The early Porsche 944 (1982 to 1985.5) has a pronounced engine shudder. On engine deceleration, the engine will drop several hundred rpm below normal idle before returning to normal. The whole car shakes. This shudder problem seems to affect the US version of the early 944 more than others.
Technical Description: On engine deceleration, the throttle switch is closed and the fuel is cut-off. This is a design feature. As the engine speed drops below 1,600rpm, the fuel begins to flow again. The engine then returns to a normal idle. This is how it is suppose to work. But.........
The early Porsche 944 engine computer (US DME) has a fuel cut-off set at 1,350rpm. The fuel flow start too late. My engine dropped to 600rpms before returning to normal idle of 950rpm. The shudder was very pronounced. The worst part of this is the shudder could not be fixed. It was internal to the fuel injection computer and swapping DMEs gets you another computer with the same problem. The early Porsche 944 Euro version has a fuel cut-off set at 1,600rpm and does not have a noticeable shudder problem unlike the US version.
Update 2002: There is now a bug fixed version of the 944 DME operating code. I have modified the code to increase the fuel cut-off point to 2,000rpm. The code has been thoroughly tested and the shudder is gone!
NOTE: Not all early US Porsche 944 show symptoms of the shudder or it is so mild as not to be noticed.
With the engine off, remove the 3-terminal connector from the throttle.
Short terminal #2 to #18. A small paper clip will do it. This will simulate a closed throttle for the DME.
Start engine and increase speed above 1,600rpm. The injection signal will be stopped near 1,600rpm. Record the actual rpm that the engine cuts out at.
In 1999, I came up with this mechanical fix for the shudder. It involves defeating the switch that closes at idle. If the switch never closes, the fuel is never cut off on deceleration. Result....... no shudder. Link to the mechanical shudder fix write-up.
© FR Wilk 2001