1994 ford ranger 4.0l engine IAT sensor voltage fluctuating... Truck only starts cold

[Jawboy]

Hey guys,
My truck has been flawless for years always taken the most care of her then all the sudden one morning...

When I start my vehicle the idle will jump between 1500 rpm and 2000rpm the check engine light will also turn on and off in synchronization with the idle. So check engine light on Idle goes up to 2000 check engine light off idle drops to 1500. Once the vehicle is warmed up the idle will continue to jump up and down and CEL on and off but at a lower rpm.
So I started ohms testing a few sensors and such (TPS, IAT, MAF, AICV) all tested fine then moved onto the wiring harness. Everything checked out fine except the circuit to the intake air temperature sensor. The voltage is supposed to read 5 volts from the harness.
The voltage will read 5 volts then as the check engine light comes on and the idle jumps up it will read 1 volt. Check engine light off, idle drops to normal, voltage back to the specified 5 volts. and the motor will continue to idle up and down and up and down.
I have tried my obd 1 code reader and the code reader says. "Vehicle not responding" when trying to pull codes.
Another thing is the truck will only cold start. Once warmed up it will crank and crank and crank but never start.
according to my chiltons book I may have a bad ecm.
Any help would be greatly appreciated. Thanks!

 

[flh700]

sounds like a temp sensor had similiar problem swapped that out and straightened it out ,it sends info to ecm telling how warm or cold engine temp is and allows for fuel air ratio , mine was sending false info wouldnt let truck start when warm.

 

[Jawboy]

Ok Ill have to start looking at the sensors a little lower on the engine.
ALSO the truck failed a wiggle test that my scan tool has provided!!

 

[1994Ranger$$$pit]

Use the CEL and a paper clip to check codes with the engine off.( how to in the Tech section)
If that don't work it sounds like the ecm is toast.

 

[Jawboy]

The paper clip CEL method worked! 6 codes!

113 IAT sensor is/was high or open
118 ECT sensor is/was high or open
123 TP is/was high or short to power
341 Octane jumper installed
513 Replace processor (PCM) (internal failure)
522 MLP Sensor Circuit open/ vehicle in gear

I assume all the other codes are because of the bad ecm.

I have new ECM on the way it will arrive friday.

The truck is 20 years old has almost 200,000 miles on it. Is it normal for your ECM to fail. I have always been meticulous about maintenance. The truck was running great I drove it everyday for years untill the other morning.

 

[1994Ranger$$$pit]

Cool.
Make sure there's isn't some place where water may have gotten to it.
My 94 has 160K on it.
BTW,where did you get the ECM from,just curious?

 

[Jawboy]

Engine control module was the fix! truck is running great again.
I ordered the part through carquest. The company that re-manufactured the ECM was A1 cardone.
Also noticed that there was a small bit a moisture around the ecm!
Ill have to figure out a way to seal that enclosure in the firewall. Any Ideas?

 

[black_demon69]

Engine control module was the fix! truck is running great again.
I ordered the part through carquest. The company that re-manufactured the ECM was A1 cardone.
Also noticed that there was a small bit a moisture around the ecm!
Ill have to figure out a way to seal that enclosure in the firewall. Any Ideas?

duck tape lol

 

[FritzTKatt]

On Rockauto it lists different part numbers (all F47-'s, of course) for the ECU, but they differ for trans, drivetrain, and emissions. I don't see any listed under manual, 4x4 and federal.

Does it make a difference to have an auto trans one on a manual?