My dad and I have been scratching our heads over this one for a while now... I have and 86GT with a 2.8L. When driving it seems to intermittently lose vacuum. The only thing it seems to be connected to is when the car is in closed loop (warmed up). We've checked all the vacuum line and components. They are all good. It is very odd because the vacuum actually goes to zero when driving and it's been getting worse. (Don't worry, I only take it out for test drives now to prevent further damage) I've checked/ tested or replaced the ICM, TPS, MAP sensor, egr valve, egr vacuum solenoid assembly, air charge sensor, fuel filter, oxygen sensor, and cap and rotor (not all for this problem just listing some stuff for a quicker response from people . ) two interesting, likely related things... The O2 sensor had a chunk missing out of it, the distributor cap had its center pin out of the middle of it on the inside and looked like it had been arcing. None of this solved the power/vacuum loss problem. I'm wondering... Could this be a plugged/melted Cat backing up the engine badly? It only has about 30kmiles on the cat.

Any ideas anyone? Hopefully you guys can save me some time, money, and heartache

Ummm... I've got to admit, I don't know what you mean. "Complete vacuum loss..."???

Does this mean you can't clean your carpets? Seriously though, you've lost me.

The engine uses vacuum to run... If I hook a vacuum gauge up to the engine and drive a little ways, the engine will intermittently lose all vacuum.

quote Originally posted by LaFierte: The engine uses vacuum to run...

I understand where the vacuum comes from and why it's required. I don't see how an engine can run with NO vacuum.

Very few of us drive with a vacuum gauge attached to our engines. What are the symptoms of low/no vacuum that you're experiencing?

[This message has been edited by Patrick (edited 04-09-2015).]

I don't understand it either haha, that's why I'm on here. I attached the vacuum gauge and a fuel pressure gauge after having problems with intermittent power loss while driving in closed loop (I test drove it with the gauges attached, so did my dad. We took turns watching them since they were sticking out from where the decklid vents normally are). Fuel pressure was fine, nothing wrong with that system, I've sense checked everything just to be sure. But the vacuum gauge would go to 0 to 4 psi during these power losses. Really has me scratching my head.

[This message has been edited by LaFierte (edited 04-09-2015).]

Where do you have the vacuum gauge connected?

Sounds to me like you have the vacuum gauge connected to a port that has controlled vacuum... like something to do with the gas tank venting or EGR system or whatever. I'm just guessing here.

There's no way that the intake plenum itself or the throttle body sees no vacuum with a running engine.

[This message has been edited by Patrick (edited 04-09-2015).]

On a tee, directly off the upper intake plenum. And I did try a second gauge just to make sure the first wasn't off. It read the same. Maybe my car is possessed? This no vacuum (or extremely little on some occasions) doesn't kill the engine, but it only lasts for a few seconds at a time, and of course I lose all horsepower then. Maybe the fact that the transmission is engaged keeps it running? I was thinking a melted cat because that would explain when it only happens when I'm driving it, and the vacuum loss too if it was backing up the exhaust.

quote Originally posted by LaFierte: I was thinking a melted cat because that would explain when it only happens when I'm driving it, and the vacuum loss too if it was backing up the exhaust.

I had a plugged cat once on my '86 GT, but I didn't experience anything like what you're reporting. I found that I couldn't get the engine to rev over 2000 RPM or so.

I have since found out that a quick way to check for a plugged cat is to temporarily remove the o2 sensor. If the extreme back pressure is causing an issue, then removing the o2 sensor allows the pressure to escape. I haven't tried this myself. I'd be kind of afraid of where the hot exhaust gases would be aimed.

Sticking valve could cause a temporary vacuum drop.

Bad ECU grounds (intermittent, such as a loose but) will also cause symptoms like you mention.

Vacuum is measured in Inches of Mercury (inHg), not PSI.....

Patrick, I may have to look into that idea. Just one question on it, since this only seems to happen under load in closed loop would taking the oxygen sensor out through off the computer enough to cause problems while on a test drive?

And valve? Are you talking head valve? Or EGR? I replaced the egr valve with a brand new one, it didn't change the symptoms.

I've checked the gounds, they all seem good, not corroded at the connections, tight, and passed a continuity test. And yes vacuum can be measured in mm Hg, it can also be measured in PSI, Pascals, etc. All three of the ones at home here read in pounds. It just came down to what was available. From what I have read most of the people that have tested vacuum on here have done it in pounds, and have gotten around 15-19 pounds on average (from my reading on trying to solve this problem). I normally run at 17, but it drops to between 0 and 4 pounds under the conditions previously mentioned.

The only way this could happen would be a very large intermittent vacuum leak, which sounds crazy......but possibly your brake booster hose is loose or cut, or the internals of the booster are damaged.

[This message has been edited by Gall757 (edited 04-09-2015).]

Just in case you didn't realize, when you push the throttle your vacuum will go near zero since are essentially creating a vacuum leak. That aside I would plug each component running from vacuum and see which is the problem. My guess is since it affects driveability so drastically, either brake booster as previously mentioned or you may have an IAC, idle air control, issue.

IAC has been checked. Swapped and reswapped back in when it didn't make a difference. Throttle position does not seem to matter. In fact it usually happens when I have my foot holding down the gas at a constant rate, not moving it now that you mention it. As far as the brake booster... My braking power is excellent. Can it still be the brake booster? I suppose I could plug the hose to it and test drive it carefully. Thank you for the ideas guys.

When your vacuum drops do you pick-up an engine miss? Also any popping?

Yes I do. After a period of power loss and vacuum loss it pops load a few times and goes back to normal. Timing is on.

I would have to say you have an intake valve sticking.

......valuable additional info......

What would fixing/checking the intake valves entail?

quote Originally posted by LaFierte: What would fixing/checking the intake valves entail?

Check everything under the valve covers first, if your lucky you'll find something there. If not it will mean pulling the heads.

This usually happens because an engine is running hot, or with low oil, and carbon deposits build up. Many shops have treatments that they can run through your intake....stuff like Seafoam, etc. and it could fix the problem. If not, you would need to take the valve covers off and look for something......maybe a weak/broken valve spring or one valve that is discolored.

Ok, so Before I go tearing into the heads and investigating for a sticking intake valve... I'm confused.... Since I'm only having the problem when it is hot and the car is under load like previously mentioned, wouldn't the intake valve idea also happen when the car isn't under load at least? (And when the car is cool as well?) I can still look, but I'm just confused on how that would work.

How to test for a plugged cat, with what you already have, a vac gauge

Hook up the vac gauge, note the vac at idle, prob around 16-19, now hold the throttle to like 2500ish RPMs and hold it there, the gauge will drop for a sec to about 10ish <-number doesn't matter, now what should happen is the vac should go back to the idle vac noted above, if the vac starts to drop off (move towards zero) you have a plugged cat.

quote Originally posted by Patrick: Very few of us drive with a vacuum gauge attached to our engines.

I do.

quote Originally posted by Patrick: I don't see how an engine can run with NO vacuum. There's no way that the intake plenum itself or the throttle body sees no vacuum with a running engine.

Yes it IS possible, I do it all the time....Can you figure out how?

quote Originally posted by sardonyx247: Yes it IS possible, I do it all the time....Can you figure out how?

I suppose you're referring to a "boosted" engine?

quote Originally posted by Patrick: I suppose you're referring to a "boosted" engine?

Also besides boost at wide open throttle you are at zero vac.


And if you want to get real technical, vacuum doesn't exist, just the lack of atmospheric pressure.

quote Originally posted by sardonyx247: Also besides boost at wide open throttle you are at zero vac.

And that's a whole lot of help in this thread... unless unknown to LaFierte he has a boosted engine.

quote Originally posted by sardonyx247: ...vacuum doesn't exist, just the lack of atmospheric pressure.

Hunger doesn't exist, just the lack of food.

[This message has been edited by Patrick (edited 04-10-2015).]

Sticking EGR valve. Remove the Valve and check for carbon buildup on it.

quote Originally posted by LaFierte: Ok, so Before I go tearing into the heads and investigating for a sticking intake valve... I'm confused.... Since I'm only having the problem when it is hot and the car is under load like previously mentioned, wouldn't the intake valve idea also happen when the car isn't under load at least? (And when the car is cool as well?) I can still look, but I'm just confused on how that would work.

Metal expands when it heats up changing clearances, so a hot engine could tighten up a valve. They also run hotter under a load.

Well I think the cake might go to sardonyx247 for this one. It didn't show too much out of the ordinary at 2500, but at around 3000 it acted up pretty terribly. (I decided to try it a little higher since my car has an 84 transmission with 4:10 gears. The RPMs run a bit higher than a normal 86-88 gt while driving.) This would make sense. If it really is the problem--- I'm staying reserved since I've thought it had been resolved before--- I'm guessing that the distributor cab and rotor failed, was about to run through arcing, which explains the missing... That back firing likely was what burnt out the O2 sensor as mentioned. And back firing enough to burn out a O2 sensor could for sure melt the cat.

I'll try to drop the exhaust this weekend and get it fixed. I'll post the results for you guys, and if anyone else has a problem like this in the future. Thanks guys!

Bit late to the game but does it smell of rotten eggs? That's a good sign the cat is clogged.
Mine got to the point the car would not run at all.
This is what he cat looked like when I opened it up:





Usual cause is running rich which causes too much raw gas to the cat. The cat goes into overdrive doing it's thing and gets so hot, it melts.
and of course, often the cause of running rich is a vacuum leak. That's what had happened on mine.

------------------
Anything I might say is probably worth what you paid for it, so treat it accordingly!

Dave

www.ltlfrari.com

quote Originally posted by LaFierte: That back firing likely was what burnt out the O2 sensor as mentioned. And back firing enough to burn out a O2 sensor could for sure melt the cat.

So the back firing is the issue and the failures in the exhaust system are the result. Perhaps it's another sticking valve?

Wow that is one ugly looking cat.

Gal, I'm going to drop the exhaust first since that last vacuum test points to it, if that isn't the problem then I'll be looking at the valves. As I mentioned the distributor cap was bad. The center pin on the inside was actually missing, so it was just arcing to run. It would back fire due to it. I'm guessing that the cat was just a victim of the original case. But I won't know for sure until I try.

Edit: my cat doesn't smell like rotten eggs, but was running a little rich before I replaced the cap and rotor.

[This message has been edited by LaFierte (edited 04-10-2015).]

Before getting to deep into this matter you may want to do a compression test on the cylinders cold & hot.

ps: This should be done BEFORE pulling the heads,...?/@#....................."just in case".

Spoon

------------------
"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

You could also screw a compression tester in where the o2 sensor bung is and measure what the exhaust is up to...,before you drop trou..

So, compression test read good across the board. I dropped the exhaust, half the cat was in the muffler, melted. The other half was still where it should be, but lightly melted. I removed and replaced. The car has more power now, but still has that same old power/ vacuum loss when it has warmed up. It seems to only be when I am holding the gas constant, without any movement. As soon as I let off or step down on it I get vacuum back (although not all of it). Interestingly, if I do not try to correct it.... it'll lose power up until right around 1000 RPMs and the correct itself. Any new ideas?

[This message has been edited by LaFierte (edited 04-12-2015).]

The charcoal canister purge valve?

Well I've checked everything else it seems... Any types on testing it?

Unplug and cap it off. You'll have to unplug the tank vent that terminates at the purge canister as well, so you don't build vacuum in the fuel tank.

Canister is good. Interesting, after changing out the ICM and the coil, and still having the problem... I found out the timing was off... But even with it timed right it is still running rich. I'll have to look into it more this weekend. In the meantime... And more ideas?