Gas tank popping again

[quattro920]

Hi everyone,

The dreaded pop is back again. It will only happen when the gas tank is less than half full.

I've already replaced the following,

1. Vent tube from charcoal canister
2. The charcoal canister itself
3. The gas cap

Where should I start looking now?

Thanks.

[Edwin NL]

Ehhh keep the tank full?

You did all i would have

[Cheyne]

Drill some holes in the bottom of the fuel tank to help equalise the pressure. It'll play havoc with your gas mileage though and you might want to avoid smokers

[chris33]

I have the same issue. I've just gotten use to it...kinda like the VANOS rattle.

[Edwin NL]

Yeah but in the long run it will crack your tank!

[chris33]

So, whats the fix? Ive tried every thing that has been recommended on the board.

[ProStreetDriver]

what about drilling an extra vent on the top of the tank with like a check valve just to release the excessive pressure


is this the original or aftermarkt/non oem tank???

[quattro920]

As far as I know this is the original tank. I also scanned the car with carsoft have a code from the DME that says "98 Activation fuel tank evaporation system" Now I have had that code before and put in a new gas cap but it doesn't solve the problem. Any chance this may be related to the pop? Also any idea why the gas tank only pops when the tank is less than half full?

Also I want to learn more about the clicking thingy next to the oil filter housing. I think that's the valve that allows vapor into the engine again. Is there anyway to check that's working and working at the proper time? Does that have any relation to the popping gas tank?

[chris33]

This TIS illustrates the most common coause: http://www.e38.org/SI.New.Evaporativ...Purge.Line.pdf

[quattro920]

Yes that has been replaced!

[chris33]

Same here...When does yours pop? Mine only pops when I first get in the car after it has been sitting overnight. Before the fix it would pop after the car had been turned off and had sat for a few miniutes.

I know that I am not getting a vacuum in the tank while the car is running as I have never gotten a rush of air when removing the fuel cap like I did before the repair. So, My thought is that maybe the cooling fuel is causing the tank to implode a little because of the permenant crease (indentation) that formed in the tank from the original problem.

[quattro920]

Mine only pops when the tank is below half full. I don't get the rush of air either from removing the fuel cap. If I manage to keep the tank above half full it doesn't make any noises.

[wyshyvanuk]

This problem sounds the same as the one I've had for the last couple of years. I've always thought it was a single backfire, but it could just as well be the tank popping when it's under a quarter tank full. Over the last three days I've tried loosening the gas tank cap prior to starting the car in the morning, and, wow, the pop disappears. Has anyone found a solution? Thanks.

[novasol]

A little hijack here:

Is it normal to not have a hissing noise when removing the gas cap? My other cars have, but my 740 never does...

[quattro920]

I didn't have a hissing noise either when removing the gas cap.

As for the popping, it's been called a "redneck" fix but I drilled a hole thru the middle of the gas cap and it hasn't popped since. You can call that fix whatever you want but it works.

[wyshyvanuk]

My 740 has a build date of 11/98 so the newer purge line shown in http://www.e38.org/SI.New.Evaporativ...Purge.Line.pdf
should have already been installed. FWIW, I've seen diagrams on RealOEM that show that the purge line is actually connected to a leak diagnosis pump on USA models. There should be an air filter linked to the pump but I don't know enough about its operation to know whether it will equalize the gas tank pressure.

[maverick040759]

My 740 has a build date of 11/98 so the newer purge line shown in http://www.e38.org/SI.New.Evaporativ...Purge.Line.pdf
should have already been installed. FWIW, I've seen diagrams on RealOEM that show that the purge line is actually connected to a leak diagnosis pump on USA models. There should be an air filter linked to the pump but I don't know enough about its operation to know whether it will equalize the gas tank pressure.

I know this is an old thread, but I want to add/input something and see if anyone has any new info. 2001 E38, My tank crumpled in (tank popped) and had a leak where it creased the metal. I replaced the carbon canister (new) and the tank (used). After firing it back up I heard a thump (after about 30 seconds). I turn it off and looked underneath. The drivers side of the tank had caved in a little(not as bad as the first time). After doing more research, I decided to see if I could blow air from the tank towards the carbon canister. I removed the purge line (the larger of the two on the drivers side of the tank). I was not able to blow air through it. I removed the tire and the inside trim and found what I deemed to be the rollover valve. I removed it from the top of the expansion tank and tried again, no luck. After removing the valve, I was able to get air flow. I also checked airflow through the expansion tank (from the carbon canister connection) and it was good. My Bentley book doesn't mention the valve. There is a picture of it on page 160-16, but it does not mention it there. The TIS talks about clogs and kinks int he line, but not the valve. Has anyone had the issue of the collapsing tank and the fix was the rollover valve? I have seen a lot of threads regarding too much vacuum in the tank. My assessment is that it is not a real vacuum, but rather the fuel that is being sucked out is not being replaced with air due to a blockage (somewhere). Please correct me if I am wrong. I have also seen threads regarding not being able to pump enough fuel in to the tank. Others have alluded to fuel tank being full and the sensors being wrong. The inlet for the fuel is at the bottom of the tank, so air will not evac through the fueling line. This also leads me back to the float valve being stuck (for some reason other than a roll over).

[acspecialists]

I was just working at the shop and saw a guy smoke test the EVAP valve. The valve should be closed so if you see smoke it is leaking, in this case the tank would be under vacuum from the intake.

[maverick040759]

I was just working at the shop and saw a guy smoke test the EVAP valve. The valve should be closed so if you see smoke it is leaking, in this case the tank would be under vacuum from the intake.

I bypassed the rollover valve, and there was still suction in the tank that I could hear through the gas fill hole. Removing the bypass completely relieved the suction. So I connected it again and removed the line from engine (vacuum) to the canister and the problem went away. I can definitely feel suction on the line from the engine. There is supposed to be vacuum there(it's a vacuum line). But I am assuming that there is a mechanism that is supposed to limit the effect in the tank. I replaced the Evaporative emission control valve(EECV) when I started this project. I am still trying to understand the mechanics of the system and if I am getting the proper voltage to the EECV from the ECM at the proper time. The Leak Detection pump (LDP) is supposed to pressurize the tank system to assess leaks, but as far as I can tell, it's not happening. In my previous post I said I didn't think it was vacuum, but I might be wrong. Now I'm wondering if the LDP is getting voltage.

[Timm]

Couple of things worth reading on this subject:

https://www.meeknet.co.uk/E38/m62tu.pdf#page=35

https://www.meeknet.co.uk/E38/E38_Fuel_Tank_Venting.pdf

[maverick040759]

Couple of things worth reading on this subject:

https://www.meeknet.co.uk/E38/m62tu.pdf#page=35

https://www.meeknet.co.uk/E38/E38_Fuel_Tank_Venting.pdf

Are these pages accurate for both US and non-US versions? I'm pretty sure that non-US version do not have a carbon canister. I have issues with a closed system right now. If it is closed there is enough vacuum to partially collapse the tank. If I read this correctly, my problem might be the TEV. Even though I have changed it, it could still be bad, or it is not getting voltage to close. I still need to see if the LDP is getting voltage. The fuse (F-17) is good, but I haven't checked it at the pump. I am having flashbacks to the vacuum issues I had with the manifold. I closed up all of them 2 summers ago. I am wondering if something I did there affected this system.