Ok, brief history of the Demon Machine. 01 GA 3.4, 110k miles.


Three months ago, we replaced the Intake Manifold Gasket due to major coolant leakage.
Shortly thereafter, we ran into problems with the engine losing power, spcifically having trouble with acceleration.
That progressed into poor shifting. On the scanner, the computer was commanding upshifts, just not when it was supposed to.
Coolant test showed exhaust gas in the antifreeze, so head gasket got replaced.
Shifting problems continued, feared tranny problem.
Discovered clogged catalytic converter. Replaced converter, shifting patterns returned to normal, car ran great.
A WEEK LATER on a road trip, engine started to drop RPM and backfire. Limped it to dealership where they diagnosed clogged fuel filter (yes, I know this sounds stupid given the backfire). In fact discovered ANOTHER clogged converter. Actually, 'melted' would be more accurate.So here's where I'm at fellas... The dealership are a bunch of morons. They tried to replace the fuel pump and charge me $600 when fuel pressure was within normal range. They also tried to give me the song and dance about pressure but no volume. They also tried to blame the burnt cat oa lean condition caused by the clogged fuel filter. But I've never seen a cat melt from a lean mixture. Suddenly I didn't trust these guys to find their own quarterpanels with both hands and a map. I got a replacement cat, and offered to put it on myself rather than have the hayseeds do it. But rather than burn a third converter during a 2 hour drive home, I busted the innards out of the current one, bolted it back on, drove it home with scanner attached.

During the bolt-on, I discovered that the upstream O2 sensor was unplugged. When and how that happened, I don't know, but it would have thrown a code and I never saw one. Car drove home well, no problems. Scanner showed that the Long Term mixture trend was being reduced by -8, so I believe the engine was running VERY rich, which would explain the burnt converter. By the time we got home, that was down to -1.

The question is 'why'?

Twice yesterday I was able to get it to reproduce the RPM drop and backfire. I need to hook it back up to the scanner and see if it's running rich again, but I'd bet money that it is.

My regular mechanic has suggested Crank Position Sensor, which we could see on the scanner IF we can get it to drop out again while plugged in. I'm already about $2000 into repairing a problem that hasn't been clearly identified yet, and I'm tired of throwing parts at the wrong problem. I need to find the cause and stop patching symptoms.

Ideas, please?

Thanks,
Aaron

Welcome to AF.

Well first off if you were leaking coolant into your exhaust that will fry your Converter. So any residue of the coolant in your pipes could cause other issues if its not cleaned out.

On the odd shifting, check your MAF wires and your IAT.
If either are shorting out, it will make your shifts irratic.

Also if your upstream O2 was disconnectd then that could be why it was running to lean or to rich. And if you cut the catalyst from the coverter, then the down stream O2 would have foul readings , so your PCM was getting all screwed up with the out of spec readings it was receiving.

110k miles, its about time to replace both the O2's if they havent already.

Thier were also some issues with Converters on 01-02's, I think GM even extended the warranty on them to 150,000 miles.

Hope that helps.


ps- find another shop if you dont trust the hayseeds :thumbsup:

Thanks, Xero.

A couple of things that I should point out here. The replacement of the cat seem to have rectified the shifting problem, and when I have free flowing exhaust, there are no problems.

I'm pretty sure the dealership unplugged the upstream O2 since that would have immediately thrown a Check Engine code. I believe they forgot to plug it back in. In any case, the O2 sensor readings are typical and cycling normally, so I'm less inclined to think something is wrong with them. But I agree that age makes it a prudent change regardless.

During the drive back with the cat substrate removed, I expected the downstream O2 to show something wrong, but it didn't. I'm wondering if that's more of a long term code.

In any case, I'm gonna plug up the scanner and see if the Crank Position Sensor zeros out when/if I can get the engine to drop out and backfire. It bugs me because it really feels like the spark just disappeared.

Maybe the ignition module....

It could be, if it's not sparking right it can dump gas into the cat and burn it up. Do you have an aftermarket remote start or secutiry system? There is a TSB about them, basically the relays in them are affected by bumps in the road and momentarily cut spark. Over time it dumps enough raw gas into the cats to make them burn up. Also check your injector PW at idle, it should be under 3mS once it's up to operating temp. If it's very far over that, try an induction flush if there's no apparent problems with anything else.