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1992 Isuzu Pickup surging

Anyone have advice on a 2.6L 4cyl. that is “surging” normal idle, then down to nearly dying then back up. The engine and fuel injectors and nearly everything else has been replaced, but the surging is completely unaffected. My mechanics and I are at a loss. In advance, thanks.

Idle rpm wandering/too high idle rpm can be tough to diagnose. Usually it is some kind of air leak allowing excess air into the intake manifold. Possibilities: Idle speed throttle actuator, idle up valve (increases idle when AC turned on, etc), intake ducting air leak, intake manifold air leak, egr leak, vacuum hose leak, vacuum device leak e.g. brake booster, PCV system leak or faulty pcv valve, Oil filler cap not sealing, oil dipstick fell out.

First monkey around with the controls that you can affect, see if any of them have an effect. AC on/off, headlights on/off, rear window defroster on/off, anything that might tend to bog down the engine at idle. After that I’d focus on the idle speed actuator, brake vacuum booster, or throttle body or intake manifold gaskets. Sometimes starter fluid can be sprayed near suspected air leaks. If the rpm goes up, there’s a leak there.

The Idle Air Control valve might be hunting for the proper idle position.,1992,pickup,2.6l+l4,1176678,fuel+&+air,idle+air+control+(iac)+valve,6072


Thank you. This problem is so iffy. Sometimes fails and sometimes normal. You’re advice is so welcome thank you. Joe

Clean the throttle body and adjust the throttle position sensor.

Will do and I cannot thank you enough

fyi, when my Corolla of similar vintage developed that problem a couple years ago, turned out to be a problem w/the idle air control mechanism. My solution was to just disable that function. The downside is I have to keep my foot on the gas pedal a little in very cold weather for the first couple of minutes after the first start of the day to prevent stalling at the first stop sign. But it doesn’t get that cold in San Jose even in the winter, so not much of a problem for me.

Thank you so much. This may help.