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post #1 of (permalink) Old 08-02-2000, 10:48 PM
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GMC TBI pressure on adjustable reg

I've got a slightly modified GMC 94 1/2 ton, 350 TBI motor, Thorley headers, K&N, off-road Hypertech chip (stage 2), TBI spacer, really free-flowing exhaust, 165 degree thermostat, timing is 4 degrees before TDC, and it runs high octane because of the lean problem caused by the previous owner's 305 injectors still in place. (I've been informed the injectors are not a problem when the pressure is turned up).

I was wondering if any one has knowledge about how much pressure to turn up the adjustable fuel pressure regulator. I've already ordered the part from Summit. (I've been told to up it a lb or two, but is there a way to mount a guage on a TBI? It doesn't appear so from the manual, so how do you go about getting it dialed in to the correct pressure? I'm sure I need to set it back to 0 degrees TDC and then turn the pressure up).
Thanks for the info.

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post #2 of (permalink) Old 08-03-2000, 11:45 AM
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Re: GMC TBI pressure on adjustable reg

I did this on my 92 a few years back. What I did was order a fuel pressure port (scrader valve?) for a TPI fuel rail. It costs about $20 from GM. I then removed the top of the TBI and made my stock regulator adjustable. The pressure port can be installed in the top casting between the injectors just behind where the air cleaner stem is. This area is thin, but can be drilled and tapped for the port. I used some JB Weld epoxy, and after the valve was in, built up the area around the valve to help support it, and also seal it. Never had a problem with it, and ran it this way for several years (5-6). As far as pressure, the factory runs 10-13 psi. Try turning it up in 1/2 psi amounts. You'll be supprised how much it will affect the motor. Be sure to reset the computer after each change. I used 13 psi with a 408 motor and a crane 252H cam. You may want to make your idle adjustable also, while you are working on the TBI. Good Luck!

post #3 of (permalink) Old 08-03-2000, 07:24 PM
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Re: GMC TBI pressure on adjustable reg


You are having to run high octane gas because the chip is cranking up the timing, on top of the fact that you have it cranked as well. I would assume you have problems with detonation when you don't run high octane. I know I did. As far as fuel pressure, I would do as the other gentleman said, go about a 1/4 turn each time, and you will be amazed at how it affects the motor. Keep in mind, each time you turn it up or down, you need to let the computer re-learn. Unhook the negative battery cable for about 5 minutes, let the truck cool down a bit, then restart. I would keep the timing where you have it. Too many variables will drive a person crazy. I would also call Turbo City if you're having a hard time. they are TBI gods. I got my TBI, and steel braided fuel lines from them.


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