joejoe

Been a month or so since have looked at this car. The last time I drove it it left me stranded for about 1 hour, drove it home and would crank but not start after about 10 minutes. Thought it would start the next day, but would not, nor the next day or since. Here is what has been done to this point:
checked fuel dampner/regulator (no gas smell so are ok?)
injectors serviced (witch hunter)
upper intake refresh
new plugs, wires, caps/rotors
wp/tb tensioner rebuilt
grounds cleaned
tps lead replaced
vacuum pots replaced inside car
heater valve replaced
a/c compressor replaced
motor mounts replaced
idle control valve cleaned
new fuel filter
ignition switch replaced
new (rebuilt) MAF
new ignition, starter, ezf, fuel pump relays
new battery (10/13)

I am wondering if crank position sensor could be causing the problem. Really wish I had addressed this when top was off, but I had not heard of any real problems experienced with them. Thought it might be the check valve (for the hot start part) but now I do not know as it won't start cold.

Today I started checking the wiring from under hood (jump post) and started on fuse panel. I do not think I am going to find anything though so I thought maybe some other thoughts are needed (I am just not sure where to go next) When jumped fuel pump relay the pump does run, not sure if that says much (or anything)

Also I have tried different computers of another '86 and car ran same, original comps. back in now.

I know it seems a lot has been just thrown at the car hoping to fix this problem, however all would have been done at some time. Essentials were first (tb/wp rollers) Any help/ideas are greatly appreciated. At least I am not frowning and cursing it anymore lol.

SeanR

The CPS can and will go bad causing this exact issue. Seen so many of them and recommend to change them when you are doing a top end. Look at it this way, it only hurts the pocket book when you change it and it should last another 20 or so years.

joejoe

Thanks sean for the fast reply. Anything else I need to replace this?

MainePorsche

Have the top end off to get the best access.
Can be difficult to remove even in the best circumstance. I took a Dremel and cut the plastic between the sensor and the bolt holding it in place - gave me 'space' to get penetrant in where I needed it - got it out easily. If you Search, some have employed contraptions with wire and hooks, etc...

Glen McCartney

When I had my intermittent non start, it wound up being the CPS. When the tach bounced while cranking, the car started. When the tach didn't move while cranking the car wouldn't start. Replaced it an no problems since.

joejoe

I know it is not bouncing at this time. I have noticed that the tach bounces on the 3 944's I have, and they are supposed to. I just never noticed it on the 3 928's, maybe it is just more subtle?

Glen McCartney

Mine is very obvious, visibly bounces off zero

depami

My CPS came out with just some twisting and pulling. Must have gotten lucky. It was a PITA to get to but all I removed was the air box.

You didn't really describe your symptoms other than no start. Fuel, air, spark, compression; all at the proper time.

Do you have spark? If so, it's not the CPS. But as others have said, replace it and you should be good for another 20 years.

Landseer

It's possible for tach to bounce, and for it to even peg, with CPS disconnected. Just happened here tonight. Not reliable to make CPS judgement on tach needle movement

depami

Thank you! I've been suspicious of that for quite some time.

outbackgeorgia

Check the CPS connector, they have a tendency to crumble and only work intermittently. Leads to a false CPS "failure" diagnosis.

Landseer

true, and same for the car-side wire leading to that connector.

But as Sean suggests, replacing an old CPS is prudent. In fact, when resurecting one of these, replacing all the sensors is the right approach.

rifitz

Sounds very much like my experience with the '85, even with a new CPS

https://rennlist.com/forums/928-foru...ssue-long.html

Landseer

I'm suspicious of the car side part of the CPS harness.
Something is going on in there.

I speculate that the ground, which is the coax shroud braid until the approach to the plug itself, may be suffering from corrosion. The only constant to solving these no start issues seems to be manipulation of that wire.

Saw similar in a seemingly possessed 98 wrangler. It 'ate' CPS. Created all kinds of lore and rumors about replacement parts, techniques, position of moon during repair, etc. Turned out being a corroded factory crimp splice wrapped deep in the harness near CPS plug. The splice gathered the CPS shroud ( ground) along with grounds for oxy sensors and iirc a tps sensor. Tach behavior led you erroneously ( with cheering from the experts) to replace the CPS. But to do so, you had to disturb the harness. See the do - loop? A fix might last a month or 2 years. But the issue had nothing to do with viability of cps.

Then again, I've got a good friend that runs a german- only shop. He claims the cps is major root cause and can fail intermittently on Mercedes and BMW, same as our sensor.

Not sure what is really going on with our 85/86, and also 84 euros ( same setup) but am dialing in on the harness, connectors and cps. After being certain the fuse box is pristine clean, newer cps, fresh relays and relevant fuses, and polished ground points as prerequisites. The other prime suspect is corrosion within the crimp and/or solder joints at the actual ground point ring terminals.

joejoe

Welllll.... crap. Update...swapped the cps today and was not terrible coming out and was removed in one piece. I did notice it was pretty dirty and had metal filings on the bottom. Also changed the check valve which was bad. Put it together and started right up! Unfortunately after letting warm for about 3 min. went on a run and it died, as if the ignition was turned off. Rolled to side of road and it almost started. After looking angrily at the motor, AND the fuse panel it started and made it through a U turn and died again. Finally it started and on the way home it had intermittent episodes of dieing and running.

Back home it will not start again. Going to wait until tomorrow when a bit cooler and start checking wiring and connections again.

Anyone have any thoughts on what to check before? Again thanks for all help and ideas.