[shboisr]

I have a 1991 C2500 Silverado EXT. Cab. 5.7 I purchased this truck, brand new, so I know it's history. In 2015 I replaced the fuel filter and at that time I took a fuel pressure reading, down-stream of the new filter. At that time the pressure was 14 psi. Now, I'm getting a pressure reading of 11.5 psi, (motor running) at the throttle body inlet. Ok, so my question is; "Is 11.5 psi fuel pressure adequate, boarder-line, or too low"? the truck has it's original tank and pump.

Thanks for any help.

 

[BackyardMech]

I think 12 is what your regulator is set at in the TBI. Your filter is that old?

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[BackyardMech]

Original pump in a 91? Lucky SOB lol...

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[shboisr]

Please see my original question. I'm not asking about regulated pressure.

 

[BackyardMech]

Please see my original question. I'm not asking about regulated pressure.

Oh I read your post...I'd say ok..Do you have a problem that drove you to ask the question?

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[shboisr]

I'm having a Slightly off idle/low speed idle problem, such as when moving through parking lots, creeping in traffic jams ect. Other than that it starts and runs perfect. I'm just curious as to just how low the fuel pressure can get, that warrants having to drop the tank and change out the pump. The engine is your standard off-the-rack 5.7 TBI, with absolutely no modifications.

 

[BackyardMech]

Low fuel usage situations shouldn't be affected by low fuel pressure. only high when you need tons and there is none flowing.

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[BackyardMech]

Vacuum leak
I can go on forever, but that's a good spot. PCV piping, brake booster, the works. get carb cleaner or ether, and spray everywhere with precision of course to see if your rpm changes.


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[Coyote-tex]

I would actually be looking for an ignition related problem with those symptoms. How are the cap, rotor
(especially) and the wires?? The key ignition wire is the coil wire, so just changing that and the rotor which are both inexpensive really helps many times. You may not need to throw a world of parts at it. Assuming of course the plugs have been changed in the last 30-50K miles,....

 

[Cowboytrukr]

Howdy and welcome!

11.5psi should not give you problems. However, 30 year old pump and distributor could be contributing.

If it was me, I’d replace the coolant temp sensor, fuel filter, and check the distributor for shaft play and rust/corrosion under the rotor on the reluctor ring.

Hopefully the pressure returns with the new filter.

I don’t like spending other peoples’s money, but a distributor and pump would be on my to do list just because they’re original.

One step at a time, though.

 

[shboisr]

Thanks for the reply . . . . All the above mentioned has been done and a whole lot more. New distributor, filter, coolant temp sensor, cap-rotor-wires-coil . . . . almost everything has been replaced and or tested, o2 sensor, anti-knock sensor, even the ECM has been replaced, all with no effect. Right now, the truck is actually in an auto repair shop, and even they can't find the problem. At times a code 42 will show up, which is a communications error code, (of sorts).

Ok, so this problem has been on-going since 2017 and has been getting progressively worse over the years. At this point, I'm thoroughly convinced that the only way find the problem is to use a full-blown, OBD1 scan tool and force the problem to occur. But, (the chances of) finding someone with this tool, let alone knows how to use it, is slim to none, at best.

 

[BackyardMech]

CODE 42? You get that when you unplug to time your engine. It's either the bypass circuit unplugged, bad wiring to the icm or ICM or wiring issue to the ECM I guess. But none of that adds up to low idle issues but runs fine driving.

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[BackyardMech]

Maybe your timing is retarded so much you run like crap until you run at higher RPM.



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[PlaywithTBI]

I'm thoroughly convinced that the only way find the problem is to use a full-blown, OBD1 scan tool and force the problem to occur.

Yes, that's the right way to go

But, (the chances of) finding someone with this tool, let alone knows how to use it, is slim to none, at best.

Not really, the SnapOn "Red Brick" can do that and there are thousands of them for sate floating around on places like Flee Bay, etc. Other ones are available too. I use my laptop, TunerPro RT software (freeware or a suggested contribution), and an ALDL to USB cable (~$60) with which I can data log, graph, and monitor all of the sensor and output the ECM sees.

Here's a couple examples from my 88 running the 1227747 ECM.