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1995 Gmc Sierra starts but won't idle

gwhuntergwhunter Member Posts: 2
edited January 2020 in GMC
This truck starts will run as long as you float the throttle but when you stop it stalls emmediately. Here is what has been changed. The egr valve, map sensor, computer, fuel pump, iac, spark plugs, wires, distributor cap and rotor, had control module checked at parts store, ok, checked fuel pump relay with meter, bench tested oil pressure switch because of it's backup circuit to fuel pump, egr solenoid, back probed all sensors and relays for proper voltages, ok, checked tps ok, plenty of spark and good compression on all cylinders. If I put external vacuum on map sensor and hold my finger on the vacuum port I disconnected it from it will run. I'm not grasping what this is telling me. Any help at this point would be appreciated. It only throws a code 43 which is knock sensor. I haven't checked that yet but would that make it not run at idle. I can't keep it running long enough to try and drive it. Forgot to mention it is a 5.7 L.


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    deanrileydeanriley Member Posts: 1
    Did you ever figure this out? My nephew has the same truck with identical symptoms
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    thecardoc3thecardoc3 Member Posts: 5,753
    edited January 2020
    deanriley said:

    Did you ever figure this out? My nephew has the same truck with identical symptoms

    Look at the MAP sensor signal and see if it matches manifold vacuum. If it is reading too high check the vacuum signal for the sensor, if it is higher than the manifold vacuum as described in the post you responded to the throttle body gasket has probably failed. They tend to go bad right where the port is for the MAP sensor and weaken the signal.

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