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Old 10-08-2008, 06:04 PM
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drivability problem, SBC

Hello everyone,

I'm trying to diagnose a drivability problem... rather, a couple problems one step at a time.

I built and installed a vortec headed SBC with L31 fuel injection in a 2004 Silverado.

LT4 Hot cam
Proform 1.6 roller rockers
Comp beehive springs

The computer was re-pinned and flashed with a V8 comp tune for that fuel injection.

I had a vacuum leak at the intake manifold gaskets due to a bummed lower intake... it has been replaced with a new manifold.

When the engine was put back together it fired up quickly without the aid of mt right foot, idled perfect... for less than a minute. Than it began to stumble, rough idle, was prone to dieing, couldn't idle in gear, tough to start and needed throttle to fire up, and if it could run while in gear it had no power.

Does this sound like the valves are too tight? like when the lifters are bled down the valves are closing but after oil pressure is built up the lifters pump up and the valves aren't closing all the of the way?

I know I need to get a better computer tune in there, but i can't start tuning until the mechanical problems are worked out. Any other ideas?

here are a couple vids. The first is the engine idling & reving with both intake vacuum leaks and assumed open vlaves.


And this vid is after the intake manifold install, after the oil pressure is built up

Cheers ~Mykk


Last edited by 04SilveradoMykk; 10-08-2008 at 06:10 PM.
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Old 10-08-2008, 07:36 PM
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Even so I`m not sure on this. First off, get rid of the proform rockers, I would run stock rockers before I ran any junk from proform. I`m sure they aren`t causing the problem now but they sure will bite you later.
I would go back over the valve adjustment. At idle at first start up it sounds like it has a skip. I would check over the plug wires and be positive the firing order is correct. It idles as if it has a massive cam, while the hot cam is bigger than stock considerably, it`s known to have a smooth idle so this to a degree points to misadjusted valves. Lastly, I`m not sure if I understood your post correctly. If the changes to the computer system were made when it had a vacuum leak then it`s running off those settings which would work for a engine with a vacuum leak but not one without, I maybe way off on this, but trying to be open. I`m sure someone else on here can give you far more info than me on this. It appeared black smoke was exiting the exhuast which hints at a rich fuel mixture which also will be in the computers tune.
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Old 10-08-2008, 07:48 PM
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Thank you for the input.

The computer, although adaptable to conditions and has learning curves, Is a series of set parameters unaffected by the condition of the engine.

Your right though, it does idle as if it has a bigger cam.

Your under the impression from seeing my vids that the miss is ignition related? I won't rule it out, but I do have header boots on the ignition wires and have double checked wire placment on the cap.
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Old 10-08-2008, 08:04 PM
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Fuel pressure is VERY critical. You need at least 55 psi to run properly.

There should be a shrader valve on the drivers side. Hook up a fuel pressure tester and see what you got.
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