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.
Hybrid Mode
