Losing Fuel Pressure

[Bobbyh68]

I have a 2005 LQ4 6.0 thats losing fuel pressure after engine shuts down. 58psi
while running and slowly drop after turning off. overnight it will drop to zero.
Im using the stock 6.0 in the tank fuel pump with a corvette regulator/filter combo.
1st. Replaced fuel pump (Walbro 255) with no change. Still have pressure drop!
2nd. Replaced entire tank pump assembly and no change. Still have pressure drop!
3rd. Removed spark plugs and put camera inside cylinders, no fuel dripping?

I give up, any ideas?

[mongocanfly]

Shouldn't be a issue and not uncommon..turn switch on and pressure builds back up..every one I've owned does the same..not a problem..even my new truck does that..pump will run a few seconds when you turn the key prior to cranking

[DLW]

Originally Posted by mongocanfly

Shouldn't be a issue and not uncommon..turn switch on and pressure builds back up..every one I've owned does the same..not a problem..even my new truck does that..pump will run a few seconds when you turn the key prior to cranking

Ditto that. Not uncommon in any LS motor I have had for the pressure to slowly drop off over time. Pump will build up pressure again as soon as ignition is on.

[Bobbyh68]

I have to cycle the key twice to build up enough fuel pressure for a smooth start. If I don't cycle the key it stumbles for a couple seconds or starts on the second try.

Doesn't seem normal.
But runs AWESOME, I don't regret The LS swap!
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[mongocanfly]

cycling the key shouldn't have any effect on it as far as a startup stumble is concerned..turn the key..pump builds pressure and then shuts off once pressure is built..is it throwing any codes?...also did you get it dyno tuned..

[Rich84]

Originally Posted by Bobbyh68

I have to cycle the key twice to build up enough fuel pressure for a smooth start. If I don't cycle the key it stumbles for a couple seconds or starts on the second try.

Doesn't seem normal.
But runs AWESOME, I don't regret The LS swap!
Posted via Mobile Device

My Tuned Port injected engine with an inline fuel pump acted the same exact way, right down to the running awesome part.

With my LS swap I went with an in tank pump and it does bleed off pressure after shutdown, but it starts instantly when you turn the key.

Maybe you have a restriction in one of the lines.

[Bobbyh68]

Originally Posted by mongocanfly

cycling the key shouldn't have any effect on it as far as a startup stumble is concerned..turn the key..pump builds pressure and then shuts off once pressure is built..is it throwing any codes?...also did you get it dyno tuned..

No codes, and its a mail order basic tune.

Thanks for the inquiry,
Bobby
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[mongocanfly]

If you like how it runs now get it dyno tuned...I picked up almost 50hp on the dyno over the base tune it had in it..300 bucks well spent..

[Bobbyh68]

Shops around here want $500 for a dyno tune.
Going to wait till I do a major change like cam and springs.
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[BR3W CITY]

Mine bleeds pressure after 20-30minutes. External 255. Never hard an issue starting.
I do go key-on for 3 seconds before starting, but thats to allow my system monitors to power up before I crank.

Fwiw tune pricing is all over, and depends largely on WHAT your tuning, WHO is tuning, and WHICH way they are doing it. Avg for a 3-pull dyno session tune is like $3-400 most places, which is acceptable.
I/We prefer to do tunes based on whats needed, not just 3 pulls. Our larger projects can see up to 100 miles between the rollers AND road tuning. This means that trans tables, part throttle and driveability concerns are also being addressed. This takes 5x as much time, but its how we can watch 1k+hp cars leave with complete confidence. On the high end for example, GenV DI Corvettes with big prochargers can run $1k-2500 to tune. Its pay to play in the big number game.

[Bobbyh68]

Originally Posted by BR3W CITY

Mine bleeds pressure after 20-30minutes. External 255. Never hard an issue starting.
I do go key-on for 3 seconds before starting, but thats to allow my system monitors to power up before I crank.

Fwiw tune pricing is all over, and depends largely on WHAT your tuning, WHO is tuning, and WHICH way they are doing it. Avg for a 3-pull dyno session tune is like $3-400 most places, which is acceptable.
We prefer to do tunes based on whats needed, not just 3 pulls. Our larger projects can see up to 100 miles between the rollers AND road tuning. This means that trans tables, part throttle and driveability concerns are also being addressed. This takes 5x as much time, but its how we can watch 1k+hp cars leave with complete confidence. On the high end for example, GenV DI Corvettes with big prochargers can run $1k-2500 to tune. Its pay to play in the big number game.

I'm going to try the key-on for 3 sec approach. Will give an update after work.

Thanks,
Bobby

[DLW]

Good to hear you are tuning on the street as well, so you can hit all/most the other load cells you can't do on the dyno, unless it's a very specific type of load bearing dyno. Non-lockup transmissions especially beneficial to road tuning. Most just do the WOT dyno tuning now-a-days.

Originally Posted by BR3W CITY

Mine bleeds pressure after 20-30minutes. External 255. Never hard an issue starting.
I do go key-on for 3 seconds before starting, but thats to allow my system monitors to power up before I crank.

Fwiw tune pricing is all over, and depends largely on WHAT your tuning, WHO is tuning, and WHICH way they are doing it. Avg for a 3-pull dyno session tune is like $3-400 most places, which is acceptable.
I/We prefer to do tunes based on whats needed, not just 3 pulls. Our larger projects can see up to 100 miles between the rollers AND road tuning. This means that trans tables, part throttle and driveability concerns are also being addressed. This takes 5x as much time, but its how we can watch 1k+hp cars leave with complete confidence. On the high end for example, GenV DI Corvettes with big prochargers can run $1k-2500 to tune. Its pay to play in the big number game.

[BR3W CITY]

The dyno is a Mustang with roadforce, so it can be "loaded" somewhat to add the drag from "real world" road interface.. In winter obv. its harder to get these things on the street (and just not safe), so at least it allows you to get a much more thorough tune on stalled auto's without doing stuff like forcing lockup on the rollers. For the 8 and 10 speed auto cars, I have a deep suspicion most of those trans tables are bone stock on canned/3 pull tunes.

[DLW]

At least you use a mustang dyno. Hub dyno would be nice too.

[BR3W CITY]

Originally Posted by DLW

At least you use a mustang dyno. Hub dyno would be nice too.

You won't find many places that use hub dyno's, especially not in the hotrod market. Its strengths are also its limitations. Because you don't use any interface between the wheels and the dyno, you eliminate issues like tire shake and spinning (bias ply big tire problems). Downside is that you can't use it as a diag tool for those sort of problems. I see hub dyno's way more often in road course cars (needing extending time on a dyno), and far more frequently outside the US.
Dyno cart (engine dyno) if the heart is out, otherwise she's on the rollers.

[DLW]

I hear you. I mention the hub dyno as that was what my tuner used exclusivel here in the bay area for my street/drag car. The biggest pro was loading it through all the kpa ranges. His is an older model and maxed out at 1K HP though, so that became an issue for me and mustang dyno/track was it after that.

Anyway, I think I hijacked enough here, sorry OP.