gm tbi

Chip Johnson

Asked by Apr 15, 2013 at 09:06 PM about the 1995 GMC Vandura G25

Question type: Maintenance & Repair

95 gmc 4.3 replace tranny with a 1972  350  thm.  throttle  body
problems ever since but vehicle had set 6 months previously.  cleaned
injectors  (kit) fppr now only one injector fires .left sidy  right side has
signal but will not open , it will open if I run a jumper wire to it .
cleaned another injector   
Bench tested w 9 v dc 300 mill amp transformer . installd would not
spray ,have sgnal but  will open what's up w signal  wires 12 volts will
open with jumper wire .?? I would appreciate it

9 Answers

Do you know how much fuel pressure there is available?

1 out of 1 people think this is helpful.
Chip Johnson

In tank going to check fuel pressure now

1 out of 1 people think this is helpful.
John Saffrahn

Its throttle body injection not carbureted and never was carbureted Roy, there is no spot on the side of the block for a mechanical pump and even if there was, a mechanical pump is only good for about 2-3 psi where the throttle body needs 10-13 psi. Chip have you tested resistance of the two injector wires? It takes very little amperage and voltage to activate a noid light and 0 amps to show voltage on an oscilloscope, if you have a slightly corroded wire in there it will reduce the amps in the circuit to where it won't activate the electromagnet in the injector but it may seem ok with a noid light or oscilloscope. With the original trans being electrical and the one you put in being more mechanical, did you do all the steps necessary to program the PCM to realise there is a different trans?

2 out of 2 people think this is helpful.

I'm wrong and living in a fantasy world in my own head here's the scoop--The fuel metering assembly contains a fuel pressure regulator which dampens the pulsations and turbulence generated from the very high pressure fuel pump. Think of the FPR as a conditioner that smoothes out the flow of fuel from the outside to the inside of the fuel metering assembly. The FPR also maintains a constant, steady pressure at the injector assembly. Dual fuel injectors are mounted over the throttle valve, synchronized, and raised slightly over a venturi (narrowing radius) throat. Each injector is controlled by the ECM through an electrically initiated solenoid (switch). The precise amount of fuel delivered by each injector is varied by the amount of time that the solenoid holds the injector plunger open for operation. A high pressure, high volume electric fuel pump is used with the TBI system. This pump is located within the fuel tank itself (which can be a PITA if you have to replace it).

John Saffrahn

Throttle body injection should be no less than 9.5 while idleing and should never go past 13. The only time its acceptable to pass 13 is when the key is initially turned on and the system is being primed, as soon as the pump shuts after priming the pressure should drop to below 13. If its anything above 11 at an idle then there is an issue with the fuel system.

2 out of 2 people think this is helpful.

A stab in the dark but is the ground good on the right side?

1 out of 1 people think this is helpful.
John Saffrahn

Fuel pressure regulator is designed to regulate fuel pressure using engine vacuum, the more vacuum the less demand for fuel, idling and deceleration have the highest vacuums. When vacuum is applied to the regulator it begins to open relieving pressure until it hits a certain point. When vacuum is released (during hard acceleration or when the engine is off) the regulator closes and full pressure is achieved so more gasoline can be injected without increasing injector pulse width crazy amounts.

2 out of 2 people think this is helpful.
John Saffrahn

Yes Roy 72 did have a Carb but the engine is usually where that's located not on the transmission.

2 out of 2 people think this is helpful.

thanks John felt the breeze as the union suit flapped open exposing my raw ass for the kickin'

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