tonnant

My 1986 is missing when the engine is cold. I've had the car for 15 years and haven't replaced the o2 sensor so I thought I'd do that and look for vacuum leaks. I found some leaks but my real trouble is the wires running to the o2 sensor. The blue wire, from the connector, across the passenger footwell and into the engine bay, is melted. The insulation is separated from the wire and the wire itself is broken right through at the firewall. At the firewall the copper is far to oxidized to splice a new wore in there. I'm trying to decide if I should try and rerun the wire along it's original course, maybe splicing in if I find a good section. Or if I should jump the wire to it's final destination. Trouble is, as read the wiring diagram (and I'm not all that confident), I don't understand why it's heading out to the engine bay at all. I read the diagram as it joining with wires for the fuel injectors and then going to fuse 42. If thats true, why go out to the engine bay at all? If any of you have any advise, I'd love to hear it. I am reluctant to cut into the wiring harness.
Wiring diagrams below;

worf928

O2 sensor wiring should be routed out of the interior via the 'hump' it should not route-out via the firewall.

Got some pictures?

tonnant

I'll attach some. I'm actually talking about the other end. Where the wires that connect to the o2 sensor's wire connectors go.


o2 sensor connector, near drive tunnel.


CLose up of burnt wire


Burnt wire entering engine bay above passengers right foot.

worf928

Well, that wiring is a f#$%ing mess.

However, since I'm usually wrong with pre-'87s I'm going to stand aside and wait for the S3 gurus to opine on exactly what should go where.

davek9

Yeah sure is, wonder what melted it, was the fuse #42 blown?
The O2 wiring is all part of the Main Engine harness, wondering if something else shorted and melted it.
As for your question, it's the Engine ground MP VIII that is picked up under the hood

I think you will need to jack up the front and look up at the harness where it enters the Firewall and see what else you can find, its not easy w/ the engine in the car

Mrmerlin

my guess is that someone dropped the exhaust /cats and did not disconnect the O2 sensor wires at the base of the CE panel first, this then let the wires insulation get sliced on the chassis,

from there its not far to turning the key on and then having 12v fry the insulation..

That said you might try to get a good used engine harness for half price at 928 INTL OR chase the O2 sensor wires till you find the wire to be in good condition.
The O2 sensor also runs on the fuel pump circuit

tonnant

Thanks. I think you may be right about the short against the drive tunnel. The rubber grommet is missing from there. Thing is, I think it may have been like this for years. The interior of the car should have filled with smoke and smelled. I've had the car since 2003 and it hasn't happened since then. I'll try tracing the harness back a bit, see if I can find a place where the wire has enough left to splice. If not, i was planning on doing an engine out restoration in a couple years. I may just decide to live with the cold engine misfire until then.

Mrmerlin

note now that you have moved the shredded wires you may have enabled the new short be careful when you turn on the key

tonnant

Went to find my damaged wire today. Found out that, although I apparently should trust my wiring diagram skills a bit more, I'm an idiot. Blue wire runs right to the fuse box. Just like the wiring diagram seems to say. On the upside I now know what the letters on the central wiring diagram mean. Looks like they are the plug blocks visible in the last picture. I really wish I had understood that before I started cutting. Now I have to figure out what to wrap it all back up with.



After CAREFULLY cutting into wiring harness. No blue wire.


Then tore rubber sleeve around wires inside the car. Notice the melted looking wire upper right.


Oh look.. melted wire going to fuse panel. Just like wiring diagram said.