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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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Have a 93 GMC !500 With a 5.7 in it. Starts fine but when it

Customer Question

Have a 93 GMC !500 With a 5.7 in it. Starts fine but when it warms up it runs poorly. Whats wrong?
JA: Will the headlights turn on? Is the battery fully charged?
Customer: Yes.
JA: Are you fixing your 2500 yourself? What have you tried so far?
Customer: Ignition module, Spark plugs, Wires, IAC, Oxygen sensor
JA: Anything else you want the mechanic to know before I connect you?
Customer: Seems like when it goes to closed loop is when it fouls up
Submitted: 1 month ago.
Category: GMC
Expert:  Edward replied 1 month ago.

Hello, Eddie here and I will do my best to help. Have you checked the fuel pressure by chance when it's acting up? Also, is the check engine light on now when it is running? If so, do you know what codes are stored? thank you.

Customer: replied 1 month ago.
Have not checked fuel pressure. Do not have a gauge. Have changed fuel filter and MAP sensor also. I have the EGR off now and tried to apply vacumn by mouth and I can't get it to move by suction but it will move by finger force.
Customer: replied 1 month ago.
Never mind about the EGR valve. mechanically it is functioning.
Expert:  Edward replied 1 month ago.

Alright, These really like 35 psi running of fuel pressure, the spec is 40 -47 psi key on engine off, and 30-44 psi at idle, it should go up slightly when you crack the throttle. I would check the fuel pressure just to be sure it isn't low, You can probably get a local auto parts store to loan you a gauge like auto zone or oreilly. I would do that first just to rule it out.

Is the check engine light on now when driving or is it out? thanks.

Customer: replied 1 month ago.
There is no codes or check engine light. It has been idling about 20 minutes now and is doing fine. Guess I'll have to take it for a drive.
Expert:  Edward replied 1 month ago.

That's interesting, has it got factory exhaust manifolds and exhaust system? Is the catalytic converter on it?