91 1500 Idle Problems

edlugz · May 31, 2010

So I'm trying to diagnose my dads truck it has problems starting then runs rough and dies if gas is not constantly applied. There's no obvious vacum leaks, the strange thing is that if I unplug the connector that you unplug to adjust the timing, it runs perfect but with a check engine light on. Any thoughts?

RageZro · May 31, 2010

Didn't see what engine you have but you are taking the ECM's timing control out of the loop when you unplug that connector. Have you coded the ECM? Plug the connector back in, pull the fuse to the ECM or disconnect the battery for 10 seconds and then reconnect, then restart the engine. (WARNING! You shouldn't have the theft prevention radio but if you do then don't disconnect the battery, just pull the 10amp ECM fuse) Now with the power off jump the right most top row two pinholes with the key OFF. Turn the key to RUN ONLY ~ DO NOT TRY AND START!!! Watch the Check Engine light, it should go out then blink once then a pause then twice, then if there are any error codes it will blink them out so write them down and post them here. It will continue to repeat the codes until you turn off the key and remove the jumper. BE SURE TO REMOVE YOUR JUMPER BEFORE STARTING!!!

The ECM is probably trying to compensate timing for some sensor error. There aren't a lot of sensors on '91's so you can try to isolate the most likely candidates. If you get no codes then unplug the spark control module and try restarting. If no change in the way it runs then plug the SCM back in and unplug the the MAP and restart. The EGR control would be the next one but you can check the EGR valve itself by pressing on the diaphragm with the vacuum line unplugged from it, if it moves with some stiffness and you hear air come out of the hose connector on it then it is probably good. I don't think the throttle position sensor wouldn't be an issue but would cause a flat spot in response when unplugged, the Idle control module shouldn't either but since shorted components can cause all kinds of strange behavior you can try them if you want. Look for damaged wiring to the sensors

As always make sure the wires, plugs, cap and rotor are in good shape to cover the basics. My bet is the MAP unless your anti-knock sensor lead is shorted that goes to the SCM.

edlugz · June 1, 2010

well its 5.7L engine and while looking at the engine i noticed a connector that wasn't plugged into anything and i couldn't find a female end to plug it into. attached should be the pics of what Ive found. now as for reading the codes i thing i found the ecm fuse it says ecm ign but im not sure of what box your referring to that i have to jump the two points at.