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Well, I am dealing with a fuel loss issue of some sort.
First, equipment:1987 chevy R10 silverado. It has a remanufactured 5.7 350 I bought a while back from city motor supply. It has a stock TBI with a stock ignition system with new cap, rotor, plugs, and wires.
The problem: When I drive it with little demand and stay pretty soft on the pedal, it runs fine. But if I try to get on it at all it will bog out and act like it wants to die. I put a fuel gauge on it and saw the pressure drop [while in park it does it too] to almost zero psi. I have timed it and retimed it so many times. I replaced 3 fuel filters, on the third one I did a blow test on it; clean. The fuel pump is in the tank and is new (stock). I replaced the fuel regulator. Tested the TPS and ohm'd good. I am going to check for vacuum leaks with a can of carb cleaner sprayed around the intake and the brake booster. Just wanted to troll the locals to see if anyone would want to check it out (in person). I have a long thread on 73-87chevytrucks.com and have not made any progress. So.....
I ain't an "expert" on your platform, but spent many years working on fleet vehicles durin' that time frame.
The thing to do here is to figure out whether you have a return line. If so, is the relief valve operating properly?
Try throttling up slowly - is there a given rpm at which the pressure starts dropping out? Once you have that determined, then start going from back to front (IOW, start @ the fuel tank), and check pressure as you approach the suspect rpm. What you're looking for is the "break point" - where the pressure loss is actually occuring. A crimped fuel line between the tank and engine may flow fine at lower fuel demands, but become a "choke point" at higher flows (think of a kinked water hose). Because of the higher demand, the pressure will drop off because it's being restricted.
To me that sounds very much like a MAF (mass airflow) sensor or an O2 sensor.. My wife had a '00 Celica that acted exactly the way your Silverado sounds. I would take the MAF out periodically and clean it with (non-chlorinated) brake cleaner and it would clear up for a while. Ultimately the O2 sensors needed to be replaced but we traded in the car before that was ever done.
I don't have a mass air flow sensor on my TBI. And I checked for crimped and kinked lines. I did have a friend tell me to clamp the return line and see what it does. Havent done that yet. But yes it does happen around the same rpm range. Right around the 3500-4000 rpm range. That is based on sound because I do not have a tach.
You say you put a fuel psi gauge on it and saw the pressure drop. To me this indicates you have a severe lean issue.
It is likely since you have an electric fuel pump, that what is happening is the fuel is bypassing inside of the pump resulting in a low pressure output.
What type of in tank fuel pump do you have? If it's a Walbro F2000169 series pump that was made around the middle of 2007, take it out and replace it.
For what it's worth I had this very same issue....I have an 87 Regal turbo (Grand National) that has an in tank pump...I had one of these Walbros in the tank...one day I started running into some mad lean issues/detonation issues....looked at my fuel psi gauge and I couldn't get the psi past 53 psi before it would suddenly drop off. (on GN's the fuel psi is 43 psi at idle w/o any vacuum on the fuel psi regulator and it's supposed to increase 1 psi per boost)
Anyways replaced the pump and all was good again. As it turns out inside of the Walbros is a pressure bypass valve that allows the pump output to increase pressure when you accelerate...during the 2007 run of Walbros, there was an issue with a piece coming loose inside of the pump which would cause fuel psi to drop off because it would bypass inside of the pump, leading to lean issues in fuel injected engines.
But I replaced the pump and it had no improvement. The pump is this Delphi I got on Amazon. I just replaced the pump and not the assembly. Do those pumps have that bypass? On my setup, required pressure is 12-14psi. Which I have until high demand is put on.
Just because you replaced the pump does not mean it cannot be ruled out as faulty. For instance the Walbros I mentioned that were built in mid 2007, some of them were faulty out of the box, and didn't show their signs until the first time you went to start the engine or a few months down the road.
Bottom line if your fuel pressure is falling off then you clearly have a fuel issue, which could be a bad pump, bad regulator or a combo of both.
And always remember, if man made it, it's guaranteed to fail at some point.
Just because you replaced the pump does not mean it cannot be ruled out as faulty. For instance the Walbros I mentioned that were built in mid 2007, some of them were faulty out of the box, and didn't show their signs until the first time you went to start the engine or a few months down the road.
Bottom line if your fuel pressure is falling off then you clearly have a fuel issue, which could be a bad pump, bad regulator or a combo of both.
And always remember, if man made it, it's guaranteed to fail at some point.
This is true. Also, there is a bit of strange info I left out. When I replaced the fuel filter, it was great and ran like brand new. But after it sat, even for a few minutes, it would go back to its old ways.
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