Fellas,
I have a stock 1985 SE 5 sp. Iron Duke. When I start the car in the morning, or if I restart the car after an hour, the oil pressure reads 40 psi or slightly better. After a normal 18 miles drive my oil pressure needle continues to drop. After 30 (+) miles, the needle finally settles down to a reading around 20lbs. Not good🤔. When I finally have to stop at a red light (or at traffic) my oil has heated up, and consequently my oil pressure needle touches the “red zone”. The red light DOES come on. At idle, I have to slightly add more more gas throttle to lift the needle off of this “red zone”. When I am having fun at an off ramp, or at a twisty stretch of road, the needle will again go down😡. I try to run 20-50 oil. My Duke seems to like this oil better. However, after many more miles, my oil pressure needle will read only read 27 lbs.(max)! It will drop again into to “red zone” if I’m in a no-go traffic condition.
My first thought is my trouble is in the rod journals. Do I have too much rod bearing clearance? Or, perhaps I need a particular oil, and or stabilizer, to keep the engine out of the “red zone”. Do I need a whole new engine? I hope not.
Your thoughts?
Kevin

quote Originally posted by kevin: Do I have too much rod bearing clearance? Or, perhaps I need a particular oil, and or stabilizer, to keep the engine out of the “red zone”.

The duke isn't exactly a quiet engine at the best of times... but do you hear any suspiciously loud rattling or knocking?

I run 20w-50 oil in my '84... along with a can of "old-school" STP. I'm sure Ogre will come along and call it snake oil , but this engine continues to run strong... for a duke! I autocrossed it for several years as well, so it's been driven hard.

[This message has been edited by Patrick (edited 04-16-2019).]

Patrick,
Thank you for responding. No, the Duke produces no mysterious sounds. That is good news. I’ll have to supply it with 20-50 and a Lucas type oil enhancer. Does anybody else have some thoughts?
kevin

It seems pretty clear your pressure is dropping when the oil heats up and thins out. There is probably an engine rebuild in your future.
If you are already running 20w-50 then that rebuild needs to happen soon.

I'd be a little skittish about running 20w-50 in the Duke unless you are in Arizona. Stock recommendation is 5w30. I'd go up to 5w40 without losing much sleep to cure an issue like yours but no thicker.
Personally I've switched to 0w- oil in the winter as I've read on BobistheOilGuy that even 5w is really too thick to adequately lubricate things on a cold start.

quote Originally posted by edfiero: Personally I've switched to 0w- oil in the winter as I've read on BobistheOilGuy that even 5w is really too thick to adequately lubricate things on a cold start.

I'm definitely no expert, but I suspect the thickness of oil required depends a lot on the engine's manufactured tolerances (in addition to subsequent wear). What works for a modern-day "tight" engine may not work as well for an ancient technology rattle-trap like the duke.

[This message has been edited by Patrick (edited 04-16-2019).]

Have you verified oil pressure with a known good mechanical gage?
If not, do so.
If you have, try straight 30w and a quart of Lucas.

Low temp flowability isn't an issue in the warm months, if your oil pressure when the engine is warmed up returns to normal (15 @ idle, 10 psi for every 1000 rpm) you'll be good to go and know that you've got some bearing wear.

quote Originally posted by olejoedad: Have you verified oil pressure with a known good mechanical gage?

Yes because Dash gauges often lie even w/ a new sender.

Car should have 5W-30 for most of U.S., 4 - 4.5 quarts depend on filter used and 36-41 PSIG
You might get away w/ 20W-50 some area where never get Freezing weather < 32°F (0°C)

Using heavy oil and additives are fixes for low pressure and often hide problems that will destroy an engine.

See my Cave, Oil and Filter

If you really have low pressure then might have a Iffy pump or pickup.
Pickup tubes often get damage when things hit the oil pan or replace the pump.
But can be other things like worn cam and/or crank bearings. (V6 used in Fiero and others are well known for this problem.)
Also a "plug" behind cam gear that seals an oil passage maybe loose or someone drill a new gear lube hole and too big. (See my Cave, Cam Gears )
There is nothing in the head that have pump oil but drains from valve train can be block and hold a lot of oil in the top faking low oil when running. If dip stick is wrong or pickup damage too then can be far worse.


------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

[This message has been edited by theogre (edited 04-16-2019).]

Your oil pressure might be fine. The pressure always drops as the oil heats up, and reacts strongly to engine RPM. You don't typically notice that in modern cars because the gauges lie to make it look impossibly stable. If you hook a mechanical gauge up to them, you'll see the same fluctuations.
The Fiero gives an honest reading directly from the sender without any "massaging". But while the gauge is honest, it's accuracy is still questionable.

First thing before you get too worried - hook up a mechanical gauge and see what oil pressure you really have. It will be more accurate than the dashboard reading.
Do not just replace the sender. They are chronically inaccurate so that won't prove anything. A mechanical gauge is the best way to really know what you have.

With the gauge connected, let the engine warm up and recreate the low dash readings that are worrying you. See what the mechanical gauge says in those conditions.

The typical rule of thumb is 10psi per 1000rpm is okay. Although I question whether that can be expected at higher revs - I think it's more of a curve than a straight line.
I don't know if there's an official spec for the 2.5L, but over at 60degreev6.com they list this for the V6s:
10psi @500rpm, 30-55psi @ 2000rpm
http://www.60degreev6.com/v...gen-1-specifications
I don't know what their source was for that. And again that's for the V6 - maybe somebody has specs for the 4cyl.

If the mechanical gauge disagrees with the dash, then you either have a bad sender, or a bad gauge/wiring. To some extent it will always be inaccurate but given a choice, it's better to have accuracy at the low end of the scale (where the reading matters the most).

==
Below info is for the V6 - the 4cyl could have differences-

You can test the dash gauge and wiring by plugging known resistance values in place of the sender. On the V6s, it's basically 1ohm = 1psi. 80ohms reads about 80psi. You can twist a few resistors together in parallel to make the resistance you want to test, plug it into the harness connector, and you should see a matching reading on the dash. Note that the current flow is high enough that you probably will need multiple resistors in parallel. If you don't spread the load across enough resistors then they will get hot, but for a quick test,you can probably get away with it.
That's for the V6 - I don't know if the 4cyl gauge works the same, I'm just guessing that it would. But the resistance scale might be totally different, I don't know.

If the gauge itself is working correctly, and a mechanical gauge disagrees with the sender, then consider replacing the sender.
After past experience, I would only use ACDelco oil pressure senders.
On my V6, I found that the original 1986 GM and 2 modern 1988-style ACDelco senders were more accurate in the low end of the scale than any of 3 1988-style BWD knockoffs were. After going through 3 BWD senders, I finally gave up on them and went back to using the Delco. Comparison with a mechanical gauge confirmed that the Delco was more accurate, and the BWDs were *all* falsely making me think my oil pressure at hot idle was much lower than it really was.

The data above and in my cave is from GM FSM
Oil Pressure most vehicles, Fiero included, is valve regulated often on the pump itself and Should Not move much outside of range specified at all RPM.

Low pressure even above the "red zone" is Not Good. Low pressure, 10 - 30 PSI, will accelerate wear for everything because less oil actually gets to parts.

Many are told use High Volume oil pump...
High volume pump at same pressure w/o moding oil map for the rest of engine is useless and just tries to dump extra volume via the regulator. Problem most Regulators in this case are pegged wide open and system sees High Pressure. Many filters can't handle High Volume pumps and filter media can fail then you hope the Filter Bypass works to get enough oil to the rest of engine. Fiero l4 and V6 Filter Bypass is in the engine and very small.
High pressure is not good either and above 60 - 80 PSI can "blow out" or otherwise make most filters fail. "Blow outs" can blow the can of course but more often warp the base or otherwise make the seal to leak like hell. Oil senders often hate high pressure too. More so w/ old type that 85-87 Fiero came w/.

The bypass only keeps oil pressure from exceeding a safe maximum - on the V6 it's limited to 80psi, don't know if the limit is the same on the 4cyl.
This is only really in play when the oil is cold. Once the oil warms up, the pressure should no longer be high enough for the bypass to be a factor, except maybe at high revs. There's nothing to keep it in a particular range other than being below that max. As such, it's normal for the pressure to change significantly with temperature and RPM.
Low pressure isn't good, but I think people tend to exaggerate how much is normal or necessary, especially at a hot idle. ~20psi @ 900rpm idle when fully heated is nothing to worry about. <10psi, go ahead and worry, but make sure it's an accurate reading first.

[This message has been edited by armos (edited 04-17-2019).]

Why would you want to run 20W50 in an engine that was designed for 5w30? The only reason I can think of is you artificially want to raise the oil pressure because the bearings are worm out. I rebuilt the Iron Duke in my '86 last year and with fresh bearings the pressure never drops below 40psi. I can see running 10W30, but 20W50 is really pushing it unless you live somewhere its always hot. I don't drive my car in the winter, but it still gets chilly in the spring and fall here in Detroit so 20W50 would be difficult for that thing to pump. I have a couple older diesel Mercedes-Benz and those took 15W40.

If you question your actual oil pressure check it with a pressure gauge. While the engine is off remove the oil pressure sending unit and threat in gauge that takes a direct reading (not like the factory transducer type). You'll get a precise reading and know for sure if you have low oil pressure.