Originally Posted by
foama
@Dabullfrog91: The carbon incident of 2016 (see first post) was a freak event. It happened to mine and several other identical cars that filled-up at two particular fuel stations. No similar problems since, and the valves remain clean ever since.
How fast? These cars don't have much acceleration, but are fast enough to keep up with the average traffic in the fast lane of the German Autobahn, and thats fast enough for me.
UPDATE September 2018:
The valves look clean, just as clean as last year.
The cause for the incident must have been a tank full of bad fuel.
UPDATE September 2019
Valves look just as clean as ever. Perfectly clean, just as inside combustion chambers.
However, ever since the car was "repaired" and carbon removed, the compression has been much lower at around 9bar (about 130 psi) on all cylinders, and fuel consumption is consequently about 10% or so higher than before.
This car has a compression ratio of 11.5 : 1 and so compression should be much better.
UPDATE August 2020: IT HAPPENED AGAIN!
Until very recently the valves looked clean as new.
It happened again! Last week the car wouldn't start. It had been resting in the garage for a few days after a long drive on the autobahn, just as in the first incident in post 1. A quick compression test showed almost nill compression on two of three cylinders, and very low on the third. Looking with the endoscope the intake valve stems were seen full of brown or black-brown gummy sticky goo and therefore could not close all way. Valves open means no compression means no start.
Since the warranty had expired years ago, I repaired it like this:
The intake manifold was removed and three new aftermarket gaskets manifold-to-head bought.
I stuffed a narrow strip of cotton rag saturated with acetone into the intake hole leading to the valves of the first cylinder and added some acetone to it about every few minutes. After a half hour or so removed the rag and saw the gunk had softened. With a bundle of cable ties wrapped together as a tool, the gunk was mechanically scratched off, adding a little acetone every minute or so while doing it. Put the rag back into the intake, repeatedly added acetone, repeated the entire procedure again a few minutes later. Did the same on the second cylinder. When turning over the engine by hand, I could tell the valves were now moving properly. Some of the goo was still there, but most gone and the remaining much softened. Put everything together quickly, started the car and drove off to fill up the half empty tank with some fresh fuel. Checked it a few days later and everything looks perfectly clean again. Compression is unchanged as after the first warranty "repair" see first post of the thread.
A chemical engineer explained to me that E10 fuel (10% alcohol) plus acidic greasy combustion products cause exactly that sort of residue. I didn't really understand what he said, but understood it had something to do with chemical reactions, the higher alcohol content of the fuel, and the blow-by reentering the intake. Although the car uses practically no oil, I ordered a little catch can that might help condensing some of those combustion byproducts. It will be intalled in the 10mm hose between valve cover and intake manifold. I wonder if it makes any difference...
