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im sorry for the long wait did you still need help ?
did you have another question opened up with your trucks concern ?
who told you to tap the ECM ?
what is your truck not doing or doing ?
We have had a series of problems with our 1993 GMC Sububan 5.7L TBI. For a while, it would turn over but not crank. We replaced the coil and distributor and it cranked but would not run correctly. We noticed that we could not hear the fuel pump run when we first turned on the key so we replaced the pump and can now hear it run initially before we crank it. It cranks now when cold and runs fine but will not idle. as soon as the RPMs drop it dies. We have replaced the IAC, MAP, and engine temp sensor and other things that I am probably forgetting. We are thinking that maybe the ECM is bad. I was looking at a similar post on your sight and the "expert" suggested testing the ECM by removing it and tapping it with a screw driver handle or soft hammer with cranking the engine. The response did not indicate what to look for or listen for to determine if the ECM is working. Any suggestions on what could be causing the truck to stall at idle?
so Rhonda the truck will start up just fine and drive fine as long as your foot is on the gas pedal is this correct ?
and as soon as you take your foot off the gas pedal it will not idle and stall out but will restart right back up like nothing happened ?
This is correct in some cases. When cold, the truck starts great but will not idle. Once warm by holding your foot on the pedal it dies and is very difficult and in some cases impossible to restart. Once cooled off, the truck starts again just fine but will still not idle. In most cases, when we turn the key, you can hear the fuel pump running. But after it dies when warm the fuel pump is intermittent. This has been a very frustrating experience for us (especially my husband). Real scenario, truck cranked fine on Sunday and tried to idle but stalled. Crank right up again (still cold) and my husband drove it for a while making sure to keep the RPMs up at 1500. After about 10 -20 mins of driving, the truck just died in the middle of the road and would not crank for a while. Finally was able to get it cranked again after about 25 mins of trying and jumper cables and made it back home.
this can be an ECM problem it can also be an ignition switch problem not sending the correct voltage to the ECM so it can operate correctly
since you noticed the fuel pump will not run sometimes when you turn the key on is very interesting
any chance you can get this to die and not start so we can see if its missing fuel or spark when it will not start ?
We replaced the ignition switch too. Forgot to mention that. I am sure we can get it to repeat the problem. My husband did undo the fuel lines in the throttle body thinking maybe they were locked and one side was pumping fuel and the other not. We replace the fuel filter also. I have also noticed that there seems to be a correlation between the buzzer when the key is turned for the 10 seconds or so and the pump running or not running. Not a 100% of the time but it seems that for the majority if no buzzer then pump doesn't run which is why we replaced the ignition switch but it still does it.
get it to not start
with the air cleaner off when cranking it over if fuel pulsing out of the bottom of both injectors ?