1996 GMC Sierra 7.4 C3500 strange engine problems.

[sonicdude10]

New 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.

[Doug_Scott]

You did not change the fuel pump so you have no idea if it was done with new parts or used parts. You need to verify that the pump is not only new, but is the correct one. Have you blocked off the fuel return line and checked fuel pressure with engine not running, key on? 20psi is simply not enough.

 

There are only so many parts involved. The problem is somewhere between the sock on the fuel pickup and the fuel injector pintle. If you did not replace a part, I would not take someone else's word it was replaced correctly,

 

The code you get regarding the MAF may also mean the air flow does not match the reading. Verify that all connections in the air intake system are tight and leak free. Adding air to the intake after the MAF will screw up the system. Note this does not mean a vacuum leak, I am talking about a leak BEFORE the throttle body, but after the MAF sensor,

[sonicdude10]

You did not change the fuel pump so you have no idea if it was done with new parts or used parts. You need to verify that the pump is not only new, but is the correct one. Have you blocked off the fuel return line and checked fuel pressure with engine not running, key on? 20psi is simply not enough.

 

There are only so many parts involved. The problem is somewhere between the sock on the fuel pickup and the fuel injector pintle. If you did not replace a part, I would not take someone else's word it was replaced correctly,

 

We fixed the fuel problem. We know the replacement pump is new, was installed correctly as we have the box it came in and checked the mounting of it, and was the proper model as the design matches the original. The pump has a sticker on it that says "for Vortec engines only". Turns out that new pump was bad out of the box as we checked the pressure right at the pump with the rest of the fuel system disconnected. It only put 20 PSI out. We checked the original pump on a home made test bench and got 80 PSI out. So we put the original back in and the engine now runs properly. It starts quickly, runs smoothly, and has 54 PSI fuel pressure at the test port all times. Not very often a new fuel pump is bad out of the box. We know it was never run before we started working on the truck as they could never get power to the pump. They replaced it thinking that would fix the no power to the pump problem. That issue was the ignition switch being bad. Replaced it and all systems got power including the pump. So we know it was never run before we replaced the switch. The new replacement pump is just bad out of the box.

 

The code you get regarding the MAF may also mean the air flow does not match the reading. Verify that all connections in the air intake system are tight and leak free. Adding air to the intake after the MAF will screw up the system. Note this does not mean a vacuum leak, I am talking about a leak BEFORE the throttle body, but after the MAF sensor,

 

We know the sensor is proper as it is the one later Vortec 7.4 engines use. I think 1999 and later. All air connections are tight and leak free. I know all parts of the intake before the throttle body are tight as I made sure the surfaces were flat and the clamps were tight. Interesting thing is the engine still throws that code even when we disconnect the MAF. Either way, the truck runs smoothly now and that's all that matters at this point. Even if that P0102 code stays in the computer we don't really care at this point. Return it to the owners as is. We had a deal to get it running and we have done that. Let them deal with the MAF problem. Take it to a proper GM service center. That is unless you know anything else that might be causing the improper reading as I know it isn't a leaking intake tract at this point.

 

I do thank you for the reply. Not trying to sound like a dick or anything. Just tired of that truck with all the little problems we fixed on it. The MAF issue is the last driveline problem on it.

[Paul Mohlin]

Idle is fine but no power when I push gas peddle 

[richard wysong]

probably bad cats