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Hi everyone,

I have a 1990 3.9 RRC. Lately I've been having issues with aggressive idle after the engine warms, and acceleration lag. Sometimes at stops the idle is smooth, other times it's fairly aggressive and shaky. Same with the acceleration, it doesn't always happen. When I get on the gas from a stop, it will go pretty slow and not respond to pressure I put on the pedal, before it picks up and gets going like normal.

Because of these issues, I went to test the alternator today and had someone rev the engine to 2k. The alternator charge was fine. Closed the hood, and went back later to start it up again. It had a really hard time starting, and immediately started to die so I gave it a little gas.
It picked right up, but the rpm jumped to 2k even though my foot was off the gas. It wouldn't settle back down, so I shut the engine off, which sounded really not right.
I checked under the hood, saw the hood kickstand had fallen down and was resting on the engine near the throttle, so I clipped it back onto the hood, closed it, and started the truck with no issues. I chalked it up to the kickstand somehow getting stuck on something and didn't think much more of it.

The truck started several times through the day with no issues. I went to get it from alignment and it did the exact same thing. Died, almost died on second try until I gave it gas, then jumped up and stayed at high rpms. Checked under the hood, this time the kickstand was where it should be. I closed the hood and tried again, it started fine.

I'm stumped on this one. Any help is appreciated. Just replaced the stepper motor yesterday as well.
 

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Hi everyone,

I have a 1990 3.9 RRC. Lately I've been having issues with aggressive idle after the engine warms, and acceleration lag. Sometimes at stops the idle is smooth, other times it's fairly aggressive and shaky. Same with the acceleration, it doesn't always happen. When I get on the gas from a stop, it will go pretty slow and not respond to pressure I put on the pedal, before it picks up and gets going like normal.

Because of these issues, I went to test the alternator today and had someone rev the engine to 2k. The alternator charge was fine. Closed the hood, and went back later to start it up again. It had a really hard time starting, and immediately started to die so I gave it a little gas.
It picked right up, but the rpm jumped to 2k even though my foot was off the gas. It wouldn't settle back down, so I shut the engine off, which sounded really not right.
I checked under the hood, saw the hood kickstand had fallen down and was resting on the engine near the throttle, so I clipped it back onto the hood, closed it, and started the truck with no issues. I chalked it up to the kickstand somehow getting stuck on something and didn't think much more of it.

The truck started several times through the day with no issues. I went to get it from alignment and it did the exact same thing. Died, almost died on second try until I gave it gas, then jumped up and stayed at high rpms. Checked under the hood, this time the kickstand was where it should be. I closed the hood and tried again, it started fine.

I'm stumped on this one. Any help is appreciated. Just replaced the stepper motor yesterday as well.
I would replace the idle control valve and clean the intake side while it is out. With a bit of searching, you can locate the GM version which will save you a few dollars. Check the connection and that the ICV is tight before replacing.

Unfortunately, you could possibly have a temperature sensitive vacuum leak – a bit more difficult to diagnose.
 

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And I check vac leaks by starting engine and then spraying carb cleaner around top end. If she surges then you;ve got a leak.
 

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Absolutely go with spraying carb cleaner or WD40 round the top end. If it alters the running, you have a vac leak. Rule that our first, then start looking at sensors.
 

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Sorry... just reading this with interest - what do you mean by 'around the top end'?

cheers
 

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Plenum chamber, gaskets, hoses - all the stuff at the top of the engine. There's not really anything to get a vacuum leak low down. Round the brake servo too.
 

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I read an interesting post where a person used a cigar/cigarette on a hand pump to blow smoke into a vacuum port (with the air intake taped off and engine off obviously). The pump action would keep the cigarette lit and pushing smoke into the intake manifold and out through any possible leak (hot or cold). I know you can have special smoke machines for the same purpose but using a cigarette and a hand pumps just sounded so ingenious.

Not sure if it works, you can probably find somebody on Youtube showing you if it does.
 
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