I've got a 1989 pickup 22rE. My idle speed seems correct while the engine is sitting with the parking brake on. The slightest touch to my brake pedal, and my RPM's drop... Then surge up again, drop, surge and repeat that process until I release the brake pedal. Does anybody have any idea what this may caused by?

 

Yeah this is super common. Your idle is just set too high, and when you touch the brake, it sends a signal to lower RPM's since your foot is on the brake. If you have your idle set too high, the two work against eachother.

So lower your idle to somewhere in the 750 to 950 RPM range. Its 850 for a 4x4.

 

Alright. Now, how else do I adjust my idle other than by using the idle adjustment screw on the throttle body? Because the screw is already very low and it is still idling pretty high. I do have a 4x4 just to clarify, but I have the most basic model. I do not have an RPM gauge.

 

If the idle adjustment screw is turned all the way in, that usually signifies that the Idle Air Control Valve is not operating correctly. This is usually because your coolant system has air in it. Make sure you burp the system with the heater on, and as I learned last week "massage" the upper radiator hose of all the bubbles.

The IAC valve likes to trap air in it, and throw your idle up high. Since there is air in it, and not getting the hot coolant, it thinks the engine is cold and kicks up the idle.

If your absolutely positive you have NO air in the system, burp it anyways. If that doesn't solve the problem (it most likely will) then your IAC might be clogged with gunk, or its something else. Maybe TPS, but doubtful.

 

I already burped the system, actually by letting my engine run with the radiator cap removed and letting all bubbles and air pockets surface and then let it run an extra few minutes after seeing the last set of bubbles.

I actually just cleaned out my throttle body the other day. Could some of the runoff of what was inside and on my throttle flap have gotten inside the IAC valve?

 

Usually, but not always, the IAC will get stuck and will cause the idle to drop too low...not high, but you never know.

I would really try burping the system again. Jack the front of the truck up to make the radiator cap area the highest in the system. The most common problem is not burping the system all the way and the bubble stuck in the IAC can be a pain to get free.

Yes, cleaning the throttle body, if really gummed up could have effected the IAC below, and cleaning the IAC is a good idea if you have the TB off the truck.

Since its easier to jack the front of the truck up and burp the system, I'd try that one more time. If it doesn't help, pull the IAC off.

Also, double check your throttle positioning sensor, and it is in spec. Also check your Engine coolant temperature sensor and that too is in check.

 

When my IAC went bad it caused the idle to repeatedly go up then cut out. If your idle screw is all the way in it would be my first suspect.

 

Is it surging with the A/C on? Mine was doing that until I adjusted the white plastic screw.