Mike C.

I'm trying to track down a strong fuel odor in the car when I shut it down after running a while (up to temp). I've checked all of the vent line connections and hoses and no 'smoking gun' found. I'm now looking at the shop manual trying to figure out how the tank venting system is supposed to work when car is running vs. not running. With the engine stopped (and cold), the vent control valve (smaller diameter metal thing) is open as I can easily blow air through it (lung pressure). If I then put a vacuum on it, it closes - so apparently engine vacuum (larger hose) is not supplied to shut off valve when engine is running (shut off valve is the larger diameter plastic thing). The shut off valve diaphram is connected to a water temp vacuum switch which tests closed in the cold state (haven't checked it warm yet). I assume that means when up to temp and running, vacuum gets applied to the shut off valve diaphram and the valve opens. The other connection on the shut off valve Tees into the vent line (going to back of car) and the vent cannister. So what what is supposed to be happening when engine is shut down (warm)? The control valve will be open and (I assume) the shut off valve will close with no vacuum applied from the temp switch (open or closed, no vacuum available). If anyone understands how the vent system is supposed to work, I would much appreciate an explanation.... I just want to figure out if the two valves and temp switch are doing what they are supposed to do. The manual is mute on testing them...

SD Porsche Fan

I can't answer your question on the valves but I had a strong fuel smell after shut down and it was due to a fuel rail leak. Also check the o-rings on your fuel injectors.

Spidey944

I too had an injector that was leaking at the barrel when shut off. Not a bad place to look the fuel rail is.

Luis de Prat

Check the hoses / tubing around the fuel filler neck, located behind the interior panel on the passenger side C pillar, below the hatch.

Mike C.

I did check all of those Luis. I tightened the clamps a bit but nothing was loose or showed any evidence of seepage. Might be worth changing the FI o-rings, at least they're cheap....

Mike C.

FWIW, the shut off valve diaphram tested bad. I installed a spare one that tests good (holds vacuum)but haven't run it yet to see if it makes any difference.

DDP

You can also jump two ports in the DME relay to run the fuel pump with the car off. You can then search for leaks.

Mike C.

Thanks Derek. Worth a try..

DDP

No problem. I can't remember off the top of my head which two on the relay, I need to see it. But clarks garage I bet would have it.

lucifer_sam

A strong fuel odor inside the cabin was the first indicator my FPR gave me when it started to fail. The hard starts, stumbling and dying in traffic came later :\

Mike C.

I can't rule the FPR out yet. The only thing I've done is to pull the vacuum line off after shut down and I see no evidence of gasoline getting past the diaphram. Were the hard starts always after shutdown? I've never had a problem starting it cold, nor does it stumble/die once running.

lucifer_sam

Cold starts were usually not a problem for me with the failing FPR, although if it failed while running, then there was a lot of cranking involved. (Middle lane interstate cranking, which is the worst kind.)

After the stumbling/dying showed up, I noticed two things that got me suspicious of the FPR - the gas smell in the cabin (which I'm pretty sure was coming from the fuel float cover in the trunk) and the whining fuel pump.

As far as I know, the FPR sits after the fuel rail
(Tank->Pump->Filter->feed line to front->Dampener->Rail->FPR->return line to tank)
and will fail closed, forcing the pump to push with all it's might against a closed door. So at this point there's still fuel in the rail, although the injectors will now intermittently fail to open because their input fuel pressure is too high. The German Engineers do what they can.

If your FPR is approaching fail, and closing sooner or more often than it should, it may be taxing your pump and causing high pressure in the feed line, which is a possible cause of fuel vapor smells. I'm not convinced this is the problem in your case, but I think the best way to check is do to a fuel rail pressure test according to: http://www.clarks-garage.com/shop-manual/fuel-01.htm

Sadly, the list of FPR related issues is large and vague. There are even accounts here on RL about people who have had continuing symptoms after swapping in a known working FPR, and only really cleared up the problem by installing a brand new FPR.
https://rennlist.com/forums/924-931-944-951-968-forum/184816-you-d-never-know-but-the-fpr-can-fuel-fool-ya.html

I don't want to get you running down the wrong path, so I'd say do the pressure test and if it looks good, discount the FPR for now. If nothing else is working down the road, try a replacement FPR. If someone is willing to let you borrow theirs, be sure that it has been purchased new within the past year or so.

Mike C.

Thanks LS, I am now debating whether to rent or buy a fuel pump pressure testing gage set. One thing I'm also doing is replacing the fuel pump check valve (fuel pump is original with 240K miles). I think Clark's mentions that this may also cause hot start problems.

lucifer_sam

Yeah they say these pumps are indestructable but 240K, hey, anything could be going. Congrats on the longevity, anyway!

The check valve would keep the fuel between the pump and FPR pressurized when the engine is off, so that you have full fuel pressure the instant you turn the key. (Probably the main reason our cars are designed so that the fuel pump doesn't even start up until you move the key to the "Start" position.) A failing check valve could definitely cause hard starts. Are hard starts accompanying your fuel odor issues?

Mike C.

Hard start when warmed up, shut off and left to sit for several minutes. It will start, then stall if I don't rev it up and 'clear things out'. I thought it might be a leaky injector but I sent them off to Witch Hunter to clean/flow them and they were all O.K.