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Fuel Pressure High - Spewing Gas at the Tank

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I could use some advice on my high fuel pressure issue. My car only sees one or two race weekends a year.

1988 924S - low mileage rebuilt engine, fuel filter/pump/checkvalve all done in the last few years, charcoal canister delete (vented hose), new fuel lines in the engine bay.

 

Symptoms:

-After running at race load, the fuel tank becomes very pressurized, to the point of spraying a fine mist of fuel out of the tank when removing the gas cap.

-Recently at High Plains (high altitude), the car was intermittently becoming unresponsive to throttle input. The engine stayed running but I had to limp back into the pits.

-During this same weekend, I came off the track, stopped the engine at the scales, and I couldn't get it to start again until it had cooled down.

 

Note that I haven't had this problem before, and I haven't changed anything fuel system related. Could it be a faulty fuel pressure regulator? Any suggestions for next steps in troubleshooting?

 

 

Many thanks,

 

Andrew

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Big Dog

It seems to me that, if your fuel tank is pressurized, something in the vent system is plugged up. I have never traced out all of the small hoses around the filler but there are a number of them. Something in the vent system must be blocked or it would simply vent pressure out. I am guessing the reason for the pressure, since you are using gas up and should get a vacuum, is higher temps from the exhaust heating up the tank.

 

You can check your exhaust system for a leak that could be causing excess heat but the vent is a separate issue to deal with.

 

There are inexpensive fuel pressure gauges that can be attached to the fuel rail to check fuel pressure. I do not have one but have been told they exist and simply thread onto the fitting at the front of the rail. Be careful removing the nut, however. There is a steel ball inside that is easily lost.

 

Big Dog

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B1BFlyer

I think I may have something similar happening. It only did it the last race of the day when it was pretty warm out, but when I shut the car off it sounded like bubbling in the fuel filler neck. I went and carefully loosened the fill cap and it vented for over a minute.

 

I didn't have any performance impacts like you, but it may just not have made it to that point before the race ended.

 

Let me know if you get this figured out, I'll see if I can find any solutions on mine and let you know if I do.

 

Cheers,

Ryan

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Thanks for posting here about your similar issue. I now have my car winterized for the season (I'm in Minnesota), so I won't do any testing until Spring. I'll follow up then, but I'll look forward to any testing you might do before then.

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Sterling Doc

I had that happen to the extent it distorted the floor above the fuel tank. I eventually had to drop the tank, and replace the vent lines on top of the tank.

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I had that happen to the extent it distorted the floor above the fuel tank. I eventually had to drop the tank, and replace the vent lines on top of the tank.

 

Maybe that's the ticket. I've had two different fuel rails/regulators and have still had the problem. I'm assuming my issue is likely at the tank as you're suggesting.

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B1BFlyer

What about drilling a 1/8" hole in the fuel cap? Would that remain race legal? Seems a lot less work than dropping the tank.

 

Ryan

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Sterling Doc

Until you are upside down (BTDT). Don't mess with your fuel system - do it right! 25-30 year old hoses carrying gas/vapors sometimes need replacement!

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brkntrxn

I obviously do not own nor have I ever owned a 944, but wanted to provide a reply to drilling the gas cap. The issue you guys are describing is somewhat common on Mustangs and Mustang Cobras. I tried lots of options over the years to include drilling the gas cap. That didn't work for me. In fact, it allowed it to leak that much easier in the corners. Getting heat away from the tank and proper vent tubing is the only real solution. Or at least it is for the Mustangs.

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Spring is here, time to get to work. I checked all the hoses by the filler and they all seem clear. I blew into each one of them. The little clear ball valve seems to work as well. I haven't check my fuel pressure yet. I'll have to source a gauge. I'll have to check and report back. Other than that, are there any other suggestions?

 

Many thanks.

 

IMAG0178_zpse9dcc62d.jpg

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Big Dog

I have not had my tank out for awhile but there is a vent line that goes to the front of the car to a charcoal filter inside the left front fender. Many times, the line is disconnected somewhere and can get clogged up. Sorry but I do not recall just which line it is but you can find it under the car and trace it back. You can disconnect it somewhere to see if it is clear or not.

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Good idea Jim. I know where that line is and will check it tomorrow, hopefully when I check the fuel pressure. I think Autozone loans pressure gauges.

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