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Discussion Starter #1
Mine running about 800RPM hot and cold, day and night. Don't see any difference of idle with the engine temp. Probably fast idle solenoid how do you diagnose that ? Truck is 1991.
 

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In gear it's 600 RPM. Check your throttle cable to make sure it's not sticking, make sure both springs are intact.

Is the plunger on the fast idle solenoid staying extended?
 

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Mine running about 800RPM hot and cold, day and night. Don't see any difference of idle with the engine temp. Probably fast idle solenoid how do you diagnose that ? Truck is 1991.
Assuming you have a tach.
Adjust the idle speed to 650 - 700.
The cold idle solenoid plunger should just be touching the throttle tab, see pic.

With a cold engine start, cycle the throttle WOT one time, that sets the cold idle solenoid plunger to idle speed 950 - 1000 RPM.
Adjust the cold idle plunger to achieve that. Adjust with a 1/4" wrench, screw plunger in or out.

When the pass side head reaches 120*F, the cold idle solenoid should de-energize and the idle should return to the 650-700 idle speed. The temp sensor is like under the alternator in the head. That sensor also controls the IP cold idle advance solenoid.
 

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750 to 800 when cold but I have to tap the throttle to get mine to kick in.

once its run for about a minute or 2, it drops back to 600-650.
 

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750 to 800 when cold but I have to tap the throttle to get mine to kick in.

once its run for about a minute or 2, it drops back to 600-650.

That's because the fast idle solenoid isn't strong enough to push the throttle lever.
 

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I don't normally have any issues unless it hasn't been run in a while. then I get that typical miss until I get it up and running and rev it just a bit a time or two.
 

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750 to 800 when cold but I have to tap the throttle to get mine to kick in.

once its run for about a minute or 2, it drops back to 600-650.
Ain't that what I said in the above post?

With a cold engine start, cycle the throttle WOT one time, that sets the cold idle solenoid plunger to idle speed 950 - 1000 RPM.
 

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Discussion Starter #10
After rediagnosis, idle speed is 600 Rpm when cold and then it stick at 800 RPM when hot, I'll check for plunger sticking. Truck is Zf5
 
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