Engine Performance problem
1993 Chevy Truck 6 cyl Four Wheel Drive Automatic 140000 miles
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My truck has a Z-code 4.3L V-6, no emission air pump, A/C, TBI, all stock. It starts and idles fine but when I'm on the road at 55-60 mph and try to accelerate it has a very noticeable hesitation and occasionally backfires. Took it to the local Chevy dealer and they said it was the EGR valve so they replaced it. It still does the same thing. Did the normal routine...new plugs, wires, checked distributor and rotor (both look fine), new air filter, cleaned the injector fuel screens, new fuel filter, verified correct timing, nothing seems to help. No codes on the ECM so I'm totally lost. I have an Actron code scanner and I did the service diagnostic test with the engine running and it indicated an "open loop" until the engine warmed up, then it showed a "closed loop" mode which is supposedly normal when the engine reaches enough temperature for the O2 sensor to feed back a signal to the ECM. I checked the input voltage to the MAP sensor and it was within range. I am at a total loss as to what to try or even check next. Any help would be greatly appreciated.

try disconnecting the EGR valve and see if it does it. You can always clear a code, but if the problem reamins you know it isnt the EGR system. Has a fuel pressure test been done?

I disconnected the electric plug to the EGR solenoid and it ran good. Plugged the connector back in and it did the spit-n-sputter again. Since the EGR valve is new would it be safe to say that the solenoid valve is faulty? I tested the vacuum on the solenoid inlet and it had good negative pressure but no negative passed through the solenoid. Also, on the outlet side of the solenoid there is two ports, one for the EGR and the other is supposed to be a vent. I noticed that there was a cap on the vent side.....should that be capped or should it be able to vent to atmosphere? What's the best way to test the solenoid?

Best way to test is to run a long vacuum line to a gauge that you can see while driving. That way you can see if it is trying to activate the EGR. If you can find an old school parts house, get a vacuum delay valve, install it inline to the EGR, it will keep it from coming on too soon causing the stumble

I tested the solenoid by removing the unit and applying 12V across the input terminals. This should have opened the solenoid allowing vacuum to pass from the manifold side (input side) to the EGR side. The solenoid did not move to allow the vacuum to pass. I assume that since it did not activate then the solenoid is defective. Since it's a relatively inexpensive part I put one on order and I will install it tomorrow and do another test run. The truck runs great when the electrical connection to the solenoid valve is unhooked so this almost has to be the culprit.

Let me know what happens

I installed a new solenoid valve and the truck runs great! Thanks for your help.