demie

New owner of a 2003 cabrio w 36k miles. just got a lot of service done at Rennwerke outside of white plains. They only service Porsches.. Yanked the engine and did new New ims / rms plus full 30k service plus new continentals. The one thing they didn't get to was this weird humming noise emanating near the gas cap. Only hums at idle. When you step on gas pedal it goes away. Owner of Rennwerke said it is likely a faulty check valve tied into the carbon filter / fuel air intake. would require right front wheel well to be opened and some parts replaced. anyone ever experience this problem?

Hurdigurdiman

New owner, first posting. Need to see photos of your car.

wwest

Gas pressure bypass flow back into the pressurized(***) fuel tank when fuel burn is minumum.

Late model cars keep the fuel tank pressurized 24/7, you might hear the pump(***) run even hours post ignition shut down.

Shark Attack

On my 99, this was the first symptom of a soon to follow, irratic idle to engine stop then no re-start. The issue was a valve in the engine compartment that turned vaccine on and off to this check valve the other guy is talking about.

Imo000

Can you please post more on this. I have this "moose" sounding noise coming from the gas cap area when the engine is shut off and if the RPMs drop too low during take off. Also have been chasing a hard start (have to crank it for 10sec or more at times) and my idle is at time incosistemt.

fpb111

Search "Hard starting after refueling" The regeneration valve is in the engine compartment. Evidently as/when it fails/starts to fail it can "Honk, Squeal, Whistle, Squawk, fly south for the winter..

https://rennlist.com/forums/996-foru...-any-help.html

Pelican has them.
http://www.pelicanparts.com/cgi-bin/...FILTER_TOOL=ON

PelicanParts.com

This does sound like a faulty vent valve also referred to as a bleeder valve. It is located towards the top of the filler neck and is fairly easy to replace. We have new Genuine Porsche vent valves here and a write-up about it here as well as a quick video about it here:

Shark Attack

Yes there is a valve in the engine compartment that turns vaccume and on off to the tank. Mine was sticking open putting a lot of vacc on the tank making it so the pump couldn't pump the fuel.

The valve comes right off the TB and it is mounted right above the plenum rail on the left side. It is about 3" long and has a 2 wire connection. it has a quick connect that attaches it to the hose that runs up to the tank

Imo000

Thanks. I was looking at it today when changing the oil. Do you know that if the electrical plug is disconnected, is it supposed go be closed?

fpb111

The one at the filler on my 2004 Mercedes ML 350 quit working, it stayed open. That valve closes when the ECU decides that the conditions are right for a pressure test (Vacuum?) of the vapor recovery system. In that case no electric meant it was open.

If you put your finger over the hole in the grommet under the gas cap I think you will feel vacuum and the "HOOT" will stop as long as you keep your finger there.

I'm pretty sure that the vapor recovery systems work the same.

The valve in the engine compartment pulls fuel vapor into the engine. I cut the Mercedes one open it had a pintle valve (like a fuel injector) that clicks when it is working. When it fails it allows too much fuel vapor into the manifold making mixture too rich for the warm engine. Hence hard starts at hot engine restarts after fuel fill, lots of vapor being pushed around.

The valve under the filler cap closes/seals the system for the integrity test. If it doesn't pull and hold xx inches of vacuum it fails = check engine (PO455 large air leak can also be the gas cap) and /or hooty noises happen.

fpb111

Put your stethoscope on the one in the engine after things are at operating temp. You should hear it chattering metering fuel vapor from the tank into the manifold. When the ECU calls for test mode engine valve opens to create vacuum, filler vent valve closes to let vacuum build in the tank/charcoal can, lines etc for the vapor recovery system test.

fpb111

The one in the Mercedes has a light spring that pushed it closed, = no vacuum to pull vapor if it was disconnected. That would stop the hoot noise but if it is the vent valve (gas cap) that is bad, replacing the vapor recovery valve (engine) might not cure CEL or Hoot.

I'm using the Mercedes as an example because it just drove me crazy until I figured out there were two valves that could cause the CEL PO455 I was getting.

demie

Thanks all for your advice. During these posts I had the car back in the shop. They ran the Porsche super computer against the car. And both valves were found to be not operating normally. Both valves (purge and one by vaccuum in engine area) replaced and lines blown out. Was so excited to get car back and not sound like I am driving an overpriced Dyson.... And then that damn sucking sound again Called back and mechanic said possibly defective gas cap that is not holding the seal. Wish I could get out of this with just a new gas cap. Drama continues.... If I can't fix problem going to reach out to Dyson for sponsorship

Imo000

Thanks for the pointers, I'll check both valves. I have replaced the gas cap gasket a few weeks ago as it was badly dracked. By the was for future reference, the 996 gas cap gasket is the same as a Dodge Caliber or Neon. Only the gasket, not the cap.

Imo000

As Test of the vapour recovery valve. I'll plug it on the gas tank side of the valve and see if the hard start issue resolves. If it does, I'll remove the valve abd see if I can clean it to bring it back to life. If that doesn't work, I'll replace it.