81 SC Dies after 18 minutes-repeatedly

[tutzilla]

I hope that someone can shed some light on this problem because I am at a loss for what the solution is.
After traveling about 10 miles from my house the other day, my 81 SC sudddenly lost power(stalled?), down shifting may have jumped started the car thus keeping it running. I pulled off the road (car was still running) and limped ahead for about a few hundrerd yards before it stalled and then wouldn't start. I had the car towed home and here it sits. I have ruled out:
Gas-Has 1/2 tank
Spark-Yes
Battery-Good
Checked grounds
Removed after-market security system
Changed fuel filter
Tested power @ Thermotime switch, WUR, o2 relay, as well as other test(By pass of fuel pump relay,etc,) suggested by the Bentley Manual- all OK.
The only suspect test was a gas volume test (low, only put out about half a quart in 30 seconds) which led me to replace the fuel pump.
Now here is the really weird part! The car will consistanly start and run fine (a bit of a cold start problem but that is nothing new) for exactly 18 minutes and then all of a sudden the idle drops out and the car dies.If I pump the gas hard prior to it dying I can keep it running for just a little while longer. Unfortunetly the closest Garage is more then 18 minutes away! Has anybody seen this before? This has given me flash backs of my 61 Spitfire (Left on the side of the road again). Any help would be much appreciated! Rob.

[SteveStromberg]

Filter screen inside the tank? Steve

[Rot 911]

How about a clogged gas vent? Try running the car again. When it dies, open the gas cap, if you hear a big whoosh, you have a clogged, crimped, blocked gas vent.

[Mark in Hermosa]

Cylinder Head Temp Sensor? (Do the 81's have these?). My 85 did the same thing. We replaced it and it ran fine (when hot).

Good luck.

[makaio]

I'm with Kurt V. on this one. Sounds like a faulty vent.

[Vipergrün]

Fuel accumulator?

[singpilot]

Good thing work is only 16 minutes away. j/k.......

I second the gas cap. Fuel pump may have died early from trying to overcome the vacuum. Are you sure you have a half tank? Could the tank have collapsed and trapped the sender at half?

See the thread on Gas Cap Vent?

[Superman]

The vent theory is good. Further, I have had similar problems with other cars where rust and dirt were clogging CIS components. Sometimes the car runs fine for a while, but rust starts stacking up somewhere and eventually impedes suficient fuel flow. At rest, the rust settles down and the passages open again. Until flow and pressure cause the rust to stack up again.

So, if the vent theory does not pan out, replace the fuel filter and disconnect all the other CIS componets. Blow compressed air through those lines and rinse out the components. Check the screen in the tank. Reassemble.

[Bill Douglas]

The other advice sounds good. But I'll still make my contribution.

I had a VW beatle that did this and it was an obstruction in the fuel line.

What I would do is disconnect the fuel filter in the egine bay, stick a bit of hose onto it and let the fuel pump run for a while while it pumped into a suitably large container (oil drum). Let it run for five or more minutes to see what it does.

[john walker's workshop]

so there's still spark immediately after it dies, and when it won't immediately restart? blue and snappy?
pop out the gas gauge, reach in and unplug the over-rev relay for test purposes. it's mounted vertically on the left inner cowl by the hood hinge. also accessable from the front trunk. 3x4" black plastic, plug on bottom, raised X on it's face. cuts out the fuel pump, and after 20+ years, i'm starting to see some failures, where we never previously had a problem with them.

[Wayne 962]

Originally posted by tutzilla
Unfortunetly the closest Garage is more then 18 minutes away!

Drive faster!



-Wayne

[tutzilla]

Thanks for all your suggestions! In response to sum:
Tank in good shape, filled 3/4 gas tank prior to first death
Head temp sensor -not till 84
Fuel filter brand new
Disconneced rev limiter as well as pulled fuel pump relay and did a bypass taking out many possible electric problems

As of this colder New Hampshire morning, once started, my car ran fine for over 30 minutes this time and then the idle dropped and the car died. During its death I opened the gas cap as well as after its death with no improvement and no whooshing sound - I believe I had tried that remedy when the car first died on the side of the road but I tried it again to confirm the fact it wasn't a vapor lock problem. I now tend to believe that it is a problem regarding a engine warm up temperature that the engine shuts down after hitting a specific temperature. Does this ring any bells with anyone - WUR (sticking or gunked up) or maybe a malfunctioning temperature switch? Because of the outside temperature change I believe the car ran longer this time- If this is true I shouln't have any problems come january! Any suggestions? Thanks, Rob.

[dean]

One thing you could try is to put CIS gauges on and see what happens to the fuel pressure when she dies

[tutzilla]

Finally an answer to what was ailing the "money pit". Bad CD box! After throwing parts at it (new fuel pump, wur, etc - I swore that it was fuel related! ), I gave up and got it to a real Porsche mechanic. After inspecting the inside of the box it appears as if a wire may have made contact with the inside of the box and had worn away the outside plastic. Hopefully this may help anyone with a similar problem. Thanks, Rob.

[vash]

bad CD box? wouldnt this have shown up as a crappy spark? or no spark? looking for something to file away mentally.

cliff

[ruf-porsche]

Originally posted by Mark in Hermosa
Cylinder Head Temp Sensor? (Do the 81's have these?).
Good luck.

NOPE, not unless someone put a 3.2 engine back there.

[smestas]

I had a similar problem with my '78SC. After alot of replacing suspected parts I finally found that if I disconnected the vacume lines from my WUR and pluged them the car ran fine. After running the car like this for over a year I finally took the WUR out to see if I could rebuild it and found that the whole inside was rusted and stuck.

Not sure if you are having the same problem but I thought that it would be worth mentioning.

[tutzilla]

I may be optimistic in saying the car is fixed because I have only run it twice since it was "fixed" but both times the car ran until I shut it off (45 minutes or so). I first ruled out spark because the first time the car died a "road side mechanic" tested for spark and assured me there was spark ( He also diagnosed that the battery was dead - Wrong! ). Because I didn't have a pressure tester I went by the Bentley Manual volume test in which I received about half of what the manual called for so I replaced the fuel pump, (not the fix). After this didn't fix it I went after the WUR. Come to find out the WUR is off a BMW and without a vac line so I replace that,(still not the fix)! I even took off the aftermarket security system (just incase). I finally did the right thing and took the car to a qualified Porsche Mechanic (Mark @ Exotech) and he diagnosed the problem immediately. He replaced my CD box with a used one and the car is running fine. After inspecting the old box it looks like a wire that sits between the side of the upper circuit and the lower circuit board may have chaffed or worn open slightly- just enough to cause a problem once the engine and the box heated up. Thanks to everyone who tried to help!

[Mookster]

Does the car buck or hiccup before it stalls? If you still have the CIS injection it could be the O2 senor relay under the passenger seat.

My old '72 would stall driving down the road. After I waited a little bit it would start and you could drive it again. This went on for 3 days The problem was the fuel pump.

I hope this helps.

[Wayne 962]

Electrical problems are always tough. This, in hindsight, sounds similar to what is known as a cold-solder joint, or similar failure.

Basicaly, what happens is that vibration from the car creates cracks in the solder joints. When the CD box (or any electrical appliance) is cold, the cracks are close together and touching - resulting in electrical contact. When the components heat up, then the joint spreads apart, and the cracks may stop electrical connectivity. This was a major problem/concern with the satellites that we worked on at Hughes.

I had the same problem happen to my Mom's 13" Sony TV. After leaving it on for about 23 minutes, the picture would just disappear, and it would become all fuzz. The solution? Take apart the TV, get out a soldering iron, and reflow some of the solder on some of the components. Which components? The ones most likely to suffer this problem are the ones that heat and cool the most (sinks, transistors, etc.) - usually anything on the board with a big terminal or a large footprint, or a heat sink. Once you reflow the solder, then there is a good chance that the problem will go away. Sometimes you can actually see the cracking if you look very, very, very carefully. Most often though, the cracks are small, and there are hundreds of components on the board so you can't tell which one has the bad joint.

Another way to check sometimes too is by banging the side of the unit (banging the TV worked for a few seconds). This is a tell-tale sign.

Hope this helps someone out!

-Wayne