L98 Engine Idle
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Instructor
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From: Miramar Florida
L98 Engine Idle
Could anyone give me suggestions on what to check? I have an 86 L98 engine that idling a little fun. The engine starts fine and runs however, when idling it constantly adjusts RPM. It idles high then idles low, then high then low. I recently replaced the head gaskets & have done a complete tune-up. While doing the tune-up, I did notice a little play on the bracket that the rotor mounts to. Is that little bit of play normal?
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From: Summerland B.C. Canada
Look for a vac leak and set minimum idle and TPS idle voltage.
http://www.lbfun.com/warehouse/tech_...ch%20Paper.pdf
Thread Starter
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From: Miramar Florida
Ok, I checked & adjusted the IAC & TPS. This did improve the idling but it is still not smooth. I started checking for vacuum leaks but didn't find anything. What I did find was that if I unplugged either one of the vacuum lines at the back left side of the plenum, it seemed to idle better. Any ideas?
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From: Miramar Florida
OK, Today I replaced the distributor, when I removed the old one from the engine, I found that it had a little play at the drive gear, the new one had no play. I re-timed the engine to 6*. There seemed to be some improvement however it still wanted to constantly adjust the idle speed - between 6 & 8 RPM. I adjusted the idle screw slightly about 1/4 to 1/2 turn and I got it to be stable at 700 RPM. It idled a little high in park (900 RPM) but ran smoothly. Is this OK? Any other suggestions.
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From: Overland Park Kansas
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Raising idle to cure hunting is probably an indication of vacuum leak IMO. Have you rechecked all vacuum lines since the head gasket to make sure you haven't missed something.
Since a bigger vacuum leak changed the symptom, I propose it made the IAC add more steps -- effectually the same as when you raised the idle.
Keep looking.
Should be OK to drive the car until you find the problem.
Since a bigger vacuum leak changed the symptom, I propose it made the IAC add more steps -- effectually the same as when you raised the idle.
Keep looking.
Should be OK to drive the car until you find the problem.
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From: Anaheim CA
Play in the distributor is normal. When you wiggle the rotor you're seeing lash between the distributor gear and the cam gear. When you take the distributor out there is some end play between the distributor gear and the distributor housing. This is also normal. Back in the "old days" the hot rodders put shims in there to reduce the end play. This is a waste of time and money, as the cam pushes the distributor gear up against the bottom of the distributor housing when the engine is running, effectively reducing the end play to zero.
Wandering idle speed is the classic symptom of a vacuum leak. If all your vacuum hoses and fittings are OK, it might be a worn throttle shaft, which sucks in air. Try pushing your throttle shaft (where the various cables attach) up, down, forward and back and see if that affects the idle speed. Another way to test for this is to spray carb cleaner in that area.
Another possibility is a dirty or contaminated O2 sensor, but that will only show up when the car is in Closed Loop mode, i.e. warmed up.
Wandering idle speed is the classic symptom of a vacuum leak. If all your vacuum hoses and fittings are OK, it might be a worn throttle shaft, which sucks in air. Try pushing your throttle shaft (where the various cables attach) up, down, forward and back and see if that affects the idle speed. Another way to test for this is to spray carb cleaner in that area.
Another possibility is a dirty or contaminated O2 sensor, but that will only show up when the car is in Closed Loop mode, i.e. warmed up.
Last edited by Cliff Harris; Nov 7, 2011 at 02:07 AM. Reason: Added carb cleaner suggestion.
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From: Miramar Florida
Thanks. I will continue looking for a vacuum leak.
Cliff, with the distributor, the play was between the gear and the pin that hold it on, looked like the hole for the pin was waddled out a little bit.
Is ther any way to identify the vacuum leak easily, I didn't see any loose hoses or anything like that.
Cliff, with the distributor, the play was between the gear and the pin that hold it on, looked like the hole for the pin was waddled out a little bit.
Is ther any way to identify the vacuum leak easily, I didn't see any loose hoses or anything like that.
Team Owner
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From: Summerland B.C. Canada
Did you ever set the minimum idle, followed by resetting the TPS ?
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From: Miramar Florida
Agent 86- yes I did set the minimum idle followed by the TPS prior to making the second adjustment. I will try to set the TPS again with the current minimum idle setting and see if that helps. Thanks for the suggestion.
Slingshot
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From: Charlotte NC
Remove the ICV, clean it and the throttle body thoroughly and reinstall (torque to specs). Reset the TPS once again. If you are still running the original ICV, consider replacing it. Let us know the results. 
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From: Miramar Florida
OK, I got the new ICV installed today and went back through the IAC & TPS setup. The new ICV didn't seem to make a difference. I ended up re-adjusting the minimum idle screw a little as I had done before and then went back & adjusted the TPS again. It is now idling around 700rpm in park & 600rpm in drive. I tried looking for vacuum leaks again also, tried spraying starting fluid around TB to see if any leaks in that area also. Found nothing. What else can I do?
