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98 Malibu idle issues, ses light

8296 Views 17 Replies 2 Participants Last post by  Starship
I have a 98 Malibu LS 3.1V6, the car will not idle when in park or nuetral. The rpms consistently jump from 500rpm to 1100rpm back and forth and will sometimes completely stall out.

Also, only when the car is cool, it is somewhat hard to start. It only wants to start if I tap the throttle. Then, once started it runs very rough, like a severe misfire. But once I rev the motor up, it clears out. It does not do this if the car is warmed up.

If the car is in gear, it will idle just fine. And the car seems to run and drive normal going down the road. No actual misfires.

The check engine light is on. It is showing codes P0300 (multiple misfire) and P0401 (EGR flow insufficient). The plugs and wires look good, I have switched out the coil pack with a known good one, and put a new EGR valve on. No changes.

I dunno what to do next, I am suspecting the catalytic converter. Seems to have a foul smell from the exhaust, and a puff of blue smoke at cold startup leads me to believe possible worn valve seals may have caused failure of the cat.

What you guys think
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The PCM expects the engine RPM to drop when it enables the EGR flow. P0401 probably just means it didn't see enough drop or even a rise in RPM. That's not surprising when the air-fuel ratio is constantly fluctuating (sounds like it). You either have a vacuum leak or something that simulates one. Have to tried unhooking the MAF sensor? Also, does the ECT sensor reading in the scan tool look reasonable?
Okay, that kind of indicates that the MAF sensor is out of calibration which can cause the kind of wild swings you're seeing. In the absence of the MAF input, the PCM goes by predefined parameters and thus things settled down. I'd try a new MAF sensor and check the air filter.
Sorry to hear it wasn't it. Still, it wouldn't hurt to try some MAF cleaner spray on the old MAF sensor. Also, if you don't have some, get a can of TB cleaner to spray into gaps and mating surfaces while looking for leaks. A decent OBD scanner is probably good to add to the toolbox.
The fuel pressure should be at least 41-47 psi. Going even lower with engine on is not good. You may want to pull the vacuum hose from the fuel pressure regulator and make sure there's no fuel leak.
No, I should be the one thanking you. I was sure you had an air/vacuum leak (which affects the engine idle the most).:eek: I'm sure a lot of folks will benefit from your experience.
Oh, man, that's brutal (just when I was feeling good).:( P0401 is tough to figure out since these EGR valves aren't vacuum-controlled. The PCM monitors the pintle valve via the feedback circuit. You may want to trace the EGR circuit wires on the following PCM wiring diagrams:

Check them for continuity and make sure there's no significant resistance. If they check out, maybe there's blockage in the EGR passages.
As far as ABS, look up the DTCs in the following charts:

And, see the EBCM wiring diagram:

It looks to me like a lousy electrical connection problem. As a first step, try reseating the connectors at the EBCM.
It's inside the driver side fender. Raise it and then remove the splash shield. Be careful with locking tabs and sliders. They tend to get brittle over time.
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