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

[dgt]

I have a 1988 951 race car, purchased 3 months ago as a DE car and converted to a full track-only race car.

When I am running 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 race with much less fuel than this to start.

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

Thanks, Doug

[nize]

water in the tank?

[dgt]

RE: water in the tank, the tank has been drained, cleaned, and refilled.

I appreciate the comments.....this will be a process of narrowing down what it could be...

[nize]

how about if the fuel pickup line has some stress fracture in it that becomes evident under high-g's, causing it to suck air?

i'm not 100% familiar with the entire fuel line routing on the 951 so i could be way off base here.

[kdjones2000]

This is the same problem that was happening to me at both AX and HPDE events. For AX I was always setting up to start with 1/2 tank, and couldn't figure out why my car was dying on RH turns.

Only after running 20min sessions in DE events did the cause become clear: During the 3rd 20min DE session the car would start to hesitate and get progressively worse, eventually leading to small backfiring before coming back into power. Fill the car up, and it behaves perfectly fine again.

Since the fuel pickup for the 944 is on the PS of the tank, and there is a significant volume of the tank in the "Upper DS", I don't see an easy fix to stop the fuel from migrating over to the DS during hard right cornering.

The only real fix for this IMO is to go with a bladder.

This all being said, I have asked several 944 owners whether or not they have experienced similar behaviour with their cars, and not one has noticed this.

No one here on Pelican could answer my query on this same issue about a year ago, either.

DGT, it sounds like you have already replaced everything obvious I can think of that might help with the problem.

Here are some off the wall ideas I can think to try:
1) Some racers have set up a "double fuel pump" system for their cars, primarily to make sure that they keep AFR's down on their high power engines. Maybe this could help, by reducing the fuel linear velocity going into the pump.
2) Try increasing the length of the outlet hose going from the tank to the pump. If this is full during the turn, it could help reduce/eliminate cavitation.

Good luck, Keith

[2Tight]

Assuming that it's a problem with fuel shifting how about installing a duplicate outlet on the DS of the tank and route over the TA it to a 'Y' right before the fuel filter? There are plastic fittings and glues to make a solid connection easy on plastic tanks.

[nize]

wouldn't that just make it suck air both ways? since air has less resistance than fuel.

[2Tight]

Originally Posted by nize

wouldn't that just make it suck air both ways? since air has less resistance than fuel.

Come to think of it you are probably right unless he ran a separate pump on the other pickup. I'm still trying to figure out why it's happening in the first place. Do you know if they are baffled near the pickup area or elsewhere in the tank? If he still has 12 gallons in the tank and it's happening that just doesn't make sense at all.

[nize]

how about jacking up the passenger side of the car to figure out at what angle the symptoms occur?

[wild man]

Not that I am very familiar with fuel tanks or lines, but could it be a bad spot in the return line that is allowing air bubbles to be shot into the tank?

[dgt]

I finally had a chance to try my first round of 'fixes' on the track yesterday and thought it might be good to update this post with the solution to my problem for future reference.

I replaced the in-tank fuel filter (the old one was in horrible condition, had holes in the mesh, so don't know how it could have stopped fuel from flowing) and replaced the O-ring on the fuel cap (cap leaked fuel with a full tank in the past) - apparently, problem solved. The same track corner than was causing me issues last year could now be taken without engine cut-out with fuel level all the way down to low fuel light coming on.

Granted, the weather was very cold (35 degrees) compared to last year when this issue came to light, but there could be other issues that will come out when the temperature increases, but for now, problem solved. Fingers crossed.

I also replaced the in-tank fuel level sender, but that should not have anything to do with the original problem.

[kdjones2000]

Doug, thanks a bunch for the tips.

I didn't replace my screen when I had the tank out the last time - I think that I will try that next time I am down in that region.

I am thinking that fixing the fill cap probably didn't do anything, as I have replaced my cap with a new one before and never noticed a change. My current also holds pressure, so I doubt that this is causing the stumbling.

Cheers,
Keith

[edbaus]

Doug,

Sorry I came in late on this one. I have had this exact problem with a couple of 944 na race cars I have owned.

The screen is the first culprit. If it were to continue (as a friend of mines did)

He replaced the gas tank complete and still had the issue. Turned out to be a pinched breather line for the tank. He replaced it and can run well into the light coming on with no issues.

Thanks

Ed

[dgt]

Ed,

Thanks for the follow-up comment. I have been able to find several people with this issue over the years (posts going back 8+ years), but solutions have been much harder to find. I'm glad you provided your solutions that have worked.

The breather is one that I was going to look at next if this didn't fix it. I did run compressed air through the breather and lots of 'stuff' came out, so this could have been part of the issue also.

So far, so good! Thanks for your reply - I'm crossing my fingers that the issue is not 'heat' related - and repeats itself as the weather warms up. -Doug

[kdjones2000]

Which breather line are we referring to here?

I replaced all of my lines above the tank, but didn't touch that check valve or the expansion tank hoses. Are you talking about those or the lines going to the charcoal canister?

Thanks, Keith

[kdjones2000]

Almost a year on, and I finally got around to checking my strainer. The issues I was having were both power loss on hard right hand turns in AX and Track Days, and I was having my fuel pump dying on me about once a year.

Last time this happened was 2 months ago, and one of the PCA guys who was there and also runs a 944 repair shop mentioned that every time he replaces a fuel pump, he automatically replaces the strainer. The idea is that if the strainer is broken then junk at the bottom of the tank can get into the close tolerance fuel pump and cause it to seize. For the $30-50 it costs a new strainer is cheap insurance.

When I took the strainer out it was not that obvious that there was a problem, but there was one indeed:





The seam where the netting is glued to the support is where these things fail:





Tested the new strainer out at the track today, and I was able to drive the car with no issues down to 1/16 of a tank.

So my advice to you is, if you are noticing power cutting out on right hand turns, or you are going through fuel pumps like Dorritos, then check your strainer. You'll be glad you did.

Cheers, Keith

[Rasta Monsta]

+1:

[Rasta Monsta]

Originally Posted by edbaus

Turned out to be a pinched breather line for the tank

Ed, can you elucidate this a bit further? Which line? I'm afraid changing the screen didn't help.

[todd Jenne]

I am having the same problem. Its very consistent when I use 10gals or half tank it starts cutting out in right hand corners fill the tank with 10 gals and the problem goes away. I just ordered a new screen filter I hope it cures the problem. will report back after next track day on the 6th of September

[todd Jenne]

Fix the fuel problem