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Old 10-15-2008, 10:07 AM
NC Benz's Avatar
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Join Date: May 2007
Location: Fuquay-Varina, NC
Posts: 485
Vacuum build up in fuel tank - Causes power loss

For a couple of week, our 84 has been having an issue.

We would fill up with fuel and the car ran GREAT! After a while the car would lose power and buck going up hills.

When we stopped to fill up, we noticed that when we opened up the fuel cap, we would get a big WHOOSH like vacuum built up inside the tank.

Filled up again and the car ran GREAT. Stop to give it a little more fuel and the cap was on so tight that it was hard to get off.

Makes sense now... the vacuum would build up as fuel got pumped from the tank.

So we temporarily took off the cap and we ran great.

So what line/orifice is clogged on this so that the pressure cant equalize?

NC Benz
Fuquay-Varina, NC
1979 300D
1983 300D Turbo 260,000 Miles
1984 300D Turbo 345,000 Miles (sons car)
OBK #31
1998 Ford Expedition 5.4l (fer Haulin'!) 145,000
1973 19' SeaRay with 115 Mercury TOWER OF POWER!
Club Car Golf Cart 36V
Ex toys:
1967 Mustang 289 (First Car)
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1980 Audi 5000 Turbo
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Old 10-15-2008, 10:12 AM
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Join Date: Aug 2007
Posts: 972
There is a fuel vent under the car. In addition to the supply line, there are two other lines that conect the fuel tank. One is the return line, the other is the vent line. Follow the vent line; there should be a bell-shaped valve on the end. It probably is clogged. If cleaning doesn't help, replace it.
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Old 10-15-2008, 10:19 AM
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Join Date: Sep 2004
Posts: 145
Your tank vent is plugged. I believe the vent on your car will be found underneath the car forward of the left rear wheel. You will see a cone shaped plastic device at the end of a metal line. You can remove the plastic piece, that is probably not the problem, you most likely have rust in the metal line. Try blowing it out with air - that probably won't work, but worth a try. Ultimately, you will have to separate the vent line where it enters the tank, underneath and snake it out up into the tank. The only way I was able to do it was with a piece of old speedo or motorcycle control steel cable in a drill, sort of a roto rooter. If you do this and it works, change your fuel filter after a few miles because it will plug up with the crud you dislodged. As a temporary fix, you can drill a small hole in your fuel cap, that will release the vacuum. Drill a hold small enough that you can plug it once you correct the problem. Good luck.
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Old 10-15-2008, 06:38 PM
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Join Date: Mar 2005
Location: Yountville, CA
Posts: 1,175
Read the previous posts as they are detailed and correct. I just had the same problem, my vent tube was blocked, and like you, had the the big"whoosh" whenever I opened the fuel cap. Car flies now, makes a huge difference! Just follow the instructions, good luck!
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