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Toolhut, ASE Certified Technician
Category: GMC
Satisfied Customers: 550
Experience:  Owner of a mobile diagnostics company assisting auto technicians with difficult vehicles
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I have a 1994 GMC Jimmy 4.3 Vortec W code. The Jimmy idles

Customer Question

Hi, I have a 1994 GMC Jimmy 4.3 Vortec W code. The Jimmy idles rough, it shakes, and idles up and down. It revs to high rpms without trouble and seems to smooth out when brought just off idle. When it idles up and down the oil gage goes between 40 and 5. When I drive it and floor the gas pedal it seems to have just about the same power as before the idling problem. when I let off of the gas to slow down it idles low enough that the brake pedal gets hard and the engine sometimes dies. The check engine light only comes on when the thermostat opens. I have checked the code (obd1) and get a map sensor low voltage code, a idle air control low voltage code, and a tps low voltage code.
I have replaced the ground strap, idle air control, map sensor, tps sensor, ecm, distributor, egr, coolant temp sensor, knock sensor, alternator, plugs, wires, cap, rotor button, ignition control module, spider injectors, nut kit, upper intake gasket, and brake booster check valve. I have unplugged the o2 sensor which didn't make a difference. I believe that there has to be a short in the wiring but am unsure what wires or wire to check that would effect all of these sensors. The only problem that I have had that is electrical so far is with the tail lights I replaced the blown taillight fuse and it seems to have fixed it. The taillights and dash lights went out when the fuse blew, I haven't been able to drive it long enough to see if it is still a problem. It blew when I went over a rough bridge......
Submitted: 1 year ago.
Category: GMC
Expert:  Toolhut replied 1 year ago.

Hi, My Name is ***** ***** for posting your question to just answer. I am a ASE Certified Master Auto Technician with 20 years experience. I am confident we can resolve your problem together.Please remember I don't know your skill level. I can't see, smell, hear, or touch the vehicle. It may take a few back and forth replies to get to a satisfactory resolution.I also try to give all of the information you may need expecting a positive rating.

Do you have a vacuum gauge?

Customer: replied 1 year ago.
I do not. I have sprayed 2 cans of break cleaner on all of the vacuum lines and around the upper and lower intake while the Jimmy was running. The engine did not idle any higher. I thought at first it may be a vacuum leak but I don't know how a vacuum leak would cause a low voltage code on the map, tps, and idle air control. If you are referring to the hard break pedal I was using that as an example as to how low the Jimmy is idling.
Customer: replied 1 year ago.
Brake** auto correct....
Expert:  Toolhut replied 1 year ago.

I suspect you have a broken valve spring or other valve train problem. Not a vacuum leak.

A Vacuum gauge is the only way to confirm for sure

Customer: replied 1 year ago.
How would a broken valve spring cause a low voltage issue?
Expert:  Toolhut replied 1 year ago.

On map voltages yes and when it runs bad it will cause other codes. Your symptom of a rough idle that smoothes out is either egr or valve train related