Weird OBD Code

Can someone help me diagnose this code I'm getting. The car is my '99 BMW M Coupe (E36). I am getting the code:

P0446 - "Evaporative Emission System Vent Control Circuit"

The code comes on and goes away occasionally. It seems to come one when the fuel tank is about halfway full, while cruising down the freeway at a steady speed.

This was happening for a while several months back and just popped back up again. I'm not exactly sure when it goes away. I'm pretty sure that it does so after filling the tank back up. But it might be that it happens as the tank get close to being empty. I'm pretty sure that filling it is what solves it.

Any ideas what is causing this and what I can do about it? Doesn't seem to actually be a problem. Car runs fine and there does not seem to be an effect on gas mileage that I've noticed.

There are evap sensors in the tank, often the system is sensitive to top filling the tank, fuel will go into the evap vent and cause a lock in the vapor system.

So... if that's what this is, how do I correct it?

I don't "top off" my tank. I only go until the pump stops.

Do you figure this is just a silly little sensor that needs to be replaced because it's getting old and finicky?

Possibly, or a wiring issue, or a vacuum line, or a golf ball...

Replace the gas cap and check the exhust routing. If you've got a aftermarket exhust be sure there plenty of room around the tank. Your likely heating the tank thus increasing the tank presure.

Also check the purge valve for the evaporative canister, if it's not sealing that could do it. There was a Mazda tech bulletin for Miatas which sat for extended periods setting that code, the cause was spiders making nests in the purge hose.

44Dwarf wrote: Replace the gas cap and check the exhust routing. If you've got a aftermarket exhust be sure there plenty of room around the tank. Your likely heating the tank thus increasing the tank presure.

Exhaust is stock. Car does not sit for extended periods.

Gas cap though... I need a new one. It's using the cap from a different BMW, that fits but is definitely different. Sounds like the likely culprit right there.

Thanks.

Yeah, the BMWs will often throw codes when gas caps are left off or if they leak. That sounds like your problem.

It sounds more like the car is looking for a vacuum to be present on the fuel sys and it does not see it. If you just wont to try parts, try the vent sol. It is most likely leaking.

BMW's "Evaporative Shut Valve"

Agree with Jensenman, that's a common area that results in this code. You'll also get a 441 with it some times, as I recall. I've seen the poppet valve stick in the seat more than once, giving this code because the system didn't purge down right.

I don't know what your warranty start date is, but you may even be able to get the dealer to take care of this one, the warranty for emissions devices is 10yr/100k miles.

Um, not quite. The Federal emissions warranty on the PCM and catalytic converter ONLY is 8 years/80,000 miles. All other emission components are covered by the base vehicle warranty only. AFAIK, Kalifornia and Nueva York have state emission laws governing emissions warranties but they do not extend the base warranty coverage, they only add some wear components like spark plugs for specified periods.