gronkX

I have an '86.5 5 speed, and am trying to figure out a no-start problem.

Sometimes, the car simply won't start. It doesn't do this consistently, but often enough so that I don't trust the car. Last summer, it died twice while driving it, both times requiring a tow. Some times when it enters one of these "no start" moods, it will start up 10 minutes later, other times it won't start for a couple of days.

I've taken it to two mechanics (there's only one guy locally with any 928 experience) and neither of them could isolate the problem, so I guess it's up to me.

It's been sitting in the garage over the winter, and last week I figured it was time to get it going again. It started on the first turn of the key, and idled & seemed to run fine (it's not licensed yet, so I haven't actually driven it). I've been starting it every day or so since with no problems, until today when it wouldn't start. The weather is cold & damp today, and the no-start condition does seem to happen more often when it's damp.

2 years ago, I replaced the CPS. I rechecked & cleaned the connection this week, and the car was running after that, although today it won't start.

Last summer I did a complete tune up: wires, plugs, rotors, caps. I also replaced all of the '53 relays.

This spring, I went through and cleaned most of the ground points, and cleaned all of the positive & negative connections on the battery. I didn't do the ground points from the engine to the body, but have verified that most of the metal parts of the engine are getting a good ground when the car won't start.

Even though the relays are new, I tried jumping #30 and #87 on the fuel pump relay, and can hear the pump running, although the car still doesn't start.

I put an inductive timing light on one of the spark plug leads, and am not seeing a spark when the car is not starting, so I'm assuming at this point it's probably not fuel.

I do notice that when it won't start, the tach does not move when cranking the engine - could it be a bad CPS? Is there any way to diagnose that without a scope?

I'm really at a loss as to what to do next - other than replace the CPS, what else can I check?

MainePorsche

I see you that you are most thoughtful about maintenance issues, but a simple yet deceptive one maybe present here. Your battery ground strap may be degraded under its insulation and this will be hidden from simple inspection. To confirm you can remove the covering and reapply some new silicon covering if the strap is good. This would be a starting point from what you describe.

gronkX

The car cranks fine, and even when it won't start all the electrics work - headlights, radio, etc. so I assume the ground is good.

Fogey1

That's a big assumption. There are a lot of separate ground points for separate systems. Here are a few links:

https://rennlist.com/forums/8938265-post3.html

https://rennlist.com/forums/928-foru...ml#post8938265
The four "deadly" grounds

https://rennlist.com/forums/2220328-post33.html
Electrical System Maintenance


Also some non-start and sudden death events have been traced to the electrical half of the ignition switch.

Will

NC928S4

Coils? Odds of both failing at same time are infinitesimal but I had one bad coils and one corroded end at the other. More likely a computer problem. Good luck.

MainePorsche

Initially you didn't say it cranked fine. That changes the differential.

gronkX

Ya, I did get a bit wordy with the description, but this has been frustrating me for a while now.

I think I've eliminated grounding as a potential cause, so if it's not a bad CPS then I'm thinking it's a computer problem. But I'd really like to figure out what's going on instead of just swapping out parts and hoping.

Bmw635

Since the car cranks means your ground is clean enough for couple hundred amps. How's the main relay or fuel pump relay?

I don't know Porsche well, but cracked solder joint in the Bosch DME was common in Bmws in the eighties.

It'd help if you state yr and mileage to help us more accurately.

gronkX

All of the relays have been replaced, and when it won't start jumping the fuel pump relay doesn't help. The car is an '86.5, and has about 138k on it.

While the car is running, I've tried going through each fuse and relay and tapping & wiggling it a bit to see if that would generate a failure. I've done the same with the various connectors into the fuse panel.

soontobered84

Remove and clean the main engine ground under the passenger side of the car. Clean the mount points also. Reinstall.

Next clean the 2 grounds at the back of the engine V. When reinstalling, make sure they are tight.

If the computers (LH and EZK) do not find grounds, they will not operate correctly. When they do find a ground, they will operate as designed. It can show up as an intermittent problem. I had my '89 flat bedded twice before I figured out the issue.

mickster

+928

gronkX

OK, I finally got around to looking at these, and they're pretty crispy - on one of them, the wires seem like they're loose, which might explain the intermittent part.

I looks like there's a bunch of wires going into each side, and that they were just crimped in and then some head shrink was applied over it. Also, there didn't seem to be a lot of extra wire in there - am I going to get into trouble if I cut off 1/2"?

Bjbpe

I have had a similar problem. Car cranks like crazy and a squirt of starting fluid will fire it up but it won't last. Sounds like the fuel pump, right? In my case I checked fuses and relays and couldn't find anything. We had a heck of a rainstorm recently and the next time I went to start it - nothing. Cursed at the car and used the Cadillac instead. Next day it started fine. Possibility of humidity problem in the wiring? I've ordered some of the good stuff that folks have been talking about for wiring. Maybe that is your problem too?

MainePorsche

That is a possibility.
There is not a lot slack in some of the ground wires to grounds 8 & 9 (I think it was to MP VIII that had the least amount of play in one of the Brown wires).
Before you do surgery there, be sure you position the wire to the ground point bolt to get an idea of how much wire you can sacrifice. If a significant amount of wire is degraded, you may have to add a prosthetic piece in order to make the connection.
Do quality work on these wires down there. They are in a harsh environment and they are the grounds for important controllers.

Alan

The length doesn't matter as long as it reaches comfortably.

Alan