US '80 SC with '81 engine - CIS setup?
 

Hi,

after learning that my '80 SC has an '81 engine, I started thinking about the implications this may have on the CIS and whether the transplant has been done properly. Actually I try to find the cause of my idle problem when cold. When I turn the key, it starts immediately but idles only when I depress the accelerator pedal. It then hunts up and down in rpm which smoothes out the warmer the engine gets. Once it is hot, the idle is constant around 900 rpm.

Browsing through the Bentley manual I noticed that the oxygen sensor system is quite different between the two years. For the '81 model year, Porsche added the cold start accelerator unit, the 35C switch and the 15 degree throttle sensor switch.

I have verified that the oxygen sensor control unit under the passenger seat is the one from the '80 model. The micro switch has been removed from the engine and the 35C switch is not connected.

Is this setup OK or is it a possible source for my bad idle? The Bentley manual lists the frequency values of the FV as 85% for the '80 and 65% for the '81 model when cold. This would mean the engine runs rich during warm-up.

Are there other things to consider when transplanting an '81 engine into an '80 car?

Thanks, Manfred

Manfred,

We are talking about a USA model in Germany ? My advice would be to hook up an analog dwell meter to the test port in the engine compartment and verify that the lambda system is pulsing the FV at the correct default duty cycle open loop and then switching into closed loop. Your symptom is common on aging US lambda 911SC's and usually can be adjusted out with a bit of knowledge and experience. If you set the hot closed loop mixture to 25-35 dwell, the ignition timing to spec, the idle to 950 rpm and the vacuum retard always plugged, the engine will have a much stronger cold idle after start up and the best WOT performance. A working O2 sensor closed loop feedback will also give you the best fuel economy. The change in the injection system in 81 was to better comply with the cold running USA IM 240 emissions test and you should be able to tune around the mismatched parts with a little effort.

Paul,

thank you for your recommendations. I will give it a try. The proposed 25-35 value for the FV, is that degrees dwell or duty cycle?

Thanks, Manfred