Question about 1987 Ford F 250

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Eng. rpm not dropping when clutching & at idle, rpms go up & down

This is a 1987 Ford F-150, 302c.i. fuel injection, manual trans.
No cold idle in morning, engine rpm's do not drop or drop very slowly when I clutch to change gears. Also at a stand still, the engine rpm's will drop down and go back up constantly.
I have had the truck to 2 different garages. So far they have replaced the Idle air control valve, oxygen sensor, and the throttle position sensor.
One shop I had it to had the truck for 4 days interchanging parts and they even did a smoke check to see if there were any vaccum leaks but all to no avail. This is getting very expensive and frustrating. Hope you can help.

Regards,

Dave

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  • dj1955 Feb 03, 2009

    Thank You for your help. I appreciate it. Can you explain the plate though and how it's installed?

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Here are a couple things to try. The idle surge is a common problem. Ford makes an adjustable plate like the one below. It allows come control over your idle. I made one and it eliminated most of the surging. I think the Ford part is about $30. Can't seem to find the ford part number though. Search online.

http://k-mansparts.com/items/performance/ford-mustang-iac-adjustable-idle-spacer-kit-50-46-f2pz9f939a-detail.htm

Try www.fordfuelinjection.com or other ford sites. Lots of great information.
My 89 seems to roar loud when cold when coming back down to idle and I am sure its the fan clutch. I have also heard the water temp sensor can cause idle problems because the EEC does not know what state the engine is in. (hot/cold, etc)
Hope this helps. I do recommend the adjustable plate though.

Posted on Feb 03, 2009

  • jetfixr Feb 04, 2009

    The plate mounts between the Idle air control valve and and the throttle body. It allows you to fine tune the bypass air around the throttle plate. This helps to establish a minimum RPM which helps to catch the engine before it goes below the minimum idle RPM. It basically smooths out the ups and downs. It may still surge some, but not nearly as bad. Mine helped a lot,

    The IAC valve on the engine tends to continuously chase the idle and seems to be a weak link on the Ford design.

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