A couple of weeks ago, my 1986 F-150 302 EFI began dropping to very low rpm’s after the normal engine speed “flare” on cold start-up. The engine would almost die then “recover” to a normal idle speed and ran fine thereafter. I also began to notice a long (3-4 second) crank time on subsequent (warm) start-ups. I replaced the idle air control value with a Motorcraft one but still get the same symptoms – starts great when cold, almost dies, then recovers. Three to five second crank time on subsequent (warm) start-ups, almost dies, then recovers and runs fine. Map sensor has been replaced twice in last twelve months so I am wondering if the throttle body needs cleaning. Any helpful suggestions appreciated.

Edit: I took a chance, removed the two intake hoses from the air filter box, and (as best I could), cleaned the butterflies and bore surfaces where the butterflies close inside the throttle body then re-assembled everything. The truck started immediately when cold (same as before), but seemed to run smoother. I drove it several miles, parked it, waited a short while, and it started on the first try with no extended crank time (these are the conditions under which I previously had the extended crank time).

It seems this partial throttle body cleaning helped so I will look into actually removing the throttle body for a thorough cleaning (have to get the plastic clips off the throttle cables without breaking them and have to get the two coolant lines off the throttle body without doing any damage).

Hope the above helps someone with a similar issue.

 

Thank you for posting what you did to fix the issue you had and as you said it may help someone else.
Dave ----

 

Try the tfi module my truck did the same thing then i replaced it and it ran fine if that doesnt work i would check the fuel pumps and fuel regulator and check to see if the fuel filter is clog

 

I recommend you/mechanic, use new gaskets if you remove your throttle body if you are not already thinking this way. Previously used gaskets have a potential to leak, especially on the intake systems, they are very inexpensive.

Keep an eye on your vacuum lines and vacuum readings as you go, you will have issues with those old brittle lines as time passes. You have a 30+ year old truck, so, you may still have components of that vintage throughout your truck.

You had some previous discussion on your MAP sensor, the vacuum line for the MAP runs thru a 4x vacuum connector on the passenger fender skirt, then around the back of the engine and up to the vacuum tree on your intake. There are 3 vacuum lines in that sleeving around the back of your engine, for the MAP, the TAD valve, and the EGR Vacuum supply to your vacuum reservoir. If you ever suspect the MAP vacuum line, you can run a temporary line directly from the vacuum tree to the map sensor.

Keep an eye on your continuous memory codes from time to time, they may give you an idea of an emerging issue. Those are the codes that come after the KOEO codes during the KOEO self-test.

Great to see how involved you are in the maintenance of your truck, and the obvious reading you have done along the way….

Here is a link to an 1986 manual that migh be of interest to you; 1986 EVTM - Gary's Garagemahal (the Bullnose bible)
Vacuum Tree diagram attached;