1996 5.7 vortec miss.

racerkyle · October 18, 2004

I have a 1996 GMC Z-71 with a new crate engine installed. All plug wires, plugs, distributer etc.. is new. A P0300 code is popping up and it causes the engine to miss, but when I clear my computer, the engine runs great then I drive it 10 or 15 miles (sometimes more) and the code pops back up. It says it's a Multiple/Random Engine Misfire detected. I don't know what to do. The truck is sucking gas really bad. And I smell gas a little when I get out of the truck after drving.But I didn't smell it around the intake. Some people are telling me it is the fuel injection stuff, some kind of spider thing on the injectors. may have to be replaced. Also after I get my truck back from getting the engine in, the transmission will not go into lock-up some times. Like half the time.The torque convertor is maybe 4 months old. Took it to a tranny man and he told me there is nothing wrong with the tranny. Maybe electrical thing. Nothing major, just hard to find. Could that be caused by the engine miss? What do I do. Help! Thanks!

covvy · October 19, 2004

That engine is very sensitive to TPS for both the engine and transmission operation. Using a DVOM make certain the TPS sweep is within proper operating range. If you can get your hands on a scan tool then check what the fuel trim is at. If the block learn is above 150 then the engine has an intake leak or faulty 02 sensor. If its below 150 then your fuel pressure is too high or the injectors are being held open too long causing overfueling or again possibly a bad 02 sensor. The fuel feed lines inside the manifold are prone to cracking so make certain that they are solid. Good Luck

Peter_C · October 20, 2004

Did you reset your cam angle sensor after installing the distributer?

racerkyle · October 20, 2004

Well my mechanic has my truck again. He talked to a guy at GM and he told him to check the timing. It ahs to be set with a computer. GM man said he thinks that what the problem is. Hopefully, nothing serious. Will let you know. Thanks for the info. anymore will help. I'll tell my mechanic what you guys told me. Thanks!

Peter_C · October 21, 2004

If your mechanic uses a Snap-on scanner and has troubleshooter it will explain the procedure for setting the cam timing under "Tests and Procedures".

racerkyle · October 21, 2004

Okay guys, yesterday my mechanic said he got shocked twice will trying to time the engine. So today, I got another brand new set of plug wires and dist cap. $61 wires. Drove it to town about 15 miles, checked my computer with my computer and it has P0300 in it again. We cleared the computer after installing new wires and stuff. If it was the cam angle sensor, wouldn't the computer have that as a code? My mechanic is thinking that I need to replace fuel injection (the plastic lines that go to each cylinder) One thing though, sometimes while setting with the key turn to on but not running. The fuel pump kicks on for a couple seconds. Is that anything? Thanks!

Peter_C · October 22, 2004

Get a rid of the mechanic and find a good technician. Get the cam angle set properly. If the code still exists let us know. As to the fuel pump kicking on for a few seconds that is normal.

budhayes3 · October 22, 2004

It would be nice to see the percentage misfire per cylinder to see if it's across the boards, or if only a few cyls are effected. Next step would be to hook up an ossilloscope and watch spark events and check the graphs for spikes and weak signals.

racerkyle · October 22, 2004

Okay, got more news. Found out that my gas mileage is like almost half. But would the timing being -17 out cause that? It is at 0 now. Haven't driven it enough to know about the mileage. Talked to GM tech, he said that I have individual injectors for each cylinder. Hope he knows. He says we need to run some tests on it. Check each cylinder with some kind of graph. It could be as simple as a EGR valve, fuel pressure reg. could be just dumping fuel in, or fuel pump. Tomorrow, I'm gonna try to get fuel psi checked by a buddy. Oh one more thing, GM man says the transmission not going in to lock up sometimes could be caused by the engine stuff going on. Anymore tips? I'll keep you all filled in about all this. I'm about sick of this truck. Ha! But I love it. Just tried of spending $$$$!

catmandoo · October 23, 2004

My 98 is doing the same and the truck has about 140k on it . The only difference is that mine will go about 700km's before doing it again. You will be sitting at the lights and it will begin to miss and then the engine light comes on. So I go home and dissconect the battery for the night and then it runs good for another 700k's. GM said my cat conerters were at fault but I suspect it is my intake gaskets.

lowflyer · October 23, 2004

I'm with Peter C on this one. Get the truck to a mechanic that can properly set up the distributor before you throw any more money at it. If the angle sensor is off, even by one tooth, it will wreak havoc on the rest of the FI system.

After the cam sensor is set right, check the fuel side of the motor. The Injection system has one injector, but eight "nozzles" placed at each cylinder port. The plastic hose that connects the injector to the nozzles (the spyder) tend to crack and puddle fuel within the manifold. Nozzles are prone to fouling and getting plugged with carbon is another common problem. Also the fuel pressure regulator is known to develop a leak and affect driveability. Most of these fuel injector problems can be identified with a simple fuel pressure guage.

Good luck.

racerkyle · October 23, 2004

Alright, got more news. Today, took to my truck to mechanic that has a snap on snanner. Not the mechanic that did my engine. Fuel pressure test on line before it goes to the fuel psi regulator. Got about 55, and stayed constant, but it did drop a lot when truck shut off. Is that okay? We spent about 45 minutes with the truck running, checking everything the scanner could check. All sensors are working right, the cylinders are all misfiring, two of them are a lot worse then others. But it's weird, I don't feel the miss. I can accel fast, slow, it don't matter really. The misfires are worse when rpms are up, thats what the scanner shows. Fuel psi regulator, can the fuel psi be checked after it? Need help still.

budhayes3 · October 23, 2004

Fuel pressure dropping with the truck off should be normal...it flows back through the return...I'm pretty sure that 96's have a return line, but don't quote me on it.

Jeremy WI · October 24, 2004

Fuel pressure should be 60-66psi key on engine off, it should hold pressure within spec after the fuel pump quits

racerkyle · October 24, 2004

Nothing new today. Hopefully get an appointment with GM Shop tomorrow. Let them do it. The scanner we used says that the misfire code will cause the torque convertor to kick out sometimes. Hopefully that is true. If the timing is set at 0 degrees. Then wouldn't the cam angle be right? So can the fuel psi be read after regulator? Would being -17 degrees timing out, cause me to have 6 or 7 mpg less? We are sure that it is 0 now though. I guess taking it to GM is the best thing to do. Thanks!