Idle issues
Thread Starter
3rd Gear
Joined: Dec 2011
Posts: 3
Likes: 0
From: Orlando FL
Idle issues
My '86 doesn't hold a steady RPM, it goes from 6 to 7 to 8 and back to 6. Just did tune up on it, and don't want to make it worse, but not sure what to check next. Any help would greatly be apprieciated.
Dave
Dave
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
Wandering idle is almost always a vacuum leak.
You can check the vacuum lines by disconnecting them and plugging the place they came out of (a thumb works). There are two at the rear of the plenum on the passenger side. The brake booster can have a cracked case or a ruptured diaphragm (a small tear can cause rough idle). The vacuum connection for the EGR valve (bottom of throttle body) is ported vacuum and there shouldn't be any vacuum there at idle. The last place to look is the aluminum vacuum tube on the driver's side that goes to the PCV valve and charcoal cannister. My car had a small vacuum leak at the place where it connects to the intake manifold (next to the cold start injector).
Another source of vacuum leaks is a worn throttle body. The butterfly shaft can wear out the holes in the throttle body and leak air.
You can also have vacuum leaks at the gaskets where the runners connect to the plenum and intake manifold. The best way to find leaks like this is to spray the places where it might be leaking with carb cleaner (which will change the idle if there is a leak).
You can check the vacuum lines by disconnecting them and plugging the place they came out of (a thumb works). There are two at the rear of the plenum on the passenger side. The brake booster can have a cracked case or a ruptured diaphragm (a small tear can cause rough idle). The vacuum connection for the EGR valve (bottom of throttle body) is ported vacuum and there shouldn't be any vacuum there at idle. The last place to look is the aluminum vacuum tube on the driver's side that goes to the PCV valve and charcoal cannister. My car had a small vacuum leak at the place where it connects to the intake manifold (next to the cold start injector).
Another source of vacuum leaks is a worn throttle body. The butterfly shaft can wear out the holes in the throttle body and leak air.
You can also have vacuum leaks at the gaskets where the runners connect to the plenum and intake manifold. The best way to find leaks like this is to spray the places where it might be leaking with carb cleaner (which will change the idle if there is a leak).
Last edited by Cliff Harris; Jul 22, 2012 at 04:21 AM.
