No doubt there'll be howls of protest at starting a new thread with a similar title to another current one, but it seems the other has arrived at some sort of resolution and, in any case, I suspect my problem is slightly different.
I had similar issues to those on the other thread (misfire, hard start, no power etc.) but in my case the diagnosis was chronic corrosion where the HT lead plugs into the coil. I replaced both the coil and the lead and everything was fine for 6 weeks. But then the old problem returned so I checked the lead again. Same corrosion as before.
This cycle of events has repeated itself four or five times in the last nine months. It appears the HT leads works loose and the subsequent arcing causes the contacts to corrode.
What causes the lead to work loose? Could it be that the position of the coil is wrong as it seems to be twisted in a contorted way to fit into the available space?
Bearing in mind it's a RHD car and the coil faces backwards on the right hand side, should the coil face forwards or backwards? The coil on the left side faces forward.
Or is there another more banal explanation?
It's driving me to distraction!

 

has the car gotten wet after you swapped out the coil and wires?
if so then you need to add some die electric grease to the boot on the coil,
and add a strip of weather strip to the area just above the coil connection on the fender edge,
this will keep water from dripping down and wetting the connections.
The right side coil should face to the rear the left to the front. viewed while sitting in the car.
Also make sure the coil wires dont rub on any part of their run,
worn MMs will also cause some wire misdirection as the engine will sit lower than it should

 

That's the problem. The HT lead rubs against the side of the engine bay because it seems to be too long for the confined space.

MMs were replaced a year ago.

The replacement bits were bought from Porsche. But the connection always feels loose. I've replaced the HT lead twice so could the coil be the wrong one?

 

Sounds like you might have the wrong coil in there.

Please remove the offending coil, and HT lead and take a picture of the end going into the coil, and a picture of the entrance of the coil.

 

Post a PIC - So that we can see...

 

It's 9 o'clock in the evening here in the UK so the pics will have to wait till tomorrow.

As I said, the new parts were bought from Porsche - a new coil and 2 HT leads. Are there variations of coil types for a '90 S4? Someone suggested that with or without a cat may play a role in this.

 

The Cat will have no effect on this.

There are two different types of coils for the 32V cars, and there are two different coil ends too.

If you have early leads with late coil it will not work at all.
If you have late leads, with early coil it will go in but feel loose, and will fail quickly as the spark will be arcing in the boot causing fast wear.

on your model year, both ends of the coil lead should look the same. (small hole in the end with a wire which applies pressure to the threaded stud to hold it in).
The coil should also have a small stud just like the connections on the distributor cap.
It is likely that your coil doesnt have this stud (early style).

 

Ah ha! That sounds promising! I've often wondered whether there are variations in coil types.

I don't know what the connection in the coil looks like without removing it. The lead has a relatively short metal connector, about a 1/4 inch with a spring clip around it.

Maybe I need to take the coil to Porsche Spares to see if they can match it up. But what are the criteria for the different types of coil?

 

There are only two types and I'm sure that someone on here would have the correct part number for the correct coil.

 

Sounds like wrong coil is the issue, but do you have the cover that goes over the wire and end of the coil?

*** I have an early car, so may not apply.

 

the connection on the coil will look like either a deep hole,
or the tip of a sparkplug with the metal fitting removed thus exposing a threaded stud.

SO you need to look inside the coil and see what you have then match the wire to the coil then make sure the wire also matches the cap connection

 

So these are the relevant photos. I get the impression the rubber boot stops the metal connector from penetrating far enough into the coil to be secure. Over time it gradually squeezes the connectors apart.

Apologies for the quality of the photos and the obscure explanation.

 

That is the correct coil lead.

Now show us the opening of the coil....

 

"the connection on the coil will look like either a deep hole,
or the tip of a sparkplug with the metal fitting removed thus exposing a threaded stud."

Mine's definitely the "deep hole" version.

 

You have the wrong coil as I predicted.

Buy the correct one for your model year and it will then work properly