I thought I would pass this on because it is so unusual and may save someone
else's a lot of grief.
About 2 years ago I purchased a very clean 1988 928 S4. Before I bought the car I took it out and drove the S__t our of it. I ran flawlessly, a very fast automatic. I went down to the southern part of SC to pick the car up a few days later after the deal was closed.
The car ran very well (yes I punched it several times on the way home) until I got within 2 miles of my house. Then as I was slowing to stop at a gas station to fill up, I started getting a very strong vibration. My heart sunk, it was violent enough that I thought that the cam belt had slipped and my engine was badly damaged. The next evening I rotated the cam to TDC and was surprised to find that the cams seemed in time. This was followed by a careful and complete compression check that also verified that this was not my problem. The plugs looked to be almost brand new and showed no signs of carbon build up.
I called Jim Bailey at 928 International and discussed the problem and we decided that the mass air flow sensor might be bad and I bought one right away for about $600.00 and installed it, later in the week. "No Joy" the problem remained and was so bad that I was very reluctant to drive the car. I posted this problem on the rennlist and got many suggestions from a faulty brain to a deflected flex plate on the flywheel. Over the next year and a half I replaced the plug wires checked the coils, replaced the fuel filter, checked the chips up at the radiator, inspected and cleaned the rotors/caps, redid all the ground points, ran every injector cleaner I could find thru the system, all to no avail.
On occasion the car would run perfectly. Then without warning the vibration would return. Now this was not just a small vibration, It was extremely extremely violent, so bad that you wanted to reach up and cut the key off as fast as possible. The vibration would sometimes be so bad at idle and low throttle that I was afraid to drive the car. Other times I would get so pissed, that I would go full throttle, just to get it over with. Each time I did this, however the engine would vibrate violently until about 3500 rpm and then go like a bat out of hell, as if nothing was wrong.
Some times the problem would come and go several times on a 60 mile jaunt.
My 79 928 5 spd had never given me a minute of trouble, in over 40,000 miles.
I continued to try this and that, spending time and money with no results.
I bought a set of Bosch crossfire platinum plugs, but when I removed the first Beru plug that was installed in the 88, it looked so good and new that I put it back in.
Then about two months later with time on my hands, I put the Bosch plugs in anyway. I was sure this was an exercise in futility, but when I cranked the car, it seemed to idle and rev more smoothly. It was late and I needed to turn in, but I thought what the heck I will drive the problem child around the block. To my amazement no vibration. Well around the block ended up being about 25 miles of putting the car through its paces.
It ran perfectly. The next day I called Jim Bailey and told him what I had found, and he told me that he had never run up on such a problem before.
He ask if the Beru plugs had carboned up, I told him they had not, in fact they appeared brand new. The plugs are type R6 715. I can find no reference to them anywhere on the Beru website. My emails to them remain unanswered.
What apparently was happening was that these plugs were failing to fire (possibly due to extremely high resistance), and with the super high voltage in this ignition system the arc was jumping to the next closest plug connection on the rotor cap until it found one that would fire. This was causing the engine to fire way out of time, producing a tremendous vibration/detonation (I am lucky it did not scatter the engine).
Apparently as the combustion chambers got more air and fuel this phenomena stopped. Most peculiar, but it happened.
I was so skeptical about this problem returning, that I have waited about five weeks before posting this on the list. But the car is still working great. Now I am wondering what German V8 tractor is missing it's plugs.
1988 S4 Auto Med Blue 70 K miles
1979 5 spd Silver