Adjusting Idle Control on 88 Chevy Van

buckyJuly 30, 2007

Hey Fellas: My 88 Chevy 3/4Ton travel van is stalling with engine hot and the air conditioner on. Idle is so low the engine just stops. The 350 engine is throttle body injected. I am wondering if the idle speed can be adjusted in the same manner as on a standard carb or is it controlled thru the Engine Control Module? Also: Can the problem be caused by a vacuum leak? I'd sure appreciate some input on this as I was driving in the mountains on the weekend and the engine quit when going down a steep hill. Losing power steering and brakes at highway speed is pretty exciting. :-)

Thanks: Bucky

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If your AC is factory installed, check to see if you have an idle stop boost solenoid. Many AC setups in that era used a little solenoid, or vacuum actuator, to increase the idle position of the the throttle plate when the AC was activated. Often, these mechanisms became sticky and did not advance the throttle setting even though the soleniod was energized. If this is the case, some throttle cleaner spray/lubricant may help. There may be a separate idle stop screw for this booster as well.

Since your problem happened at altitude, the throttle stop might need to be increased to compenstate for the thinner air. But, you would need to set it back after returning to lower altitudes else the idle speed would become too fast.

    Bookmark   July 30, 2007 at 4:00PM
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Thanks for the response Jemdandy. The A/C system is factory. I am aware of the idle boost solenoids on the old standard carb set-ups but I can't see one on my throttle body injection unit. I'm thinking that the idle control might be controlled by the engine control module but I'm not sure. I have a vacuum leak that I have been unable to find and am wondering if the problem is related. Any further thoughts on this would be appreciated. Bucky

    Bookmark   July 31, 2007 at 1:11PM
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What threw me was that you said you had a throttle body injector and most of those were just one step up from a carburetor. But you mentioned an engine module controller. If you have an engine module controller, likely you also have an IACS ( idle air controller). The IACS with the engine controller adjusts the engine idle speed. A serious vacuum leak can upset this system and this may be your problem. Find that leak.

Many of the little plastic fittings in the vacuum hoses are restrictors - these have small calibrated holes to restict the amount of air flow. One such restrictor is in the vacuum feed to the heater/AC system that operates the various actuators in the air plenums. Thus, if any of these hoses gets pulled off, the amount of vacuum leak is restricted. Lean out problems may occur if any of these restrictor fittings have been replaced with standard straight-through fittings.

Places for vacuum leaks:
o Tubing
o Fittings
o Vacuum bottle (often on or near the fire wall)
o Power brake
o Vapor recovery system
o Any AC vacuum operated parts such as a hot water diverter valve located in the heater hoses.

If you can hear the leak, this helps in your search.

I am not familar with the system for '88 Chevys. I had an older 1980 Chevrolet Citation, V6 2.8 L and I recall that it had a peculiar vacuum control. The vacuum was limited to say 16 inches. When the vacuum exceeded this value, a valve allowed air to enter the intake which leaned the mixture severely and stopped combustion. This was done to prevent backfiring in the exhaust and blowing off the converter. This came into play mostly when slowing down or comming down a hill with a closed throttle with the engine was in engine braking mode. This never gave any trouble, but could have if it stuck open.

Good luck. Let us know what you find.

    Bookmark   August 1, 2007 at 1:16AM
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Idling slower, and slower until it stalls is a classic complaint on that engine system. It would be throttle body injection, with a completely computer controlled idle speed. I wrote an article on diagnosing just this kind of a problem in '92. Basically something is causing the fuel injection computer to think that the engine is idling faster than it really is. Common causes are a cracked magnet in the "pole piece" GM's name for the center shaft of the distributor that creates the RPM signal in the pick-up coil, or a poor/noisey ground between the distributor and the computer. A good automotive electronics technician, with GM experience will recognize this symptom right away, and will set out properlly diagnosing the failure.

DON'T throw parts at it, have someone who recognizes the symptom test and repair it. Often times, there is more than just this one thing wrong, and the shop may have to do several repairs to work their way back out and restore your truck to the way it was built.

    Bookmark   August 1, 2007 at 8:29AM
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Thanks John G and Jemdandy. Very good understandable information. There is always more to these problems than meets the eye. Question for you John? You say that the idle is completely controlled by the ECM. Does this mean that my set-up does not have an IACS? Thanks: Bucky

    Bookmark   August 1, 2007 at 8:02PM
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John G : Would it be possible to see the article that you wrote in 92 on diagnosing this problem. I always try to improve my knowledge and would be interested in learning a bit more about my almost classic 88 Chevy. :-)

Thanks: Bucky

    Bookmark   August 1, 2007 at 9:04PM
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The "IAC" is the stepper motor that the PCM controls in order to control the idle speed. Your truck does indeed have one. There is also a minimum idle speed setting, which is adjusted by blocking off the passage that the air for the IAC flows through. The reason just trying to make an adjustment there will not help is the ECM is still misscounting engine RPM and is feeding too much fuel, because it triggers the injectors one time for each "ignition pulse".

Bucky, As far as reading that article, I might be able to find a copy online somewhere. But really it's badly outdated, and if you don't have a stong electronics background, and you dont own and are comfortable using an occilliscope then it's going to read like a TV repair lesson. Suffice it to say, if you have a real scan tool that displays the engine idling speed, and you have a hand held tachometer that reads out very accurately (1rpm), you can compare the scan data engine speed to the speed the engine is really running at. They should be within 25rpm of each other at all times. In the failure described in this post, you will see the scan tool reporting 900-1000 rpm, meanwhile you hear, and see the real rpm dropping lower and lower until it stalls.

    Bookmark   August 2, 2007 at 5:32AM
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John G. : From what you've explained about the 92 article I'm pretty sure that my eyes would just glaze over and I'd get a headache trying to understand it. Electronics were never my strong point. I have enough of a challenge with things mechanical. Thanks again for the insight and information you've provided me. Always appreciated.

    Bookmark   August 2, 2007 at 4:07PM
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John G: I've had a few other mysterious glitches with the old van for the past few years and I'm wondering if they could be related to the stalling issue. When I have the air conditioner on with the dash air vents open and require heavy gas pedal when climbing a hill, the dash vents close up. After a few minutes of flat driving they will open up again. The cruise control also gets weak after 5 or ten minutes of highway driving and requires a slight push on the gas pedal get it operating again. I was thinking that both these issues were related to a vacuum leak somewhere. Could a bad EGR IAC valve cause these things to occur. I've checked all hoses and connections and have found no leak. Thank a lot: Bucky

    Bookmark   August 3, 2007 at 11:06AM
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FOUND THE CULPRIT..VACUUM LEAK: Following John G and JemDandys suggestions I spent the last day and a half sleuthing out the vacuum system on my old van. I started at the engine and worked my way back to under the dash. Replaced several vacuum hoses while I was at it and checked for a free moving EGR valve diaphram. I finally discovered that the small plastic rotating diverter that controls all of the little plastic vacuum lines under the dash had broken. Thats why I was not only having idle stalling problems but also dash vents opening and closing mysteriously and cruise control kicking doing weird stuff. I picked up the part at the local auto wrecker and replaced the vacuum diverter switch. Cruise and air vents now work perfectly...YAHOO... and I've got my fingers crossed that the vacuum leak was also the cause of my hot idle stalling problem. Thanks again for the great advice and suggestions fellas. It feels darn good to finally find my problem and get it fixed. This forum is invaluable to an old car guy like me. Cheers: Bucky

    Bookmark   August 12, 2007 at 10:33PM
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the scanner that i hook up to my van show nothing was wrong does it mean the idle need to be ajusted

    Bookmark   April 3, 2011 at 2:01PM
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