pressure build up in fuel tank

**trout1105** · 9th October 2017, 12:47 AM

The filler is closed by a threaded plastic cap which screws into the filler neck. The cap has a ratchet mechanism to
prevent over tightening and seals against the filler neck to prevent the escape of fuel vapour. The filler cap has a valve
which relieves fuel pressure to atmosphere at approximately 0.12 to 0.13 bar (1.8 to 2.0 lbf.in 2 ) and opens in the
opposite direction at approximately 0.04 bar (0.7 lbf.in 2 ) vacuum.

This is from Rave So I am pretty sure that my problem with the vacuum in the tank is caused by the vent in the cap not functioning correctly and explains the leaking balloon like noise at times.
This should be an easy fix

**Pedro_The_Swift** · 10th October 2017, 07:08 AM

Originally Posted by bee utey

A good way to get atmospheric moisture inside the tank to rust it and ruin the pump.

no doubt you are right,, and probably the reason I havent done it,,,

still IF the cap ever vents inwards thats what will happen anyway,,

anyone ever purged the system using software?
or know what causes it to purge?

**bee utey** · 10th October 2017, 08:04 AM

Originally Posted by Pedro_The_Swift

no doubt you are right,, and probably the reason I havent done it,,,

still IF the cap ever vents inwards thats what will happen anyway,,

Yes but the amount of water vapour ingested is much reduced if you contain the fuel vapour pressure during the warm part of the day. Constant evaporation cools the fuel to below atmospheric temperature and promotes condensation in the tank.

**Pedro_The_Swift** · 11th October 2017, 04:10 PM

and any air ingested around here will have a lot of SALT in it,,
even better!

DiscoMick · 11th October 2017, 06:06 PM

Originally Posted by trout1105

This is from Rave So I am pretty sure that my problem with the vacuum in the tank is caused by the vent in the cap not functioning correctly and explains the leaking balloon like noise at times.
This should be an easy fix

New cap would be my first thought.
Does your tank have a breather hose from the top running up to near the cap? Could it be blocked?

**trout1105** · 11th October 2017, 09:24 PM

Originally Posted by DiscoMick

New cap would be my first thought.
Does your tank have a breather hose from the top running up to near the cap? Could it be blocked?

To be quite honest I don't know if there is a breather hose there or not, I will have a look tomorrow.
I only get this problem on a long run where I have used 3/4 or more of a tank in one go, the thirsty V8 sucks about 15-16ltrs an hour at 100kph.

**jh972** · 12th October 2017, 11:19 AM

Read the post from Trout, above.
The cap is meant to retain pressure & vacuum to a point.I suggest it's perfectly normal. Mt D2 did this all it's life - never caused a problem.
Retaining pressure (or vacuum) stops unnecessary venting, reducing moisture intake & fume exhaust.

**Stanleysteamer** · 12th October 2017, 08:36 PM

Originally Posted by jh972

Read the post from Trout, above.
The cap is meant to retain pressure & vacuum to a point.I suggest it's perfectly normal. Mt D2 did this all it's life - never caused a problem.
Retaining pressure (or vacuum) stops unnecessary venting, reducing moisture intake & fume exhaust.

To all posters.
If you go for a long run you must stop from time to time, why not just vent the tank then, if you are bothered? I once had a neighbour with a strimmer/weed whacker that stopped for no apparent reason, from time to time. He had no luck with official repairers. So he gave it to me. I ran it until it stopped running. As soon as I vented it, it restarted. So the cap was not venting at all. Lesson learned.

New cap or shift the blockage somehow. But, as others have said, it may actually be functioning OK. If you have a pressure and a vacuum gauge you may be able to test it as per Rave.

Surely a new cap is cheap enough? Best of luck.

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