The more my fuel drops, the more sluggish the car gets, and the engine vibration increases a bit (things start to rattle). I usually fill up the tank to full when it drops to 1/4, and everything goes back to normal - no rattling, accelerates significantly faster and overall the car feels more 'nippy'. The only explanation I can think of is gunk and/or water from condensation in the fuel tank. Second opinions on what is causing the issue and tips on DIY solution are very welcome. Thanks!

 

PS The car was on full service 3 months ago, but the fuel filter wasn't changed, because it sill has a lot of time and miles left.

Blocked tank breather causing a vacuum to build up in the fuel tank as you drive, test for this by opening the filler cap after a longer drive and see if air is sucked in.

I have same issue on my Octavia. How do you unblock it?

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Kinda depends on what's blocked really.

The vent is in the wheel arch I think

 

The breather pipe seems to be clipped to the O/S/F of the fuel tank and runs back to the charcoal canister, the diesel engine may not have the canister in which case the breather will run directly back to the filler neck.

I found a similar picture http://www.partsbase.org/skoda/fabia-fab-eu-2001-20105-fuel-tank/ and it has "Breather line (petrol) / Gravity valve (diesel)". Is this valve, and the line to it, the likely culprit in my case? If that is the case - can anyone provide links to a guide on how to remove and/or clean them?

 

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The breather pipe seems to be clipped to the O/S/F of the fuel tank and runs back to the charcoal canister, the diesel engine may not have the canister in which case the breather will run directly back to the filler neck.

An alternative to sepulchrave's suggested 'long drive then look for hissing/vacuum as you remove the cap' might be remove the fuel cap after any journey, say your commute home tonight. Replace it immediately.

Then remove it again in the morning before you start the car, and listen for any pressure equalisation.  Just the temperature difference between end of journey in evening and start of journey in morning ought to be enough to give you a hiss if the gravity valve is really blocked.

If this was the case I'm pretty sure you would have heard the pressure release when filling up?

Next time you start to feel the drop in performance stop and open the cap,if performance does not get better after opening cap it's not a breather issue.

If the above don't help I would stick a can of additive in the diesel tank to remove any water.

An alternative to sepulchrave's suggested 'long drive then look for hissing/vacuum as you remove the cap' might be remove the fuel cap after any journey, say your commute home tonight. Replace it immediately.

Then remove it again in the morning before you start the car, and listen for any pressure equalisation.  Just the temperature difference between end of journey in evening and start of journey in morning ought to be enough to give you a hiss if the gravity valve is really blocked.

 

I filled up to full yesterday and did a 30mile trip this morning. I just opened the fuel cap - no noise at all. I will give it another try in a few days, as I am fairly certain I hear it hissing every time I fill up the tank. However, I might be getting confused by the tank pressure relieve valve (when fueling, I usually press the small button behind the cap to relieve the pressure and squeeze in another liter or two).

 

By the way, my fuel gauge doesn't go down evenly. I fill up to full and it takes twice as less miles to drop to the 3/4 mark than it needs to drop from 2/4 to 1/4. Basically, the lower it gets, the more miles it takes to go lower and I make the exact same 30mile route at the same time every morning and evening. I pay attention to my trip MPG and it is pretty much the same on every trip. Could that be related?

Edit: @coskev Are you sure that an additive can get rid of water? I was told that adding in a couple liters of petrol when I full up will do a much better job, as petrol can mix with water and it will get burned off in the engine. Haven't tried it though, as I wanted to get a second opinion before I put any amount of petrol in an old diesel car.

Edited 14 January, 201610 yr by xlr8

Yeah plenty of additives list water removal on the labels.

I would bang some additive in,then run it right down low before refilling.

Edited 14 January, 201610 yr by coskev

http://m.ebay.co.uk/itm/PROTEC-Common-Rail-Diesel-System-Cleaner-P2101-Removes-Water-Cleans-Protects-/181456648256?nav=SEARCH

Pretty sure petrol will not remove water,as water/condensation in the tank can affect petrol cars too.

When I thought I had this problem (on an Alfasud in 197x) I drilled a 1mm hole in the plastic fuel filler cap and had no more trouble. A colleague told me of a drive he did from Calais to Frejus when he found his fuel tank seemed to hold less and less fuel on each fill up - it was - his pump was collapsing the tank (Triumph Dolomite !).