Sten

A friend has just bought a 964 race car, and the fuel tank is imploding! Undoing the filler cap releases the vacuum formed in the tank, and it returns to it's normal shape. I suspected a problem with the tank venting valve, which is being pulsed as the engine is running (switches 0v from pin 5 of the ECU). On my 964 the vent valve doesn't pulse, it's got a steady 0v so the valve is open when the engine runs. The vacuum line is clear. I've tried another ECU in the car and it's exactly the same. Anyone know if there's other another input to the ECU that would influence the tank vent valve?

elbeee964

huh -- this sounds interesting.

DAVISRILEY

Never seen this on our cars, but have seen it on a couple of sc's. Was caused by just that, a screwed up vent. Pretty amazing how much the fuel pumps can pull the tanks in.

Andy Roe

I think there's a breather pipe on the fuel filler neck. Have you checked to see that it's not blocked too? Or is this the vacuum line you mention?

jamesallum

It sounds like a carbon canister which I believe is just placed near the breather pipe. Had the same problem but was lazy and got my garage to sort it. About 1 hrs work and £80 for the part from memory. I’m sure someone more technical will be along in a min and tell you exactly what part it is.


Cheers
James

DavidI

Apologies for resuscitating an old thread, but was this ever resolved?

My '90 C4 was fine on the short drive to work Tuesday morning (4 miles). I left the car sitting for 30 minutes or so then got back in for the drive into town - about 8 miles in traffic, should have taken 45 minutes.

The car started OK but wouldn't hold an idle. Pretty much every time I came to a stop the car would stall, and I ended up doing the 'into-neutral, left foot brake right foot keeps some gas on' dance to save cutting out every minute. Driving was seriously erratic - I needed 3k revs to get away 'smoothly', and the rough idle became an even rougher power delivery higher up the rev range. I never got above 4k.

After a few miles I noticed an unusual sound which from the car appeared to come from the scuttle behind dash but closer than the luggage compartment in front. It was a kind of mechanical creaking, like old heating pipes warming up or cooling down. At one point I'm sure I noticed a puff of smoke from the back of the hood (closest to the windscreen) and pulled off the road and parked up. I only had a few minutes to look for causes/damage, but couldn't see anything, although the creaking continued, albeit much quieter now the engine wasn't running.

I got a cab to my meeting and picked the car up a few hours later. It was now idling but hunting badly and power delivery was still rough - not as bad as when I had pulled in, but still pretty awful. I crawled home, expecting to look for the usual culprits of DME relay etc. today.

I swapped DME relays even though the one in there is a few months old and started the car. I'd removed the front luggage bay carpet, spare wheel etc. in the hope of tracking down the noise or seeing what could possibly have smoked. With the engine running I could see the fuel expansion tank caving in on itself! I turned the power off, and the tank gradually reformed. There are two bolts at the front joining expansion to main tank, and one bolt had pulled through the plastic on the tank (it's only in a flat plastic area, no danger of a leak) and was now fixed solely to the main tank, showing the amount the expansion tank had misshapen. Perhaps interestingly, the bolt was missing a washer that appeared on the other side, so maybe this had happened before my ownership too.

I've searched the forums and the web and been really surprised at the lack of similar incidents, unless my searches are really poor! Going through Adrian's book it looks like there is a valve by the fuel filter (I can't see it) and a carbon cannister and vent above the left rear wheel. There's a pipe that exits the expansion tank by what I think is the solenoid (pulls out a little, right hand side, top, of the tank) and drops down behind the main tank, and I expected to find that free but it appears to dive off to the back of the car and I don't want to put too much pressure on it to try and pull it up. It seems the tank is not venting, though some air must be getting in to get the tank back into near-normal shape after the engine runs, but not enough to make up for the speed fuel is pulled through.

Anyone have any ideas? I'm hoping it's a fairly obvious one in which case I can get the car around the corner to a non-Porsche guy I trust, otherwise it's a tow job to the nearest specialist.

Thanks for any thoughts,
David

DavidI

A little bit more info:

With the petrol filler cap removed the car runs nicely (perhaps a fraction lumpy? Can't be sure as it's cold). Wind the cap on and within a few moments the expansion tank starts getting ugly. Release the cap, deep intake of breath and all's well again.

I think I'm going to ask the local mechanic to look at it, but if anyone knows where the blocked pipe is likely to be I'd appreciate it!

DavidI

On the off-chance this ever crops up again and could help someone out, it turns out that the fuel filler cap is designed to NOT be totally air-tight. I've never noticed any complex parts or machining (though I've never been interested enough to look) so perhaps it is just sized to allow enough air to squeeze in. Changing the cap for a new one has solved my problem. The thread on the old was definitely a little corroded and I assume had become too tight a fit.

What a stupid problem and almost as daft a resolution. I'm just pleased I used the local mechanic and didn't tow the car to the specialist....