bureau13

I was reading your thread about that module last night. From that, and other searches, it seems they don't fail on their own too frequently. I haven't messed with mine (I didn't even know what it was!) and the car seems to run fine when it runs, and then not at all when it doesn't. I think this is probably not as likely a culprit as some of the other items people have mentioned, but I've added it to the list.

Regarding grounds and weather, it was maybe very slightly cooler here, but not significantly so. The first things I will do are test for spark and injector activity, followed by swapping out some new relays when they get here. The next on the list (to be done whether the problem is fixed or not) I think should be those grounds, and then a CE Panel R&R.

Landseer

Grounds and ce panel first. MAF wire has a shield wire that can pierce other insulated wires within the bundle. You likely, on a new-to-you 928 will have inherited a series of intermittent start issues. That's my takeaway at least after 7 years and wrenching on a dozen of them. 85/6 seem to be absolute worst years for this random no start stuff

Landseer

I had one where mouse pee corroded the valley ground wires as they entered the ring terminal that bolts to the valley. Took a while to find.

bureau13

Mouse pee! Damn. I know this car was sitting for a while. PO did an intake refresh and replaced vacuum tubes, CPS, fuel lines and a few other assorted bits, but any number of things could still be lurking. Whatever it is is definitely happening more frequently now. I was thinking if any of the stuff I did could have caused it. I suppose you never know (Distributors, thermostat, gauge temp sensor are the main ones). The distributor would seem to be the most obvious candidate, but I did that first and didn't notice this issue until later.

dr bob

On the other thread, I recommended that you replace the critical relays with brand-new known-quality relays. I haven't been following along closely (darn clients taking up my time...). Have you done that yet?

Meanwhile...

Things that potentially fail as they heat up:

-- Relays. As the contacts age and resistance build, current flowingthrough them adds heat and warps the little arms that hold the contacts. As they age more and try to work with warped arms and more arcing, time-to-fail gets shorter until the no longer pass enough current to run their loads.

-- Ignition Coils. Failure is most typically caused by poor plug wires, worn plugs with higher firing voltage requirements, and heat. The internal windings short to each other whenever the internal electrical leakage path is shorter than the path through the wires and plugs. Internal arcing adds heat of course, plus the engine bay heat is added progressively as you drive the car. Know that most spark-test protocols that include holding a plug wire near a ground point can easily cause a weak coil to fail. Hence the recommendation from our experts to use a spare plug with the shell grounded. One interesting feature of ignition coils is that they will always find a path to dissipate the seconday voltage. It should be via the cap/rotor/wires/plugs of course, otherwise it's back through the internal windings to ground somewhere. FWIW and in my limited experience, most "failed" coils are replaced as a troubleshooting step, and in fact are still fine. If you see oil leaking out of the coil, it's already toast. Absent that symptom, it's a crapshoot.

-- Fuel Pump(s). The pump motor may have a tough time turniong worn rollers in teh pump, especially if there's crud binding them up. If left that way for a while, the motor will overheat and the pump may fail. Failure doesn't usually follow the 'works OK then fails after a while' pattern you share, but that's not an absolute.

-- Electronic modules (ignition and fuel). These -can- show the failure mode you are experiencing. The only reliable test is replacement, since diagnostics along the roadside when failed are inconvenient. However, the failures are time- and temperature-dependent, so you can as easily recreate your symptoms sitting in the driveway with the car idling. Your 'noid light will help with this, as will your spark tester (with a helper to crank the starter as you test for fuel and spark.)

-- Ignition switch. They do go bad. But the same '15' circuit on the switch that runs the ignition/fuel/etc also provides power to dash indicators. If you get a dash full of indicator lights (oil press, charge, etc) when you experience the engine stopping, or you can see the tach boince a little as you crank the starter, it's most likely your ignition switch is OK. Replacement of the electrical part isn't a big job if you want to eliminate that as a possible cause. Buy your replacement from one of our vendors to make sure you get a good one. This part is shared with a lot of other Porsche models, enough that there are cheap knock-offs floating around, even branded with genuine Bosch and Porsche labelling. More than a few folks have discovered the substitutions the hard way.

bureau13

Good info, thank you. Regarding the relays, I have ordered the replacements but they haven't arrived yet. I expect to see them in the next couple days.

I didn't order enough to replace all of the 53s, unfortunately, but I can hit the important ones. I got cocky when I thought I had fixed it by removing whatever was in the kickdown spot, but really I gather I should be replacing those 53s with known good ones anyway.

dr bob

That's my recommendation. Relays are too cheap and cause too many stupid problems. I've never been stranded beyond my garage or driveway, and the times I was it was for a worn-out battery, and for a weak fuel pump relay. For the relay, I made it all the way down to the end of the driveway (about a quarter mile) before it gave up. I had the jumper with me, as well as all the spare relays, but I could hear the pump running so didn't initially think of that relay. A fuel pressure gauge and a jumper showed full pressure, and the car started right up. New relay replaced jumper, car started right up. Hmmm, something to this 'relay-relay-relay' thing after all!

bureau13

I got my relay goody box...2 53s and a fog light relay. I replaced the LH and EZ relays with the new ones, and jumpered the fuel pump relay, no luck (but the fog light relay fixed my non-working fogs problem, so yay). However, with the fuel pump relay jumpered I heard no fuel pump activity, either with ignition on or otherwise. I swear I heard it before, but given the intermittent nature of this issue, maybe when I heard it it wasn't a failure condition. If the jumpered fuel pump isn't running, then that would seem to be a pretty good clue that I either have a bad pump or a bad connection to the pump, yes? (I did check the fuel pump fuse, it was good) I had to do some other stuff, but next time I'll check to see if I am getting +12V at the pump. What ground is used by the fuel pump? It could be that the pump isn't getting a good, solid ground I suppose.

Hilton

Did you check the fuel pump fuse too?

If the fuse is ok, then remove the fuel pump cover, and you should be able to hear a slight hum from the motor as it fails to turn.

So it appears likely the pump is either dead or jammed. Remove the pump, and then use some wires and a car battery to alternately apply voltage to the fuel pump forward and backwards, to see if it spits something out of its inlet and un-jams. Pumps (even new ones) can jam if something makes it past the in-tank pump (due to a failed hose), or if a PO fitted a strainer, due to a rip in the strainer fabric.

soontobered84

Are you sure you are jumpering the correct relay? (4th from the right on the top row) Relay XX. Not trying to be a smarta$$ or anything, but just checking that you're in the right place.

bureau13

:-D That is just the sort of thing I would do, but in this case I'm sure. I have Alan's handy PDF of the CE panel printed out and in front of me at all times. In this case, it's easy to see because it's right next to the reverse lights, which are already fitted with a jumper. In fact I just pull that jumper and move it over one spot.

Hilton, the fuse was still intact. That's a good idea, with the pump. However due to the way it sometimes works, sometimes doesn't, it still feels electrical in nature to me. My money is on the connection at the fuel pump or a ground. However you may well be right, and those sound like some good tricks to try.

soontobered84

That is strange that jumpering the fuel pump does not make it run. It should run from the second you jumper it until you unjumper it. Key on OR key off. Something is amiss there.

Look carefully at the terminal where the fuel pump relay and/or jumper is. Check the Relay panel where each terminal of the relay would plug in. Sometimes, the female connectors can be pushed out the back of the Central Electrical panel and the male connectors may or may not make good contact.

bureau13

Good call, will do

Landseer

Fuel pump ground is in the hatch area.
Up thru 86, its buried under the interior quarter panel near the taillight.
But IIRC 86.5 had ground more accessible, maybe near the spare tire?

bureau13

I'm going to cross my fingers and hope for the more accessible option. Either way, I smell victory in the not too distant future!