89 L98 Idle issues
Thread Starter
Intermediate
Joined: Jul 2011
Posts: 30
Likes: 6
From: Naperville IL
89 L98 Idle issues
Hi New to the forum and was looking for a little help I just bought the car (1989) it runs good, good acceleration etc.. but idles rough like it has a miss and when I come to a stop light it idles about 900 to 1000 rpm sometimes it will drop to about 700 or 600 rpm sometimes not. The PO removed the EGR and AIR and had 24LB injectors, so I replaced with 22LB injectors and put in a chip to bypass the EGR and AIR i know EGR/AIR dont affect idle but figured it needed to be done. I have done a search and checked the noted things in the archives vac leaks etc but have had no luck. I did try to set base idle but when I follow the instructions found on this forum when I start the car after unpluging the IAC and timing connector and removed the jumper the car idles at about 850 to 900 rpm I cant get the idle down to 425-450 so I can not set the TPS. Anyone have any ideas.
Thanks
Thanks
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
You need to make sure the IAC is completely extended and closing off the air bypass passage. Sometimes you have to turn on the key, unplug the IAC, turn off the key, plug the IAC back in, turn on the key, unplug the IAC. You didn't mention it, but I assume you've got pins A & B shorted on the ALDL connector while doing all this.
Then you disconnect the ignition bypass connector by the windshield wiper motor. This eliminates the advance added by the ECM. You set the timing to 6 degrees. Leave the bypass connector disconnected.
After all of the above, you turn the screw on the throttle body to get 450 RPM. If you can't get down that low then there is an air leak somewhere that's allowing extra air to get past the throttle body.
The prime candidates for air leaks are worn bushings on the throttle shaft, broken vacuum lines or connectors and bad plenum or manifold gaskets.
Then you disconnect the ignition bypass connector by the windshield wiper motor. This eliminates the advance added by the ECM. You set the timing to 6 degrees. Leave the bypass connector disconnected.
After all of the above, you turn the screw on the throttle body to get 450 RPM. If you can't get down that low then there is an air leak somewhere that's allowing extra air to get past the throttle body.
The prime candidates for air leaks are worn bushings on the throttle shaft, broken vacuum lines or connectors and bad plenum or manifold gaskets.
Le Mans Master
Joined: Feb 2005
Posts: 6,477
Likes: 565
From: Clearwater Florida
.......... Sounds like a vacuum leak ... I suspect the brake booster ........... Follow the instructions in the Factory Service Manual for setting the minimum idle ... they are very specific , and very important to get it right .............
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
You need to unplug the IAC while the ignition is on.
When you turn off the ignition, the ECM sets the IAC to the park position. It presets the IAC so that it's in the correct position when you start the engine the next time. You don't want that to happen. That's why you unplug the IAC while the ignition is still on.
When you turn off the ignition, the ECM sets the IAC to the park position. It presets the IAC so that it's in the correct position when you start the engine the next time. You don't want that to happen. That's why you unplug the IAC while the ignition is still on.
Thread Starter
Intermediate
Joined: Jul 2011
Posts: 30
Likes: 6
From: Naperville IL
I do have A&B grounded and did remove IAC plug while the ignition is on, I am going to try the turning ignition on and off a few times to be sure it is fully extended. C-409 If the brake booster is bad wouldn’t I be able to hear a vacuum leak? I will let you know what I find
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
When the diaphragm in my brake booster ruptured I didn't hear anything unusual. It just got REALLY HARD to apply the brakes.
When mine broke about 15 years ago I bought a rebuild kit from GM. It was about $70 back then, so if it's still available it's probably at least twice that much. The studs that mount it to the firewall are REALLY hard to get to, up near the brake pedal. I also had trouble getting the booster apart because the 10mm nuts are deep inside holes that are very small in diameter. I had to buy a 1/4" drive deep socket.
When mine broke about 15 years ago I bought a rebuild kit from GM. It was about $70 back then, so if it's still available it's probably at least twice that much. The studs that mount it to the firewall are REALLY hard to get to, up near the brake pedal. I also had trouble getting the booster apart because the 10mm nuts are deep inside holes that are very small in diameter. I had to buy a 1/4" drive deep socket.
Thread Starter
Intermediate
Joined: Jul 2011
Posts: 30
Likes: 6
From: Naperville IL
SES will not flash
I started over again setting base idle and figured out why the IAC will not close, the problem is the SES light does not flash I though it was the first time but I guess not, so I went through chart A-2 from the FSM and it shows replace ECM and if that does not fix it replace mem-Cal.
My question is on the FSM Chart A-2 number 3 says to back probe ECM CKT 451 with a test light to ground do they mean using a normal test light grounded and touching the back of the CKT 451 at the ECM connector which is what I did and the test light went on but no change to the SES light which means the ECM is bad, just want to be sure I did the test correctly befor I buy an ECM.
Any one else have any experience with this issue or have any other ideas
Thanks
My question is on the FSM Chart A-2 number 3 says to back probe ECM CKT 451 with a test light to ground do they mean using a normal test light grounded and touching the back of the CKT 451 at the ECM connector which is what I did and the test light went on but no change to the SES light which means the ECM is bad, just want to be sure I did the test correctly befor I buy an ECM.
Any one else have any experience with this issue or have any other ideas
Thanks
