Coke Bottle Noise / Fuel Filler Howling Noise

[Lowski]

Earlier this year my 2002 2.7L Boxster developed the howling noise from the fuel filler area. It went from being very intermittent to almost constantly present at idle. I’ve scoured the various Porsche forums trying to understand not only the fix, but what the cause is. I want to share some of the things I learned and see if others can help fill in the gaps I haven’t quite figured out.

The following information is for Boxsters and 996s with Onboard Refueling Vapor Recovery (ORVR) which is option code M664. The fuel evaporative systems on cars without ORVR is similar, but the there are differences in the installed hardware and system function. See this link for a quick primer on ORVR. https://en.wikipedia.org/wiki/Onboard_refueling_vapor_recovery

The Boxster/996 fuel evaporative system operates in a few different modes. The system captures vapors during fueling (ORVR), it captures fuel vapors when not fueling, it has a tank/system leak check function and the captured fuel vapors are sucked into the intake when the engine is running to clear out the vapors from charcoal canister.

The noise is common to part of the ORVR system. The following page from the After Sales Training document – P25 Advanced Fuel & Ignition Diagnosis documents how the ORVR system behaves. The schematic really helps understand how the vapors get from the tank to the canister and the intake.



Before we get too deep into the details I want to add some excellent pictures I found in the following post from JSG1901. I find it much easier to understand the system with them.

Removing EVAP components and charcoal canister...?









The noise emanates from “D”, a vent hole in the tray in the fuel filler area. The vent hole “D” can be seen with the fuel filler door open at the forward end of the tray. The hose from vent “D” leads to the valve assembly in the R/H wheel well behind the liner. This valve assembly contains the 996.605.213.01 Fuel Tank Vapor Control Valve “F” for ORVR use and the 996.605.201.01 Shut Off Valve “G” which is used during the fuel tank leak test. The hose “C” connects the valve assembly to the canister and hose “E” is an open ended vent as far as I can tell. I believe it runs to the rear of the car, but I have found very little reference to where this goes. However hose “E” becomes important to us as we move forward.

When the noise is present the engine is running and the evaporative system valve on the air intake is open. This intake valve goes by many names regeneration valve, tank venting valve, but its part number is 996.110.129.06. When the intake valve is open the engine pulls a vacuum on the canister through hose “A”. As hose “A” pulls a vacuum the canister needs a source of fresh air so the vapors can be sucked out. This source of fresh air is through hose “C” to the valve assembly and then hose “E” out of the valve assembly.

The noise is caused when the vacuum is great enough in the valve assembly that it pulls open the normally closed 996.605.213.01 Fuel Tank Vapor Control Valve “F” and pulls air through the vent hole “D”. Why does the valve get cracked open? It is either caused by a weak spring on valve “F” not keeping it shut, a blockage in the vent hose “E” causing higher vacuum or a combination of them both. When troubleshooting on my car I could make the noise stop by disconnecting “E” from the valve assembly, opening valve “F” by triggering the reed switch in the filler neck with my finger or plugging the vent hole at “D”.

An additional wrinkle is that my car also developed code P0446 after the noise started. My Durametric had a fault code for the shut off valve “G” associated with the code P0446. Based on the P0446 code and the noise I think my vent line “E” was partially plugged. The lack of fresh air during clearing of the canister caused the code and the increase in vacuum to crack open valve “F”.

I blew out hose “E” with compressed air, but since I don’t know where hose “E” goes I couldn’t verify the blockage and if any blew out of it. I hooked it back up and the noise stopped, so I think it did something positive. I also changed valve “F” 996.605.213.01 and valve “G” 996.605.201.01 with used ones I bought while I was in there (I’m tired of taking the liner out…).

My recommendation would be to start with hose “E” if you have a problem. Other cheap solutions would be to disconnect “E” from the valve assembly and add a short hose or just plug the vent valve at “D” while driving and remove for fueling.

[JDMsurprise]

Excellent information. Thanks for sharing!

Sent from my SM-G781W using Tapatalk

[spinjockey]

So I used this thread to fix the howling in my new to me 2002 boxster. The previous owner put tape in the fuel vent. It didn’t come up during inspection but once I found it when I tried to fill up with gas I knew what was going on. The symptoms were a howl that was intermittent under a variety of conditions from idle to steady cruising at 3000 rpm in 2nd and 3rd gear.

My first step was to replaced the two valves F and G and tried to blow out line E. The result was it made that howling worse. With the engine running and howling, I disconnected E and could feel the vacuum pulsing and eventually the vacuum became constant. This made me think that possibly the solenoid valve on the manifold was sticking. I made sure to clean out the housing for F and G.

I then replaced the solenoid valve 15. This definitely quieted down the howling. It was softer, would still occur at 3000 rpm but would stop on a regular interval.

I figured since vacuum followed a path of least resistance that maybe there was too much vacuum in the evap system possibly due to a plug in the charcoal canister or maybe the filler neck valve was malfunctioning. So got those parts on order. I also ordered a CO2 drain gun because it has a nice rubber cone that seals against hose orifices for blowing them out.

Ok round 2 with F and G. I figured maybe I didn’t blow out E good enough the first time. I had hunted up the parts diagram for line E. It takes a very circuitous path through the car following the fuel line past the engine to the rear bumper. Lots of things could get stuck in that line, how I don’t know but certainly a spider nest or something could also plug the end. I used the drain gun to blow out line E. The first couple of shots, there was a fair amount of back pressure but with the 3rd shot there was a major reduction in back pressure. The fourth shot could be emptied into the line with little blowback. Ok maybe I cleared something out. Here is the diagram for the path taken by E.

https://einmaligparts.3dcartstores.com/Fuel-system-201-05_c_7839.html

Line E is item 6 in the diagram.

After a series of very lengthy test drives the howling seemed to be mostly gone. I did have an intermittent how for a second or two but that would be it. It would occur at speed typically 3000 rpm.

So the charcoal canister and filler neck arrived this week. I decided I would go ahead and install them because the car was pretty old and by this time I’d renewed almost the entire EVAP system. First part to go in was the charcoal canister. The old canister was the same part number as the new one but the old canister weighed about a pound more than the old one. Maybe it was partially plugged and reduced the overall effective capacity of the evap system which I presume operates on a fixed vacuum so the total volume being evacuated could be reduced leading to some excess vacuum through E. Ok also replaced the filler neck valve in this session. I realize up until this point I was doing pretty good science changing only one thing at a time but I’m sick of taking my car apart. The old filler neck looked quite work, the plastic was dull, but blowing through the holes it seemed to operate like the new one. I’ve read that there can be cracks in these valves. The gas cap puts strain on the filler neck and people sometimes crank down on their gas cap. Swapped out the filler neck. Oh and I blew out E a few more times since I had the whole fender liner out.

Went for a test drive. Nothing, no sound. I think I got it fixed. The evap system is quite a Rube Goldberg. I’ll point out that the system in the 987/997 is much simpler and no one complains of any howling in those models.

I know some people put tape over the vent. This does stop the howling. Surprisingly since the bowl is caused by a vacuum leak there is no check engine light. Since the leak occurs at a part of the system that isn’t monitored, the fresh air leaks through F and D instead of the correct fresh air source E. I think I had a couple of parts that had failed or were failing. I wonder if putting tape over the vent hole causes some longer term damage to the solenoid valves since their springs are being worked by the excess vacuum making the whole system leak more. This may eventually produce a check engine light but not for a while it seems.

I couldn’t live with the tape. I would forget to remove it when pumping gas and then gas would backup all over the filler. I also replaced the gas cap with a genuine Porsche gas cap. The old one was an aftermarket. Replacing the gas cap did nothing btw.

Biggest thing I learned is the drain gun is a handy tool for blowing out the lines because it allows you to use a fixed charge of co2 and feel the change in back pressure.

[spinjockey]

So I used this thread to fix the howling in my new to me 2002 2.7 boxster. The previous owner put tape over the fuel vent hole. It didn’t come up during inspection but once I found it when I tried to fill up with gas I knew what had been going on. When I removed the tape, the symptoms were a howl that was intermittent under a variety of conditions from idle to steady cruising at 3000 rpm in 2nd and 3rd gear.

My first step was to replaced the two valves F and G and trie to blow out line E. The result was it made that howling worse. With the engine running and howling, I disconnected E and could feel the vacuum pulsing and eventually the vacuum would become constant. This made me think that possibly the solenoid valve under the manifold was sticking. I made sure to clean out the housing for F and G.

I then replaced the solenoid valve 15. This definitely quieted down the howling. It was softer, would still occur at 3000 rpm but would stop on a regular interval.

I figured since vacuum followed a path of least resistance that maybe there was too much vacuum in the EVAP system possibly due to a plug in the charcoal canister or maybe the filler neck valve was malfunctioning. So I got those parts on order. I also ordered a CO2 drain gun because it has a nice rubber cone that seals against hose orifices for blowing them out.

Ok round 2 with F and G. I figured maybe I didn’t blow out E good enough the first time. I had hunted up the parts diagram for line E. It takes a very circuitous path through the car following the fuel line past the engine to the rear bumper. Lots of things could get stuck in that line, how I don’t know but certainly a spider nest or something could also plug the end. I used the drain gun to blow out line E. The first couple of shots, there was a fair amount of back pressure but with the 3rd shot there was a major reduction in back pressure. The fourth shot could be emptied into the line with little blowback. Ok maybe I cleared something out. Here is the diagram for the path taken by E.

https://einmaligparts.3dcartstores.com/Fuel-system-201-05_c_7839.html



The path taken by line E is very long. It follows the same path as the fuel line but continues past the engine to the rear bumper. In my opinion it is unnecessarily long and could very well be the source of most people's issues. In the link line E is part 6. There are lots of places for something to obstruct line E although the most likely could be some kind of bug nest. Its not likely that line E gets crimped as it follows the same path as the fuel line - you would notice the fuel line getting pinched. The line is also the same material as fuel line and very durable.

I went for several test drives. The howl now was just a few seconds around 3000 rpms in 2nd and 3rd gear. My howl was much improved but not gone. The charcoal canister and filler neck valve arrived. I installed both of them today because I'm tired of tinkering with this problem. The old charcoal canister was the same part number as the new one except the old canister weighed about a pound more than the new one. Since vacuum follows a path of least resistance there is certainly the possibility that the vacuum capacity of the system was reduced so the excess vacuum had to go somewhere. The evap system doesn't really know how much vacuum is being drawn over the charcoal canister. It just operates with some regular frequency to purge the system based on feedback from the tank pressure sensor. I also replaced the filler neck valve in the same setting. I realized I was really clean about my science up to this point since i was limiting my changes to one at a time but I had the whole wheel well liner out and it was a pain to take out. The old filler neck was the same part number as the new one but the plastic was clearly old as the orange had been faded. The valve seemed to operate the same as the new one but that valve takes a lot of abuse from people cranking down the gas cap so there could also be a small crack which has been reported by others. I also blew E out again for good measure. Then put the car back together.

I took the car for a test drive. I heard nothing. The EVAP system on the 986/996 is a Rube Goldberg. The system on the 987/997 is much simpler and no one, no one complains about howling on those cars. I realized I basically rebuilt my EVAP system. But I think putting the tape over the vent hole isn't a good solution because I think the system will continue to degrade. I think the excess vacuum over time weakens the springs in the solenoid valves, coupled with the fact that a failing charcoal canister is possibly spewing debris into the system under the excess vacuum and plugging up the solenoid valve under the manifold.

I know lots of people tape over the hole. The reason I couldn't live with the tape is that I found it annoying to remove it when I pumped gas. I think anyone who has this problem should start by blowing out line E, then replace the solenoid valves on both ends of the EVAP system, and possibly replace the charcoal canister if there is absolutely no improvement. I don't know if the filler neck is a true source of any of the howling but it may make its own unique sounds when it fails just not through the filler vent hole. I also replaced the gas cap with a genuine Porsche unit. That did nothing.

The drain gun was a really good tool for this repair. It let me discharge a fixed amount of CO2 and monitor the back pressure by feel so I could tell if the line was cleared out. It also helps that CO2 is denser than air so a couple of discharges into the line gives a little more pushing power without blasting the line with excess compressed air.

[spinjockey]

I should mention that I had no CEL. I had also recently passed emissions and had no problem with the car entering a ready state after a reset and quick drive.