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Truck starts okay but then wants to die out

Tiny
MRROBER
  • MEMBER
  • 1996 CHEVROLET SUBURBAN
  • 7.4L
  • V8
  • 4WD
  • AUTOMATIC
  • 73,000 MILES
Keep in mind that this truck sat over the winter with no engine in it. I recently replaced my old engine with a low mileage replacement. Due to parts worn on old engine I've replaced the cam with a High Energy Comp Cam and new roller rockers. I had to replace the distributor because of the worn gear and there wasn't any gears in town. It started right away, but then wanted to die after a few seconds. So I checked the fuel pressure and it was idling about 45 then crept up to 50 to 53 lbs. Anyway, I replaced the fuel pressure regulator - no difference in it's running. I then replace the fuel filter - no difference. I then thought it had to be the fuel pump if everything in between the tank and injectors had been replaced - no difference. The last thing in the system were the injectors so I put in some of my spare injectors - no difference at all. Right now I'm thinking in tight circles and just need a different and fresh point of view. By the way all tune-up parts have been changed also (plugs and wires - all correct parts).
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Thursday, March 17th, 2016 AT 8:02 PM

7 Replies

Tiny
MRROBER
  • MEMBER
  • 9 POSTS
I forgot to mention that you can keep the truck running by pumping the throttle just like you would on a old carburetor engine.
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Thursday, March 17th, 2016 AT 8:05 PM
Tiny
CAR-MAN145
  • MECHANIC
  • 321 POSTS
It could be so many things. Are there any codes? Is it running lean or rich?
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Thursday, March 17th, 2016 AT 11:26 PM
Tiny
MRROBER
  • MEMBER
  • 9 POSTS
No, there aren't any codes. That's part of what is baffling me. I found just one new plug was a little rich, but the new wires took care of that problem. Pumping the throttle is so strange. It pops right off, runs smooth, when you take it upstairs to about 2500 RPM to break the cam in, it just tries to die just like it was running out of fuel. It will then go down to idle speed and start blubbering and has a hard time idling but you can get it back up in RPMs by doing as I stated before. I can't set the timing or the break in the cam due to this problem.
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Friday, March 18th, 2016 AT 4:36 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,513 POSTS
The replacement cam you installed, does it have stock lift and duration specs?
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Saturday, March 19th, 2016 AT 7:32 AM
Tiny
MRROBER
  • MEMBER
  • 9 POSTS
This is a Comp Cams High Energy which has a slightly higher lift and greater duration. It's no where close to being a radical cam. It's designed to give a little better pulling capabilities and possible a bit better mileage while doing that. I've ran this cam in my dually that pulled the car before so I'm dead sure it isn't that. Keep thinking though bc I'm willing to consider anything.
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Thursday, March 24th, 2016 AT 9:25 AM
Tiny
MRROBER
  • MEMBER
  • 9 POSTS
By the way it now shows a code P1325 - reference to ignition. The timing gear marks are spot on and the distributor is in correctly as I checked that again for the 5th time. I changed the EGR valve and the EGR switch. Before I installed the latest motor, I swapped out the Crank Sensor because it was fairly new and the other motor ran fine except the lifters kept pumping down and I got tired of the rattling. So I don't think that the crank sensor is the problem BUT all things go bad and seeing as how that directly connected to the code, I'll check it out. I just have to start thinking out of the box. I've got so many things going on and it's tough right now so
i really appreciate your input. Like I said, keep thinking.
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Thursday, March 24th, 2016 AT 9:40 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,513 POSTS
Hey Mrrober,

First disconnect the EGR vacuum line and plug it until we get the engine running okay, next remove one spark plug wire and rig an old spark plug to it and ground outside the engine, next start the engine and observe the spark as the engine dies, see if the spark stays the whole time, also I'm thinking it could be a massive vacuum leak, either intake manifold gaskets or just under the throttle body. (these gaskets are prone to failure) here is are some guides to help find the leak.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

and

https://www.youtube.com/watch?v=xN5pcM-5bso

Please let me know what you find
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Thursday, March 24th, 2016 AT 10:10 AM

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