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Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 1685
Experience:  GM Grand Master Technician 2007. 14 year experience.
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I have a older GMC sierra K1500. The truck is a 1993 with

Customer Question

I have a older GMC sierra K1500. The truck is a 1993 with the 5.7 liter 350. When I start it cold it goes on high idle right away than settles down about 300 to 400 rpms after about 10,15 seconds. After running for about 3 to 4 minutes the idle starts raising and lowering and idling rough which stops after a couple more minutes. When the truck gets to operating temperature on the gauge it starts stumbling and and wants to kill. I have to do the brake throttle at the same time to keep it running when at a light or stop sign. I have replaced the idle air control valve twice the throttle position sensor both coolant sensors, map sensor, egr valve and solenoid, oxygen sensor, fuel pump and fuel filter. Have also removed and cleaned the throttle body and replaced the regulator plus a new distributor, plugs and wires. Runs excellent off idle when driving and has great response. I am now wondering if the ECM could be a problem but runs so great off idle. Any suggestions at this point would be very appreciated.
Submitted: 1 year ago.
Category: GMC
Expert:  Gary replied 1 year ago.

I am sorry your having this problem

everything replaced was to correct this issue ?

no codes set ?

any access to a scan tool to view live data ?

is the idle screw plug removed where you can see the idle screw to put a torx in to turn it ?

Customer: replied 1 year ago.

A friend has a mechanic he knows and the only code that came up was for the fuel cap. Don't know of any problem there. Don't know about any plug for an idle screw on this throttle body?

Expert:  Greg Jensen replied 1 year ago.

Hello, My name is***** will see if I can help you. As you know you have replaced pretty much all the normal items that can be the faults. If you and your friend have reviewed it with no codes and you have checked the fuel trims as well to make sure they are within normal range. I would be sure the egr valve is closing all the way when this twould he idle issue is present. I reviewed all the old GM tsb's and alldata tsb's and nothing matched there. I do know that these ECM's did present some odd type issues that create situations like yours. these also have a PROM chip inside the ECM as well which could be your source of fault.

Customer: replied 1 year ago.

I really don't think your answer is satisfactory for payment. I have already came to the conclusion of the ecm which also includes the prom chip. After having tried everything I have the ecm is only a last resort. I don't think Just answer has come through this time.