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So my car has developed an issue where the rpms will surge up and down while idling in park and drive. the rpms also jump occasionally while driving down the road in overdrive at 2000 rpm. You can also hear it missing in the exhaust when this happens
Its a 89 305 TPI w/ 155k miles. Everything was running great before this issue as it is my daily driver.
a lot of the sensors and injectors plugs and wires were replaced last year when I bought the car.
I tried resetting min idle using the guide but it didn't help.
Any ideas would be appreciated because this one has me stumped. I didn't see anything besides the flickering rpms when I took a datalog w/ tunerpro but i'm no expert. I could attach that if it would help diagnose the issue.
Wow, that’s really bouncing around. Check Engine Light? I had something similar right after I replaced basically My entire ignition system...my buddy and I forgot to put one of the plug wires on the spark plug. Mine would bounce the tach bad with that and could hear it in the exhaust. Another idea is a massive vacuum leak, those can wreak havoc on the TPI
Thanks for the reply. There are no codes from the check engine light unfortunately and I know the bulb isn't burnt out. There's a lot of things in those two lists to troubleshoot.
I think i'm going to start by checking the spark plug wires, fuel pressure, and vacuum leaks when I get home from work today. I can also check the MAF sensor while i'm at it even though it was replaced last year.
The BLMs from my last datalog were around 128 so I don't think its my oxygen sensor. Does that sound like a reasonable conclusion?
That sounds like a great place to start. Seeing as how you replaced injectors, check the torque on the plenum bolts and the throttle body. That caused idle surge in my car and my friends built 04' GTO but not as severe as yours. You mentioned some sensors were swapped out, was the IAC valve one of them? When I replaced my MAF the only one that worked right was the Delphi, the parts store stuff did not work properly. Better yet have you ever taken the TB and IAC off and really gave it a good cleaning? With the miles you have it could be nasty in there. A full deep clean of my TB was the final thing to straighten out my idle.
Yeah the IAC TPS and MAF were all replaced last summer. I also just recently put new bushings on the throttlebody shaft and deep cleaned the passages. So the throttlebody is in good shape. The MAF was a delphi if i'm remembering correctly. I think TTOP350 is probably on the money on this one. Ill be troubleshooting after work today and Ill snug up the plenum bolts as well. Thats a good suggestion
Sounds good, lot of ppl run AC Delco for cap and rotor, I do and no complaints. Heard mixed things on a lot of the other aftermarket stuff. Keep us posted
I got some troubleshooting in tonight. I checked the fuel pressure first. 36 idle, 42 wot. Then I checked the resistance of the spark plugs which were fine. I checked for loose connections but there were none. Then I started to check the plenum bolts like Reddragon suggested and I stumbled across this.
The coil mounting bracket has two bolts that hold it in and it looks like the back one is gone. Looks like these bolts also hold in the intake manifold as well. I'm not sure how i haven't noticed this before to be honest. must've happened recently. I'm not sure if that's connected to the surging issue but it needs to be dealt with I think.
How did the cap and rotor look? Here’s an idea, try spraying down the intake area with TB cleaner in the area you mentioned and around the base. If your idle changes etc you found your problem. When you did the injectors did you re-torque the intake when everything was off? When I did mine they were a bit loose, I followed the how to guide on this site and the author mentioned doing that and thread sealer on intake bolts.
The Cap and rotor looked fine. No corrosion. I didn't re torque the intake manifold bolts when i swapped injectors. I wish i did though. I'm gonna try the TB cleaner technique and then tighten the bolt up and find a replacement for the one that is missing. hopefully it didn't break off.
OK i sprayed tb cleaner around the area that was loose and there was no change. So i tightened everything up to spec.
I wanted to double check the cap so i took a picture of it so i could get a second opinion. What do you guys think of this. the cap is about 9 months old since i replaced it last. all of the terminals look like these ones.
My 89 LB9 with 120,000 miles had a shot timing chain that caused this. It got worse over time. I changed everything until I finally checked the chain. That fixed it.
I thought when the ICM went it usually wont even start or will have to cool down before it will run again? I wonder if the pick up coil/distributor is on its way out?
FWIW, when mine starting going, it would run fine and then it was like you turned the key off for a split second. It did it idling, it did it cruising. And when it did, the tach would jump. Not saying that's what's wrong with yours, but I think it's worth investigating.
Also an auto parts store can typically test those for you as well, easy enough to remove and just get it tested. I actually drive around with a spare in my car because my wife’s uncle who has owned an 88’ L98 IROC since new said it’s the only part that’s ever stranded him and he’s replaced a few in the 120,000 miles or so on his car.
One thing I have noticed is that the jumpy tach doesn’t happen in the morning on the way too work. Then when I go get lunch three four hours later it’s jumping all over and won’t settle down till I park it for the night.
I think I’ll bring the ICM to a parts store to be tested. For the timing chain I’m not really sure how to test that without tearing into the engine which I’d like to avoid since it’s my daily.
If that's the original ICM, I would replace it anyway. I would recommend buying an AC Delco replacement. And make sure to put a nice, smooth, consistent layer of thermal paste where it meets the distributor.
I replaced the ignition control module and i had a warranty on the cap so i got a new one for free. The flickering has stopped and hasn't come back yet so I think its safe to say it is fixed. If it comes back ill update this post but hopefully I wont need to. Thanks for all the help guys.
07-12-2020, 06:59 PM
