1994 GMC Jimmy idleing high

First thing to do is check for a vacuum leak. Pay particular attention to the EGR valve area, in intake manifold.

Let me know how the vacuum hoses and connections are and I will follow up.

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EGR valve is located center, upper engine area, front of intake manifold, passenger side of water outlet, mounted in intake manifold.

EGR valve would cause rough idle and stalling in most cases, not high idle. However, to check EGR valve function, have someone idle the parked vehicle with the brake on while you examine the plunger shaft to see if it is stuck open. Also, while engine warm, rev up engine and watch to see if valve is moving.

High idle can be caused by several things. A list of possible causes and corrections are below.

Ensure throttle linkage and/or throttle position sensor do not stick or bind.
Check park/neutral switch circuit. Ensure park/neutral switch is properly adjusted.
Check exhaust sytem for restrictions, such as a damaged or collapsed pipe, muffler or catalytic converter.
Ensure initial ignition timing is properly set.
Disconnect MAP sensor. If condition is correct, replace sensor.
Check engine idle speed (both base and PCM idle).
Check Idle Air Control (IAC) system. Check for foreign material in IAC bore.
Check power steering pressure switch circuit.

Information about backfiring can be found at the link below. .

https://www.2carpros.com/first_things/why_does_an_engine_backfire.htm

After checking out these steps let me know if you still have the problem

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Yes, clogged fuel filter will cause backfiring. Wires should be OEM parts and recommended size. If they are smaller than recommended size then yes, they could cause backfiring. Plugs will not receive enough to ignite all of the fuel which in turn will cause backfire. I would start with swapping out those wires before replacing the fuel filter.

Oh okay, that would be even better. I was just trying to save money but if you can afford it then I would definitely replace it all. Even if fuel filter isn't the problem it will give you better performance and much better fuel mileage with a new filter.

First check your ATF (auto tranny fluid). If fluid is burnt or low then replace it and fill properly. If the fluid is good then the tranny will more than likely need overhauled. Having to rev the engine to get the transmission to move sounds like a worn out hydraulic front pump or something is really wrong with the gears themselves.