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Caterham

Member Since: 06 Nov 2008
Location: Birmingham
Posts: 6265

2011 Defender 110 Puma 2.4 XS CSW Stornoway Grey

interesting theory (thanks) but pretty sure that's not the case for me (and not just because we don't get 50 deg C) but because this problem can be when the engine is hot or cold.

19th Mar 2012 1:39pm

Merlin

Member Since: 30 Oct 2010
Location: Newmarket
Posts: 980

Could be a miss fuel problem. Petrol and diesel separate out and hence have different performances. There is an additive you can put in to clean the fuel system.

Merlin

19th Mar 2012 5:19pm

Caterham

Member Since: 06 Nov 2008
Location: Birmingham
Posts: 6265

can anyone confirm if you fill up with fuel use say half of the tank then remove the filler cap is there a whoosh of air into the tank ie like a vacum has formed?
although it doesn't suprise me as i've had this with several previous cars it kind of suprises me on the basis that the fuel tank has a breather and therefore I wouldn't have thought this would happend?

27th Mar 2012 9:44pm

blackwolf

Member Since: 03 Nov 2009
Location: South West England
Posts: 16892

2007 Defender 110 Puma 2.4 DCPU Stornoway Grey

If the tank breather is blocked, the pump will struggle progressively to lift fuel from the tank due to the partial vacuum created as the fuel is removed. Since it is inevitable that air will eventually leak into the tank and destroy the vacuum, in this case it is highly likely that the engine will run better with a half-empty tank than with a full tank.

The reason is that in the case of the half-empty tank, the air present in the empty half will expand, and although the pressure in the tank will be lower than atmospheric it won't be much lower. The pump will cope, although as the fuel level lowers it will get progressively more difficult.

If you were to try to pump fuel out of a full, perfectly-sealed tank, in practice one of two things will happen - either the pump will fail, or else the tank will collapse (and I have actually had this happen to me on an HGV with a blocked breather), because there is no air to expand and you are attempting to create a full vacuum.

A simple test would be to drive for a fair distance, preferably at a fairly constant and reasonably high speed, then stop and immediately remove the filler cap. If the breather is blocked, you will hear an inrush into the tank (or in extremis you may be unable to remove the cap).

28th Mar 2012 1:59pm

Caterham

Member Since: 06 Nov 2008
Location: Birmingham
Posts: 6265

blackwolf. so it would seem i have a block tank breather and my problem is fuel starvation.

so just to confirm. when you take your fuel filler cap off there isn't an in rush of air ? because thats what I get and this is presumably due to a blocked breather?

another trip to the dealers. only this time it would appear I have something for them to aim for and resolve !!

28th Mar 2012 2:17pm

BigMike

Member Since: 13 Jul 2010
Location: Lancashire
Posts: 2253

If you get a big rush of air and it's hard to remove the filler cap then yes, tank breather. Just a short "phut" of air then it's unlikely. Just get under it and take it off if you can and have a look.

28th Mar 2012 2:22pm

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