Persistent engine knock.

jmckeefery · 2006-04-12T16:23:40-05:00

Porsche 968 1993 engine knock. I Love the car but it has always had a persistent engine knock. The cylinder compression has always been very high. I am the third owner and do not know of all the work that may have been done to the car. I have the full set of Porsche shop manuals and the compression spec is 8 bar and over for a new motor and 6.5 bar is considered low enough for a rebuild. I have 17.5 bar. In PSI they are as follows. #1 = 245 PSI #2 = 250 PSI #3 = 256 PSI #4 = 238 PSI The engine knock is worse with lower octane fuel and with higher engine temperatures. Adding octane boost helps but not much, and not nearly enough. I was worried about lean injectors, so I checked the fuel rail pressure and it is good at 45 PSI. The plugs look sooty but even across all four. These indicate too rich or too cold. This makes no sense, I would have guessed the opposite in both cases. Oil burning is not seen and not likely with compression like that. A poor spark could cause all of the above conditions by allowing so much carbon buildup that the compression is increased. I have wired into the engine computer (I am an electrical engineer) a bright blue LED that flashes when the knock sensors detect a knock. Hard acceleration is always fine. Cruising at 3200 RPM with light load is where the knocking is. If I hard accelerate to 5,500 RPM and let it sit there as the car stabilizes speed the knocking is extreme. So my hypothesis so far are: 1. Bad spark, lots of carbon, increasing compression, then pre ignition due to carbon hot spots and high compression. The solution to this may be to buy a higher energy ignition coil and see if the condition improves. I also need an additive that you would recommend to get the carbon out of the cylinders. 2. Either the timing belt or the cam to cam chain is mis-adjusted, causing very high compression. There is no fuel available that can run at 18 to one compression ratio. The solution to this is to remove the timing belt cover and valve cover and do a lot of measuring. It's a lot of work. I am hoping to avoid this. Thanks for you help, James McKeefery 408-946-8550

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Persistent engine knock.

flash
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#21

06-18-2006, 02:07 PM

yeah - that's more like it

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