Thread: Won't idle; missing above idle if you can get it started

1998 Grand Prix GT; 205.9k miles

I can usually let the car sit all day or overnight (to cool off?), it will then start, and run great for about 15 minutes. After that, it requires quite a few attempts with some accelerator to get it running again (without waiting overnight). It's firing on every key turn. It usually takes a little accelerator to keep it running. TRAC off light comes on at random times while trying to get it started.

Only one current DTC before starting troubleshooting below: P0141. Scanner reported no freeze frame data available.

I can post FSC stored DTCs if necessary. If memory serves one was some sort of O2 low voltage, another was P0300, random misfire, and I can't recall the third at the moment.

What I've done:

• Cleaned out clogged up EGR valve and replaced gasket
• Replaced plug wires
• Pulled each plug wire and ensured spark from each coil terminal
• Cleared DTCs
• Cleaned battery terminals and chassis ground; suspected chassis ground was disappearing causing the engine to die
• Temporarily removed front O2 sensor to create exhaust leak in case catalytic converter was an issue
• Replaced fuel pressure regulator
• Replaced IAC valve due to heavy buildup and showing shorted with DMM
• Removed and cleaned MAF sensor with MAF cleaner
• Removed throttle body and cleaned with carb cleaner
• Looked into upper intake and observed what appeared to be deformed (melted) plastic near the back side of the plenum. Upper intake was replaced at approximately 110k miles.
• Replaced Crankshaft position sensor
• Rescanned at various points with no DTCs being set. Scanner reports no freeze frame data available.
• Checked fuel pressure at various points; pulls down very near 40psi when starting, recovers to 48psi when it starts, and then returns to 43psi while the car idles. This is all within the first 15 minutes of cold start. I' m not seeing any difference in fuel pressure when it won't start.

I'll post more if I recall additional steps that were performed in trying to fix this car.

Thanks in advance for any assistance.
JR

Maybe check the ICM that sits under the coils. I could see that causing a random misfire. Have you tried unplugging the MAF and then trying to start it?

Also, I hope you really didn't clean the TB with carb cleaner. Unless of course it is approved to work w/ the modern TB's as it may eat the coating off the inside.

I had not heard that one.. I did use regular ole carb cleaner from O'reilly.


Update: The car still starts when cold, but it takes a few tries and consistently misses after the crankshaft sensor replacement.

I am headed to O'reilly shortly to have the ICM tested. Pretty sweet machine.

ICM checked out good two times at O'reilly.

Fuel filter was changed in December and then again right after this problem started (a week from yesterday). Both filters were WIX brand.

I took the car off the jackstands tonight (still jacked after crankshaft sensor) and decided to run it while misfiring. Surprisingly it smoothed out after a minute or so. I drove it around and returned home in time for it to die again as before the new crankshaft position sensor. It then started it's routine of firing, but immediately dying. I unplugged the MAF sensor and it started with a little accelerator. It was able to idle while pulsating between approximately 700 rpm to maybe 1500 -1700 rpm. After another minute or so it settled down and idled around 700 rpm. I ended up driving it better than 30 minutes with a couple of dies thrown in, but it at least started back up so I wasn't stuck as before. I returned home, plugged in the MAF sensor let it run 5 to 10 minutes, and then took it for another run. Like clock work in a few minutes it died and would not start and run. Unplugging the MAF sensor on the road allowed it to restart and get home. I'm assuming this means the MAF sensor is bad unless it's reacting to something else in the equation.

Thoughts?

Changed MAF sensor and it runs now. Getting engine light and codes since changing the crankshaft sensor and MAF sensor. One code is the P0141 I had from the beginning. Another is the P0300 (random misfire), the other two were MAF / MAP / vacuum codes. The MAP sensor fell apart when I pulled it out of its socket so I replaced it.