Why is my truck loping at idle?


Asked by Jan 16, 2015 at 09:16 AM about the 1988 Ford F-150 XLT Standard Cab SB

Question type: Maintenance & Repair

When I first crank my truck it idles fine for 15-20 seconds then drops lower and starts to lope. Even giving it a little nudge on the pedal will not smooth it out. After about 15 minutes of driving it smooths out, but I have to  two foot it when coming to a stop or it will die half the time. Any advice would be greatly appreciated.

5 Answers


check your IAC valve

1 out of 1 people think this is helpful.

Also check the throttle body, make sure it is clean and the air rate is set to specs.

1 out of 1 people think this is helpful.

A quick and easy way to troubleshoot this is by spraying some gumout around the base of the throttle body while veh.is running. If the engine idles high, throttle body may be loose or gasket faulty.Replacing the inline fuel filter is a good idea and a cheap way to see if it is the issue too.Also check all hoses around the throttle body especially the pcv line and pcv valve.Make sure it is sealed when inserted into the manifold.As already mentioned, the IAC can be removed and cleaned,with gumout , then reinstalled. Whether new or reinstalled, the IAC has to be reset.This can be accomplished by, reinstalling the IAC and turning your ignition key 3 times to run,then off again, without starting the engine. That should reset the IAC to factory default.


Thank you for all your suggestions, I will put them to use. I know now how I will be spending my Sat. Results posted after.

Andrew Olsen

These trucks are known for vacuum leaks from the upper intake gasket and the factory hard vacuum lines cracking and creating a leak ad well. Same for the iac. They get gummed up and need cleaned

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