2000 Pontiac Sunfire: stumbling, black smoke

Last week I tried posting this to alt.autos.gm and alt.autos.pontiac.

The responses were not really very revealing, so I'm re-posting it here below, in the hopes some tech in this group might have some ideas.

--------------------------------------------

A guy I know has the above mentioned vehicle with the 2.4L four (the one with two tailpipes and two mufflers) and a manual transmission. He's got about 100K miles on it. There are no modifications to the car or its wiring.

He knows next to nothing about cars, and I'm not familiar with GM vehicles, so I'm hoping for some knowledgeable advice from the two groups I've crossposted to.

When this car is cold, it emits black smoke, enough to discolor the bottom of the bumper. Plus there is a stumble or hesitation when you press the gas pedal. His MIL is often on, and sometimes it even flashes, which I understand to mean that there is an emissions problem that is severe enough to damage the catalytic converter. There is no hesitation or visible smoke when the engine has warmed up.

He has taken the car to an independent garage. We do not know what specific DTC(s) were pulled from the engine's computer. We do know that there was at least one code for a misfire; that's all he was told. In an attempt to cure the misfire, the independent garage replaced some or all of the coil packs (is that what this car has?). This did not help at all.

My acquaintance tells me the problem seems to be worse in damp weather, and that there is the occasional cold start where there is no hesitation.

I did some Googling in the hope that this may be a common problem, but it appears not to be. Any ideas?

Reply to
Tegger
Loading thread data ...

Bad pressure regulator. Or a bad injector dumping extra fuel. Pull the vacuum line to the brake booster and take a sniff. If you smell raw gas then the regulator likely failed.

Reply to
Steve W.

Did they do a fuel pressure test:

and also leave the fuel pressure gauge on for a few hours. That will tell them if there is a fuel pressure leak down.

My guess is that there is a leaking injector or more, that fills the intake runner with fuel after a few hours. Which makes the car run like the cylinders with the leaking injectors are flooded.

I hope this helps.

Reply to
Refinish King

This is the quad 4? The coils and icm are integal to that foolish cover. Makes it tough to do ignition diagnostics. I've seen a lot of them with no fuel pressure port. Makes it tough to do fuel diagnostics. The pcm is in the right front fenderwell were it gets covered with road crap and makes it tough to backpin for circuit testing. I've seen these engines jump cam timing on the exhaust cam only, where it wont show on a scope of the ckp and cmp. All in all, a real POS.

Reply to
Steve Austin

Steve Austin wrote in news:47555acc$0$4999$ snipped-for-privacy@roadrunner.com:

I don't know.

Doesn't sound good at all.

I told this guy to take it to the dealer, hoping the dealer might have more intimate experience with this model and any problems its systems may have.

I'll pass this along, Thanks for the reply.

Reply to
Tegger

"Steve W." wrote in news:fj1fd0$lf8$ snipped-for-privacy@aioe.org:

Thanks for the tips. I'll pass them on to my acquaintance.

Reply to
Tegger

"Refinish King" wrote in news:eT55j.7330$gs.2412@trndny08:

So do I. Thanks.

Reply to
Tegger

Tegger wrote in news:Xns99FC6356D7059tegger@207.14.116.130:

An update: The car came back from the dealership last night, and I have more information.

The engine is indeed the 2.4L Quad-4. It has about 100,000 miles on it.

The dealership did this:

1) Scanned for codes, P0301 and P0303 stored. 2) Fuel injector "flush and balance". 3) Throttle body clean. 4) New fuel filter. 5) Replaced spark plug boots and plugs. Apparently the old boots were cracked and there was carbon tracking down the (two-week old) plugs.

They did not check the fuel pressure.

Taxes in, the cost was $470.

My acquaintance reports that the car appeared to behave well at first, but at one point this morning it stumbled like before (but this time when full-hot) and the Check Engine light came on again, even flashing at one point. After the original service by the independent garage the car also appeared to behave properly at first.

I took the car myself for a 10-minute test drive this AM. The car had been sitting for 45 minutes in 15F weather. The engine felt fine. No stumbling or bogging.

I have advised him to get the code read at a parts chain.

Reply to
Tegger

Tegger wrote in news:Xns99FD83BA36B12tegger@207.14.116.130:

In case anyone's still following a thread this old...I have a final upate on the stumbling/black smoke problem, which has now been solved.

Since the MIL came back on again, the dealership took the car back for more troubleshooting. The tech ended up driving the car for a few days as though it were his own, which turned out to be an excellent idea. This enabled him to see the problem in action.

The culprit? The ignition module. Apparently this puppy was intermittently failing to operate the coil, leading to a misfire and the related DTC. The malfunctions were sometimes severe, sometimes not, explaining the erratic symptoms.

The ignition module was replaced last week, and the problems and DTC's have entirely disappeared.

Thanks for everyones' help here.

I'm wondering why a faulty ignition module would not itself have set an error code. Don't GM's engine computers monitor igniter output?

Reply to
Tegger

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.