94 Grand Prix SE 3.1 V6 127k backfires barely runs

[fritcr]

I have a 94 SE 3.1 and it will "hump up", backfire through the intake. It just throws a fit. Sometimes I can get it started but it will just barely pull itself, then after a minute or so will backfire and die. It sounds like it is missing really bad when it does run. I have checked compression and it's good. Took valve covers off and all is fine with the rockers. New plugs and wires and triple checked the firing order. Different known good coil pak and module off running car. 2 crank sensors and 2 cam sensors. Pressure tested coolant system for leaks and head gasket/intake gasket problems with nothing. I'm at a loss...I'm out of ideas and so is my mechanic. Oh yeah and he also tested the injectors too. I bought it like this and thought it was something simple like a bad coil pak. I was WRONG! It looks like someone has been "messing" with the ecm too. My mechanic don't think an ecm would do this. It is like the engine is WAY out of time. If you unplug the coil paks (so it can't fire) then the engine turns over smooth like a normal 3.1 sounds. Plug that puppy in and it has a FIT!
Anyone have any ideas for me or him to check. He has been messing with it for almost 2 months and I would like to get it back and get it running.
Rob

[tblake]

Still havn't got it fixed, huh?

What is the Partnumber on the PCM (or ECM rather)?

I know Rich mentioned a PCM stock ECM part number that is very troublesome.

What are your compression readings approximate?

[richtazz]

what's your key-on/engine off fuel pressure readings?

with 127k, you could also have a stretched/jumped timing chain. I would pull the valve covers and make sure the valve actuation is in time with the crank position as you roll the engine over by hand.

Finally, since the ECM controls ignition timing, fuel injector on-time and many other parameters, a bad one can cause all sorts of issues.

[lefty1120]

Plugged Cat??

Neighbors '01 Grand Am was doing this...punched out the guts of the Cat......fired right up and ran.

[fritcr]

Ok thanks for all of the ideas, I'll try to answer what I know.

I don't think he has checked the cat. I'll have him check that or take the exhaust off BUT I don't think it would be that because it humps up turning over etc. but if you unplug the fire to it then it turns over fine. I would think that would not make a difference if it was the cat.

I'll have to drive over and get the part number off the PCM and get that for you. It is at his shop. Also, I'm not sure he has checked the fuel pressure but will ask him. I also would not think that would make it backfire etc. The jumped/streched timing chain makes the most sense but he said he took the valve covers off and checked that. Last I talked to him he was going to pull the timing cover off and check it but have not heard anything.

He checked the compression and said it was good on all cyl but I didn't ask what it was.

I'm leaning toward the PCM or the timing chain. I'll try to get that PCM number for you.

[tblake]

Fuel injector resistance?

[doctorhrdware]

I had an old GMC pickup many many moons ago. The timing had jumped, it was a real bear to get it started. It would sputter and cough through the carburetor. It was also running much warmer then it would normally. Talk about a Pia to replace the timing and gear. Had to drop the oil pan in order to get the timing chain cover off.

[fritcr]

Tblake the mechanic said he was going to "check" the injectors. I assume he was talking about the resistance but i'm not sure. I will try to give him a call later and see what all he has done. Give him these ideas too.
Timing chain jumped would sure makes sense with all of this.

[tblake]

Yeah Jumped Timing does sound like a possibility which is why I wonder what your compression readings are. About 160-180psi after 6 huffs of air would be about right. (if my memory serves my right)

[richtazz]

160-180 is maybe a little high for a motor with 127k on it, 135-150 would be more realistic, but you were in the ball park Tim.

[tblake]

Thanks for the clarification Rich.

If he has a motor with the timing chain off, what would he expect as a reading after 6 huffs of air? Maybe 90-100 max?

I know I've worked on OHC motors that have skipped time on one cameshaft. One van inparticular, it still ran, but stumbled at initial accelleration. Once you got the RPM's up it was fine. Bring it into the shop, on the back cam find 160, 155, 160, and on the front, 140, 140, 130..... I knew right away what was up.

[fritcr]

I called him this morning and he is a 2 man shop and was very busy today. I actually called to get him to inspect my chevy truck. He worked it in but when I asked about the old gp he said he had gotten started pulling the timing cover off and then got interupted. He had to go so I didn't get to ask more about compression readings and such. I do know he said that the compression was "good" and even on all cylinders. That's why he/we didn't think it has jumped time but now he is back to checking that next.
Rob

[richtazz]

A compression test won't always finger a jumped chain, as long as both valves are closed when the piston reaches near TDC. Compression will still read in acceptable range, even though the actual phasing of the cam and crank are off so far that it causes backfiring.

[fritcr]

Ok, well that's very interesting Rich. Now more than ever I'm thinking that is probalby what is wrong with it then. BUT would it still turn over fine and sound fine etc. with the spark unpluged?

[richtazz]

It will turn over fine with the spark disconnected even if the timing is way off. The only time this is not true is in an interference design engine where the valves will hit the piston if it's out of time (which your engine technically is not). This is because there is no force from the ignited mixture pushing against the piston as it is on it's way up, or making it's way past the still slightly open valve (which is what is causing the backfiring) because it's firing way early or way late.