QuestionI have a 1994 Chevy Z71 5.7L 250K mi. My speedo and tach are going nuts at idle and the speedo is always maxed out while driving, the tach works fine at cruising speed. My scan tool reads the rpm and the speed just fine with no unusual signals or jumping around.
It idles fine, but upon take off, and while trying to pass it intermittently hesitates, stumbles, and backfires, and every now and then it will do the same at road speed. Under these conditions the Scan tool says the fuel trim is 158 demanding more fuel and the exhaust oxygen is always rich.
Within the last 3 days I've replaced the ignition module, cap, rotor, plugs, wires, coil, fuel filter, and coolant temp sensor. I've also inspected the EGR and the distributor shaft for cracks and sprayed carb cleaner all over to check for vacuum leaks and found nothing wrong.
Every time I've got done working on the truck it would run perfect for awhile (10-15mi) then all of a sudden it would go back to running rough. Today I've also noticed that its kicking itself back into open loop after a 15mi drive and the coolant was at 195. The only thing I can think of left would be the fuel pump getting weak but that wouldn't have anything to do with it going into open loop, or the ECM is taking a dump. Any info would be helpful for my possessed truck.
Thanks
AnswerHi Jeremy, I guess the first thing I would do is check the fuel pressure to make sure it is good...10 to 13. The next would be to replace the 02 sensor for a bad reading. Be sure to check the engine timing to make sure it is set correctly. Haven't seen to many ecm's go bad on these trucks.
Hope the helps
Have a Nice Day Mark
