fiveohcalloway

So, my friend has a 95 mustang gt and is having problems with the car idling and driving crappy. So far he has done a complete tun up on it since buying it 2 months ago. In the morning when he starts it up it idles at 1100 rpm and sometimes jumps up to 1500 or 1700 and stays there for a second or two and then comes back down to 1100. He says it doesn't do it all the time just sometimes. The other issue he is having is if he drives the car before letting it warm up to normal operating temperature it spits and sputters while giving it gas. He said its like choking and doesnt want to go. If he lets it warm up then its fine nothing wrong. He changed the fuel filter this weekend to see if that would clear it up and it hasn't. He has checked for a vacuum leak and hasn't found one either. So any ideas will be much appreciated.

dawson1112

First thing that comes to mind with a car not running properly when its cold is the engine coolant temp sensor. This is the sensor that tells the ecm what temp the engine is so that it knows where to set the air/fuel ratio and timing. A false reading will cause this exact problem. I have seen these sensors do some pretty odd stuff such as giving a false reading of -40 degrees F when it starts to warm up a bit the reading becomes more acurate . A false reading such as this will cause the ecm to dump way to much fuel . Im guessing that when it does act up like this it smells like unburnt fuel ?

petrock

The temp sensor has no effect on the air/fuel mixture. None at all.
What I think your thinking of are O2 sensors. Those do effect the air/fuel mix. However, when the motor is cold or at wide open throttle (open loop mode), the ECM ignores the O2 sensors completely. Its only when the motor gets hot that the ECM starts listening to the O2 sensors (closed loop mode).

Since his problem only in open loop mode (when the motor is cold), its not the O2 sensors. When in Open Loop mode the ECM determines how much fuel to put in the motor based on the readings from the Mass Air Flow sensor. Yours probably needs a good cleaning.

dawson1112

So your saying the ect has no effect on determining air/fuel ? If that is the case what does the ecm look at to determine engine temp?

Since our efi engines dont have a choke the ecm needs some referances in order to maintain the idle and operation of a cold engine.
The O2 sensors dont come into play untill they heat up. Once they heat up the ecm looks at the ect again , if it is not up to operating temp it will remain in open loop mode. If the ect is telling the ecm that the engine is still cold It will assume that it needs a richer air/fuel mix. If the engine is actually at 140 degrees and the ect says 40 degrees you end up with to much fuel . Same goes with the intake air temp sensor. Colder readings get more fuel.
Now once the ecm goes into closed loop the ect is resposible for telling the ecm to turn the fans on and off.
Maf tells the ecm how much air is coming into the intake at all operating temps . A dirty or bad Maf will cause problems throughout the operating temp ranges not just when its cold.

Ecstasy

The other things could be ignition related, a faulty ICM(ignition control module) can cause that, and it's not expensive to get a new one and worth it anyways, along with the ignition coil. Then if he has a bit of money I'd get a new Idle air control valve($75?) and make sure the voltage is correct on the throttle position sensor after you change the iac since you're right there.