Thread: Intermittent Idle Surge/Rough Idle

The car in question is 99 Grand Prix GTP.

Half the time it will crank up and idle just fine, other times it will crank up, idle fine for 30 seconds and then idle surge for another 30 seconds or so. The surging is anywhere from 200-500 RPMs. Last night it did something completely new though. I drove the car for around an hour and this time it didn't idle surge until near the end of my ride when I came to a stop before getting home. It continued to idle surge until I got home and turned it off.

A few things I've noticed when it is idle surging:
The vacuum gauge reads around 11 in/hg (down from 15-16 in/hg)(It will go back to normal when surging stops)
The car seems to be sluggish when pushing on the gas
The surging is normally 200-500 RPMs
Sometimes the idle will raise to 1100 or so but continue to surge

Things I have tried so far to resolve it:
Junkyard MAF
Cleaning the TB and MAF

IAC is a year old so I haven't tried a new one.
Also, no codes are being thrown

sounds leak a vac leak to me.

Originally Posted by Scottydoggs

sounds leak a vac leak to me.

Even though it's intermittent? Some days the car will crank and run just fine.

Originally Posted by Sven The Moparman

Possibly a sticking egr valve if you're still running one.

I still have one but I'm pretty sure the code for when it's malfunctioning has been deleted. If the code is deleted does that disable EGR? If that doesn't disable it, can I disable it by simply unplugging the EGR valve?

spray the lines injectors, egr tubes, tb gasket blower and lim gaskets with carb cleaner, if the idle jumps up you found the leak. move the lines while spraying them too if nothing shows at first.

Check for vacuum leaks and did not locate any.

So I decided to check a few other things. The fuel pressure was at 42psi and went up to about 50psi after disconnecting the fuel pressure regulator. IAC typically goes over 200 a few times when the issue is occurring but goes down to around 60 when it finally smooths out. Short term fuel trim was at -14% while the issue was occurring but drops to 0% when the issue goes away. O2 sensor was reading .9V when the issue was happening and .6V when it wasn't.

All that being said, it appears it is running rich when the idle issue is occurring.

Any other ideas of what it could be?

Well the car finally decided to throw some codes. It is throwing P0134, and P1133. Both of the codes are pending faults and not current faults though.



Is it possible for a failing O2 sensor to cause surging?

pending is as good as a cel on.

check fuse #21 marked ING. if its good you need a new o2 sensor.

Awesome. I'll check it out, thank you

Fuse was good so I replaced both O2 sensors and that solved the issue. The front one was definitely bad judging by the two codes i had.