924 Breather

[johnturner]

Hello My first post.
I have a low mileage 1985 Porsche 924
that runs and drives wonderfully well, and is a pleasure to drive.
Other than at times being reluctant to start.
I have noticed that when I open the petrol cap that there is a gush of air entering the tank so there is obviously a vacuum in the petrol tank.

I am guessing that this will hinder the cars starting and that there must be a breather for the tank somewhere.
Has anyone had similar experiences and can anyone offer advice, any help would be appreciated.
Thanks
John

 

[russmw]

Hi and welcome,

There are two answers to your question and one or those is a huge positive. The 924 tanks are metal and known for corroding then leaking, as yours is holding a good vacuum it's clearly not cracked or split, result.

As for the vacuum the fuel system is sealed so should you roll the car it won't leak fuel every where but excess pressure should be let out and there's a breather pipe which runs down the side of the tank on the filler cap side. I presume the pressure isnt there when the car is cold, just after a good run?

Is the starting problem when the cars hot or cold?

Thanks
Russell

 

[s999sws]

I think you’ll find that it’s not a vacuum, but under slight pressure. It’s easy to think that it’s air rushing in, when in fact it’s escaping.
Cheers
sws

 

[vitesse]

The fuel tank should be pressurised-this & the fuel pump check valve help with hot starts in particular-also the K Jetronic Bosch fuel system includes a fuel accumulator situated after but close to the fuel pump -this includes a diaphragm with a powerful spring internally which is largely intended to maintain fuel system pressure ,acts as a buffer & over the life of the 924 ,was altered several times to improve hot starts ,along with other various tweeks to the fuel distributor.

Worth checking the fuel pump check valve & this accumulator.

 

[pse_SC]

They all seem to do this, usually when car is warm/hot. Never saw it as a problem, though the rush of pressure can be quite surprising if you’re not used to it, when you release the fuel cap at the petrol station []

 

[johnturner]

Thanks for the reply Russell
The car starts ok Cold and Hot but after a run of an hour or more if I stop for a coffee or whatever say 45minutes it is then when I have problems starting and then that I have noticed the air gushing in or possibly out of the petrol tank.
Thanks again
John

 

[johnturner]

Many thanks to everyone who replied to my post you've given me lots of advice and options to look at
Thanks again
Cheers
John

 

[scm924s]

The tank breather system passes through an expansion tank up by the filler tube, hidden under a cover, there are a couple of non return valves in there which are to prevent spillage if the car overturns and can sometimes stick preventing proper breathing and causing tank pressure. A PITA to get at.