Hi I'm new here, just joined the forum. I'm having idle down issues with my 1990 F350 460 zf5. I've looked and chased and done all the possible vacuum leak tests. Have replaced IAC, TPS, MAP, O2 sensor, intake air charge sensor, both temp sensors, fuel pump. The engine does 2 different things separately. If it idles right then it will not idle down between shifts or if I rev it over 2k rpm. It will literally take 10- 60 seconds to return to idle, and yes throttle body is closing properly. Sometimes when I start it it won't fire unless I give it a little throttle, then it idles very low , as in so low it sometimes stalls just by turning the steering wheel but it drives perfectly and idles down as it should. It will only remain like this for a few minutes then return to its normal state of perfect idle with extreme idle return delay. Also when I'm driving it during the low idle/ proper idle return status I can feel when it changes back to the idle delay status because it feels like I just punched the throttle, i can feel a noticeable sudden increase in throttle. I'm at a complete loss as to what is causing this, it seems as if somehow the injectors are not shutting off when they should or something along those lines. Any help would be greatly appreciated.

Also it smells like it's running rich when it's in the delayed idle return status

 

Do you have the egr disconnected? An old work truck we used to have had all the emissions stuff removed and it did the same thing.

 

i just recently deleted to smog pump and tubes to the cat but this issue was existent before I did that. The pump was bad so I thought it possible that something in the smog system was the culprit so I eliminated it from the equation and the problem remains. Egr is still intact and operational.

 

I suggest to run the KOEO tests and stored code display. If you get System Pass for both (11/11) then try cleaning the IAC. Or perhaps do both for good measure.

 

i have ran the tests multiple times, if it's idling low it throws codes 116 and 411 412 which are engine coolant sensor and idle speed system not controlling idle properly, either too high or too low and will not complete the test. If it's idling properly but not idling down it'll complete the test with a 412( idle speed high)
. This is the 2nd brand new IAC I've installed on it.

 

Those are KOER codes. I suggest you start with the Key On Engine Off (KOEO) tests and stored code display before going to the KOER tests.

Very interesting your 1990 truck is displaying three digit codes. Ford did not start transitioning from two to three digits until 1991. Even then it was usually the trucks with E4OD transmissions. I suspect someone has swapped in a later computer at some point.

Stranger things have happened.....

Another observation is Code 116. If the truck was fully warmed up before running the KOEO or KOER tests then you may have your root cause of the high idle. Code 116: ECT out of self test range 0.3 to 3.7 volts.

 

I have done both KOER and KOEO . KOEO is systems check ok or 111 I believe it was, it will intermittently throw a CEL for egr valve not opening but it never stores the code. I have ran both tests with it cold and warm and I have replaced the ECT sensor but I will test the voltage, could be an electrical issue. Also I wasn't aware of the 3 digit codes not being used in 90', I'll look into that as well but it seems to me that shouldn't make a difference but at this point I'll try anything. Is it possible that I have a pcm for an automatic ? And would that potentially cause my issue?

 

I realize this thread is older but was the problem solved? If so how was it corrected?