2000 gmt400 3500hd 7.4 vortex low power

[jtbarnes67]

Hi folks I'm new here and this is my first post. This should be the right place for questions for this truck.



Truck has 90k miles with a 12' flat bed. Fuel pump had been replaced before it was parked a couple years back. When got it up and running had low power, wouldn't hardly pull itself and would stahl at a stop. Replaced fuel filter. No help. Fuel pressure was a little low at 52 psi. After shutting off it would lose pressure at different rates. One time it would hold pressure as should and next would very slowly lose pressure. Then would fall more gradual. Live data showed fuel trims defaulted for bank 1 stft 50% ltft 25% or visa versa and bank 2 was a little high but ok. I figured FPR.



Pulled plenum off replaced fuel pressure regulator. Distributor cap and rotor. Plugs and wires was new not long before parking so inspected wires and cleaned the gas fouled plugs.



Now after back together the fuel pressure is up to normal range 65 I believe with key on engine off. 58 at idle. Around 65 snapping throttle. The issue now is the fuel pressure drops immediately to 0 after shutting off. I had a hard time getting the snap ring in place holding the FPR in place. Its possible I got the small 0 ring not seated proper. I say this cause it came out a couple times when struggling with install. Or the extra pressure gas cause the slow bleed off to become a fast bleed off. It's not attempting to hold at all. One other thing that changed is the rear tanks pump seems to be coming on. I'm not sure if that's what I'm hearing or not due to I haven't had an extra hand to work intuition while I crawl under and listen. I had only been using the forward tank and hadn't had any thought about that pump but I had not been coming on.



I know there's a test procedure for finding fuel pressure bleed off and determining whether it's FPR, pump foot, or injectors but I'm not sure of it. Could someone help with procedure? Is there anything I'm missing. ? Thanks for the help ahead of time. !!

 

[Carlaisle]

You have a new symptom and only changed one thing that could conceivably have affected that symptom, so that should be where you start your diagnosis. The rear auxiliary tank only transfers fuel to the main tank. The pump in the main tank is the one that provides pressure for your injectors. It doesn't sound like the injectors are to blame for your current issue, but the stockers are junk. You should plan to replace them.

 

[Schurkey]

What are the fuel trims AFTER replacing the regulator and achieving appropriate pressure?

Verify ALL sensors and computer outputs--EGR, ignition advance, etc.

How plugged is the exhaust system? Catalytic converters melted/fouled with soot?

 

[stutaeng]

You need to figure out where the fuel is going after you shut it off. Can your scanner perform an injector balance test? No codes I assume?

You can try to temporarily block the fuel return line to make sure fuel isn't being returned to tank. Maybe you got a faulty FPR?

Agree, check fuel trims now that you changed the FPR.

Or you can try disassemble the the entire fuel injector system and test it with fuel pressure to see if you see a leak, just be careful with fuel.

 

[jtbarnes67]

Thanks for replies. I did not get an alert for your replies I will figure that out so I can keep up with thread.

Fuel trims are relatively the same with bank 1 defaulting once circuit closes.

My scanner will not do a injector balance test. No codes present or before.

 

[jtbarnes67]

You have a new symptom and only changed one thing that could conceivably have affected that symptom, so that should be where you start your diagnosis. The rear auxiliary tank only transfers fuel to the main tank. The pump in the main tank is the one that provides pressure for your injectors. It doesn't sound like the injectors are to blame for your current issue, but the stockers are junk. You should plan to replace them.

Thank you for your reply. I started to replace injectors while I had it apart I just need the truck crossed my fingers FPR was the issue.

 

[jtbarnes67]

What are the fuel trims AFTER replacing the regulator and achieving appropriate pressure?

Verify ALL sensors and computer outputs--EGR, ignition advance, etc.

How plugged is the exhaust system? Catalytic converters melted/fouled with soot?

Fuel Trims are the same defaulting on bank 1 .
Cats have been hulled out .

 

[jtbarnes67]

Ok so here's what I've learned the reason I had such a hard time installing the FPR to begin with is the new aftermarket fpr had a different shaped plastic spacer that didn't allow me to get fpr deep enough to let the ring clamp seat properly so when I pinch off return in blew out.

Took back apart used old plastic piece install went as should have. Put back together and now I've got a steady drop kinda like I had before I started. Pinched off return still leaking down. I'm kicking myself because I did not check fuel trims before I decided to take apart and get a visual to see if injectors are the cause of the leak down they are not leaking a drop. It shouldn't be the foot on pump as pinching it off should have found a problem with pump. Not the injectors per visual. And so the new FPR seems to be the culprit. Right ? Wait when pinching off return would that not only stop the foot from bleeding down but would pinching the line off not also stop a leaking through FPR as well. I think I've read there's another procedure to isolate just the FPR to check for bleeding off.

So I'll need some input on this before I got further. Wish I had checked trims before taking apart. Help

I'm gonna give all the fuel lines a once over to make sure there's no one leaking somewhere.

 

[jtbarnes67]

I'm gonna have some time to work on the truck this weekend so I'm updating what I've done.

I thought the $15 cheap aftermarket FPR could have been bad out of the box so I had intended to buy a Delphi but no one had them in stock so I got a GP Sorenson for $85 that I feel confident with.

Installed new FPR now getting 50 psi KOEO and 48-50 needle unstable psi at idle and around 58 snapping the throttle . Still have slow bleed off .

Fuel trims same as before at idle defaulting 50% stft 25% ltft bank 1. Bank 2 around 16% stft 0-5% ltft at idle. After Giving throttle for a minute both banks come into normal range.

The thermostat seems to be stuck open causing low Temps so ill have a new one installed soon. I then will see what happens when I pinch off the return line. To try to isolate the

 

[east302]

On the small block, you would put a valve in place of the filter, prime the pump, close the valve and check for a pressure bleed down. If it held, it’s the pump. If it drops, it’s an injector, external leak or leak back through the return.

You’d then open the supply, prime system and block the return - there’s a union on the return line adjacent to the filter for installation of a valve. If it holds, then it’s leaking back into the return. If it drops, then it’s an external leak or an injector.

Your injectors are external, right? You pulled each injector with system pressurized?