Cerridius

Hey everyone. I just got my car back and it's running terrible, almost like its running on 5 cylinders, which I assume it is. I guess this has progressively gotten worse, a month ago when I would start the car in the morning it would run extremely rough, but with about 20seconds of holding the throttle at about 2500rpm it would clear up.

But now it's constantly doing it and now it finally threw code 33. I guess because this it's caused my #1,2,4 cylinder plugs to foul. I touched the header primarys on those cylinders and they were cold. I pulled #1 plug and it was completely black.

Anyway. I'm not sure how to continue from here. I changed the MAP sensor and that didn't help, and I tested the wires on it. It's getting 5v like it should. But the signal wire which is suppose to be sending back 1.0-1.5v is reading 3.94-4.10v. I looked and I don't see any vacuum leaks, could the PCM be bad?


Thanks

Cerridius

The car is bouncing from 0-5" of vacuum at 1000rpm. Is the 3 fouled cylinders causing the low vacuum which in turn would cause the MAP sensor code?

Injuneer

Think of MAP as the "opposite" of vacuum. High vacuum = low MAP, low vacuum = high MAP. You can actually calculate the vacuum from scan data. Vacuum = MAP - BAR, where BAR is the barometric pressure.

DTC 33 sets when throttle angle is less than 3% and MAP is greater than 80kPa. When the code sets, the PCM defaults to using throttle position and engine RPM to calculate an estimated MAP value.

It's likely the misfires are causing the high MAP (low vacuum). It appears you put a vacuum guage on it to confirm that there is a real problem and it's not a MAP sensor problem or a PCM problem. 5"Hg vacuum would roughly correspond to 4.0V MAP reading.

Is it possible you have a valve adjustment problem? You've got to track down the source of the misfires.

What did you just get your car back from? What work was done to it?

Cerridius

Just got it back from having the transmission rebuilt. But before I gave it to them the car was slightly doing this in the morning for about 20 seconds. I guess it just happen to finally get to the point where it constantly does it when they had it.

I haven't touched the rockers in a while but it's possible. What would cause the cylinders to foul the plugs? Rockers too tight or too loose, which causes a foul of plugs?

So this slowly got worse over maybe 2 months. And yea I agree about the 3 dead cylinders causing a low vacuum and causing the MAP code to go off. I'll change out the plugs when I get the chance and then test the MAP sensors signal wire again.

Then I'll have to figure out what caused the cylinders to foul?



Edit: makes sense about the 5" of vacuum being about 4v. I got 3.9-4.1v from the MAP signal wire while I had the 5" of vacuum

Injuneer

Plugs foul from running rich.

Cerridius

What's more likely to cause that, rockers too tight or loose?

Injuneer

If you have a leaky exhaust valve, you're going to be blowing air into the exhaust on compression stroke. The O2 sensor sees the air, says "running lean" and the PCM starts adding extra fuel it doesn't actually need.

I can't think of too many things that would cause 3 cylinders, located on both banks of the engine, to run rich, and only toward the front of the engine.

You might want to use the 94/95 OBD-1 free software (sticky thread at the top of this forum) to run a data log. That will tell you if the PCM is cranking up the fuel.

Cerridius

Only thing I can think of is maybe the rockers were just slightly out of spec on those cylinders from when I did them a while ago and just slowly fouled the plugs. Because I also have an occasional intake backfire and I've tried everything with that besides messing with the rockers again.

Cerridius

Thanks for the help Injuneer and everyone else. Found my problem, while it may not be the same for other people who have this code.

Heres how I went about figuring this out, all my plugs were fouled which I thought was the reason why a few cylinders were not firing and why i had only 5"hg of vacuum. So i changed all the plugs but this did nothing so next thing I tried was checking the fuel pressure. Hooked up the gauge and I have the proper pressure, about 43psi. I had already check for spark by getting shocked. So I thought maybe it was the fuel injectors. Ever fuel injector plug was getting 12.2v and the injectors themselves had 16.3ohms. I unplugged a couple while it was running and I had 0 change in how it idled. The couple others I unplugged caused the engine to almost stall. So the next thing i tried was a noid light and that checked out.

Last step was a compression test. I did #1 cylinder since I knew it wasnt firing and the easiest to check, and boom found my problem, 0 compression for Dry and Wet test. I decided to check cylinder #3 to be sure and it only had 120. I didnt bother to check any others since its pretty obvious from here. Just to be sure I pulled to valve cover to be sure the valve spring was intact and It was fine.

So having no compression in 3 cylinders caused my "low vacuum" issue which in turn, set off my Code 33 map sensor DTC.