Hopefully someone can answer this question. Sure one of the experts can.

When my car doesn't have much fuel in it, it runs OK. But when it has a full tank of fuel it runs great...

Any ideas why?

Ok I really know SFA about engines but maybe there is some residue that has built up in the fuel tank over the years, when the fuel gets low, some of it starts to block??

hehe, don;t take my word for it though..

Wez - i think you been learing stuff hanging round here.

Andy - that would be the first thing i do. A REAL good clean of the tank and a change of filters

Any idea how to get into the fuel tank? And is it possible to save the fuel in the tank or do you suggest doing it when it is almost empty then draining it completely?

http://www.vw-resource.com/fuel_tank.html 

have a read here

(save this site - it REALLY does answer most questions)

Will do. Thanks all.

my car does that. I thought it was the extra pressure from the weight of the fuel making it run better. That's based on absolutely no factual information by the way.

I would have thought that. Except the fuel tank is at the front, and it is a rear wheel drive. So doubt it would make that much difference.

I read how to do the removal and refurbishment of the fuel tank. Sounds like something I could do but need to buy a lot of replacement seals and the like. Plus it sounds pretty time consuming with a lot of waiting around for stuff to dry. But a man's got to do what a man's got to do.

I also read the one about replacing the fuel filter(s). Which sounds like it might be an idea.

What I don't understand with what Wez & Bizarre said is that why would the rust and what have you only try and get through the fuel line when it is getting lower in fuel? Wouldn't it always try and get through?

Either way, I'm on uni holidays at the moment and not working a lot so I should try and get it done in the near future.

If I opened up the fuel tank where the float goes in when it was almost empty would I be able to use a torch to see what the condition is like to make sure it is the problem before I go through the whole process and find out it did bugger all?

This might sound a shade paranoid, but if you really want to look inside the tank disconnect your battery, and put the torch in a sealed clear plastic bag. Petrol vapour can go boom.
Another easy way to tell if your car is sucking crud is to check the filter if you have a paper one. I always have crud residue in mine. I really need to clean the tank.
My carby float tanks had over 1/2 cm of mud in them too. Yuck!

I might get under the car tomorrow and check that filter. Then maybe replace both of them before I do the fuel tank. But it sounds like because it only runs shit when its low on fuel that it's probably the tank. Looks like were both up for this job.

I looked under the car. I couldn't find a fuel filter anywhere... But I replaced the filter next to the engine. Cut the old one open and it didn't look too bad but it was far from perfect. Seemed to run nicely when I started it up even though it was cold. Although it does have just about a full tank of fuel so will see what happens when it empties a bit.

might be a pressure thing? As tank gets lower there is less fuel pressure. Dodgey fuel pump????

Nah, it got a new fuel pump for the roadworthy just before I bought it.

Just have to wait and see if it still runs well when closer to empty. The filter may have fixed it. I put in new spark plugs as well which made it generally run a bit smoother so it may just have been a combination of little problems.

Yeah, its probably just the rubbish in the tank then. Like you said, rubbish would be going through the engine, even when full. But (depends on the rubbish) if the rubbish is willing to float, it won't come down and get sucked into the engine until the level of petrol in the tank becomes low.

And as fuel is sucked out of the tank by the engine, the rubbish in there is going to come together and get more concentrated as the petrol is gone, and therefore collects up to become of higher concentration. Therefore, the < amount of fuel left, the > the concentration of rubbish in the tank, therefore the > amount of rubbish to get sucked in.

Well, thats just a thought, hopefully you might be able to get my drift.

Wateva way it is mate, I hope u get on top of it. Goodluck, im sure u will.

Oh, and the full tank would increase the pressure of the fuel going into the engine, therefore its easier to suck in. And when the level of fuel decreases, there is virtually little or perhaps no pressure and the pump has to do all the work instead. So having a fuel engine helps it go into the engine, not sure how well though. But I would be pretty confident to say that it would have some form of effect.

Yeah, its not really a major issue. I just noticed it was slightly better when full. So I'll just see what happens. But I do understand what you mean, I didn't think about it floating before.

cheers

The pressure will change quite alot, but the fuel pump should even the pressure out over the range of the tank being full then going empty. If you are using ur car as a daily driver, how much crap can be in the fuel tank??? If the prob stay around its most likely a pressure thing.

Well I am using it as a daily driver now, but it has rarely been driven in the last 3 or so years before I bought it.

One other thing that I thought would make no difference but might. When I was doing some rewiring in the boot I noticed a small amount of petrol leaking from out of the cap in the centre of the fuel tank. There was 5 screws on it so I tightened them up. They all required about three turns to be tight. But the ability for air to leak into the tank would make absolutely no difference would it? I don't really understand how it works, but I figure it probably doesn't. Air could probably get in through the fuel cap quite easily anyway.