Sporadic, erratic idle

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Dec 15, 2019 | 07:52 AM

Bob90F250

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Joined: Dec 2019

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Sporadic, erratic idle

New member here. I’ve searched the forum, but can’t find a direction to begin trouble shooting my issue (with the truck) I have a 1990 F250 with the 5.8. It’s got a few oil leaks that I will be addressing, so I will be addressing an idle issue as well. While driving around town or on back roads the truck runs great, but if I take it on the highway for any length of time, >20 minutes or so the idle will drop off when I exit to the point of almost stalling or stalling with the a/c on. As soon as I can accelerate a few times it’s fine again. I do believe it needs the EGR addressed as it gets some spark knock under light load like pulling a small hill. It goes away when I get on it. So I’m looking for a direction so as not to just throw parts at it.
Thanks in advance,
Bob

Dec 15, 2019 | 10:22 AM

mary-kate

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Joined: Jun 2004

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I think you've hit on something with the EGR. If it's stuck open it can dilute the mixture and cause stalling. You can pop it off and clean it out, just to see if it makes a difference. You can also apply vacuum to the diaphragm and see if it is moving the valve at all.

My '91 had an electric solenoid control the vacuum to the EGR, and a couple feet of hose to go bad too.