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Discussion Starter · #1 ·
Hey guys, I'm new so i can't post in the B5 section. So i noticed a thumping noise in the rear left hand fender well when the purge valve (N80) pulses that is very noticable at idle when stopped. Heres a video of my problem --



The video shows somewhat inside the rear drivers side rear wheel well. the noise occurs when the N80 valve switches on.

Some history:

Two months ago the charcoal canister became clogged and the car wasn't taking fuel at the pump. I deleted the charcoal canister and replaced it with a simple "T" and the problem was fixed.

About one month ago the car was running rough and stalling at idle so i replaced the MAF first, then the turbo inlet pipe because it had a hole in it. It still idled horribly and didn't have any more vacuum leaks

I then replaced the purge valve, the N75, every check valve under the hood, the suction jet pump, the pcv breather hoses, and some of the old brittle hoses. I even replaced the diverter valve because I was getting compressor surge at mid boost levels.

After all that, it was running very well for a while then recently I noticed it idling periodically rough. This sound seems very strange and it think it might point to the problem. Is my leak detection pump bad? Is the fuel tank breather valve bad? Today my girlfriend got gas and there was positive pressure in the tank when she took the gas cap off. No CEL. Please Help!
 

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The EVAP system broke so you took some of it out and left some of it in and now that system is behaving strangely. I wonder why. :poke:

The thumping noise from the rear wheel well is probably the leak detection pump running. I don't know why yours is now always running. I do know that system is monitored somewhat by the ECM and if the ECM thinks something is abnormal, you'll get trouble codes and a CEL. If you have emissions testing in your area, you may need these systems operating enough to keep the ECM satisfied so that you can pass emissions.

If you want to delete a system, first learn enough about that system so that you can modify it without screwing up things that rely on that system or monitor that system.


 

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Discussion Starter · #3 ·
I forgot to mention that this was happening just as frequently before I deleted the charcoal canister. I understand how a charcoal canister functions and deleting it should should leave the leak detection pump unaffected. I don't think it's the leak detection pump always running. The sound occurs when the pruge valve is running; it sounds like the gas tank is resonating with each pulse of the purge valve.

You can pass emissions with the charcoal canister deleted. The system doesn't distinguish if the fuel vapors are scrubbed by carbon in the canister or if they're not. Is the leak detection pump or the fuel tank breather valve causing some kind of vent bloackage?

also to note that the line running from the former charcoal canister to the purge valve is free and clear.
 

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You've now completed your first two posts. As soon as the system recognizes this, it will grant you ability to post in other areas of PassatWorld. I'm going to move this thread to the B5 Garage forum where it will get better views and hopefully productive responses.
 

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If you want to delete a system, first learn enough about that system so that you can modify it without screwing up things that rely on that system or monitor that system.
Modifying any part of the emissions system is illegal, regardless.

My suggestion would be to go back to stock. A charcoal canister from a junkyard or a part-out is your best option.

Regarding what you said was happening two months ago, the correct fix would have been to test and clean/replace the EVAP canister purge valve (aka N80), not deleting the canister.

If you total up all the headaches and extra costs in deleting these components, it's a near certainty you'd come out ahead by simply maintaining or replacing them. There are enough sources for cheap B5 parts out there.

That video posted by Steve is great BTW.
 

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The thumping noise from the rear wheel well is probably the leak detection pump running. I don't know why yours is now always running. I do know that system is monitored somewhat by the ECM and if the ECM thinks something is abnormal, you'll get trouble codes and a CEL.
That was my thought as well. When I first got a B5, I was puzzled by the infrequent episodes of rhythmic thumping from the gas tank. Amusingly, the VW dealer claimed not to know what it was. Turned out, of course, to be the leak detection pump.

(GRIN) Since alfa has re-engineered the systems on his car, stock advice doesn't apply. He should consult his engineer for advice.
 

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Today my girlfriend got gas and there was positive pressure in the tank when she took the gas cap off.
Is she 100% sure it was positive pressure and not suction? When you open the cap, one will sound about the same as the other.

The sound occurs when the purge valve is running. It sounds like the gas tank is resonating with each pulse of the purge valve.
It sounds like your EVAP system might be suctioning your gas tank. Even a small amount of suction over enough square inches of area can create a strong force.




The purge valve meters how much flow occurs from the EVAP system into the intake manifold. Flow is induced by engine vacuum. There should be a check valve that keeps boost pressure from ever flowing through that path and pressurizing the EVAP system. When the system is flowing from EVAP to manifold, there has to be an air inlet somewhere in the EVAP system to allow a vacuum break. Find that and confirm it isn't blocked. A good diagram of your system would come in handy.
 

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Discussion Starter · #8 ·
This sound was occurring just as frequently when i still had a charcoal canister. It's happened ever since i got the car.

"When the system is flowing from EVAP to manifold, there has to be an air inlet somewhere in the EVAP system to allow a vacuum break." --Wouldn't that be the fuel tank breather valve?

Deleting the charcoal canister is a common thing to do. You will still pass emissions with no CEL. I am not "re-engineering" anything by simply deleting a can of activated carbon.
DIY: Gas tank problems Evap Canister Leak Detection pump
 

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Discussion Starter · #9 ·
Modifying any part of the emissions system is illegal, regardless.

My suggestion would be to go back to stock. A charcoal canister from a junkyard or a part-out is your best option.

Regarding what you said was happening two months ago, the correct fix would have been to test and clean/replace the EVAP canister purge valve (aka N80), not deleting the canister.
I'm not worried about legality. The charcoal canister had clogged and wasn't allowing me to get gas. By deleting it, I was able to get gas.
 

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Looks as though the intake filter for your LDP is severely clogged (see video at 8 seconds in). Thus when your GF uncapped the fuel tank, it was under vacuum.

Also, I believe the charcoal canister has an integral one-way check valve built into it to allow air in but no VOCs out....thus when you deleted it, all of the air for LDP checks is now going through that poor clogged ($45) LDP filter.

Try to locate a used charcoal canister to undo what you've done and see how the system operates.
 
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