Vacuum in the fuel filter?

[TDI4evah]

've been having a weird problem lately with the engine being starved for fuel. Sometimes it works fine, sometimes it gets bad enough to kill the engine completely.

When I pull the thermo-tee fitting in the fuel filter there's a short sucking sound. It did it tonight with a brand new fuel filter, so I think it's safe to rule a plugged filter out as a source of the problem.

Could this be a plugged tank vent ? (just thought of it, haven't checked if there's a vacuum on the whole tank) The only other things that come to mind are a plugged fuel pickup or a plugged supply line.

I'm stuck at a motel and hoping to fix it in the morning. Ideas very welcome.

(moved from the MK4 section)

 

[csstevej]

I would lean toward the fuel pickup in the tank....maybe the check valve sticking closed.
You could also leave the fuel tank cap loose and drive around and see if symptom returns, at least this way you will know if vent related.....but I don't think it's that. That's all I got....no coffee yet.

 

[compu_85]

Some vacuum in the fuel filter is normal... after all the IP is what sucks the fuel from the tank.

Does your car have a clear line from the filter to the IP? Is it full of bubbles while the engine is running?

-J

 

[TDI4evah]

After sitting overnight there was no vacuum in the fuel tank. I left the cap loose.

Additional symptoms, might be the same problem, might be a different one, but yesterday afternoon when it was starved for fuel I pulled over and swapped the fuel filter. I didn't hear the vacuum then, though it was a pretty noisy place so I could have missed it. The thing was that it was starved for fuel, but there was fuel both in the filter and some that drained out the port that feeds the IP. IE, it should have had fuel going to the IP, but it was starved for fuel. On that filter I got ~100 miles before having more problems. *that* makes me think of the vane pump on the IP having issues, but the vane pump not working and a vacuum in the fuel filter don't go together.

It started a little hard this morning, then idled fine with no bubbles in the clear line. I'll go after some coffee and see how it runs.

I'll update here when I find out what the problem is.

Thanks for the help and suggestions!

 

[PeterV]

1. miles on the car it helps diagnosis.
2. Transmission we need to know it helps diagnosis.
3. You use any additive?
4. its not freezing in Maine yet BUT you might need the secret H mod! If you do not have the H fuel sending unit.
5. 10+ year old car you may have fouling of the in tank sending unit .. Yes tank grunge.
Were you at the TDIfest 3 weeks ago at Sunday River we had many of the best gurus there that could have helped for free.
Any mods to the car?

 

[TDI4evah]

More data....

It didn't seem to have any air bubbles in the fuel line when idling and first started this morning, so I took off. As soon as it needed more fuel to climb some hills it was starved again. I got it turned around, parked outside a car parts store, and pulled the fuel sending unit. Clean as a whistle, no gunk, float valve working fine. I blew down the intake tube and it bubbled cleanly. No blockage of any kind that I could see. I tried to blow the supply line clean and got some fuel out, but don't know if I fully cleared the line.

With the car running normally now - low speed, around the parking lot, back to the hotel - it goes fine. Every time I turn the car off and pull the thermo-tee I get the sucking sound. I'm thinking that this is probably normal, a result of pulling through the check valve.

Much more importantly, when just idling, streams of fuel alternate with streams of air. It's easy to imagine that getting more air is responsible for fuel starvation at higher loads.

Could this just be a small leak in the fuel supply line? Where else could the air be coming from?

Peter, if it helps, it's an 11 mm pump and an 02m. No tank pump installed (yet). Stock fuel filter. Rebuilt engine with ~10K on it. And I wanted to go to TDIfest but couldn't make it - bit wasn't having the problem them so no one would have helped fix it

 

[DanG144]

Spoke with him on the phone.
Very puzzling.
But it does seem that the vacuum in his fuel filter is much more than normal, something is plugged in the supply line.
He is investigating that, and some get home strategies if required.

 

[TDI4evah]

Dan and I explored a lot of possibilities, and one of his suggestions got me here today.... I expect it'll get me home tomorrow as well. Thanks Dan!

The problem is certainly something in the fuel supply line between the tank and the filter. I believe it's the check valve, but won't know for sure until I get a chance to really take things apart.

Dan's suggestion was to string a long piece of fuel line between the outlet on the tank and the fuel filter, and that's what I did. Out the passenger window, under the hood. No problems driving today.

I'll update the thread on Sunday or Monday when I isolate the problem.

 

[TDI4evah]

Today I blew out the fuel feed line from tank connection to the filter connection with some compressed air. I collected a couple of black specks, but didn't see anything obvious.

The high pressure air flowed pretty freely, so I went back to stock. Started the car up seemed to run OK - took it out for a spirited run and it seemed to run fine.

I'll keep a close eye on it for a few days and post back here if there's an update to be done, but for now consider it some blockage in the intake, a really weird symptom, and that no news is good news.

Thanks for all the help and responses, this is a great forum!