fuel pressure low

[airbrushart]

Okay so my truck will start, and idle fine... you can even pull away slowly and cruise.. but if you get into it.. it will spit and sputter and almost die..

I tested fuel pressure at the filter and i was getting 12psi..

All of the lines were waaay rusted, so i decided to replace the entire fuel system components..

Sooo I've replaced the hard lines.
I replaced the sending unit
I replaced the fuel pump
I also installed a fuel gauge T'ed into the line by the TBI so i could monitor it

And this is what i go...

Truck starts and idles fine.. pressure at idle is 12.1 or so...

Pressure when driving seems to be irratic and the needle is bouncing between 12.5 and 11.8 if i really stomp the gas, it will drop into the high 11's...


Sooo I thought the pressure regulator was bad being that was the only thing that was left... so i swapped it out, and the idle pressure is just a hair bigger, but its still at 12.25 ish when getting into it... which the engine goes lean and spits and sputters...

So after that i swapped out pumps.. same pressures

So at this point i have no idea what to check?

Also does anyone know what idle fuel pressure should be?

Also does anyone know what accelorating fuel pressure should be?

I heard or read somewhere that is should be 12 at idle but should jump to 15 under heavy load.>?

thanks for the helps!

[airbrushart]

oh and how is fuel pressure regulated? what would cause the pressure to change under load, if it indeed does fluctuate under load?

[AverageJoe]

I had the spitting and sputtering and all i did was take a wire brush, and removed the fuel pump ground, cleaned it real nice and shiny, and bolted it back. Like new. May not be your problem though

[ChevyTech]

You do not say what you are working on but it must be a TBI truck if it runs a 12 PSI fuel pressure.

On TBI trucks the fuel pressure regulator does not vary the fuel pressure when the engine has a load like the port fuel injected systems do. The ideal pressure on TBI trucks is a constant pressure in the 11 TO 12 PSI range. The spec is 9 to 13 PSI but trucks that run near the low end of the specification usually have a fuel system problem.

I think you should look elsewhere for the cause of the running problem.

[airbrushart]

awesome thank you!

Well this is what i got... i had a local shop double check the ignition timing and they are saying that its up around 86 degrees to get it runing right.

Not really sure what they mean by that... but he said it should be around 26 ish

Soooo they currently believe that the problem might be that the timing chain has jumped.. they are trying to think of a way to check the chain without ripping apart the front of the engine...

Oh he said that if he sets it to dead nuts TDC, he can't get the truck to fire up at all... i think he said that its back firing through the intake with it set that way...



Sooo any thoughts?

and sorry yes its a 93 silveraldo with auto 4x4 5.7l tbi...

I'm leaning to the crank timing being way off... but sounds weird...

computer?

[ChevyTech]

The outer ring on the front pulley can slip and make it so the timing will be wrong when checked with a timing light.

There is a set timing connector that must be unplugged to check the ignition timing.

Timing chains don’t tend to jump on these engines. They have a steel sprocket on the cam with no plastic on the teeth. I would guess a flat cam lobe before a timing chain jumping.

[airbrushart]

i believe they are setting the timing with it unplugged ( i would hope) but i will ask them tomorrow...

Thanks Chevytech... if i have to rip this engine apart, i think i might just find one on craigs list and do a swap and then rebuild this one later for some go fastness.. lol


Thanks again sir... much appreciated...