# (Starting problems 96 GMC 3/4 7.4L



## rusty (May 10, 2004)

Need help troubleshooting my 96 GMC, It has the 7.4L which makes it extremely difficult to access the cap and distributor. My son was using it on Prom night and it died on him. I've checked the fuel pressure and it is in spec. 60 psi, and I've pulled the #1 wire and have spark. I'm getting a bad feeling about the timing chain here. The truck has 80K and has been running great. ANY SUGGESTIONS??


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## rusty (May 10, 2004)

I just pulled the valve cover and the timing chain isn't the problem. I'm planning on renting a diagnostic tester from the part store to see if there are any errors I can read. One other question, the service engine lite , is it supposed to turn on then off before its started or does it only turn off once the engine is running??


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## rusty (May 10, 2004)

Hooked up the diag. tool and scanned the OBDII it has no errors on it. Please help me on this one.


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## karl klein (Jan 28, 2001)

i have a 97 suburban with the 7.4ltr. and had same thing died while driving down the road. i had fuel pressure also i would check it again now that its been sitting for a while. can you hear the pump running? if you dont hear the pump when you start it up i would replace it. it worked for me on my 7.4ltr. i also have about 85,000 miles all though this was my second pump. good luck.


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## rusty (May 10, 2004)

I checked the fuel pump and it seems fine, I still have 60 psi at the rail to the injectors. I've purchased a new PCM but am reluctant to install it YET!! Would really like some path to follow here. I'm in the process (as soon as the rain stops) of trying to figure out the crank shaft sensor signal and what the duty cycle is a representation of i.e. is duty cycle a reference to frequency?? and if there is a way to read it with the meter I have. I really would rather not have to purchase a meter that is very simular to the Fluke mdl 73 I have but includes a duty cycle reading. My thinking here is that maybe the crank sensor isn't feeding back to the PCM to adjust the firing of the injectors , it sounds like the system wants to fire but when it does the timming appears rough.


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## GMC Jon (May 22, 2004)

Is this a throttle body or port fuel injection?


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## rusty (May 10, 2004)

Port fuel injection. Since my last post I've changed an ignition control model and there is no change in the problem. I just purchased an crank position sensor and intend on trying it tomorow. Any suggestions would be GREATLY APPRECIATED.


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## GMC Jon (May 22, 2004)

If you have spark, then the crank sensor is ok. Inop crank sensor would give you a no spark condition. The biggest problems on these have been - #1 fuel pressure regulators. You would have to remove the coil bracket and distributor cap to get at it. Pull the hose off the regulator. There should be NO gas in it. If it is real bad it can foul the spark plugs. Check the plugs to see if they are gas fouled-black in color. #2 the distributor caps burn thru internally. I have seen them burn thru and give it a misfire and also if the one for the coil burns thru it won't start. Take the cap off and hookup an ohmmeter between all of the terminals inside the cap. I would start by hooking 1 lead up to the coil terminal and then to each terminal for all cylinders. Your ohmmeter should show No continuity between these terminals. If it does, it is burnt thru internally. From there I would test the injectors and circuits to the injectors. Also check all the fuses to make sure you have 12v going to all the circuits for the ECM. Check these at the under hood relay center. There were some ignition switch problems and 1 circuit might not be getting 12v. Hope this helps.


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## rusty (May 10, 2004)

I appreciate the help, hope Napa takes the crank shaft sensor back without a receipt as it blew out the window today. I've already visually and using an ohm meter checked the fuses. How do you suggest I check the injectors. There is a connector right upstream of the pressure test point that has a 5V signal going to it. What does it control?? and what should I see there when its turning over??


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## GMC Jon (May 22, 2004)

That should be the map sensor. It does have a 5v reference signal to it. Testing the injectors is not easy. Usually I do it with a Tech2 scan tool. You prime the fuel system and turn on each injector with the scan tool and watch for a pressure drop in the system. If the pressure drops for all 8 then the injectors are working. Other wise you need an injector tester that plugs into the injector harness. You power up each injector this way. The plug is next to the coil bracket on the drivers side. There is also a tool called a noid light that plugs into the injector harness. Simply unplug an injector and plug the noid light in, crank the engine and watch the light. If everything is ok, the noid light will flash. You should have 12v to each injector, the ECM pulses the ground to make them work.


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## rusty (May 10, 2004)

Just out of curiosity, would the computer see any errors if the map sensor wasn't working? I rented a scanner and checked the error codes and there wasn't any errors present. So here is what what I've eliminated, the fuel pump to the the injector rail, pulled the #1 plug and had some spark ( funny thing was when it sparked it tried to fire) but sounded like it was firing on the wrong stroke it almost started running i.e. a few seconds before it died. Pulled the valve cover to verify the chain was attached and the valves were moving. Question for you: with the timing controlled by the crank pos. sensor ( I think) if the chain slipped a tooth would the ecm adjust for it?? I've read there is also an cam pos. sensor, is this a double check or is it an input to the ecm??


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## GMC Jon (May 22, 2004)

If the computor sees any errors in any of the sensors it will still run. The only sensor it can't run without is the crank sensor. The cam sensor is used to tell the ECM position of the cam so it can put fuel into the motor sequentially. If you lose the cam signal, it will still run. If your chain is off a tooth,your valve timing will be off and there is nothing the ECM can do about that. Take a compression test. That will tell you if your chain slipped. I have not seen a slipped chain in a very long time and I have worked at this GMC dealer for 19 years.


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