96 Silverado new everything, but sputters at hiway speed ...

Asked by Jun 08, 2017 at 06:00 AM about the 1999 Chevrolet Silverado 1500 LS Extended Cab RWD

Question type: Maintenance & Repair

I  have a 1996 5.7L Vortec TBI, Silverado, brand new CRATE Engine form the factory, new distributor & distributor cap rotor plugs, wires, fuel injection conversion, all professionaly installed.Runs great, however when I am at 70Mph and back off the accelerator slightly, the truck miss fires or sputters, enough that it jerks as if the power to the coil was shut off and back on again.It doesnt seem to be an issue at lower speeds 35 to 45 MPH.  Also just had the tranny professionaly rebuilt and shift kit installed. Connected to my ODBII and get a miss fire or cross fire code

1 Answer

85,295

Since this is a new engine, the miss-fire code should have told you what cylinder to look at for the miss-fire. Concentrate on that cylinder for problems, electrical connections, check the wiring, check the spark plug, check the compression in that cylinder, it should be around 155 lbs. maybe a valve adjustment. You computer would have needed to be reprogrammed also to run the MPFI from the TBI set-up. I am assuming the intake manifold and throttle body are new? Because if it where not, possibly the Throttle Positioning Sensor may have a dead spot in it. Another thing that would have had to be new would be the EGR valve. Just go over the engine to look for problems, make sure the EGR system is hooked up correctly. I don't have much time this morning, have to work, but would be back later today.

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