Help: 1988 5.7 TBI GM Engine - high idle

Hi,
The idle on this engine is high. My shop mechanic says it is the gasket between the tbi and air intake. I replaced it and see no difference. Any
other ideas?
tia
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wrote:

start spraying around all gaskets and vacuum lines w/ some carb cleaner... when the idle drops, the spot you just sprayed is where the vacuum leak (and the high idle) are coming from.
-Bret
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The TBI gasket was a good place to start, but not the only thing that could give you a high idle. The next thing is the IAC (Idle Air Control) valve. The IAC allow air to bypass around the main air flow path when at a zero throttle condition - it's position is controlled by the ECM depending on a number of factors, which in turns adjusts your idle up or down. On a motor that old, it probably has some crud on the pintle or valve seat not allowing it to seal completely, giving you a high idle. Either that or it's worn and won't seat, another age related condition. A new IAC valve will run about $60.00 and take around 20 minutes to install. Remember to clean the seat in the TBI before installing the new IAC valve.
After the IAC valve, check your coolant temperature sensor. A faulty sensor giving an abnormally low reading will make the ECM think the motor isn't warmed up yet causing it to idle high.
Good luck - Jonathan
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Jonathan A. Race
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Thanks for the advice, I will check this out.
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Why would it be that gasket? Engine vacuum is present after the throttle plates not before.
Why is it a vacuum leak at all? Do your plugs show a lean condition? Have you run through an emissions test station and they say it's lean?
Ted
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A leak in the gasket between the TBI and the intake manifold allows additional air into the intake system outside of the control of the butterfly valves or the Idle Air Control valve. More air = higher idle. In 1988, the gaskets were fiber based and known to split after 10-15 years or more, causing the air leak. Although it was a good idea to install a new gasket - it probably was due even if it wasn't split and leaking already - but I would have checked the IAC valve first, tested the coolant temperature sensor next, then moved on to the new gasket. The vehicle in question is a 1988, so I'm betting it's the IAC valve. I put close to 200,000 miles on my old '88 Chevy C1500 pick up with the 350 TBI, and all three needed replacing over time before I sold it.
Cheers - Jonathan
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Jonathan A. Race
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The throttle body base gasket is a common problem on those trucks. Trick is (engine off) to climb on top open throttle plates and look down into throttle body with light. Normally you can see the gasket sucked into the intake area.
I agree the TB base gasket would be the first place to check on those trucks. If that looks good then proceed on from there. Now does the engine idle high all the time??? Did you check of all the vacuum lines make sure there not cracked or broken. Take a pliers and pinch them off one at a time right where they connect to the intake to make sure there is no leaks etc.
The IAC on the side of the TBI can also act up. If old the shaft can gum up over time etc. Remove it and clean it with carb cleaner and a tooth brush etc. When you take it out note in what position it is in i.e out or in.
Have you checked codes?? If you jump A&B on the ALDL and turn on the ignition the engine service light will flash codes. Also on old GM TBI's when you jump A&B it cylcles the IAC.
Stop by a parts store and pick a service manual for 15 bucks it has lots procedures for those old truck.
As for throttle body fuel injection that old GM set up was a pretty good one. WE didn't make a lot of money off of them. The base gasket problem and bad distributor cabs is all I can think of that was a common failure.
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Thanks Hayman,
I'll try it this weekend.

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Thanks, Jonathan, I'll try it this weekend.

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Thanks everyone. The TBI gasket fixed the problem. Easy and cheap fix.
Regards
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