7 replies to this topic

[M111OWW]

I noticed this driving back from Dover to Wolvs last night,
The car started running a little rough on part throttle cruise, I though I'd stop at a service station to fill up and take a look around it.
I couldn't get the fuel filler cap off..!! When I did manage to prise it off, there was an almighty sound of air rushing into the tank!!!!
I know the vent line runs to a charcoal canister and finally into the inlet via that solenoid valve for fuel vapour emissions, but does that also allow air back into the tank to account for fuel used??

I had to stop every 30 to 50 miles to relieve the vacuum inside the tank.

Ps that also cured the rough part throttle cruise....
I'll be honest and say that I don't know the first place to start looking for this....

[FlyingScot]

Check the solenoid valve and for collapsed hoses in the system, that's the most possible causes.

FS

[M111OWW]

Ah, ok...
How would I check the solenoid valve? Could you apply a 12 v charge to it? Or would I need a code reader?

Would I be right in thinking that the tank would suck air back in to replace fuel used through the solenoid/purge valve and fight against the vacuum sucking from inlet manifold?

[FlyingScot]

I would need to check, normally MEMS provides an "Earth" to activate controls. I would try a 9V battery on the solenoid.

Look for crushed or flattened pipes going to/ from the valve, the hoses fail over time.

FS

[xrocketengineer]

While you are at the solenoid valve, you could try disconnecting the line going back to the tank. Blow (not suck) through the line with the fuel tank cap off. If you can blow through and/or helper by the filler neck can hear you blowing, then the line should be fine.

Edited by xrocketengineer, 21 September 2015 - 04:27 PM.

[M111OWW]

Should you be able to blow through the charcoal canister too...

I'm also wondering why there is a two way valve mounted on the tank and vent line, surely that's just an open valve all the time, or would it work on slight negative or slight positive pressures...

I still don't quite understand how air gets into the tank. A vent out through the inlet manifold I get, but not the route in...

[spiguy]

As far as I understand, the cap allows air in. It is sealed against pressure (pressure caused by fuel vapour in the tank will open the vent valve which is connected to the charcoal canister) but under vacuum it can allow air in. I have never studied the cap to see how that works though. So I would guess that if you are experiencing vacuum in the tank, there must be an issue with the cap.

[M111OWW]

That makes more sense having the cap let air in...

I did run a test on the two way valve last night, it took a higher than expected pressure to get it to open in either direction, in excess of 20psi....
I will check out the cap tonight.......

What I need is another two way valve on the bench where it's easier to see and test.

The cap and tank system is original 20 year old parts, maybe just worn out!!