after a recent resurrection of my truck, i ran into some more problems, my truck doesnt stay idling after it has been off for a while, the engine also searches while idling if it doesnt stall. ive already cleaned the IAC and MAS, im planning on replacing the fuel filter and spark plugs when i wont be at risk freezing my hands to the frame. the tranny also needs a band adjustment, so any ideas on what it could be

 

Well every thing hit the fan at once, I got an apointment to have it looked at, i have this nagging feeling that the tranny is trashed, my life loves to take steel toes to my teeth when I go to smile

 

IAC(idle air control) Valve is most likely bad if you have cleaned it already.
Computer reads RPMs from crank sensor.
It has "target Idle RPMs" programmed in memory.
It opens and closes IAC Valve to maintain those target RPMs.

The IAC Valve has a Step motor, or stepper, this is an electric motor that can be turned in "steps", there is usually more than 100 "steps" the computer can call up.
Each "step" can be called using a unique voltage pulses, like a morse code.
Step 20 might have valve opened 1/4", step 50 might be open 3/4", ect.............
Computer learns what "step" it needs to call so engine idle at 700rpms, and it can relearn this as well.
But when idle surges up and down it could mean the motor is not reading "step pulses" correctly, or you have a vacuum leak.
So computer is trying to stabilizes idle when the system is unstable.


Yes, 16 year old automatic is probably do for rebuild.

 

Thanks i have an appointment with a guy who does transmissions, and I found a new IAC at my parts store for 50 bucks, thanks for your help

 

So I replaced the IAC, it seams to idle a bit better, however when I went to turn it on this morning, it reved up then immediately died, then I turn it on again, it ran for about 30-40 seconds. The last 10 seconds the rpm was below 500, and it was putting trying to stay on. So now im not sure if its the shear cold affecting something like the spark plugs or wires, or possibly the fuel filter.

 

The computer uses the ECT(engine coolant temp) sensor to determine engine temp, the computer has a Choke Mode it runs when engine is cold, this is a high idle, rich fuel mix and advanced spark timing.
The ECT sensor is a TWO wire sensor located on the lower intake just above the thermostat housing.
There is a ONE wire Sender located near it, that is for the dash board temp gauge.

ECT sensor is easy to test, it is a simple resistance sensor.
Computer sends it 5volts on one wire and gets back .5v to 4v on the other wire
4v would be very cold, below 32degF
.5v would be warmed up.
You can use a volt meter set for DC volts and a sewing pin to test
Key on engine off
Use pin to pierce one of the ECT wires(leave it connected to ECT sensor)
Put Red meter probe on the Pin and Black meter probe on the engine(Ground)
If you get above 4.5volts that's the 5v power wire from the computer.
Switch pin to the other wire
If engine is Cold you should get 3v to 4v on that wire.
If engine is warm you should get .5v to 1v

If the sensor is faulty or if the connector has some corrosion in it, it will reduce the voltage going back to computer, so..........computer thinks engine is warm and if it is cold then it won't stay running, because a cold engine needs to be "choked" until it warms up.

If you don't have a volt meter then you can try just unplugging the ECT sensor, cold engine, and see if it starts and stays running.
With no ECT connection computer should turn on CEL but it will also run a cold engine start up routine, this is a "fail safe" routine.
If cold engine starts better and stays running, replace ECT sensor.

 

thanks for all your help, i finally got it running good, i bought a new sensor but havent put it on yet, when we went through a warm spell my truck ran better, so ill be putting the ECT in when i get the time.