DSOMC6

My 06 twin turbo C6 is losing fuel pressure upon shut-off. When the engine is running the fuel pressure is rock solid 58-60psi even under boost.

Fairly basic fuel system: OEM fuel pump and fuel rail, Siemens 60lb inj, KB bap.

I verified the fuel pressure loss with both my electric and a manual gauge at the fuel rail. Upon shut-off its slowly losing fuel pressure but not completely bleeding down to zero (in the hour or so I monitored it with the manual gauge).

At times the car will fire-up with reduced fuel pressure, and other times it will crank but not start. (I’m assuming perhaps the fuel has bled down to zero therefore it fails to start?)

Question: If the fuel pressure at the rail is zero, once you crank the engine, wouldn’t the fuel pump re-pressurize the line and allow it to fire-up if the fuel pump is getting correct voltage?

Another note worth mentioning…when it failed to fire-up a couple times, I put a jumper between the + and the grey fuel pump wire (basically bypassing the relay that the bap is wired into), I heard the fuel pump spin, it re-pressurized the line and fired-up.

I checked the bap output and it reads correctly at 17v.

I have a ZO6 pump assembly I can drop-in, but before I do;
Could possibly an o-ring in the crossover tube, in-tank fuel line or in-tank oem fuel pressure regulator suddenly go south causing loss of pressure upon shut-off? I'd hate to replace the fuel pump assembly, button it up and then have to go back in for something that I missed.

Has anyone come across a similar issue and what was the resolve?

TIA for any advice.

07MontRedcp

I believe there is a back flow check valve that is part of the C6 fuel pump assembly. I had a forum member contact me a few months ago asking if I could help with an external check valve setup so he wouldn't have to pay a big price for the dealer to replace the pump. He said the dealer told him that the back flow check valve internal to the C6 pump had failed. I couldn't come up with a setup that I was comfortable in recommending. I did, however, come up with an excellent oneway check valve setup for my custom RSI Stage 1.5 fuel system.

BJK

DSOMC6

Interesting...according to the fuel system diagram there is a check valve located in the fuel pump assembly between the fuel feed pipe to engine and filter.

Someone else mentioned that it is normal for the fuel system to bleed down over time? I was under the impression that the system should be fully pressurized all the time?

Lastly, I'm curious if the pump is just going south. Considering when it failed to start on a couple ocassions I jolted the pump directly with 14v and it fired-up?

walhan_qtr

I have the same problem and also was told because I dont have a check valve. mine cranks for a while before it starts. but not sure if its the same problem as yours!

DSOMC6

Thanx Bruce and Walhan.

It now seems likely there are two separate issues.
Bad reverse flow check valve letting pressure bleed off. Which swapping the fuel pump assembly will fix.

Secondly, this morning I found a loose/bad ground connection where the fp/bap and scavenge pump are wired in. Thus explaining why I was getting intermittent voltage to the fp and could hear the scavenge pump slow at idle on occasion. The fp seems to be functioning fine now and the car seems to be holding 14v consistently at idle now.

07MontRedcp

Sounds good. With regards to the stock pump SLOWLY bleeding down while the car is off, I guess it would depend on how long it sits, but it should only be a momentary thing before the fuel pump brings it back up.

BJK

old motorhead

My truck with Walbro intank pump bleeds down. Too lazy to fix it. I just turn the key switch to the run postion for a couple of seconds. Fuel pressure goes up to normal in a few seconds. Starts right up. I can see where doing that in a C6 would be a PITA. I guess you could hit the start button without depressing the clutch (M6) or with trans in gear (A6). I believe that would energize the pump and get your fuel pressure back to normal. Depress the clutch or shift to park after a few seconds and it should start normally........or fix the pump

turbotuner20v

On the stock pump + BAP on hobs switch, my fuel pressure at idle is around 62psi, but when I turn the car off, it will go down to ~50. I just figured it was normal. It never starts hard, and it's always making full pressure as soon as the car turns on.

DSOMC6

Yeh pump upgrade is on the to do list. Hopefully it will be on a non STAT basis.

Interesting, thats similar to what I'm seeing. What puzzeled me was I don't recall seeing the fp gauge move below 60psi in the past? Maybe I didn't pay attention prior.

The Cobalt elec fuel press guage shows it continuously at 54psi upon shut-off. However with the manual guage I watched pressure drop as low as 40psi? Either way I would expext the pump to repressurize the line. Hopefull the poor ground connection resolved the issue for now.

TRINIC5

I know my tuner had that same issue, he did a one way check valve and it still was losing pressure when shut down and sit for a while,it only started after he did the injectors but they did played with the fuel system the same time.
My tuner check the fuel system properly, check every possible things that are related to the system and then realized it was in the tune with the injectors.
I am not a tuner so I don't know what he did and who advice him what to do but he said it was in the tune and today the car works very well without having to change the fuel system/ nor pump. Just my 02 cent.

greerracing

My car has been having start up problems recently. i wonder if my external fuel system blew the stock check valve apart.