Questions and Answers

Audi A4 Has Vacuum In The Fuel Tank Q. Hi Vincent, I have a 1996 Audi A4 1.8 liter petrol, manual transmission, 160,000 miles, fuel injected, non turbo. Problem first evidenced when occasional erratic tickover where the revs would just dip or rise a bit very now and then. Secondly I noticed hissing at filler cap for fuel tank. Found vacuum in tank, put paper over flap and it was sucked against flap.

Leaving the cap off cured the problem so I bought a new cap but problem remains when cap is on.

When I disconnect wiring to activated charcoal solenoid valve the problem stops and I get perfect tickovers. I think the problem must be either the fuel tank vent, I'm assuming this should leave air in as opposed to being a pressure relief valve, or that the activated charcoal solenoid is letting too much air flow.

What should I do? Easier to replace the solenoid as easy to access but would I be better starting with the vent? If I leave the fault to continue is there a risk of the fuel mixture being too rich and damaging the catalytic converter? Also I thought the ECU was programmed to not allow the solenoid to open during tickover, is this so, and if yes, do I have an ECU problem?

Sorry so many questions, answering any will be of great help to me.

Many Thanks For Your Time,
Bill

A. There is supposed to be some vacuum in the fuel tank. I can't tell if yours is excessive or not. It is very possible there is nothing wrong with the EVAP system. There are a couple of tests you can do to the Canister Purge Control Valve to see if it's good. The first is a resistance test.

Checking resistance:

• Disconnect EVAP canister purge regulator valve harness connector.

• Measure resistance of EVAP canister purge regulator valve. Specified value: 20 to 28 ohms

If the specified value is not obtained:

• Replace EVAP canister purge regulator valve.

To do a voltage test you would need the Audi scan tool. I would assume you have checked the fuses first and the gas cap is the correct one for the car.