiero w/o taking other things apart to clear the radiator neck. Part shops can have "loaner tools" to rent or loan w/ deposit you get back.
This way can pressurize the system when cold, on a lift, etc, looking under the car or other places.
Ok, I know it's leaking coolant, and I still don't know where but for now that's a secondary problem. I'll try to explain why.
Like the subject says: Engine temps are all over the place. I honestly think it's losing coolant because of this. And not the other way around; that the temps are all over the place because it's losing coolant. Reason being that even when I refill the coolant, and making sure there's is no air in the system and the overflow tank is filled, engine temps are still all over the place.
I refilled coolant yesterday. Did the thing where I parked the car on an incline, opened both the thermostat housing and radiator, removed the thermostat, topped of coolant (at the thermostat housing) until coolant flowed out of radiator, closed caps, run engine for a few minutes - repeat. Run the car until it's hot, filled overflow bottle, removed thermostat cap, open radiator cap to let air out, refill if necessary at thermostat cap.
Sorry for the Cliff's Notes, but I did all I can to make sure coolant system is filled and no air is trapped. So I drive it for 10 minutes and it overheats. Few minutes later temperature drops to normal, only to rise again after a few minutes. This repeats itself for about 20 minutes, after which the temperature settles at normal (below 90 degrees) temperatures. After driving it for a while, if I check level at thermostat housing, it's at the top. If I open radiator cap, no air escapes - it starts spewing coolant immediately. Overflow bottle is still filled.
Odd thing I have noticed: if I do a sharp turn to the left, it overheats. Sharp turn to the right and temperature drops again.
So the only thing I can think of is that the thermostat is the problem. But I've recently replaced it and that did not help. Could still mean the thermostat is at fault because it's either junk or I got another faulty one.
I'm surprised The Ogre didn't chime in on the thermostat modification. I suspect there's more going on with your cooling issues than what a small hole in the thermostat might rectify, but I didn't think it would hurt to at least try.
[This message has been edited by Patrick (edited 12-27-2022).]
I didn't read all of this thread for several reasons.... Most Often Cliff and most others in recent treads has No Problem w/ temp swing driving in cold weather.
If the engine really is over heating, the coolant system has problems w/ crush pipes, crap plugging the rad, heater, or both, or worse problems. And big temp swings to boil over will keep happening when drive in warmer weather like ambient > ~ 40°F/4°C
Pulling the T-stat Often Proves Nothing when testing for problems in most vehicles. T-stat is a Metered Opening even when Full Open and keeps the Heater Loop Working. If/When have Heater loop w/ a plugging problem, this maybe disables that but still doesn't really say heater is your problem or where the problem is most times... plugged core? plugged/crush pipe(s)? F'd fitting in some areas? Heater Pipes are Aluminum and easy crushed. See my Cave, Heater
The "T-stat mod" does Not do what said the archive post and many others. Because Those had no clue how the coolant system works. So adding adding ~ 1/8" hole or metric equiv doesn't matter.
"But Everyone said most other had this."Wrong. Very Few T-stats for any other engines Do Not have "air bleeder" "Weep holes" and whatever other name for this. Most T-stats main openings are not "gas sealing" and enough trapped air will pass thru them just when filling the system.
If you see anything w/ this "mod" is often because: just band aid for real problems like heater is "plugged." Weather and/or driving changed and system doesn't show temp swings right now.
Even w/ Super stat, the engine temp can swing some when driving in ~ 0°F/-18°C or lower but doesn't swing so widely as OE T-stat types. Again, Fiero has 2 to 3 times the coolant capacity then Front Mounting Engines including many big V8 engines and This Is What this T-stat was design for including Super Stat.
"Air" trapped under the T-stat only happens maybe first time after a flush etc that let a lot of air in the system. Most Engine Setups and When the coolant system works right, will Self Purge All Air Out after 1 or a few heat cycles. Even the Radiator will purge air out to reach 100% wet when overflow and cap seals the system. See my Cave, Coolant Fill (Very Few engine will benefit by pulling a vacuum to fill the system or do other things to "burp" the system.)
Every Time you Open many radiators including Fiero you let air in the system. Bottom of the port neck of Fiero and others spills some coolant formerly filling the top tubes. The system will then repeat the self purging but air will not get in the engine. Air or or worse engine fumes in the engine then has big problems. Get/Have Coolant Exhaust Testing done and to see if test fluid/strips changes color. Because often Exhaust gets in coolant before coolant gets in the cylinder(s) Air can suck into the WP Seal and some other places w/o leaking coolant. Worse when have the Wrong Rad Cap and parked "nose down" even a small grade because Coolant is draining into/thru the overflow tank.
Most Often Cliff and most others in recent treads has No Problem w/ temp swing driving in cold weather.
It actually gets worse in cold weather.
quote
If the engine really is over heating, the coolant system has problems w/ crush pipes, crap plugging the rad, heater, or both, or worse problems.
It is really overheating, but only during the first 10-15 minutes when I drive the car. After that, temperature stabilizes at "normal" values.
quote
The "T-stat mod" does not do what said the archive post and many others. Because Those had no clue how the coolant system works. So adding adding ~ 1/8" hole or metric equiv doesn't matter.
Apparently, it's a mod used on many cars. And it makes sense. Not only to prevent a gas bubble forming under the T-stat, but also to allow some coolant to flow even when the T-stat is closed, causing the temperature difference on both sides of the T-stat to be smaller which in turn will prevent huge temperature swings.
Which is the reason I thought it was worth a try.
quote
Every Time you Open many radiators including Fiero you let air in the system.
When I open the radiator cap, air escapes until after a while coolant comes out. So I'm pretty sure it's not sucking air in.
Let me repeat what happens each time I drive the car.
After a few minutes of driving, temperature rises quickly up to the point where the needle is pegged to the right and the overheating warning light comes on.
It stays this way until after a few more monutes, the temperature drops in just a few seconds to below normal temperatures.
It then again rises quickly, again pegging the needle to the right and the overheating warming light lighting up
This repeats itself for about 15-20 minutes until the temperature drops to "normal" values where it stays for the remainder of the trip. So from that moment on, there are no temperature swings for however long the trip lasts.
Now to me, it's obvious that the reason it overheats, is because coolant is not reaching the T-stat causing it to stay closed for too long. After a while it warms up enough (through hot air, or maybe because the t-stat housing warms up) for it to open. Very cold coolant now flows through the T-stat causing it to cool off and close again after which the cycle starts all over again. But the temperature difference of the coolant on both sides of the T-stat will become less and less with each cycle until the point is reached where it will not close again, and coolant is allowed to flow normally through the coolant system.
This also explains why the problem is worse in cold weather. Because the coolant behind the T-stat cools off significantly faster than in warmer weather. So it takes much longer for the temperature difference to become low enough that the T-stat wil remain open.
Now the question is, why isn't the coolant reaching the T-stat? Not even after I add coolant? Is there a big air bubble trapped somewhere that moves to the T-stat when the engine is running?
I have replaced the water pump two years ago and it's working properly. I tested that by opening the T-stat cap immediately after starting the car in the morning. Which, I might add, was a stupid idea. It was spraying coolant like a geyser. Also, if the water pump wasn't working (properly) engine temps would never stabilize. It would overheat and stay overheated.
Check the coolant temperture with a scan tool or data logging program to verify that the dash gauge is reading correctly. If it is I would have the coolant system pressure tested.
------------------ formerly known as sanderson 1984 Quad 4 1886 SE 2.8L 1988 4.9L Cadillac 1988 3800 Supercharged
When I open the radiator cap, air escapes until after a while coolant comes out. So I'm pretty sure it's not sucking air in.
That's just not normal. The coolant in the radiator should be right up to the cap. That air has to be coming from somewhere. If it's not being sucked in, then I suspect it's being forced in from one or more combustion chambers due to a head gasket issue... but this is beyond my competent knowledge level.
[This message has been edited by Patrick (edited 12-27-2022).]
This smells more and more like a head gasket. I have not done it but there are chemical test kits that will detect the presence of comustion gases ine the coolant system. Google for more info.
------------------ formerly known as sanderson 1984 Quad 4 1886 SE 2.8L 1988 4.9L Cadillac 1988 3800 Supercharged
[This message has been edited by sanderson231 (edited 12-26-2022).]
Cliff, when I had a temp gauge cycling problem similar to yours, I got an inexpensive dial thermometer and used it to measure the temperature directly in the fill neck. I'd stop engine when the gauge pegged high, then pulled off the cap, yanked out the stat, and dipped in the probe, which proved the stat to be grossly inaccurate. I had hesitated to replace this new stat because the resistance readings were correct per the manual. In view of the hard temperature data from the thermometer, I got another stat and the gauge then worked perfectly.
Very Likely You don't have Air in the system but Exhaust. Because Cold Engine can let crap parts leak until whatever heats up. Even when warm likely still leaks but less.
Cylinder Pressure just doing basic pressure tests reaches 150-170psi or 10+bars. Engine runs gets way higher Cylinder Pressure.
Vs. 0-15psi or 1 bar coolant pressure w/ that limit set by rad cap.
"Air" then tries to find the lowest flow area that is the Radiator and why pull the rad cap you get "Air" even when T-stat tube is full. Ounce Exhaust pushes a bit of Coolant, more exhaust just exit thru same tube and O-tank. IOW the coolant system is acting sim to a normal purge cycle but one that never ends.
Iffy Head Gaskets etc Does Not = Milky Oil in a lot of cases and for same reason... cyl pressure vs coolant pressure.
If that test fails Then to see what/where is leaking... Use a coolant pressure tester that's covered above too and bore scope in the spark plug holes to watch for tiny coolant leak(s), 1 or more piston cleaner then others, etc. May get lucky and 1 or more spark plugs are cleaner/different then others. Bore scope may show even w/o the coolant system pressurized if has "clean" piston(s). Or may get lucky just w/ cyl pressure test but don't hold your breath if engine runs fairly well. ⚠️ Warning: If the pressure tester even pumps up to 15-17psi, do not push pass that... If test w/ T-stat adapter and rad cap on you can't. But tester to replace the rad cap then can and blow up the rad or whatever is weaker > 19-20psi.
"T-stat Mod" is 100% crap. Same BS is still pushed for people w/ bent up T-stats when cars really had bad heater plumbing. And the Fiero T-stat like many others can't trap air bubbles under the valve. Valve etc may look tight to you but air finds any tiny place to get out. Often Doesn't even stop all coolant flow around the valve seat because how made and often other small holes were parts meet. The O-ring is there to keep the thing in places and easier to make because tube and T-stat don't need tight tolerance to work. So if "air" seems trapped there then often is Exhaust not air trying to get thru a closed T-stat.
[This message has been edited by theogre (edited 12-27-2022).]
In this thread: https://www.fiero.nl/forum/Forum2/HTML/146435.html the head gasket break shown below resulted in coolant loss that manifested itself in fluctuating gauge temperatures. I think the gauge was reacting to intermittent splashes of overheated coolant and no coolant contact. Bubbles would flow in the coolant reservoir that I think were from combustion chamber pressure. The final evidentiary gasket issue symptom was coolant squirting through the broken exhaust manifold joints, lol! Replacing the head gasket solved the issue; no sensors or gauges were changed. (Also replaced the broken-joint exhaust manifold...)
I start the car in the morning and within a few miles (2-3) temperature goes up to 230F. It then continues to rise to about 250F, stays there for a few minutes, then drops in seconds to 140. Again it stays there for a few minutes, then temp rises to 230F-250F again. This cycle repeats itself for about a half hour, after which the temperature drops to 175F-195F at which it stays for the remainder of the trip.
Sounds to me that there is air in the block that is keeping the thermostat from opening, and when the air finally warms up the thermostat enough it opens, lets cool coolant in and closes again. I drilled an 1/8th inch hole (3mm) in my thermostat quite a while ago to solve this kind of problem. While it won't solve the problem of where the coolant is going, it will solve the problem of the highly fluctuating temperature.
Head gasket can end up like above but Often leaks exhaust coolant or both before complete failure.
Because Often isn't the gasket failed but side effect of iffy head(s) block or head bolts.
Example: Many Dukes had iffy Head Bolts when new and any could fail more so bolt between #2 #3 cyl on exhaust side stretch or break and first sign was head gasket leaked. Why? That "#2" bolt (order in tightening head bolts) gets hotter then rest because in between #2 #3 exhaust ports and weak bolts fail. If caught early, is/was a way to remove the broke bolt and replace that w/o lifting the whole head. Not a good plan for many but even GM push this fix to save Warranty Cost and payout to the Dealers.
Poof? Kit also came w/ long normal drill bit and easy out to get broke section out. From TSB Number: 87-6 (GAS)-94 Date: 7/89 BROKEN CYLINDER HEAD BOLT/COOLANT LOSS Subject: (NEW BOLT/GASKET) 1985-87 6000, FIREBIRD 85-86, GRAND AM AND FIERO MODELS WITH 2.5L ENGINES
[This message has been edited by theogre (edited 12-27-2022).]
Originally posted by Cliff Pennock: Haven't checked the plugs (they haven't been removed for over 20 years, I'm afraid they will break off).
It can happen, but it's also really important to have top-notch plugs. I'd at least check the ones facing the trunk. If those break access is much easier.
I thought I was going to have to pull the head for my #5 plug because it wouldn't move and I thought the plug was loose in the hex. Turned out to be that there WAS NO HEX. It had completely rusted away. Gap was double what it should be. I used a deep bolt extractor and gently worked it back and forth.
Crap. If it's a head gasket that needs replaced, then I'm SOL. There's no way I can do that myself (because of a lack of space to work on the car) so I would need to bring the car to a garage. And that's probably going to cost me more than the car is worth...
I will see if I can find a kit to test for exhaust gases in the coolant.
Crap. If it's a head gasket that needs replaced, then I'm SOL. There's no way I can do that myself (because of a lack of space to work on the car) so I would need to bring the car to a garage. And that's probably going to cost me more than the car is worth...
I will see if I can find a kit to test for exhaust gases in the coolant.
Usually exhaust gases from a bad head gasket will cause the reservoir to spew. At the very least you can do a compression test.
It all sounds like you’ve got complicated issues with slow coolant leaks combining with air pockets in the Fieros gigantic cooling system and the way the thermostat is designed. In my experience with it and other cars air pockets can linger for days/weeks after purging the system. I just replaced a heater core on mine and it took over a week to get decent heat from it. No flow, the hoses going to it just cold with the rest of the system fluctuating.
Usually you can look at the gauge and tell when it’s hitting the thermostat temp and it opening, even that AND the fan cycling on/off. If it’s hitting that thermostat temp and not dropping it’s an air pocket, bad thermostat or some other reason for no flow. My Sonic likes to eat waterpumps every 30k or so.
On my 3800SC swap I got it with the fan programmed obnoxiously low and no thermostat, heater core bad and bypassed. It fluctuates a lot, up to about 210 where my thermostat opens and back down to about 120 with all the cold coolant circulating into the engine and the fan running. The cycles of it fluctuating get less and less extreme the longer you run it as it takes a while for the huge amount of coolant to stabilize between the thermostat opening several times and the fan running etc.
Should NEVER let the reservoir run dry. ALWAYS guarantees an air pocket which on a Fiero is difficult to deal with. I know from owning a turbo Chevy Sonic, which is designed to leak coolant at any given opportunity. On mine you can fix every hose, every possible leak and it’ll still vent a little coolant from the cap/reservoir so there’s always some loss. Never on the ground, on it it’s just enough to see and smell some slight coolant vapor from time to time. In any case on your engine you should find every hose, every connection, know where it all is every know and then feel the connections for wetness. Check the coolant level at the res every time you drive. Don’t necessarily question the level or why it needs more but top it off if it needs it. On my Sonic, they designed it to overheat and warp the aluminum head, so many end up in junkyards. It’s designed to run hot for fuel economy, like 220-240 constantly, has bunch of plastic pieces in the system, hoses that go out frequently and bulge, waterpumps that go out regularly. Reservoir that cracks and leaks every 20-30k. I’ve only kept it on the road by being diligent with it, up to 150k now. It’s not like my Fieros Ive had or my S10, can overheat a few times and keep going. Even once on it and it’s done.
If I were in your shoes, I’d remove the thermostat and drive it for a couple weeks, checking all the connections for leaks, keeping the fluid topped before/after every drive. It appears stable after that then put it back in. Topping off coolant shouldn’t necessarily be a big deal. If it’s a head gasket problem so bad it’d cause these kind of problems you’d likely see it in your exhaust, spark plugs, compression test, etc. If you’ve got lingering air pockets the level will go down all the time until you get them resolved, and you could just get stuck in a cycle of letting the reservoir run dry and creating more.
[This message has been edited by sdgdf (edited 12-29-2022).]
Also to add, pressure test the cooling system with the plugs out, if it drops pressure use a bore scope to see if you’ve got any coolant on top of the pistons….
Btw if it’s a 4cyl doing the hg shouldn’t be completely horrible.
Here's is what I do to get the air out of the system when filling:
1) Remove the fill cap and thermostat 2) Remove the radiator cap 3) Slowly fill the system at the thermostat neck until coolant comes out the radiator 4) Fill reservioir with about 3" coolant 5) Replace the radiator cap 6) Slowly fill the system pausing to let air gurgle out 7) When the system appears full rerplace the fill cap 8) Run the car for about 30 seconds to circulate some coolant 9) Remove fill cap and top off 10) Repeat steps 7 and 8 two more times 11) Take the car on a drive without the thermostat allowing the system to get up to normal temperature 12) Fill reservior to normal level 11) Let car cool. Top off level at fill neck and replace the thermostat
If the car is normal, the coolant temps will vary as the fan cycles on and off but that is a slow process. The coolant temperature should not spike at all.
I also use this method on my 3800 and 4.9 where the fill is a Moroso in-line tee in coolant outlet hose (no Fiero type fill neck). On these cars I have to leave the thermostat in place but there is a small whole drilled in the thermstat so that the engine can vent while filling with the thermostat closed.
------------------ formerly known as sanderson 1984 Quad 4 1886 SE 2.8L 1988 4.9L Cadillac 1988 3800 Supercharged
If you suspect a head gasket failure, you can do a leak down test before you end up taking it into a shop and paying for it. If you are losing that much coolant, I would suspect you would be getting white smoke or milky oil. Leak down test is easy enough to do. It won't be definitive, just a tool to help you out. Compression test wouldn't be a bad idea either.
Just an update, in the past two weeks it has only overheated once. I have no idea why - it just stopped overheating. The single time it happened, I wasn't driving the car differently than normal. It just heated up until the overheating light came on, after which temps dropped to normal within seconds.
It is still loosing coolant somewhere though. But it's just weird the overheating stopped.
I believe I had the same problem as you or at least something very similar. I posted my readings somewhere on this board of some drives. Will post them here if I find them. I have a 2.5L though and the problem is still really unresolved... I believe it's either an intake manifold gasket leak or a head gasket leak.
EDIT: If I remember correctly I actually had to flip the signal, I think this was the proper circuit:
Just another observation: I noticed the other day that, after having driven the car for an hour or so, the radiator cap was still cold. And I mean outside temperature cold. It's currently 38F outside now and that's how cold the radiator cap feels.
So I've been checking this for the past few days and it's always cold, it never heats up.
If I feel the top of the radiator, that's cold too and it feels warmer the lower I check.
Just another observation: I noticed the other day that, after having driven the car for an hour or so, the radiator cap was still cold. And I mean outside temperature cold. It's currently 38F outside now and that's how cold the radiator cap feels.
So I've been checking this for the past few days and it's always cold, it never heats up.
If I feel the top of the radiator, that's cold too and it feels warmer the lower I check.
do you have, or have access to, one of those touchless temperature reading devices? I got one of Wish years ago and it's handy for being able to read surface temperatures easily.
Sounds like you've got some kind of block in the lines.
Here's what I would do - drain the system as best you can. Pop off the thermostat housing cap. Remove the drain from the pipe on the opposite side of the car. Get a hose, put it in the thermostat housing opening, and turn on the water, nice and gentle. Watch to see what happens. If water starts coming out of the drain, block shouldn't be from where you're pouring water, down to where it's coming out.
Or, pull the radiator, and start pouring water down from the outlet, it should come out the inlet easily.
If I feel the top of the radiator, that's cold too and it feels warmer the lower I check.
It's been mentioned at least once before (but I don't believe you addressed it)... are you absolutely positive that neither one of the two stainless steel coolant pipes that run the length of the car has been crushed?
[This message has been edited by Patrick (edited 01-20-2023).]
After thinking about this some more, I urge you to make sure that the coolant system is properly filled per the procedure I posted previously and also do the sniff test to make sure that there are not exhaust gases in the coolant system.
------------------ formerly known as sanderson 1984 Quad 4 1886 SE 2.8L 1988 4.9L Cadillac 1988 3800 Supercharged
I had installed a remanned 3.4, and was doing the "drive it easy to break it in" thing for 500 miles. I suddenly started losing coolant. (I forget what the gauge was doing, but obviously it was doing something strange. Not important to this post.)
It got to the point that I could top off the coolant, warm it up, let it sit in the garage and idle, and it would behave normally. As soon as I drove the car and stomped the gas to accelerate, it would blow massive quantities of coolant out of the overflow.
A compression test showed all cylinders within spec. Then I hit on a method... The hose from my compression tester was threaded the same size as my air tools. I was able to attach an air tool fitting. I removed all the spark plugs, and started with #1 at TDC. I removed the thermostat cap and thermostat, and made sure the cooling system was full. My plan was to plug the compression gauge hose into each cylinder, and then hit all the cylinders, in order, with the air compressor, moving each piston to its compression stroke before connecting. When I connected to cylinder 1, I got lots of bubbles from the thermostat housing. Good enough. Engine had to come out, anyway, and go back to the rebuilder. But at least the mystery was solved. (Ended up being a broken head bolt, that allowed the gasket to blow.) At first, I questioned the wisdom of hitting my cylinders with the full pressure of the compressor, but then I remembered that the compression test produced more PSI than my compressor did.
To do this outside of a garage, you'll either need a portable compressor, or a portable air tank. But it should be do-able. And since yours seems to act up when it's cold, it shouldn't be a problem to test it the same way. (Not that I agree that you have a head gasket issue. Just another way to rule that out, if nothing else works, first.) Good luck.
Note: my problem did NOT get better as the engine got warm. It got worse, as expected.
[This message has been edited by Raydar (edited 01-22-2023).]
What Raydar described is similar to a leakdown test:
That is where I got the idea. My test didn't "measure" leakdown, exactly, but it did tell me that compression was leaking into the water jackets, which is what I had to find out, and already suspected. Once I described what I did, and the results, the vendor (The Fiero Factory) warrantied the repair.
