[FiremanDave]

I have a 1997 Malibu with a 3.1L engine. I just replaced the intake manifold gasket, and now after the engine heats up, it will idle rough while the car is in drive at a stop light or sign. If I slip the transmission into neutral, it runs just fine. There is only one code, but it's for the catalytic converter, which is scheduled to be replaced within a week. It almost feels like there's a detonation when I'm idling at a stop. Any suggestions?

I work for a gas station with an attached garage, and they could only suggest that the torque converter clutch or associated solenoid could be the issue.

Thanks

 

[Starship]

Not knowing the history or the shape it's in, it's hard to tell. What they suggested, is it after they looked it over? What kind of diagnostic tests have you done so far?

 

[FiremanDave]

It has ~176,000 miles, plugs, wires, coil packs, ignition control module and intake manifold gasket replaced 2,000 miles ago. No, they didn't inspect the vehicle, I simply described the symptoms as I have here. I checked the compression after changing the intake manifold gasket and they were all in tolerance. The plugs are Autolite platinums, 8mm silicone wires. The only DTC is P0420, "Catalyst System Efficiency Below Threshold (Bank 1)". Other than that, the car runs like a champ.

Beyond 12,000 miles ago, I don't know what kind of abuse the car may have suffered, I purchased it used.

 

[chevyguy8893]

The catalytic converter could be the cause of the rough idle if is becoming clogged. P0420 code is basically just replace the converter, which you are already doing. I would wait till after the cat is put in and then see how it runs and go from there.

 

[FiremanDave]

Fair enough. Thanks for the suggestion. The only reason I was asking is it doesn't run rough at all unless it's in drive or reverse and the car is stopped. I would think a cat getting plugged would cause hesitation throughout the acceleration curve. But IANAM.

 

[chevyguy8893]

It would normally run bad throughout, but always expect the unexpected. It was just a suggestion to fix one thing before trying to diagnose another.

 

[Starship]

I concur on concentrating on one thing at a time. Still, it wouldn't hurt to check the level and conditon of ATF. Also, check the fuel pressure (key-on-engine-off vs. idle) and injectors, and maybe hook up a OBD scanner and take snapshots of operational data in the problem state vs. in idle/neutral. The recent cold weather may be amplifying errors in some temperature sensor.

 

[FiremanDave]

chevyguy8893: I understand completely. The question was more for my own clarification than anything.

Starship: I'll do that the next time I'm at work. My OBDII scanner needs an update to view freeze frame data from my car and I lost my cable. >_< I'll check the ATF today and have the mechanics check my fuel pressure when I go back to work. Thanks for the suggestions.