The Mighty 258 > Engine Electrical Systems.

Random high idle

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MIPS:
Started happening midway through the week. I would be coasting, at a light or just parked in the driveway and the engine idle would randomly go up a few hundred RPM and then go back down again without touching anything. Found that the solenoid side of the sol-vac was kicking in and out. I've verified it's not the headlight switch or rear defroster switch doing it so it has to be the computer. I disconnected the vacuum switches, coolant temp switch and air cleaner temp switch and it's still doing it so it isn't a faulty sensor. Unplugged and jumpered the spark control on the ignition module to verify it wasn't that freaking out and wasn't it either. Currently feeling around the harness for a rub-out but I'm not seeing anything so far and the last step will be to pull the computer and test it. Any ideas?

AMC of Houston:
Is it the vacuum side or the electrical side of the Sol-Vac kicking up the idle?   That'll help point to a culprit.

Taylor:
Could be the computer.
Question, does the solenoid have any voltage at idle? If it does. Does that voltage change? Could be that it is sticking?
Does the vacuum change?
Just a thought.

Illeagle1984:
Is your A/C on?  It may bump up the idle when the clutch kicks in, then back down as it turns off for a bit.  Just covering the bases...

MIPS:

--- Quote ---Is it the vacuum side or the electrical side of the Sol-Vac kicking up the idle?
--- End quote ---
On further testing with the live jig, it's both. I can see the idle solenoid and the Vacuum switching solenoid operating.
I don't have AC so its just the lights and defroster. I can verify both are off, so it's the computer calling.
An unplugged vacuum switch assembly and coolant switch assembly shouldn't cause this. The Thermal Electric Switch on the air cleaner will. I can replicate the condition by shorting the pins (switch closed, air temp is cold) but as soon as I remove the jumper it settles down again. When giggling wires I found that the engine freaked out again when I lifted the air cleaner off to inspect a whistling (the manifold is probably leaking) but I also found that the grounding for the computer was about to fall apart, so I'll need to replace the grounding eyelet and clean the TES and test again.

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