Mysterious intermittent fuel leak
 

I did a track session yesterday with my '92 968, drove home, and just as I was about to pull into my driveway, I smelled fuel. I looked underneath the car, and saw that it was coming from the right side of the bottom of the tank, right where the fuel pump is located. So I pulled it into the garage, with plans to investigate today. So I put it on jack stands (rear only), and removed the cover to expose the fuel pump. Sure enough, there was still a small puddle of gas inside the cover which spilled out as I removed it. So I jumpered the DME relay to activate the pump without turning on the engine, jumped under the car, expecting to see a shower of gas spewing from the pump or a fitting or hose, and ... nothing. Not a drop. So I fired up the engine, and still nothing. I rev'd the engine, and still no leak. Finally, I lowered the car back on level ground, fired up the engine, and still nothing.

Any ideas how a significant fuel leak (it created quite a large pulled on the ground yesterday) could disappear overnight? This one has me stumped, and I'm afraid to drive it until I get it figured out. Maybe it's my imagination, but I do seem to have gotten an occasional faint whiff of fuel smell over the past few months coming from the back, but this is the first time I've seen an actual leak, and it was anything but subtle. Thanks in advance.

Hmm.. That is an odd one...

Maybe the G-forces from running on the track were flexing a connection enough to cause a leak? With the car running, try pushing/pulling on connection points to see if you can get it to leak.

But the strange thing is that it wasn't leaking while I was on the track, or when I pulled into the pits with the engine still running, or on the drive home, for that matter. It just started as I was about to pull into my driveway, and then disappeared.

Sorry I misinterpreted your post.. Really, odd, then...

No other advice from me except 'monitor to see if it happens again'...

Is your evap system functional? I wonder if a little pressure built up in the tank due to heat, and remained in check until you stopped consuming enough fuel to compensate. If the 968 is the same as the 951, there are a number of rubber hoses and clamps -- including the main one to the pump -- that could be good enough to seal with no pressure and bad enough to leak under a little pressure.

Tom,

That's an interesting theory, and it does make sense. About a year ago, on a very hot Texas summer day after a track session, I went to fill up the tank, and when I removed the cap, there was a loud Bang - pretty scary. It only happened that one time, but it does make me wonder if there is a clog in my 25-year-old evap system, and, as you say, a 25-year-old rubber hose is springing a leak. Funny how it's intermittent, though. OK, I'll have to educate myself as to how the evap system works on these cars - never worked on that part of a car before. Any tips are welcome.

I'll check the diagram in the workshop manual when I get home, but looking at images of the evap system online, it has an awful lot of hoses and connections, and I'm sure they're neither very accessible, nor cheap. Since the car is over 25 years old (and doesn't get tested for emissions anymore), and is only driven on the street to get to and from the track, I'm wondering if there isn't a cheap and easy (but safe!) alternative. In other words, couldn't I get rid of the charcoal cannister and all its plumbing, and just install an appropropriately-sized check valve into the vent line coming from the tank, and vent the fumes into the atmosphere? Thanks.

When I had my 968, I was chasing a fuel leak and ended up disconnecting the fuel fill rubber coupling from the tank to the fill line. When I put it back on it wasn't seated correctly and ended up spilling fuel all over in the same place that you mention, but that's because I didn't hook it up correctly. If it was ripped, it could have a similar problem. Not likely, but just wanted to share my experience.

Drill a small hole in the gas cap? I'd want to find the leak too though. If a couple PSI of pressure cause it to leak, seems like just a matter of time before it leaks all the time. And, of course, it may have nothing to do with tank pressure, but caused by temps or bumping around or whatever....

I had thought about that, but it would sure be nice to get rid of all this plumbing, and relpace it with a check valve. The leak may be difficult to track down, because it is so intermittent, and even if I found the leak, I'd still have to track down what's causing the leak (I like your theory of something, like the charcoal cannister, being clogged). So I'd be looking at a lot of expensive tubing and components to replace, when it's just a track car. But drilling a hole in the gas cap would at least buy me some time until I came up with a permanent solution.

If you are looking at a track car only, no inspections. Do away with the evap system. Saves a couple of pounds. I placed a filter on the vent line back by the fuel tank to keep debris from getting in the fuel tank.

Bob,

Yes, I had the same thought. If this were a street car, I would troubleshoot and fix the problem, if for no other reason than out of concern for the environment. But given the car's track-focused nature, I want to do what is simplest, quickest, most reliable, and if it saves a few pounds, that's a plus as well. I've never worked on the evap system of a car before, so do you mind if I IM you with some questions as to how you did what you're describing? My immediate questions include:

1. Do you have to pull the tank (which means pulling the transaxle) to do away with the evap system?
2. Will I get a permanent check engine light?
3. Dumb question, but where is the charcoal canister located?

Thanks.

Answered your PM.

Question 1: No
Question 2: Don't know I am running a stand alone
Question 3: Under the front left fender, behind the wheel well. No such thing as a stupid question.

Part 30 is where you place the filter. All the rest can disappear

Thanks very much - much appreciated. I replied to your PM before I saw your response here, so ignore the redundant question I asked there - you answered it here. It was about the CEL.

Does anybody know if removing the cannister and associated plumbing will trigger a CEL with the standard ECU?

Here is a picture of the filter. It is sitting just in front of the rear axle carrier tube.