5.0 high idle... help plz

[4 bangin]

ok i got this problem at work making me feel stupid.

93 f150 5.0 high idle at all times.

not the iac, no vacuum leaks its been smoke tested.

just figured out today that if i unplug the ect the idle goes to nomal.

new ect didnt fix issue >,< still if left unplugged idles fine.

cant get either of my scanners to comminicate with this eec4 system

any input? ect wires shorted? ecm? i have personally never seen an eec4 ecm go bad.

help me figure this one out so i can get off tomorrow and go to the lake!

 

[RonD]

yes, reads like ECT circuit is bad.

ECT is the "Choke" for fuel injected engines, sets high idle and run rich conditions until engine is reported as warmed up, so Open Loop until engine is warm and EEC can go to Closed Loop.
You should also get a CEL if engine stays in Open Loop too long, the EEC has a timer for that.

If you are not getting the CEL after engine warms up and its still in Open Loop then I would suspect the EEC.
You can test the ECT circuit and sensor with a couple of pins and a volt meter.
Pierce both ECT wires with the pins, one each , turn key on.
Hook up volt meter's red probe, set for 20v DC, to one pin and ground the other voltmeter probe.
One pin will be 5v, the other is the voltage return and it should have about 3v if temp is 70degF, 2v at 100degF, 212degF(operating temp) should show about .5v, this voltage is what the EEC sees to determine engine temp.
So yes, if wires are shorted together then EEC would see 5v return so always cold engine, like -50degF cold, lol.

The EEC has a separate reference ground for this circuit, at the EEC's location, if it is loose or not connected then it could react as you have found, with ECT disconnect then EEC sees 0v on return wire, so HOT engine and low idle.
If there was a short between the wires the EEC would still see the 5v with ECT disconnected, so no idle change.

here is a good reference for ECT testing:
http://oldfuelinjection.com/?p=28