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fuel pressure dropping, can't hit boost target and rpm barely rises

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I am a newbie so please excuse my lack of understanding of what my problem is. The likely culprits are waste gate solenoid, fuel pressure regulator, fuel pump, restriction in fuel line, restricted air flow, or some combination. I have a 2012 Chevy Colorado with a 6.0 liter LS swap sloppy mechanics style with a 7875 turbo and all the supporting mods including 2100 cc injectors. I have a 4 inch exhaust with a LoudValve boost controlled exhaust cutout and then a 4 inch Y pipe going to two 3.5 inch mufflers. I should be able to make 800whp on pump gas but I can't seem to make much more than 500.

I am a novice at all things mechanical and just starting to figure out the nitty gritty of how engines really work. While I can tinker with the truck a little bit, I had to have this all professionally built. I started with HP Academy out of frustration as I can't get any satisfaction from the builder. I have just started learning tuning to apply to this truck which is running on the Holley Terminator X Max platform. I am starting blind, don't know what a waste gate solenoid looks like, if the fuel pressure regulator is functioning properly or if my fuel system even has a return line. I also wonder if the exhaust cut out is opening completely and if there is a functioning knock sensor. Check out the attached log. My first immediate questions are: Should the fuel pressure drop this much? Would it help to add duty cycle to the injectors? I can use plenty of help so any advice is welcome.

Attached Files

That's kinda a bit jump for a 'newbie' - I wish you luck and no expensive learning leassons.


The fuel pressure dropping is one of the easiest ways of wrecking an engine, especially a forced induction engine - you need to find out what the problem is and what you need to do to remedy it. You don't mention any replacement pump, so that and the supporting fuel and electrical systems will need looking at, so you at least know your start point. I would expect you will at least need to fit a higher capacity pump, filter and use wiring that will support the current draw.

If you can provide the invoice summary of what parts you've been charged for (assuming they were fitted) and some pic's, it'll give the chaps something to work with, as it isn't really my field.

I intend no aspersions against your builder, but there are a lot of 'professional' folks in all businesses that are cowboys.

The fuel pump was replaced, several times in fact and should have the capacity. From your response, it sounds like dropping fuel pressure is not expected so it would be good to know what could cause that condition. Should there be any drop in fuel pressure?

After the first dyno session, the truck displayed some fueling issues while it had twin 425 walbro pumps but it made 791whp with the exhaust cutout open during that dyno session. The truck ended up with two Walbro 525 pumps in series although my fear is that there is some issue with the plumbing of the pumps. Here is a partial build list:

6.0 liter Gen IV engine, used, from a Chevy Express

Holley Terminator Max

VS Racing 7875 Next Gen T4 1.25 AR Turbo

3" intercooler

4" LoudValve boost activated exhaust cutout

4L80E transmission with HD rebuild kit

2 Walbro 525 fuel pumps

Bosch 210 injectors

LS7 Lifter set

Engine gasket set

Trick Flow Track Max hydraulic roller camshaft for GM LS TF@-30602003 (duration at .050 of 228/230, 0.585/0.585 lift, 112 LC lobe separation)

Push rod set

Custom drive shaft

PAC 1218 valve spring set

Jake's triple billet torque converter

8 AN Fuel line kit and steel braided line

Ford 8.8 rear end with 3.73 gears

LS swap radiator with fans

Correct, the fuel system should be able to hold stable pressure under any circumstances - referenced or not - with the fuel pressure regulator being the only thing to affect it. If it drops, there is a problem somewhere.

Little busy for now, some things that are sometimes overlooked are the need for up-rated wiring, etc, as most aftermarket pumps draw more current, filtration being up to the job and clear - you have checked the filters? - not forgetting any sock fitted to the pickup in the tank, fuel line diameter allowing free flow at the volumes used - not so much a problem with EFI as it's a pressure rather than vacuum feed - basically anything to do with the fuel flow.

I'm not sure why you have them in series, they should be in parallel - you want to check that, as it is basically wasting one and not adding to fuel volume. If you needed a LOT of pressure, fair enough, but they are good for anything you'll need and it's fuel volume that matters.

Thanks for the response. I may have that wrong about sequence or series. I believe they are in parallel and that I just had the term wrong.

As Gord has mentioned, the fuel pressure should be relatively consistent and definitely what you're logging shows the fuel system can't keep up.

This log however seems to indicate you've got a huge amount of converter slip? It's a bit hard to be specific since you haven't overlaid TPS but it looks like the rpm flares straight up to 5200 and stays there while the road speed increases through the log. With that turbo you shouldn't need a particularly loose converter.

Good catch, Andre!

Depending on application I'd suggest having the stall speed at peak torque, or a little below for initial load transfer before reaching maximum torque on launch and risking 'blowing the tyres off'.

I say that because it's the torque that ultimately applies the accelerative force to the track and at maximum power that can be significantly reduced from that.

Sometimes (usually) it's power we're concerned about, but initial launch is the exception.

Oh, Jeff, the reason we're both emphasising getting the fueling sorted first is because EVERYTHING you're going to be doing is going to rely on that being stable and, underline this, a weak fuel supply can (will) lead to leaning off just when you REALLY don't want it - high rpm, high load, high boost!

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