Wednesday, March 18, 2009

BMW repair: your charcoal canister and you

I wanted to comment on something that I keep seeing: cars where the owner or previous owner has disconnected the charcoal canister. If you haven't seen one recently, it's the little can that normally sits under the battery tray on your BMW Bavaria, 3.0CS, 2002, etc. and maybe in other places on cars like the E12 5-series. It has two lines that plug in to it. Here's one that, of course, was removed for no good reason:

The fuel tank ventilates into a small plastic expansion tank in the trunk. From this tank, there is a blue plastic tube that runs through the length of the car and into the engine bay and then into the charcoal canister. From there, a second tube runs to your air cleaner (or intake manifold on fuel-injected models), where the fuel vapor from the expansion tank is eventually burned by the engine. The charcoal in the canister serves two purposes: first, it stores the hydrocarbons from the fuel tank ventilation while the engine is not running (that is, while engine vacuum isn't pulling the vapor into the air cleaner or manifold), secondly, it is a safety mechanism: a backfire in your intake won't make it back into the fuel tank.

Now sure, after many years, the canister looks pretty nasty (mostly due to battery acid eating away at it). Replacements are readily available new (example: "activated carbon filter") and they're not that expensive. I've seen the damn thing disconnected by the previous owner of every car I've owned or helped a friend buy and there's never much of an explanation offered as to why:
  • Sometimes the owner is confused, the last one handed me the car's original Zenith carburetors in a box and in there was the canister. He asked me, "what is this thing anyway? I took it out".
  • Sometimes it's "I was taking out the smog equipment and this seemed part of it".
The canister does not decrease the performance of your car or otherwise cause you problems. It's not part of the EGR system or some other smog stuff that causes the engine to burn exhaust gases.

On the 2002 we have, as a bonus, the fuel tank vent tube was plugged with a bolt by some previous owner -- that's really bad, as pressure in the tank will then keep building up with nowhere to go.

If you're really set on the idea of taking all this stuff out, or you have nowhere to connect the line because you've converted to Weber side drafts and have no air cleaner to speak of, look at how it was done on European-market cars: they simply ran the vent line from the expansion tank down under the rear bumper, but there was still a vent line. On even earlier cars, there was no expansion tank but the fuel filler cap or neck were vented to the outside.

Please also consider this when you complain of fuel vapor inside the car. Perhaps this small set of tubing that is supposed to vent out the fuel tank into the air cleaner is no longer hooked up, venting fuel vapor into the engine bay? Or did someone disconnect the line at the expansion tank, causing vapor to leak into the trunk?

In addition, on later cars like the E12 528i and E28 five series, there is a thermal/vacuum-operated switch in-line with the tubing from the canister on its way to the intake manifold. This additional step keeps the fuel vapor from being burned before the engine is warmed up (and then is able to burn it better), which decreases emissions. This is probably overkill for early cars that never had it, but that's what it's for in case someone is wondering.

(thanks to Tony and Keith on the SSR list and to john_a on the 2002 FAQ message board for additional details).


Neil said...

Thanks for clarifying the charcoal canister function. After years of complainig about the fuel spell in my renovated trunk, I am undertaking to renovate my canister. I was hoping to be able to disassemble it and load new charcoal (think aquarium). I can't seem to get it apart. Any insight?

andrey said...

Hi Neil. I don't know about pulling them apart but why not install a new one to be on the safe side? They're readily available and not very expensive.

Neil said...

I guess after 36 years it could use a renovation and I don't know if a junk yard piece would be an inprovement - $130 for the part seems a bit spendy.

andrey said...

You can use one intended for a newer car, like an E12 5-series, and they're pretty cheap.

Rob said...

Please Help: Here is what I wrote on Bimmerforums with no answers given.

" just purchased a 72 Bavaria (automatic) and the charcoal canister was unhooked and I know gas smell is an issue with these cars, but I think I have more than is tolerable. Any idea where the line from the gas tank into the canister should run and where on the air filter housing does the outflow go? I also have a vacuum line from the transmission dipstick that runs across the firewall. This is hanging in the breeze at the driver side, the origin is off a T-fitting, one part to the transmission dipstick while the other is open ended. What connects to this??? I have original shop manuals but no answers to these ?'s, Thanks, Rob

andrey said...

Hi Rob. There's a blue plastic line on the firewall which comes through the firewall padding, can you find it? From that line, there should be a hose running to the charcoal canister (under the battery). From the canister there should be a vacuum hose to the air cleaner (which has a small fitting on the underneath) and that often gets hooked into the vacuum lines on the carburetors on the front.

On US automatics, BMW had to add a transmission breather expansion tank, it's normally bolted to one of the strut towers, like this:
...there's simply a hose from the dipstick tube to that tank.

I hope that helps. Please let me know if you have any more questions and please consider joining our mailing list (if you haven't yet), there are instructions on

andrey said...

Rob, also here's a quick list of other things that cause fuel smell on the Bavaria:
- under the hood where the interior fan is (black plate covers that), there's a compartment. It needs to be sealed (check the parts diagrams and compare) and there are rubber drain tubes that need to be present.
- any smell in the trunk will drift to the cabin. Check your trunk seal, tail light seals, and there's actually a seal behind the huge silver panel in the back. Check the O-ring on the fuel level sender unit in the fuel tank and check your fuel lines.
- from the fuel filler neck, there's a line that runs to a plastic expansion tank in the trunk (above the passenger side shock tower). That needs to be connected and from that tank is where the blue charcoal canister line originates.
- obviously do check any rubber/braided hoses under the hood (fuel supply under the brake booster to the pump, pump to the carburetors, etc)

Rob said...

Think my first response is in cyberspace.

Andrey, many thanks. I finally found the blue plastic line after asking myself what it would look like at the age of 38 (black). I will do all the other suggestions as well. Do you recall which port is the input and output of the cannister, they are different lengths but no lines were attached so I cannot tell? There is a line from the bell housing to the tranny dipstick that is missing, but also no vent container for my tranny? Hey thanks for your responses, In New England, none of these cars are left and nobody who knows how to fix them. Rob

fuel trailers said...

I recommend OEM replacement parts. Very affordable compared to original parts. In this economy, we take what we can save upon.