Fuel tank venting and starvation

Sorry in advance for the length...

I'd like to hear the GRM opine and any additional suggestions before I drop the tank yet again.

OK - so... S52 powered E30 race car with stock fuel tank ( a saddle type over the driveshaft), carbon canister removed and vent line routed out behind rear tires. Car was starving in right-handers with stock fuel pump. Dropped tank, added typical secondary fuel pump to driver-side to transfer fuel to the pump side. Plumbed it right into the swirl bowl in the tank to keep it right at the primary pump pick-up. Replaced stock pump with Walbro 280GPH pump.

In the first race weekend - I started with half a tank in practice to see how far I could go. Good news... I could run 30 minutes without issue and thought the problem was solved. So, filled it to the top and went out in Qual... DOH. It started starving after 4 laps when full. I immediately thought it had inadequate/blocked venting... so I drilled a hole in the cap to test the theory. Worked like a charm... car ran the 1st race start to finish without an issue.... except.... it was spraying fuel 10' across the track from the hole in the cap in left handers. This does not win popularity contests with the folks immediately behind me... In the 2nd race I took a rag and put it under the cap as a baffle to stop the spray - and got all the way to 5th place in lap 5 before starvation made me limp it home in 11th place 25 laps later. Needless to say that sucks figuratively (if not literally).

I believe have some issue that is causing suction to starve the pump and using the filler neck as a vent is not a great idea. I think the existing vent is blocking with fuel or something and then allowing it to get sucked flat by the pump. I can't prove this - but a full tank isn't actually starving the pick-up no matter how hard the cornering force. It has to be vacuum.

The plan:

I will drop the tank. I will add a large diameter AN fitting to the top with a roll over valve. I'll use some kind of hose/pipe that can withstand 55psi of suction and remain open. I'll route it up high in the trunk, loop it and vent it out the rear. I will relocate the filler neck to the trunk on the center-axis of the car and put a sealed cap on it.

Is there anything else I can do to ensure I never end another top-5 run with fuel starvation or any other best practices/safety items that need to be addressed while the tank is out?

Defintely agree that you have a vent problem. Not sure that you need to reloacate the filler. I think that your vent idea will take care of the problem.

Have you monitored fuel pressure at all through this? Just curious how you can be so confident it's lack of fuel causing the issue. Not doubting, just making sure you've considered the obvious before considering the not-obvious.

Bryce

Nashco wrote: Have you monitored fuel pressure at all through this? Just curious how you can be so confident it's lack of fuel causing the issue. Not doubting, just making sure you've considered the obvious before considering the not-obvious. Bryce

No fuel pressure gauge in the car to look at - but... set to 50psi on the regulator. The symptoms are:

• No gas cap... runs fine
• With gas cap... not so much.
• Symptom matches previous starvation in right-handers exactly... except it happens everywhere at full throttle.
• The last thing I touched was adding an auxilliary fuel pump to counter right hand starvation.

So, atleast to me - I obviously have a fuel issue. I just don't know the exact root cause but I have a plan I hope will address any/all potential sources.

I wish I had one of those expensive electrical fuel pressure gauges on the dash but the best I can do is to put a hose on the mechanical one so I can see it from in the car temporarily. Unfortunately, the car is not road legal, has a 3" open exhaust and needs to be at full throttle for an extended period to see the problem so... iterative problem solving is not an option. I need the big hammer.

To clarify, there is no material on the face of the earth that will withstand 55 psi of suction. A perfect vacuum is about 14.7 psi of suction. Also, your regulator pressure setting has nothing to do with actual fuel pressure.

Like you said, it seems like you've got a problem with vacuum in your fuel system. However, it could be something as simple as a failing high pressure fuel pump, flaky wiring/relay, or restricted fuel lines. The first thing I thought of reading the story above is...have you tried a new (unmodified) gas cap? They are almost always designed to eliminate vacuum in the tank but allow pressure in the tank (to prevent fumes from destroying glaciers). You did well to punch a hole in it to finish a race successfully, but from the sounds of it you never put a new one in afterwards. There are ways to test the cap without driving the piss out of the car for an hour if you were so inclined. If you're worried about finishing the race, you could always put a rubber stopper or a ball valve or something in place that you can yank with a cable if you start having fuel issues in your next race. If it starts to hesitate and feel like lack of fuel, then you yank the cable and if it immediately fixes itself, you've verified the actual problem and were still able to successfully complete the race. Combined with a fuel pressure gauge (mechanical with a long hose works fine!) this would be extremely useful to help properly solve the problem. It may turn out that the real problem was a pinched fuel line that worked the failing fuel pump too hard (or similar) and you reworked your fuel tank for naught.

Or you could throw hammers at it and hope one of them is right. I'm just not a fan of hammers, especially with fuel systems. Good luck!

Bryce

Nashco wrote: To clarify, there is no material on the face of the earth that will withstand 55 psi of suction. A perfect vacuum is about 14.7 psi of suction.

Haha! I know, I know... You know what I meant though... so the pump could not suck the soft rubber flat.

Nashco wrote: Also, your regulator pressure setting has nothing to do with actual fuel pressure.

Well... it does on how much pressure the pump has to push - since the reg. is on the back of the rail there is 55 psi of pressure on the line from the pump to the back of the rail... after which the return can "de-pressurize" to whatever atmosphere available at the tank. The symptoms do say vacuum... or a pump that struggles to maintain pressure at volume or a little of both combined. I have ruled out bad grounds and low voltage conditions across the rev range.

In any case - better venting that I am sure of won't cost much or hurt anything so I'll start there and also make the pressure gauge visible from the cockpit. The hand-operated bleed-off should be unnecessary with proper venting but is a cool idea. Hopefully that will address it - otherwise there is only one component left... the pump.

A new cap is in place now. It was not available at the track.

Giant Purple Snorklewacker wrote: Well... it does on how much pressure the pump has to push - since the reg. is on the back of the rail there is 55 psi of pressure on the line from the pump to the back of the rail... after which the return can "de-pressurize" to whatever atmosphere available at the tank.

My point was that just because your regulator is for 50 psi doesn't mean you have 50 psi of fuel pressure...your regulator could be going funny when it gets hot and you could drop down to 30 psi for all you know. You could also have a slightly pinched line that allows 50 psi all day long at idle, but when flogged starts to slowly reduce pressure to 40-30-20 psi even though you have a 50 psi regulator. Just saying, when stuff this weird happens, pretty much all ASSumptions are bad assumptions. A couple of gauges would go a long, long way in diagnosing the problem.

Bryce

For venting run a line out the rear of the car. That way fuel slosh won't be a concern. Loop it as high as possible. And put a small filter on it.

it does sound like fuel starvation due to vacuume. If it were carbed car, I would think empty bowls.. but as you are injected, it has to be vacuume

E30 S52 here too. I had the same problem when I converted my car to standalone. The car would run great for a while but then sputter and die. And you could even hear the pickup pump laboring. I popped the gas cap and the tank took a deep breath. Turned out that since I didn't make any use of the emissions valve, I effectively blocked the tank vent.

I just left it hanging open in the engine bay and didn't have any trouble with it.

I have since started my turbo build and it hasn't run yet so I'm hesitant to comment with it as my basis but don't forget that the higher flow the pump, the bigger the vent needs to be. With the bigger pump you added, you're probably overwhelming the stock vent. I'm using an A1000 and it specs at least a 1/2" tank vent. I'm just going to use that fat line on the passenger side which normally goes to the vapor tank and filler neck, and loop it past the filler neck, removing the vapor tank.

You mean to tell me no one in the pits had a spare PCV valve? They make great temporary vents.