Diagram of Vacuum System

[Marccus]

'89 SHO, 410,000 miles


A while back, my idle was hunting, speeding up and then back to normal, etc,

Someone suggested looking at the intake manifold for leaks and I found plenty of them there.

I installed new runner hoses and sealed the intake manifold to the gaskets with silicone sealant and then the gaskets to the block to seal the whole thing.

It's now sealed tighter against leaks than a frog's butt in a lily pond.

All was well, and it idled beautifully.

But now I am getting the same symptoms for the idle, only not as bad.

My question is where are all the possible sources of vacuum leaks, so I can test all of them.

I have the huge binder that I bought when I bought the car for the smog control system but not with me as it gives a complete description.

I read the previous posts on how to find vacuum leaks with carb cleaner or propane (actually I have butane) so I understnd that. The first time around I just sprayed water near the intake manifled to find the leaks in the hoses.

 

[sdpatt]

If it is intermittent as you have stated, it was likely not a vacuum leak, but a failing idle air control (IAC) valve. That unit has the responsibility to control the amont of air bypassing the throttle plate for idle speed control. When the solenoid portion wears, it behaves as you have described. The Wells TV200 is about $60 from AutoZone and is held on to the inboard portion of the intake mainfold split with two Allen screws. Those screws can be accessed with the now-siliconed manifold still in place.

 

[jthod]

http://alfitz.com/sho/index.html

The book is PDF format, and does have a vacuum diagram.

This has about everything you nedd to know about the motor, spec, and parts wise.

 

[Marccus]

If it is intermittent as you have stated, it was likely not a vacuum leak, but a failing idle air control (IAC) valve. That unit has the responsibility to control the amont of air bypassing the throttle plate for idle speed control. When the solenoid portion wears, it behaves as you have described. The Wells TV200 is about $60 from AutoZone and is held on to the inboard portion of the intake mainfold split with two Allen screws. Those screws can be accessed with the now-siliconed manifold still in place.

Thanks. But I've replaced that IAC about three times in the last two years. Twice when I received codes for the IAC malfunctioning. A third time when the idle started acting up, but I didn't realize at the time the vacuum leaks were the problem.

I guess then if it isn't a vacuum leak, there must be a problem with the wiring because I replaced the connector to the IAC solenoid.

Coincindentally, I have a short on the fuse that is on the 15 amp circuit for the internal lighting (doors, roof), the clock and the radio, and ????. I had this a LONG time ago (10 years or so), found the problem, fixed it, and now it's come back.

This short seemed to pop up the same time that the idle starting acting up - coincidence? Perhaps.

I'll trace down that short first and if the idle problem continues I'll try to troubleshoot the IAC wiring.

 

[SHOZ123]

Might be the throttle position sensor too. If there is a bad spot or any abnormal resistance in the circuit it would confuse the PCM.

With the key on and engine off there should be ~5V at one pin, ~.98V at another and a ground. If these numbers are correct then slowly and smoothly open the throttle and watch the meter when set on either ohms or volts. Any jumps in the readings would indicate a bad spot.

If this is good then trace the TPS output wire (lowest voltage one) check the reading at the PCM connector.