Power cut-out, hard right turns (fuel starvation?)

dgt · 11-14-2009, 10:06 AM

I have a 1988 944T race car that this is occurring on, but know this problem has been noted on 944 NA's also, thus requesting this boards help.

When I am racing with a full tank of gas, no problem. But as soon as I have used 6-7 gallons, the car cuts out for 1-2 seconds when coming out of hard right corners and hard right corners only, then picks back up and runs fine again until the next hard right turn. Fill up the car, runs great again until 6-7 gallons are used, then it cuts out in the same corners again. This does not happen sometimes, but all of the time that this scenario is met.

In an October 31st 3 hour enduro race, this cut-out occurred every 35-40 minutes, requiring a pit stop to top off the fuel tank with 6-7 gallons at that time, then it would run fine again for another 35-40 minutes.

The car has had this problem since I first tracked the car in a late September PCA regional race, and has happened at each track event since as soon as the fuel level is 6-7 gallons below full. Keep the car full, and it runs great - but not an option to run competitively carrying 80 extra lbs of fuel, and not an option at all in enduro races (required 4 pit stops to finish the race).

I've visually checked the inside of the tank/tank filter, replaced the fuel filter and fuel pump (twice), replaced all vacuum lines, spark plugs, spark plug wires, rebuilt fuel injectors/seals. Nothing has changed so far.

I have also researched this problem on the PCA tech site as well as the Pelican forums, and have seem this problem noted a couple of times over the years, but the 'fixes' mentioned have mostly not been real fixes (ie replace the fuel tank, or run with a full tank and you'll be fine). There were some posts mentioning electrical issues, but the consistency of this only happening after I use 6-7 gallons of fuel seems to lead towards a fuel starvation issue - but how with 14 gallons of fuel left? Obviously, many 944/951's start a race with much less fuel than this so they end near empty.

Anyone with similar issues or ideas on what to check next?

Thanks, Doug

Rasta Monsta · Rasta Monsta's Garage

That is odd. I wonder if your vapor system is creating excessive vacuum in the tank? I would also consider doing a flow volume test on the pump, and checking pressure at the rail.

dgt · 11-14-2009, 03:23 PM

Rasta, thanks for the reply. I'll check the vacuum as you mention, but why would vacuum change only on very hard right turns? This does move the fuel away from the 'pick-up' side of tank. I thought also about the fuel pressure regulator, but again, why only hard right turns?

The other posts I have seen - from many years ago so this obviously is not common - were identical except the fuel level was around 1/2 tank, not 2/3 where my problems start.

Rasta Monsta · Rasta Monsta's Garage

I dunno, I just thought if the pump was marginal, little things working against it might be enought to push it below pressure threshold (also applies to slosh away from the pickup).

Vacuum issue easy enough to test. . .pull off when it starts cutting out and crack the cap.

924RACR · 11-15-2009, 05:04 AM

I agree with Rasta; while yes, you're cornering in the wrong direction for the sump - these tanks do have a very effective sump, and I've not heard of them running into cutout issues with more than a tank, at least unless something else is bad.

So, as for fuel-related issues, what he said... and it's likely fuel-related. Does your gas tank cap when loosened suck air in hard? That or vice versa is usually a good sign of bad venting.

One more final conclusive proof of it being a fuel level issue, not anything else (like time running) would be to head out with only 1/2 gal more than your bogey, 14 gal, and see if the problem shows up within a few laps - before anything else has had a chance to get hot enough to cause any possible heat-related issues.

Sorry I can't be of more help, but it seems like you're looking at the right bits, may just have overlooked something...

dgt · 11-15-2009, 06:55 AM

Quote:

Originally Posted by 924RACR

I agree with Rasta; while yes, you're cornering in the wrong direction for the sump - these tanks do have a very effective sump, and I've not heard of them running into cutout issues with more than a tank, at least unless something else is bad.

So, as for fuel-related issues, what he said... and it's likely fuel-related. Does your gas tank cap when loosened suck air in hard? That or vice versa is usually a good sign of bad venting.

One more final conclusive proof of it being a fuel level issue, not anything else (like time running) would be to head out with only 1/2 gal more than your bogey, 14 gal, and see if the problem shows up within a few laps - before anything else has had a chance to get hot enough to cause any possible heat-related issues.

Sorry I can't be of more help, but it seems like you're looking at the right bits, may just have overlooked something...

RE the gas cap, the gas cap seal is actually bad - just ordered a new one. When running track sessions, gas actually leaks out the cap and goes down the side of the car. Not major amount of fuel, but enough to be seen when dirt/dust/rubber dust sticks to it showing streaks on the side of the car. So, if anything, I'm guessing the fuel tank is venting through the cap not allowing vacuum build-up. Very good point to check the normal vent for clogging - possibly there is reverse pressure as you mention. Good point.

I have also narrowed it down to not being heat related - doing 20 minute sessions, this was my original thought as when the problem occurred, the track session would be over prior to me getting filled up and back on the track. But the 3 hour enduro put that theory to rest with the problem occurring every time 6-7 gallons was used, then fill up and the problem was gone with continuous track time.

My takeaway - I'll replace gas cap seal and check proper venting of tank.

oliver88 · 11-15-2009, 01:57 PM

had the same problem...pull the tank and yank the "sock" off the fuel pickup....Mine has never done it since the removal.

dgt · 11-15-2009, 03:42 PM

Brant,

Can you elaborate further? You are the only person I can locate that has actually experienced this problem.

Did you just remove the mesh filter covering the pick-up? That can be done w/o removing the tank I believe. Was your application on a track car or street? Is that the only change you made to correct the problem?

Rasta Monsta · Rasta Monsta's Garage

[QUOTE=dgt;5012948That can be done w/o removing the tank I believe.[/QUOTE]

Deep 17mm.