filler cap question UPDATE

hanse1bd · Post by **hanse1bd** » Sun Dec 05, 2010 12:16 am

Hello All! I'm a new owner/member from Michigan.
In September I bought a 1981 Rabbit 1.6NA 5 speed, 106K miles. Runs good enough to get me 60 miles round trip to my job.
I have already learned alot from this forum and from my Bentley Manual which was recommended here.
My first of many questions is, when you fuel, do you relieve vacuum pressure when you remove the filler cap? My car does. Sometimes the vacuum is strong enough to make the cap hard to remove.

I thought this was odd, but paid no mind until the other day. I drove to work and during the 30 mile trip, the fuel gauge kept going up. Maybe it was the miracle I had been praying for, but just in case it decided to get fuel before I drove home just to make sure. I had 300 miles on the current tank and I average about 41mpg. That means I should have had about 2.7 gallons remaining in the 10 gallon tank. Well, the car sat at work all week (I'm a truck driver). I started the car to let the engine warm up while I was fueling my truck and finishing up my paper work.
I noticed that the gauge was still reading full with 300 on the trip. I revved the engine a few times after it had been idling for about 20 minutes and it would lag, then rev. Then in sputtered and died. I assumed I had run out of fuel so I siphoned a gallon out of my truck. When I removed the fuel cap, there was the vacuum sound. I added the gallon of fuel and the engine started after only a few extra cranks. I then drove 5 miles to the gas station. It only took 6.5 gallons to fill the take to the top of the neck. That means that with the gallon I stole from work, and the bit of fuel burned on the drive to the station, it took about 7.3 gallons to fill the tank. So I wasn't out of fuel after all. I had at least 2.5 gallons when it stalled.
Since then it has run 100 miles with no problems. So what I am wondering is if the vacuum in the fuel tank is preventing the fuel pump from pulling a healthy amount of fuel? What do you all think?

Thanks,
Brian

coke · Post by **coke** » Sun Dec 05, 2010 12:23 am

I'm going to hazard a guess and say that removing the in tank screen and checking it for blockage would be a good place to start. It would also be advisable to get a new fuel filter, especially if you have no idea of the condition of the current one.

No, its not normal. I've never had one release vacuum when removing the cap, but it certainly merits further investigation. Pull your in tank screen and see what it looks like, and change that fuel filter too. Just places to start, I'm sure others will chime in with more detailed information.

hanse1bd · Post by **hanse1bd** » Sun Dec 05, 2010 12:40 am

Thanks for the input. I didn't think the vacuum was normal either. Never happened on my semi or on the diesel pickups I have owned.
I changed the fuel filter when I bought the car, and carry an extra one just in case.
I haven't looked into the pickup screen. Can I get to it through the hole under the back seat, or do I need to drop the fuel tank?

coke · Post by **coke** » Sun Dec 05, 2010 12:44 am

On the rabbit, I do believe its under the back seat. If you can't find anything there, I'd look in the trunk somewhere. On my jetta, which is obviously a much larger car its in the trunk.

vwkook · Post by **vwkook** » Sun Dec 05, 2010 11:32 am

I had this problem with a jetta 10 or so years ago. The pick-up screen would get caked with rubber residue from an inferior hose I must have used on the fuel system somewhere.

hanse1bd · Post by **hanse1bd** » Sun Dec 05, 2010 8:28 pm

So I think I found the problem. The gravity valve on the vent line between the tank and expansion bottle. I don't know where it was mounted from the factory, but I found it tucked high up in the wheel well on top of the other vent lines. The mounting bracket had rusted through, and someone stuffed it up there. It was sitting side ways and must have been stuck closed. Well I broke a nipple off trying to pull it out, so for now I just bypassed it. Will have to find one to replace it.

I didn't go fishing for the pickup filter yet. I'll have to add that to my list of things to tinker with. Right now I have to get back to reading Hagar's saga..
Thanks everyone!

Post by **Fatmobile** » Sun Dec 05, 2010 11:08 pm

A clogged in-tank screen will cause a high vacuum in the fuel line
but will not cause high vacuum in the tank.

That's caused by a clogged vent.

You can collapse a tank if the vent isn't working.
Take a look under the car and see if it's collapsed a little.
Glad to hear you found the problem.