High idle

Hi all it’s been a very long time on here.lm trying  to repair My 1986 Bullnose 5.0 mfi. My second driver.
My question: proper curb idle adjustments. I do know it’s been messed with😎 The rpm,s take off very very high when first started hot or cold. 30 seconds later rpm,s return to an expectable level. I’ve messed with set it a few years ago w/ no luck. I believe the timing is set (I’ll have to recheck). New idle air valve. No vacuum leaks. I think I checked and replaced the TPS. It’s been a couple of years. Will check for DTC,s. I’ve read that the ECM. controls the idle circuit. ANY THOUGHTS 💁. I’ve got to get this repaired the main bearings don’t like the very high rpm,s (knock)😂 It’s my daily driver now. My other truck needs head gaskets. Thanks for listening.

Goose57 wrote

Hi all it’s been a very long time on here.lm trying  to repair My 1986 Bullnose 5.0 mfi. My second driver.
My question: proper curb idle adjustments. I do know it’s been messed with😎 The rpm,s take off very very high when first started hot or cold. 30 seconds later rpm,s return to an expectable level. I’ve messed with set it a few years ago w/ no luck. I believe the timing is set (I’ll have to recheck). New idle air valve. No vacuum leaks. I think I checked and replaced the TPS. It’s been a couple of years. Will check for DTC,s. I’ve read that the ECM. controls the idle circuit. ANY THOUGHTS 💁. I’ve got to get this repaired the main bearings don’t like the very high rpm,s (knock)😂 It’s my daily driver now. My other truck needs head gaskets. Thanks for listening.

Its been a while since I messed with these systems and for good reason, from what I remember the butterfly valves should be essentially closed or just slightly open as at idle the IAC is controlling idle speed.  If the butterfly valves were cranked open then it will be like holding the throttle open.  What you can do is turn the stop screw to allow the plates to close more if they are cranked open some.  One problem you may run into how ever is having to adjust the TPS as it may go below minimum voltage for the system and may or may not throw a code for TPS low voltage.

Since you cant read live data from OBD I systems you will have to back probe the wire going to the ECM for the voltage return and loosen the TPS and rotate it till it is above minimum value then lock it in place.

messing with the idle screw is also messing with the tps setting which is basically telling the ecu that you want to go. if the tps is above a certain value then the ecu will add fuel and timing. you are not just adding air. on another note, we have had a few problems like this when replacing tps on these models. enough so that we now wait for a motorcraft brand part to be delivered even if it takes a few days longer. I cannot explain in detail but many times now having the motorcraft specific part was the answer. and yes, motorcraft costs more. but not more than doing stuff twice.

Thanks Matt, I’ll try a Motorcraft it may be a while before getting back I’ve got others broke down too. Luv the forum😀