L98 with low idle speed!?
Thread Starter
Burning Brakes
Joined: Aug 2000
Posts: 1,026
Likes: 0
From: Gravois Mills, MO
L98 with low idle speed!?
My son's 1990 suddenly developed a low idle condition that we thought was a bad PCV valve. After the condition first appeared, we were checking various things and pulled the PCV valve out and shook it, put it back in, and the idle was restored to ~1000 RPM.
It was fine for a few days, and then we decided we should replace it on general principle. BAD idea! After installing a new one, the low idle speed condition returned (400-450 RPM, barely running). We put the old one back in, and it's still low. Do we have 2 bad ones in our possession? Is there something else wrong that coincidentally crops up when we mess around with the PCV valve? Any other ideas? BTW, no codes are being set....
Also, we have recently (within the last 2 months) replaced plugs, wires, cap & rotor, coil, fuel filter, and tested the injectors.
[Modified by RedWhiteandBlue, 5:01 PM 5/6/2003]
It was fine for a few days, and then we decided we should replace it on general principle. BAD idea! After installing a new one, the low idle speed condition returned (400-450 RPM, barely running). We put the old one back in, and it's still low. Do we have 2 bad ones in our possession? Is there something else wrong that coincidentally crops up when we mess around with the PCV valve? Any other ideas? BTW, no codes are being set....
Also, we have recently (within the last 2 months) replaced plugs, wires, cap & rotor, coil, fuel filter, and tested the injectors.
[Modified by RedWhiteandBlue, 5:01 PM 5/6/2003]
Drifting
Joined: Aug 2001
Posts: 1,550
Likes: 27
From: Minneapolis MN
Re: L98 with low idle speed!? (RedWhiteandBlue)
I would probably remove and clean the throttle-body and IAC, and then re-set your base idle and TPS voltage. You can get a new IAC at Autozone for like $30.
https://www.corvetteforum.com/techti...D=55&TopicID=2
https://www.corvetteforum.com/techti...D=55&TopicID=2
Racer
Joined: Oct 2000
Posts: 460
Likes: 0
From: Lincoln NE
Re: L98 with low idle speed!? (RedWhiteandBlue)
These are the things I would check or do:
1. Remove the throttle body, take off the Idle Air Control valve, and clean everything up. Carbon builds up in the idle air passages, you could have a restriction.
2. Reassemble and reinstall the throttle body. Adjust the minimum idle air setting per the instructions in the Tech Tips section.
3. Check the adjustment of the Throttle Position Sensor.
Let us know how it goes.
1. Remove the throttle body, take off the Idle Air Control valve, and clean everything up. Carbon builds up in the idle air passages, you could have a restriction.
2. Reassemble and reinstall the throttle body. Adjust the minimum idle air setting per the instructions in the Tech Tips section.
3. Check the adjustment of the Throttle Position Sensor.
Let us know how it goes.
Melting Slicks
Joined: Apr 2002
Posts: 2,118
Likes: 0
From: St. Charles, MO Route 66 Corvette Club
Re: L98 with low idle speed!? (RedWhiteandBlue)
How many miles do you have on the O2 sensor? If you have 40k miles or more on it, try replacing this sensor as it can cause idle problems. Try to put in a GM part as I have had better luck with this brand. I also set my TPS at .65 volts (GM recommends .54 volts) and this yeilds a better idle and better throttle response. Let us know what you do to fix the problem!
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Re: L98 with low idle speed!? (RedWhiteandBlue)
Since shaking the PCV was worth something initially, make sure the hose is in good shape and not partially plugged or torn. Verify all vacuum lines connections too, particarly EGR lines at the switch and to the throttle body. You might want to check vacuum too with the gauge hooked up to the manifold PCV port. 450 is the minimum air requirement, so it may be that the IAC is fully extended & not working at all (though usually the ECM will advance timing to raise the idle speed). I'd scan it to determine what the ECM is targeting for idle speed & the # of IAC counts. 1000 rpm is too high for an L98 though - should be closer to 650. If power is down, check timing - doses idle improve if you advance the distributor? If so, verify that the #1 valves are closed and/or that the #1 piston is at TDC when indicated by the timing marks.
