High rpm at idle then

#1

1987 Ford F150, automatic, 5.0 v8, 121k miles.

I had some fluctuations in the rpms while driving a while back and the mechanic replaced the throttle sensor and that has gone away. Recently, it’s been having high rpms, between 1400 and 2000, after I drive a few miles to work and put it in park. I noticed yesterday and tested it again this morning that if I just tap the accelerator before or after parking, it idles at the normal range of 600-800 rpm. Before I took it to the mechanic, I very thoroughly cleaned the throttle body (it didn’t have much build up anyway). Any ideas what might be causing it?

#2

Sticky throttle sensor maybe? Something the the system from the accelerator pedal to the throttle body is hanging up. When a problem appears after a repair, suspect the repair. It may “wear in” and go away or it may be binding a bit. Check there first.

#3

I haven’t had a chance to drive it much it inspect it, but even when stopping at a light or sign, I can keep my left foot on the break and tap the accelerator with my right to bring the idle rpm back to normal. I’m not having any luck researching this specific issue unfortunately

#4

The high idle might be caused by a faulty Idle Air Control valve.

https://www.rockauto.com/en/moreinfo.php?pk=1111891&cc=1111464&jsn=491

To find out, start the engine when hot, and while the engine idles, take the handle of a screwdriver and tap on the IAC valve. If the idle speed changes, replace the IAC valve.

Tester

1 Like
#5

Okay great idea, I will give that a shot

#6

My F-150 done the same thing… it was the linkage what snaps on the tb… the plastic sleeve had a crack in it and hung on the spring

#7

Have your shop check these parts: dc motor idle speed actuator & idle air control valve

#8

Ah good call, I’ll check on that too

Thank you all for replying

#9

I believe we have a fix. My mechanic is in his 70s and is the local Ford guy, worked at the Ford dealer shop in the heyday of my truck. The iac valve was getting a bad signal to/ from the computer. He said even back then it used to happen and they’d just unplug it and keep it unplugged.