Fuel tank "porous"?
 

I noticed recently lon filling up the fuel tank that the top third seems to be disappearing pretty quickly on a run. On examining it the garage couldn't find a leak , observed that the tank was plastic and surmised due to some staining that it had become" porous", presumably at the top third of the tank. Has anyone experienced this or is it that they just didn't find the leak?

I don't think a fuel tank could be so described, it's solid plastic.

How much more fuel do you think you are using?

Richard; I'm afraid this is all too vague to be meaningful. You mention the top third. How do you know when a third of the capacity of the tank has been consumed?

Secondly what does "pretty quickly" mean? :shrug: Have you performed a brim-to-brim calculation?

Thirdly, if you had a fuel leak you would smell it!

High fuel consumption is usually explained by one of the following:

• Inaccurate measurements and calculations.
• Boy racer driving style. :}
• Traffic jams and/or frequent short journeys.
• Engine failing to reach normal operating temperature due to thermostat fault.

Simon

Possibly a fuel gauge problem?:shrug: (sender unit in tank?)

That is a very common problem, especially if you have a fuel gauge that moves around a little bit, you can check the sender units by going into Ipk diagnostic mode 19 subsection 6.2 and looking to see if the first digit being displayed is a 1-2 or 3 (1) would indicate both send the units are sending a signal, as for the potential of a fuel leak it’s possible for it to leak at the back by the fuel cooler, I have seen several leak in the area
Mick

Do you mean you only noticed this effect recently or has the rate definitely changed? My juice also seems to go faster from the full to two thirds full mark than the rest. I put it down to the way the gauge works. You have a linear fuel gauge operating on a variably shaped tank, so maybe that's the cause. And there's all sorts of hidden things going on all the time in these tanks, especially in the later petrol version. See: https://www.the75andztclub.co.uk/for...d.php?t=108535

A lateral thought on staining of the tank (which definitely has not become 'porous').
I'm assuming this is the 2003 1.8Turbo, which needs the Fabled Orange Clip to secure the in-tank filter module. Fitting that means opening one of the tank ports on the top. Refitting the port cover can be tricky and a significant percentage of tanks end up with a leak from the seal. A long-standing leak causes the top surface to become 'stained' by the film of fuel, which slowly darkens as the film combines with road dust and evaporates. Also, there's usually a smell of petrol around the rear wheel arches, but it will depend on the severity of the leak. You may not notice a very slow weep. Maybe yours has had the FOC fitted and resulted in a minor fuel weep?
Only a visual check can answer that and if there is no FOC, then fit one, maybe replacing the rubber seal at the same time.



TC

Over a thirty five mile drive it consumed a third of a tank, with the rapidity of usage stopping at almost exactly the two thirds mark again, I repeated this on refuelling and it was the same.

the drive was on open hill roads through the Scottish borders

Given that I'm a fifty five year old man driving a rover estate ,I think we can reject the boy racer driving style theory

there was a faint smell around the rear part of the car with a full tank, examination of the back of car in a flyby revealed nothing untoward leaking.

the report von it by the local garage posited that some sort of internal lining had perished. the mechanic used to work for a rover dealership so I presume he knew any quirks.

thank you- very helpful I'll have those two areas checked out along with the seal point helpfully described by T-cut

The fuel tank is a actually multi layer blow moulding. The outside is High Density polyethylene with a barrier layer inside, these layers are bonded together in the blow moulding process . The barrier layer is prevent hydrocarbons in vapour form from migrating through to the outside. Even if the barrier layer were to be damaged it would not leak liquid such that you would notice a significant change in fuel consumption . Plastic tanks have been around for 40 odd years I haven't heard of them becoming porous.The main areas of leakage in fuel system are joints, so that what I would be checking first. I am assuming the vehicle is a petrol, if its a diesel the tank has less layers so unless damaged it is unlikely to leak .

Thanks for coming back to me Richard. I think you're referring to the fuel gauge having rotated sixty degrees from the vertical 'full' position. Gauges are very approximate so cannot be used for consumption calculations. Their movement is also non-linear. For these reasons it is extremely unlikely that you used one third of a tank of fuel during your 35 mile drive. That's about 7 mpg!

If you really want to know what your average consumption is, fill the tank until the petrol pump nozzle shuts off and note the mileage. Drive until the low fuel warning shows then refill to the brim again noting the new mileage and the number of litres purchased. Divide the latter by 4.546 to obtain gallons and you'll have your average mpg.

There's nothing wrong with your car Richard. Relax and enjoy it. :}

And take what some garages tell you with a pinch of salt!

Simon