Some comments (archive)

I haven't seen a 3 port canister but I would assume the intake vacuum port on the canister may be the trigger to open the other 2 ports. If so then you won't feel vacuum at the fuel tank port. Instead when vacuum is applied to the intake vacuum port, a path should open between carb and fuel tank ports which would allow the vapors to pass into the engine. So why is the carb port plugged? You may not have any purging going on since the carb port should be plugged somewhere into the intake. To verify you should apply a vacuum to the intake vacuum port and see if the carb and fuel tank ports open, and then close when you remove vacuum. The intake vacuum port should not leak. If it is then there is something wrong with the internal valve.

I have a similar thing happening to my 85 745. I've got a Mercedes 2 port canister where one port is connected to the fuel tank and the other port to intake vacuum via the ported vacuum switch on the thermostat housing. So basically, when the engine warmed up fuel vapors were being drawn into the engine through the tiny vacuum hose. I always wondered why my fuel tank was negatively pressurized and I have finally realized that this setup has turned my fuel tank into a giant vacuum bottle. Whenever I removed the gas cap I would get a rush of air getting sucked back in. Not good. All it needs is a 3-way air valve similar to the one on the 735 and the vacuum connected to ported vacuum instead of intake vacuum and it should be fine. And I'll bet that my engine vacuum readings will go up as well.

demet

Demet,

Thanx for your response. I didn't think anyone was going to share any information concerning this problem.

As I mentioned, the canister seems to not create a vacuum, leading me to think that if "it should" that the canister is bad and needs replacing. If this is true, then I assume I can simply plug vacuum line so I have no vacuum leak until I can replace canister. So, is there a way to tell if canister is bad??? I really don't want to spend the money to replace if not necessary to engine performance.

Thanx again,

Mark

I don't think that you should connect vacuum directly to the canister. The canister venting into the engine should be controlled via some type of 3 port air valve that is triggered by a vacuum source to the air valve's trigger port. This separates the purge function from engine vacuum. If you need a diagram for the purge system I can pull it off the parts CD.

demet

I have an '83 745i with the "add on" fuel tank purge canister. My question is if this canister should be a complete vacuum when connected properly.

With three outlets on the canister, 1; to carb - plugged 2; to intake vacuum 3; to fuel tank. When I remove fuel tank line and put finger over connection - there is no vacuum. When I plug incoming vacuum line - my vacuum goes up from 10in at idle to about 15 in at idle.

IS THIS CANISTER SUPPOSED TO HAVE A CONTROLLED VACUUM LEAK?? IS IT SUPPOSED TO PULL IN AIR ALONG WITH FUEL TANK FUMES??

If not, can I assume this canister is bad and there should be a complete vacuum in this system? If so, can I simply plug vacuum line to canister without running into any type of recirculating system and not run into any complications?

I am trying to make sure I have no vacuum leaks before I try to increase fuel and boost to engine. Right now, when I plug this vacuum line, vacuum goes up, idle goes down. If this is a true leak, I can then adjust idle to correct RPM,

Thanx for the help in advance,

Mark

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