Glenn R

I’m at VIR this weekend. It is nice, but hot. Twice now I have come in from a session and heard a sound from the rear of the car that sounds like boiling.

When i open the gas cap, vapors hiss out but don’t stop. It just keeps spewing and spewing gas vapor.

Is my gas boiling? Is it bad? What should I do about it?

Glenn

wildcat077

Does your tank have a vent tube ?
Perhaps it might be pinched or blocked if you have a no return valve in the tube ...

Glenn R

It is not trapped vapor. The gas is actually boiling. I've found reference to similar problems in 928's, but not 944.

MAGK944

The 944 has a sealed fuel system like almost every other car on the road today so it will build up a certain amount of pressure depending on fuel level, temperature and altitude. There are a series of vacuum solenoid valves under the hood by the fuse box than will open on certain conditions to relieve the pressure by directing it to the charcoal canister. If the pressure isn’t relieved the fuel may boil and even after the pressure is relieved the fuel may continue to boil for a short while until it cools sufficiently. I’d be looking at and around those solenoids for the solution.

Fwiw a while ago I saved this description of the operation of the EVAP system on our cars from onZedge on Pelican, hope it helps:

“The EVAP system prevents fuel vapor from the fuel tank from leaking into the atmosphere. It consists of four parts. 1) The vapor canister, which accumulates vapor from the fuel tank. 2) The plastic EVAP shut-off valve (CVI), which momentarily opens, only when the engine is hot, to purge the vapor canister of accumulated vapors. 3) The metal EVAP control valve (CVII), which controls the vapor flow rate when CVI is open. 4) The temperature controlled vacuum switch (Thermo Switch), which activates the EVAP system only when the engine is hot.

The vacuum canister is located in the wheel well on the driver's side. The two valves (CVI & CVII) are located in front of the brake booster next to the fluid reservoir on the inner fender. The temperature controlled vacuum switch (Thermo Switch) located in a small coolant line under and between intake runners 3 & 4.

The 2-1/2" plastic valve (actually a vacuum operated check valve)is called the evaporative emission shut-off control valve (CVI). The inlet to this check valve connects to a tee which connects to both the fuel tank vapor line and the vapor canister. That is how excessive fuel tank vapor pressure is accumulated and condensed in the vapor canister. The motive force for CVI is through the vacuum line that goes to the Thermo Switch. The Thermo Switch has two nipples on it. One points up and the other one points up at an angle. The vacuum line from CVI goes to the nipple that points up. The nipple that points up at an angle connects to the upstream throttle body nipple that points forward at an angle. (Angled nipple to angled nipple.)

The job of CVI is to momentarily purge the vapor canister to the engine when: (1) the engine is hot via the Thermo Switch. And (2) the throttle plate just comes of idle when vacuum is momentarily high upstream of the throttle plate. After that moment, at part throttle or WOT, there is not enough vacuum to keep CVI open. Hence, if the vacuum line from CVI is disconnected, CVI "should" always be closed.

The 2-1/2" metal valve bolted to the inner fender is the evaporative emission flow control valve (CVII). The job of CVII is to regulate the vapor flow rate, through CVI, to the intake manifold via the j-boot. The motive force for CVII is through the vacuum line that goes to the downstream throttle body nipple that is shared by the Fuel Pressure Regulator and Fuel Pressure Damper.

To troubleshoot this, first locate CVII that CVI is connected to with a short fat vacuum hose. Disconnect the OTHER fat vacuum hose from CVII that leads to the j-boot and plug it. Assuming that the hose you just plugged is good and it idles better with it plugged, it is because the plastic CVI is bad and fails to fully close when it's supposed to or the short fat vacuum hose between CVI and CVII is leaking.

If either the hose from the vapor canister to the tee, the vapor line from the fuel tank to the tee or the tee itself connected to CVI is leaking or the canister is damaged, it WILL eventually be a source of fuel odor in the engine compartment, but it WILL NOT be a source of an engine vacuum leak IF the rest of the EVAP system components, including hoses and vacuum lines, are sound.”

thomasmryan

You might pull the charcoal canister and give it a shake. Sometimes the pellets can plug up the system if the canister has failed internally

If it is backed up, you should have a hard time filling the tank. The old mercedes have a fuel to freon heat exchanger on the suction side of the compressor to cool the fuel before it returns to the tank.

if you have ever distilled any party liquor, it boils at 180F at sea level.

Glenn R

I will check this system. But if I remember correctly, wouldn't increased pressure raise the boiling point? Therefore, releasing pressure (venting to charcoal canister) lower the boiling point and make it more likely to boil?

MAGK944

Ha good catch, what I should have said is “negative pressure” or “vacuum”. As the fuel is used in a sealed system you build up a vacuum in the tank that is usually prevented by the EVAP venting system. However you bring up a good point, I wonder if a sudden release of positive pressure through either the regular EVAP system or by removing the gas cap could cause boiling, especially on a very hot day at sea level.

V2Rocket

if there is vacuum in the tank, which could contribute to early boiling, it could indicate your "fresh air inlet valve" is not functioning.
there is a one-way valve that serves to let in fresh air specifically to avoid having a vacuum in the gas tank, it's in the snake bundle by the filler neck.

Glenn R

I'm off to Mid-Ohio this weekend. I won't have time to dig in to the vent stuff before then. If it happens again this weekend, I'll check the temp of the tank and some of the fuel lines.

harveyf

See my PM

Tom M'Guinn

Drill a hole in the gas cap until you get it sorted.

curtisr

I suspect that this is why dragsters have a fuel cooling tank. The gas is sent through coils surrounded by ice. I thought of using this because my 924s would bog down after one lap on the track. It turns out that there was a filter insert in the inlet pipe of the fuel pump which was rather plugged up and had trouble with the expanded gas.

thomasmryan

I would still look at the vent/roll over valve being obstructed but I lean towards the tank being exposed to engine vacuum as the prime culprit. the charcoal/venting system is plugged up on my '01 Subaru and it sounds like someone is hitting a bong in the back seat when you start the car.