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😀

Well I driven the truck around for a while since my last post and setting the base idle as the procedure (it barely runs or dies when IAC is unplugged),hoping the ecm would relearn the idle control rpm’s. Here’s where I’m at
about 3,000 rpm when cold then comes down during the warm up cycle. Start up after hot , idle goes up to about 1,600 - 1;800 rpm’s then quickly comes to about 700 rpm’s. Checked for vacuum leaks hot and cold ( used a lot of carb cleaner searching) none. No codes normal #11. Newer TPS voltage, at closed 0.7v. ISC sol. Ohms 13.2. 5v running. KOEO 12v. Timing 10 btdc. At 550 rpm’s. Spout out. Spout in timing advances. Increase idle and the timing mark jumps around. Probably a loose chain. Wouldn’t think causing high idle problem. Any thoughts? ECM? Going to pull throttle body off for a closer inspection.

well, how jumpy is jumpy. if it is jumping around wildly across a large range that's different than just not advancing on a smooth scale. they rarely advance smoothly. especially in neutral.no load. as for the timing chain it can be checked rather simply and I have replaced many at 150k intervals as a high mileage service. this is my own choice but have put many miles on v8s without ever breaking the other seals on engines. and I have found some extremely sloppy chains in doing so. I don't think this is a "wait til it breaks" item.
if it is able to run up to high rpm so often on its own it is certainly getting the air to do so. have you eliminated the truck from the engine to see if it has a system vacuum leak.

Thanks the base idle screw is set at about 600rpms w/ ISC disconnected. When I back it out at that setting it usually dies and cold starts are still really high. I did try a test when hot with my multi meter hooked up to the TPS. Just off idle the motor stumbles and hesitates ( just like a vacuum leak but I know better🤔.) but the TPS volt reading is 7.4v. I was expecting it to be jumping all over the place but know luck. Checked ohms and ref.v. On the CTS by the thermostat. Maybe I’ll pull it and check the ohms in and out of hot water. Pulled vacuum on the EGR. stumbled and died. Checked vacuum reading and it reacts to WOT. EGR position sensor has ref. voltage. I do believe the EGR and the EGRPS have been replaced. I m beginning to think I have two problems 3,000 rpm’s when cold start up possibly ECM😂. And the throttle body itself acting like a vacuum leak just off idle. I’m sure the chain has some wear on it but I don’t believe that’s causing the very high idle when cold for that matter hot, I still believe it shouldn’t be 1,600-1,800 rpm’s hot. As for isolating the truck maybe your on to something. vacuum lines going to HVAC system, trans ? Everything in the engine bay I soaked down w/carb cleaner.

7.4? at idle the tps signal return should be .5-1 v I generally like them adjusted to .85-.9 v. reference voltage should not exceed 5 volts from the ecu and tps cannot make it greater than that. please test that again. if you have greater than 5v reference going to your sensors then you may need a new ecu

First of all I would like to thank everyone on here who helped me on my truck. Truck is running better after replacing ICM,ECT.ACT,O2S and cleaning TB and resetting curb idle. Also I added a $30.00 bottle of catalytic converter cleaner to the fuel. I’ve been chasing this high idle for some time. I disconnected KS due to low oil press. Knock. I did take a 10sec. video of a cold condition start up but I can’t figure out to embed it to my post. The rpm,s on cold start up are 1,500 -1,600. Seems a whole lot better. I sincerely apologize for maybe misleading everyone on stating the rpm,s were 3,000 maybe I misread inductive tach. But in my defense I swore that it read 3,000 rpm,s at one time on cold starts. I’ve had the tach hooked up on and off many times. I’m excited about my truck doing better I would like to tackle some more issues. So I’d like to ask u guys for some more advice. First: I have historic plate on my truck so no need for state or emissions test here in Missouri and would like to remove the two converters yes count them two converters and the air pump and associated plumbing from my truck. I do have a single wire O2S on my truck. Second: where to buy decent door and window weather stripping. Third: anybody repair or replace the digital clocks on these trucks? Thanks Again Gus.