onceover

A little bit of a background first,

Ever since I've owned the car it has idled at 1100rpm. It has always bugged me a little, but never enough to try to track down the root cause.

When I bought it it had a failed FPR and needed a little bit of maintenance, so I did the following:
FPR
Fuel filter
Cleaned AFM
New O2 sensor
Cleaned ISV

While all of this did help improve the way the car ran it had no affect on the idle.

Recently I realized that if I pressed very lightly on the gas pedal the idle would drop to 850rpm. Turns out I was pressing it enough to disengage the idle switch and not open the throttle plate.

I repeated this by disconnecting the idle switch at the connector. Sure enough, rock solid 850rpm idle.

If I unplug the ISV the idle starts to hunt around which leads me to believe that its functioning properly.

Any ideas?

Rocket Rob

Have you inspected your throttle cable installation? With the engine off, can you see if the throttle cable is slightly slack and that the throttle is pressing the idle switch? If not, you need to correct it so that it does.

Recently, my cab was in an accident and the body shop removed my engine to repair. When I picked up the car, the idle was off. It took me a while to find the problem. I found a vacuum hose that was off and the throttle cable sleeve was not fully inserted into the support bracket. As a result the cable was pulling the throttle open just slightly, enough to keep the idle closed switch open. It took me 3-4 hours to correct both but now the idle is rock steady as it was before.

onceover

Yep the throttle closes all the way and engages the idle switch. This is what allowed me to figure out that disengaging the idle switch drops the idle to where it should be

Rocket Rob

I would then look for vacuum leaks. But I think its odd that the problem would go away when you disconnect the idle switch. Do you have a stock DME chip?

onceover

Any common places to search for vacuum leaks? Would an DME adaptation potentially help?

I'd be curious if someone with a proper idle unplugged their idle switch to see what happens, anyone?

Not sure on the DME chip, I'll have to pull it out and check

Rocket Rob

Have you had any recent engine work done?

In my case, the body shop missed a vacuum line on the left side on the interface tree behind the heater booster blower. It was a 6mm ID hose.

-nick

Definitely check to see if you have an aftermarket chip. The fact that it lowers with the idle switch unplugged, sounds suspiciously like someone raised the idle in the chip.