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Chevrolet 3500 96 Chevy 3500 7.4L. Problem is when starting

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96 Chevy 3500 7.4L. Problem...
96 Chevy 3500 7.4L. Problem is when starting it has rough idle and it turns off, unless I give some throttle then it will sputter while giving throttle but after a few seconds it will idle by self. Not to smooth but will stay on. It idles around 7-800 RPM after the rough start up. I think that's a low. I cleaned throttle body and had check engine light read by scanner at auto zone. Had two codes in system both same code? P0102. Ive replaced both mass air flow sensor& air charge sensor.
Submitted: 6 years ago.Category: Chevy
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Answered in 4 hours by:
9/9/2011
Chevy Mechanic: Jerry Newton, Chevy Technician replied 6 years ago
Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 4,832
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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Things to check: fuel pressure needs to be up to spec, and not leaking down with the key off. I don't know if you have a fuel pressure gauge, but you can borrow one at no cost from Autozone or Advance. Fuel pressure should be 55-61 psi for this engine.

Pull the vacuum line off the fuel pressure regulator, make sure there's no fuel in the vacuum line. If the inside of the vacuum line is wet, you need a new pressure regulator.

P0102 suggests that the mass air flow reading is much lower than anticipated by the PCM. Make sure that the duct that runs from the MAF to the throttle has no leaks in it. If it's sucking air due to a cracked boot or missing piece, this is the cause of your problem. You may need to take this duct off and look at it thoroughly with a flashlight to make sure it's not cracked.

Those are the things I would check first, if your truck were in my shop.
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Customer reply replied 6 years ago
Ok thanks,
I will get a fuel tester tomorrow.
I played with it a little bit after reading your response and also did some searching on fourms.
Seems the regualtor is a common problem. What I found so far is If I leave the key on for about 10 seconds before starting it starts fine. Still can fell a slight miss.
Also I pushed valve on conector for tester and there was no fuel comming out after it had sat for about 3hrs or so.
Should it have the 55-61Psi all the time even shut down? Also it does smell rich when idle or after turned off.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 6 years ago
I think you're going to find that you have a bad regulator. Fuel pressure should hold close to 55psi for at least ten minutes or so. There's not an exact specification for that, but the idea is that if it drops off as soon as the pump stops running, that's bad.

I have a customer with a truck just like yours that's been complaining to me about the same problem, but I haven't had it in the shop to pinpoint the issue yet. I suspect that I'll find a fuel system problem with it, though.
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Customer reply replied 6 years ago
Ok. I have now replaced fuel pressure reg, rotor, cap, wires, spark plugs, upper intake gaskets and re set timing. The fuel pressure reading after all work was done was 50-55 and bounced up to 55 then down to 50 the whole time. Not a constant reading. When first turn on key it would be at 60psi. But drop down a lil when running.
If I turn on key and start with out a wait, it will start and then die. Not sputtering or any type of misfire like before but won't stay on.
Now if I leave key on for say 5-10 sec before cranking it will start normal and run fine. It has a slight miss to it but might be the harmonic balancer not sitting tight on crank. When Turing to find TDC I noticed the bolt was stripped and would not get snug. Had to turn flywheel to get TDC.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 6 years ago
I'm looking through a database of repairs for this concern, and the common link seems to be fuel pumps. This symptom sounds like a fuel pump concern in my gut, and the database confirms my suspicion, coupled with the erratic readings you're experiencing on the gauge.

My recommendation on this is to replace the fuel pump.
Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 4,832
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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