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1996 GMC Sierra 7.4 C3500 strange engine problems.
sonicdude10 posted a question in Ask the GM TechnicianNew guy here with engine problems. All parts are stock. No modifications have been done to engine and engine management to our knowledge. With that said, here is what is going on. Truck is a 1996 GMC Sierra. Crew cab, long bed, C3500, non dually. Has 7.4 454 Vortec and 4l80e transmission. Driveline has 147,000 miles and engine has good compression. All cylinders read 120 to 140 PSI. What's going on is the engine refuses to run properly. First up is the MAF sensor. Had the engine light on the dash so I used our OBD2 scan reader on it. Got P0102 which is Mass Air Flow sensor low output. Replaced the sensor with a Bosch unit and cleared the code. Next engine start that same code came back. I checked all the wires and plugs that section of the harness goes through. It has a good run to the computer. No frays or breaks. I even reset all plugs thinking there might have been some resistance in the connectors. No dice. Code comes back every time I clear it. Any ideas? Next up is the really serious problem that prevents the engine from running properly when cold. Low fuel pressure. According to our gauge it only has 20 PSI max and leaks down as soon as the pump turns off. On the fuel system we have a new pump in tank, fresh filter, and new regulator. That was a pain to install as the top of the intake manifold had to come off. Still have the low pressure and leak down. We thought the injectors were leaky but the thing is lean at idle as we sprayed starting fluid in the intake tube and the idle evened out. When cold it doesn't want to idle at all but will even out at 650 to 750 RPM when warm. Any idea what is going on? We have no idea what type of pump is in the tank as the people who own the truck put it in before we were tasked with fixing it. Still have the stock one. We plan to pressure test it and if it can go up to at least 80 PSI we will just put it back in and see what happens. So, any ideas on what is going on? I can record videos if any of you want to see the engine and hear it running. If anyone can figure this out I would greatly appreciate it as this goes beyond my knowledge of fuel injection problems.