Surging idle
Thread Starter
Intermediate
Joined: Oct 2017
Posts: 41
Likes: 1
Surging idle
I have an 87 coupe that has a wandering idle. It wants to vary between 1100 and drops to 7 or lower, then back up again. Also sometimes stalls when coming to a stop sign. It's a stock L98 with the 4+3 trans. No codes showing....what could it be?
Safety Car
Joined: May 2001
Posts: 4,720
Likes: 1,625
From: Fairfax Virginia
Sounds like you might might have a bad vacuum leak there.
Clean everything around the throttle body really well with some spray carburetor cleaner and let it dry. Then I would verify that your Throttle position sensor and Idle air control are working properly and then move on to the vacuum side of things.
Check the fuel pressure on the rail and verify that your FPR is working properly. There should be no fuel smell coming from the vacuum line that goes to the FPR.
You might want to verify that your EGR valve is there and operating normally. A faulty EGR can cause idle issues as it uses vacuum to operate. The vacuum solenoid for the EGR is connected near the thermostat housing on my 1988 C4. On mine the ends of the hoses were all broken so I snipped off the last inch and reconnected them. The vacuum system is over 30 years old and it is probably time to check all of it. I had another leak on my Cruise Control unit where it leaked slowly.
This might help identify the problem, using the charts in the FSM might be the best way to find resolution to your car's issues.
Good Luck with your car!
Clean everything around the throttle body really well with some spray carburetor cleaner and let it dry. Then I would verify that your Throttle position sensor and Idle air control are working properly and then move on to the vacuum side of things.
Check the fuel pressure on the rail and verify that your FPR is working properly. There should be no fuel smell coming from the vacuum line that goes to the FPR.
You might want to verify that your EGR valve is there and operating normally. A faulty EGR can cause idle issues as it uses vacuum to operate. The vacuum solenoid for the EGR is connected near the thermostat housing on my 1988 C4. On mine the ends of the hoses were all broken so I snipped off the last inch and reconnected them. The vacuum system is over 30 years old and it is probably time to check all of it. I had another leak on my Cruise Control unit where it leaked slowly.
This might help identify the problem, using the charts in the FSM might be the best way to find resolution to your car's issues.
Good Luck with your car!
Team Owner
Joined: Jul 2004
Posts: 28,331
Likes: 3,245
From: Hartford WI
I prefer to get a scanner so I can see what is happening from the ECM point of view. I would double check the timing first. If that is out, all bets are off. I want to see if the CTS is accurate when I verify it with the infrared thermometer. That can change the idle. I don't care what the coolant temp gauge and the tach says since it can be inaccurate. I also want to make sure that the chip isn't altered and the desired idle is wrong for the setup. Checking by IAC counts also tells you may be having an air leak. If you turn the screw out and it still is at 0 counts, I suspect an air leak. Target should be about 20-30 counts.
