Hello All,

Newbie to the forum looking for help diagnosing my son's car problems. Unfortunately, this may take a while. It's a 99 Cavalier 5 spd manual. All stock, 112K miles and was well maintained until.....last winter. He got stuck in a snow bank and, not being sure exactly what he was doing, hooked the tow rope to the closest thing he could find under the front of the car. That turned out to be the main engine control wiring harness for the PCM and needless to say it was torn away. Had it towed to a nearby "repair facility" and $521 later it was allegedly cut, soldered and shrink wrapped back into shape.

A week later it started running poorly so we took it back and was told it was a bad MAP sensor and it would be an additional $100. OK, done. It ran well for a while and then it just started going downhill this past summer. It started stalling so we took it to a local place and they couldn't get it to do it. No codes, but a clogged fuel filter needed to be replced. $96 later it's still stalling. It completely shutdown not too long after that. Would not crank. Had it towed to a place where I thought, I could trust a guy. He says the alternator locked up and wouldn't allow the engine to turn over. Odd occurence indeed, but he replaced it ($300) and it ran but still stalled. He took it back in the shop and diagnosed it with yet another bad MAP sensor. He claims the other shop took a shortcut and didn't install theirs correctly. Another $260. I took the car from him and will never return. Replaced the TPS myself. No help.

The car recently died in Ann Arbor so we went out to get it last night and I drove it home. I turned the key and waited 3 seconds and heard the fuel pump so, I assume, that's working. It started but wouldn't idle at all so I had to pepper it at the lights to keep it running. Once on the freeway it seemed to run fine but I noticed the fuel gauge going from 1/2 to full and back (it never really worked right) and the digital odometer flipping out. Right when I was thinking it was running fine it started buckiing like it was running out of gas. and continued to do so the rest of the way home. It would completely power down under load and stall, but restart. It was an interesting ride. I found that if I shook the car violently back and forth while it was running it would clear up for a little while.

Sorry for the novel and thank you if you've actually read this far. It's otherwise a good little car and we don't have enough money to buy him a new one so, I'm determined to fix this problem. There's a check engine light on so, I'm going to O'Reilley Auto to check it. My questions are these:

Does this sound like a clogged or clogging catalytic converter to anyone?

Would the fuel/pump sending unit be going bad as well?

Are there any obvious Cavalier issues I should be checking?

Thanks for any help.

Mike

The fact that the wiring harness was "repaired" is the biggest red flag here. After time, any of those repaired sections of wire could corrode and cause all sorts of crazy things..

If you wanted to fix this yourself, id recommend finding out what codes are stored, and report back here. Chances are they're going to be the same ones that have led all the other mechanics on a wild goose chase, but we'll see.

Thanks Kris,

Agreed. That is always going to be a consideration and I'm not sure what recourse I have.

I limped it up to O'Reilly's and the kid there got 2 codes off it:

P0171 Fuel Trim System Lean and P0107 MAP Sensor. There's no way it's the MAP sensor as it's recently been replaced. The service manual says that low fuel pressure would throw that 1st code. I wonder if it's pointing back to that fuel pump/sending unit. I see a few "How-To's" out on the web for replacing these. Was it an issue with these cars?

Thanks again.

A lean system P0171 can be caused by not enough fuel, too much air, or even misfires. These conditions can actually be CAUSED by your ECM's decisions for fuel trim. Its decisions are made using inputs, such as your MAP.

p0107 is MAP sensor signal low, which is important to include in its description. That means the voltage that the computer is getting (if any) from the sensor is lower than what the ECM considers to be reasonable.

This can be caused by a bad MAP sensor, which is what most shotty mechanics will blame before doing any real tests, OR the wiring between the MAP sensor and the ECM. If it is corroded or shorted, it will affect the voltage at the ecm.



I can take the time to help you through diagnosis, but it involves a multimeter and some common sense. =P

Interesting. Thanks!

I have a multimeter somewhere, can't guarantee the common sense.

Hello again all,

Here’s an update/question on the problem I’m having with my son’s car. After all the causes that I thought it could be I decided to start with dropping the fuel tank and replacing the fuel pump/sender, but before I did I thought I’d try disconnecting the MAP sensor just for grins.

Amazingly, the car ran fine. Started fine, idled fine, ran fine except for the eventual CEL.
So this tells me that all the other stuff I thought it would the causes are fine. The fuel pump pumps, catalytic converter isn’t plugged, etc.

So I decided to swap out the MAP sensor (3rd time’s a charm right?). I got the exact AC/Delco replacement part, removed the throttle body and used a new throttle body gasket when I reassembled. Not sure what the sensor is that I removed. Perhaps the other two “repair facilities” used a generic brand. No part numbers, etc.

Now, the car runs good, sort of. I took it to O’Reilly’s and erased the code. We’ll see if a new one comes up.

The problem I’m having now is that when I pull up to a light or a stop sign, or even slow down to turn, the engine shuts off. It’s pretty frustrating, not to mention unsafe, especially in traffic or on a hill since it’s a manual transmission.

It starts back up immediately and I can drive on so this tells me that something is turning itself off during normal driving and only resets itself when I have to restart the car. I have the factory service manual so I’m going to start looking at the IAC valve. Does anyone have any similar experiences with this going bad?

Thanks again for reading,

Thanks Bob,

How many miles did your car have on it? Wouldn't it seem like the car would run bad all the time if it was the timing chain?

My car had 128,000 miles on it and yes it would seem like it would run bad all the time but it didn't. For 2 days it would die everytime you stopped. Then for a week it would just idle bad. The whole time it would run OK except at idle. That's all I can tell you. Just the slack in the chain was doing it. The valve cover isn't that big a deal to take off and you should be able to see the top of the timing chain guide, if you can't it's broke off. You can turn the cams with a 24mm wrench to see if you get slack, you shouldn't have any, I didn't think I had enough slack to screw things up but after doing the job there was a difference, the chain is tight now. You are probably like me and will try everything else first because you don't want that to be the problem. I changed the plugs ICM Coils and all that hoping it would clear up and finally had to bite the bullet and start taking it apart looking for something else. It wouldn't throw any codes except a 172 and if you cleared it then it would take 50 - 80 miles before it would throw it again. My gas mileage started going way down also.

It's just something to check that could be the problem, maybe it isn't your problem, but it was mine. My car is running better than when I bought it new now.

If you have any other questions just ask.

Ok, I'm probably wrong on the timing chain, my car is an 04 and just saw yours is a 99. Different engines. I would go and look where they did the work to the wiring harness, check the wires and make sure they don't pull apart and make sure none are bare or touching anything including grounding on the frame or something. Just depends on how well the guy did the job, I had to rewire the main harness on a car I had awhile ago because I had a leaky fuel injector and let it go too long until the engine caught fire and burned up the harness. I took my time and soldered everything well and insulated it well and didn't have a problem with it but that doesn't mean the person that did your car took the same amopunt of time because after all it isn't their car. Isn't that how it works ? Like I said if it is shrink wrapped give each wire a tug on each end and make sure they don't come apart, if it is soldered well you shouldn't have a problem pulling on them.

Thanks Bob.