85 won't idle
Thread Starter
Racer
Joined: Oct 2004
Posts: 278
Likes: 23
From: Rock Hill, SC [RCG] KANUCK
85 won't idle
I know this has probibly been answered before somewhere on here but I am at my wits end. My 85 will not idle stable unless it is running at least 850rpm. If I try and set the idle to 550 in drive, it will jump anywhere from about 450 to 700 and the motor will want to die.
Have taken off the throddle body and cleaned it, have also installed a new iac motor and none of this has had any effect on it. And yes, I am using the factory procedure to set base idle.
Have checked compression, it is good, have replaced spark plugs and wires also.
Also have replaced the injectors.
Anyone out there have any suggestions as to what I can do to make it run right.
Have taken off the throddle body and cleaned it, have also installed a new iac motor and none of this has had any effect on it. And yes, I am using the factory procedure to set base idle.
Have checked compression, it is good, have replaced spark plugs and wires also.
Also have replaced the injectors.
Anyone out there have any suggestions as to what I can do to make it run right.
Pro
Joined: Mar 1999
Posts: 678
Likes: 0
From: Oslo, Norway
Hi.
I have an 85 4+3, but basically it is the same with respect to the idle. The only difference is the rpms to be set at min. idle air adjustment.There are some more things it could be.
1. Check for vacuum leakage. That will make the engine to hunt. Check all the vacumm plastic lines under the plenum. They are very brittle on the 85 if not changed. Check also the lines to the active coal purge. An easy test is to spray some start gas over the manifold and places where leakage is to be suspected. If there is a leakage the engine will increase the rpms immediately.
2. Check the EGR valve. If the vacuum membrane is faulty it will not operate. Apply vacuum and see if it can hold the vacuum for about 20 secs. If not the vaklve must be changed.
3. Check the PCV valve for proper operation.
I had both vacuum leakage and the EGR valve was faulty so I changed it. Aftyer this the idle were ok. Be aware of readjusting the Throttle Positioning Sensor after adjusting the min idle air.
I have an 85 4+3, but basically it is the same with respect to the idle. The only difference is the rpms to be set at min. idle air adjustment.There are some more things it could be.
1. Check for vacuum leakage. That will make the engine to hunt. Check all the vacumm plastic lines under the plenum. They are very brittle on the 85 if not changed. Check also the lines to the active coal purge. An easy test is to spray some start gas over the manifold and places where leakage is to be suspected. If there is a leakage the engine will increase the rpms immediately.
2. Check the EGR valve. If the vacuum membrane is faulty it will not operate. Apply vacuum and see if it can hold the vacuum for about 20 secs. If not the vaklve must be changed.
3. Check the PCV valve for proper operation.
I had both vacuum leakage and the EGR valve was faulty so I changed it. Aftyer this the idle were ok. Be aware of readjusting the Throttle Positioning Sensor after adjusting the min idle air.
Pro
Joined: Oct 2002
Posts: 586
Likes: 0
From: Schmuckville SA
Probably a vacuum leak..
Your intake manifold gasket could be the issue here..
Any changes made recently?? Did it run ok before?
Is this a problem you have run into after winter hibernation?
Could be old fuel maybe....
Your intake manifold gasket could be the issue here..
Any changes made recently?? Did it run ok before?
Is this a problem you have run into after winter hibernation?
Could be old fuel maybe....
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
It is likely you have a vacuum leak. The intake gaskets on L98s are notorious. I assume the IAC passages were well cleaned when the TB was off. The "idle screw" is used to set the minimum air adjustment and not the idle speed. The idle speed is burned into the chip and is maintained by the ECM regulating the IAC.
The EGR is NOT supposed to be open at idle, therefore if it is inoperative for any reason, it will not affect the idle. If it IS opening at idle for some reason, that will cause idle problems. It becomes a vacuum leak.
RACE ON!!!
Originally Posted by jan-erik
2. Check the EGR valve. If the vacuum membrane is faulty it will not operate. Apply vacuum and see if it can hold the vacuum for about 20 secs. If not the vaklve must be changed.
RACE ON!!!
Thread Starter
Racer
Joined: Oct 2004
Posts: 278
Likes: 23
From: Rock Hill, SC [RCG] KANUCK
Hey guys, thanks for all the help. just to let you know, I bought this car a little while ago and it has been doing this since I bought it.
I thought it would be no problem to fix,lol, boy was I wrong. Have checked the egr valve to make sure it is working. Also, it seems to me that it is running rich at idle, you can smell it.
Was wondering, could it posibibly be a problem with the cold start valve.
About the only thing I have not done is pull the intake off to see what the gaskets looks like, guess maybe that will be my next step. Is that a real pain to do or is it something that be could be done over a weekend.
I thought it would be no problem to fix,lol, boy was I wrong. Have checked the egr valve to make sure it is working. Also, it seems to me that it is running rich at idle, you can smell it.
Was wondering, could it posibibly be a problem with the cold start valve.
About the only thing I have not done is pull the intake off to see what the gaskets looks like, guess maybe that will be my next step. Is that a real pain to do or is it something that be could be done over a weekend.
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