Scanner shows problem codes for throttle position sensor, coolant temperature sensor and idle control circuit. Coolant sensor checks out OK as does the IAC (Idle Air Control)solenoid (which measues 10 0hms). However, in probing the wires at the IAC terminal with the Key On Engine Off (KOEO), I measure less than 1 volt instead of the 10.5 volts my Haynes Manual says should be there. When I remove the connector from the IAC solenoid, the voltage measures 11.5 volts. This is acting like a "high resistance open" someplace. I checked the engine compartment wiring and all appears OK, with 11 volts going into the PCM (computer)connector. I'm beginning to suspect a bad PCM. Ouch! Anybody confirm the Haynes measurement expectation or offer another diagnosis?

 

Id be more inclined to suspect a ground problem . I beleive there are 2 ground pins / wires for the computer ( Pin 60 ) What are the other code numbers you got ? With the engine running does the engine rpm drop if you disconnect the Isc wiring from the Isc ? Also if you can back probe the Isc wiring , connect your Dvom meter & set it on the 20 Volt Scale , start the engine & vary the rpm & the voltage should vary . Let us know what you find .

 

Since last time, I've done more work on my "engine runs rough, stalls at idle problem". The IAC appears OK. I removed, cleaned and tested it and all looks fine. With the engine running, the voltage at the IAC terminals measures about 7.5v at 2600 rpm and 5.25v at 1000rpm.

I have scanned for codes quite a few times now, and each time there's something a little different. With KOEO I always get 23-TPS out of range (OOR); sometimes I get 21-coolant temp OOR, and 24-air charge temp or vane air temp OOR.

My Memory codes have been: 11-System Pass, 51-coolant temp sensor voltage too high or open ckt, 63 TPS voltage too low or grounded, 87- fuel pump primary ckt failure or temp comp pump solenoid OOR.

My KOER codes have always shown: 12- can't control RPM during self test, 21- coolant temp OOR, 23 TPS- OOR, 42- No exhaust gas oxygen switching detected- always running rich, 18- spark output or spark advance word failure.

My KOER codes sometimes show:13-cannot control RPM during self test or idle speed control inoperative, 72- insufficient MAP/MAF/Boro change during goose test, and 27-Manifold Air pressure or Boro pressure sensor OOR.

Since I had never done it in 110K+ miles, I replaced the Oxygen Sensor. No improvement.

I have examined the TPS, coolant sensor ECT, IAC control, MAP sensor. I have measured the reference voltage to be 4.99v and found all the sensors to be functional. I "looked at" the TPS and ECT from the ECM connector (the way the computer would see them)and found them to be within the resistance limits specified in my Hayes manual. I pulled a 20" Hg vacuum on the MAP sensor and saw the frequency output change from 500rpm to 300 rpm.

Yet, the computer is telling me, through the codes, that these sensors are out of range (OOR).

When I look at the wiring diagram for my truck I see that a number of these sensors share the same "signal return" line. Thinking there might be an "open" someplace in the engine compartment wiring, I measured the resistance from "sig ret" pin of the ECM connector to the TPS "sig ret" terminal and it was a respectable 1 ohm!

The way this truck acts, I'd first suspect a vacuum leak someplace.....but I haven't been able to hear, see or find one. (The engine runs too rough to be able to detect changes as "carb cleaner" is sprayed at possible vac leaks.) And that wouldn't account for all these error codes showing sensors OOR.

A strange thing happened this afternoon as I was doing some testing work. I started the truck (it always starts easily!), and it immediately started running at 2000rpm. If I stomped on the gas pedal I could get higher RPM, but it would not go lower. I examined the throttle linkage and it was closed against the stop. When I turned the ignition off and restarted the truck, it went back to its previous "stall at idle" operation. I don't know if this is a clue or a quirk!

I'm back to suspecting the computer (ECM) again unless someone can point me in another direction.

 

Let me ask just to make sure ... Youre doing all the test when the engine is at operating temp , Correct ? Its kinda puzzling all the sysmptoms you describe . It sounds like you are dealing with 2 or more different problems that all look like they are the same . To begin with , I would test the Tps Sensor to make sure its readings are correct & consistant all the way thru its operating range . Next disconnect the Ect wiring , hook your Dvom to it & see what readings you get from a cold engine to operating temp . There may be a point where it is out of range which in turn , If I remember correctly , will keep the O2 Sensor from going to closed loop . Have you checked your fuel pressure ? The only thing that may link all the symptoms is either unmetered air entering the engine or excessive fuel pressures . Try to solve one problem at a time unless it looks as if they all have a common link .

 

 

i think the resistance at the iac is supposed to be 15 ohms. iam not positive but this seems to be what i read in specs somewhere.

 

I don't know if this will help, but my father had a '90 bronco and it ran rough and we went through trouble codes and tried different sensors and so on. Come to find out the gasket that goes between upper and lower intake manifold had deteriorated and let the engine pull in fresh air around manifold and not through the air intake sensor, making it idle either rough, high then low (as computer adjusted because of a higher O2 content in exhaust). I hope this helps in any way.
Todd
BB79RANCH