umc276

I have owned my car ('83 928s manual trans. 90,000 miles) for past 25 years and with the help of this forum have never needed a mechanic. I love the simplicity of the Ljet.

However, for the past couple of months the following problem has developed and I can't find a solution.

It starts instantly and runs great during warm up and first 20 minutes after that.
If I turn it off it restarts easily then begins missing badly (not just a one cylinder miss) especially under load. Completely temperature dependent - worse on hot days. Never over heats and has never overheated.
The miss feels like bad plugs did in the old days.

Compression good all cylinders.

I changed the plugs and ignition wires, - fat blue spark hot and cold. Closed gap temporarily on plugs to 0.20 - no change.
Cleaned injectors (all were good) and checked for vacuum leaks. Also checked Temp II sensor many times at sensor and computer plug (even tried a substituting a 250 ohm resistor for the sensor), and checked all terminal values at computer plug.

I have spare AFM, ignition module, and computer. Tried switching them all but no change. Green wire is like new, and connectors at both ends perfect. Pick up coil distributor readings hot and cold within specs.

Spare coil - no change. Voltage reading at coil terminals hot and cold OK.

Fuel pressure and flow good- same when hot and cold. No fuel in vacuum lines at damper or regulators. Fuel pressure holds after shut off so injectors and cold start valve not leaking. All injectors clicking when hot.

Replaced cat converter with "test pipe" no change.

New O2 sensor, disconnecting and reconnecting O2 sensor - no change.

Relays appear good but think a bad one would cause no run not a miss.

Timing good - same hot and cold. Battery voltage while running cold and hot good.

Disconnected tachometer terminal at dash pod - no change.

Live in the desert so no corrosion problems but did check grounds many times.

I found this thread at another site (mechanic with similar problem but he never posted the fix): [URL="http://forums.pelicanparts.com/porsche-928-technical-forum/769268-84-s2-missing-stumbling.html"

I wish it would just stop running - then fix would be easy. I'm thinking some wire, somewhere, maybe loose but where?

Any other ideas?

GlenL

I'm going to go with green wire. Is it routed to be away from the block as possible?

andy-gts

can you hook up the sharktuner,,,,hall sensor ????

The Forgotten On

L jet cannot connect to either as it doesn't have programmable eproms. It does have a hall sensor inside of the distributor but not much can be done with it.

Mrmerlin

green wire is suspect, and usually accounts for most running issues that have them

umc276

I thought about green wire but it looks like new. Cleaned connections at both ends even though they looked fine. Also enclosed entire green wire in a plastic sheath and routed it away from high tension leads.
Is there anyway to test it besides checking the resistance of each lead?

Mrmerlin

what color are the ends of the connectors of the green wire?
if brite white its new and then you may have a coil going bad.

if tan or brown then its old and should be replaced the shielding will crack and let in moisture this changes the resistance inside the wire

The Deputy

I was thinking coil earlier when he talked about it only doing it after warming up, but he mentions changing that. How about those resistors inside the driver side wing/fender (next to rad) or the supply wire for them...any chance that could be the issue?

Brian.

umc276

Thanks.
I'll check color of green wire connectors tomorrow morning.
Thought about resistors changing value when hot so I checked voltage at both coil terminals when cold and hot and voltages are correct.

Mrmerlin

as far as the coil goes,
is this a direct replacement Bosch coil?
IE the one specified for your year?

umc276

Same year. Got it from friend who sold his Porsche for parts.
Tested it when I got it (my car was running well then), it worked so I kept it as a spare.
When I tried it this time car acted exactly same as before the switch.
I also tested the spark when hot (both coils) - fat blue spark.

While searching the internet found that green wire fault could cause tach to bounce around when missing.
Unfortunately, I'll have to take my pod apart again to re-fix circuit board ground to tach. Not related to this problem as tach hasn't worked for a couple of years.
Have been wanting to fix it anyway so it might be a useful test.

The Deputy

Does the 83 have a CPS (crank positioning sensor)? If so...have you checked the connection or ohm-checked the sensor itself? I've had them where water runs down inside, along the terminals, and ruins them from inside...without any visible proof on the outside of the sensor. Or maybe a faulty plug or wiring issue?

Also, do you know if 83 was the last year for the notorious green wire?

Brian.

umc276

No CPS.
Looks like crazy green wire '79-84 cars:
Think I'll end up changing green wire after I fix tach and see if it bounces around.

Fogey1

OR ... you could patronize one of our community's vendors, at least two of whom have it cheaper:
http://shop.928intl.com/Distributor-...28-602-907-00/
https://www.928gt.com/p-1048-distrib...ire-77-84.aspx
I can never find anything on Roger's site
http://www.928srus.com/ But I'd bet my bottom $ he's got one. And cheaper.

granprixweiss928

Ive had similar issues, hot running missing. especially on hot starts on my 81.

Same scenarios with parts in my book, dry climate, changed and tested alot of parts. I have replaced green wire, but the issue remains, its not every time however.

Ive got new coil, wires, plugs, cap/rotor, done same fuel tests, etc.

how are your head gaskets? mine seep. makes me wonder, if when hot, and under pressure, there is coolant getting in and making the plugs wet.