High idle in 88 chev pickup

[judyjackse]

I need to know how to set the idle on my 1988 Chev 1/2 ton pickup. There is no idle screw and I understand that the idle is vacuum controlled. But I do not know where it is.

[Wrenchbender Ret]

Usually when one of those idle fast it is caused by a bad gasket under the throttle body leaking vacuum.

[68Gold/white]

There is an adjustment screw. It is seldom needed to be adjusted. There is a small freeze plug on the front driver side of the throttle body. The plug has to be removed to gain access.

But, I'd check the IAC motor first, make sure the tip is not all carboned up, if so, clean up the tip and the seat in the throttle body itself. AND the throttle body bas gasket can be a problem w/ a vacuum leak, especially if it'd got some miles on it...............

[judyjackse]

Your talking to a novice as far as these newer engines are concerned--if it was a model A I could fix it but I have no idea what an AIC motor is concerned or where it is. I will check the throttle body and intake manifold for leaks .

[hugger6933]

Welcome The IAC is the plug with four wires at the back right side and has a big nut on it. The best thing to do is to take off the throttle body to clean it out underneath. There is a lot of carbon buildup underneath the TBI and on top pf that the mounting gasket probably has a leak .I owned over a hundred TBI trucks and although it has been a while I remember their quirks. Take off the TBI I have seen several times the clogging so intense under the TBI that the whole PVC system be rendered inop because of carbon plugging. I once had it so bad on one truck an ice pick wouldn't clear out the carbon and I used a drill bit[all be it on a hand drill] to "drill" out the carbon. Get a couple cans of carb cleaner and spray it around the base of the TBI unit. Thiswill tell you if you have a bad base gasket the engine will change idle speed once the spray is directed in the general area of the vacuum leak.
Then, allow the truck to cool then take off the TBI unit and get to cleaning. See there are vacuum passages underneath the TBI unit that is pretty darn important to the way things go on that motor. The EGR has has one and so does the PVC valve. Like said I've had them so stopped up that the would soon punch out the vital gaskets from internal pressure build up. Jim

[Ol Blue K20]

My 89 Suburban developed a high idle and it was a bad egr valve

[judyjackse]

Again I need to know what is the TBI and EGR and PVC. Like I said if it was a Model A I could fix it with a par of pliers. Thanks for your info and please excuse my ignorance .

[special-K]

Welcome to the site. Whoops! You posted an 88-98 question in the 67-72 forum. There's an 88-98 forum for questions related to those years

[hugger6933]

Ok, back to basics here I will try to help you into this. Take off the air cleaner[breather] and the part under that , well it is the TBI witch stands for Throttle Body Injection. The valve cover on the passenger side[Stage left looking from the front] is where you will find the PVC[ witch stands for positive crankcase ventilation valve] and it has a little hose that runs to the center lower part of the TBI. Ok, the PVC valve helps the engine "breathe" inside and helps remove the pressure inside the engine from all the things turning and going around inside the crankcase. Without it the3engine would build up a good amount of pressure blowing out ALL of the gaskets and eventually oil from the motor. It is really needed. The last on the list is EGR [witch stands for Exhaust Gas Recirculation valve.] Its function it to take unburnt exhaust [with unburnt fuel]and run it back through into the motor again to give it another chance to burn thereby reducing emissions and pollution.Now after taking off the air cleaner , and spraying the carb cleaner around the base looking for a leak like I spoke of in the above post and you do find the vacuum leak, you need to remove the TBI unit from the truck to do some cleaning. This is where you tube would come in real handy. look up something like TBI cleaning or TBI removal and see what come up . Those things are very helpful in showing how to do something. I couldn't put into words the best way to do this without writing a small novella. Best off luck in doing this and remember this is a hobby in that a lot of people do this for fun. Jim

[judyjackse]

Originally Posted by 68Gold/white

There is an adjustment screw. It is seldom needed to be adjusted. There is a small freeze plug on the front driver side of the throttle body. The plug has to be removed to gain access.

But, I'd check the IAC motor first, make sure the tip is not all carboned up, if so, clean up the tip and the seat in the throttle body itself. AND the throttle body bas gasket can be a problem w/ a vacuum leak, especially if it'd got some miles on it...............

IAC motor ? where is it and what is it's function?

[1968aj]

If you look into the throttle body you will see round flap that opens when you step on the gas. They get gummed up over the years and dont close properly.spray some carb cleaner around the edge of it and use some fine sandpaper. Sometimes this will make it run better

[hugger6933]

IAC= Idle Air Control. It has the main function of allowing the throttle blades to be opened by means of a stepper motor screwed into the back passenger side near the throttle blades. Like all the other things under the TBI unit, it gets all carboned and gummed up. It takes a big socket or something like a crescent wrench to remove it. Once it is out you can scrape away the carbon and clean it real good to make it operate properly again. Once you get in under the air cleaner and start looking at the TBI a lot of this is gonna make more sense. Like I said before find a you tube vid or something like that pertaining to TBI's and then you will be better prepared to take that on. Jim

[Fitz]

Edited by staff