Surging while driving

Under PSI · January 23, 2005

1993 GMC Sierra 5.7 TBI A/T

New: plugs, wires, cap, rotor, eprom, air filter, egr control solenoid. All GM parts

While driving at hwy speeds, there is a surging in the motor. Also, it will missfire badly while reving in park to aprox. 1500rpm. I know I have narrowed it down to a problem with the egr system because none of the symptoms happen if I remove the vacuum supply to the egr valve. I have removed the valve and it is not sticking, and it is not leaking. I have sprayed throttle body cleaner into the ports of the intake manifold where the egr valve bolts up and they do not appear to be clogged. I have run 1 bottle of GM top engine cleaner through each of the throttle openings. If I have it in park and try to maintain a steady rpm of 1500 at normal operating temp., the engine will missfire badly and I can see the egr valve opening wide open. If I remove the vacuum hose at the egr valve, or disconnect the egr solenoid valve, the missfire clears up. I always thought that the egr did not operate in park. I have used a scanner to verify that the ecm is seeing that the trans is in park (so I know the park switch is ok) but it shows that the egr command is 100% during the missfire and while driving when the surging occurs. I installed an updated GM eprom and that did nothing. Any ideas? Thanks

BBSilverado · January 24, 2005

1993 GMC Sierra 5.7 TBI A/T
New: plugs, wires, cap, rotor, eprom, air filter, egr control solenoid. All GM parts

While driving at hwy speeds, there is a surging in the motor. Also, it will missfire badly while reving in park to aprox. 1500rpm. I know I have narrowed it down to a problem with the egr system because none of the symptoms happen if I remove the vacuum supply to the egr valve. I have removed the valve and it is not sticking, and it is not leaking. I have sprayed throttle body cleaner into the ports of the intake manifold where the egr valve bolts up and they do not appear to be clogged. I have run 1 bottle of GM top engine cleaner through each of the throttle openings. If I have it in park and try to maintain a steady rpm of 1500 at normal operating temp., the engine will missfire badly and I can see the egr valve opening wide open. If I remove the vacuum hose at the egr valve, or disconnect the egr solenoid valve, the missfire clears up. I always thought that the egr did not operate in park. I have used a scanner to verify that the ecm is seeing that the trans is in park (so I know the park switch is ok) but it shows that the egr command is 100% during the missfire and while driving when the surging occurs. I installed an updated GM eprom and that did nothing. Any ideas? Thanks

<{POST_SNAPBACK}>

I had the same problem a few years back on a 4.3L S-10 Blazer. What solved the problem for me was the IAC (idle air control valve)

It threads into the side of the throttle boby and has a square four pin wire connector on it.

Under PSI · January 25, 2005

Thanks, I'll check that out.

Under PSI · March 25, 2007

Thanks, I'll check that out.