94 K1500

[mrjeremyj]

1994 Chev K1500. Idles fine but upon exceleration and passing it intermittantly backfires through the exhaust and every now and then during highway speeds. My tach and speedo go crazy at idle but at hiway speed the speedo maxes to 85 and the tach works fine. My scanner shows accurate speed and rpm at anytime. When the truck starts surging, the fuel trim is 158, exhaust oxygen is rich, and as of yesterday the truck started kicking back into open loop with the temp at 195 after a 15mi drive. The MAP and TPS check out also. During the last 3 days I've replaced the ignition module, cap, rotor, coil, plugs, wires, fuel filter, and coolant temp sensor. I've also inspected the egr valve, sprayed carb cleaner all over to check vacuum leaks and inspected the distributor shaft for cracks, checked the injectors for uneven spray and found nothing wrong.

 

Every time I got done working on the truck I would run perfect for about 10-15mi then all of a sudden it would start running lean again and backfire. Looking for more ideas before I buy a PCM.

 

Thanks

[90GMCZ71]

1994 Chev K1500.  Idles fine but upon exceleration and passing it intermittantly backfires through the exhaust and every now and then during highway speeds.  My tach and speedo go crazy at idle but at hiway speed the speedo maxes to 85 and the tach works fine.  My scanner shows accurate speed and rpm at anytime.  When the truck starts surging, the fuel trim is 158, exhaust oxygen is rich, and as of yesterday the truck started kicking back into open loop with the temp at 195 after a 15mi drive.  The MAP and TPS check out also. During the last 3 days I've replaced the ignition module, cap, rotor, coil, plugs, wires, fuel filter, and coolant temp sensor. I've also inspected the egr valve, sprayed carb cleaner all over to check vacuum leaks and inspected the distributor shaft for cracks,  checked the injectors for uneven spray and found nothing wrong.

 

Every time I got done working on the truck I would run perfect for about 10-15mi then all of a sudden it would start running lean again and backfire.  Looking for more ideas before I buy a PCM.

 

Thanks

 

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Have you checked fuel pressure? Specs are 9 - 13 PSI. If the pressure is low, it may be fine at idle but can run lean at higher RPM.

 

//90GMCZ71