CIS Running Good but - a question ?
 

3rd Porsche but new to CIS - I have all the books including the "Troubleshooting Bosch CIS Systems" which has a lot of great info - and read thru all the threads, absorbing arcane knowledge, lore, and helpful hints.

My '80 SC runs great. Fires up on the first try, idles solid, runs great with plenty of power, and seldom goes over 180 deg. on the oil temp guage.

What's not to like ? Cold driveability could be better. And YES - pull the oil cap off, the idle RPM drops a bit - to about 750-800. Put the cap back on, idle goes back to about 950.

So here's my question - there is a fair amount of popping from the exhaust on overrun (coasting) - NO backfiring under load. I do not see this symptom listed in any of the troubleshooting charts.

Am I getting the dreaded P-car obsessiveness ?

Thanks for any info!

http://farm4.static.flickr.com/3347/...93ec3889d6.jpg

Cold driveability problems are often a sign of a too lean mixture during warmup. There are several reasons for this, but often it is an out of spec WUR w/ a too high cold control pressure during warmup. Having said that, depending on the symptoms, sometimes a surging from a too rich mixture can occur.

The overrun popping is often from a too rich mixture. Sometimes it can be an exhaust leak.

We could use some more data from you.

In the meantime, have I got a thread for you! If you search my name and CIS you will find I recently had similar symptoms. In my case, if I got the engine to start correctly, then the mix was too rich and I got that popping.

There seemed to be several things causing my problems, including a small hole in a vacuum line (AAR and AAV), some incorrectly routed vac lines to the thermovalve, and a WUR that was not functioning.

Based on your description, I would start by making sure all your ignition components are up to spec. Then I would look for vacuum leaks. I would check my fuel and control pressures, and the mixture.

It's very possible someone has enrichened your mixture to make up for an out of spec WUR and/or a vac leak.

I believe the deceleration valve is there to prevent closed throttle richness (popping). Does your car have one? Having said that, there are "deceleration valve haters" and here they come....

The decel valve is one of the CIS components that can fail producing a vac leak. Try plugging the line from the throttle body to it and see if it helps.

nice looking motor!! i used to have that popping deal going on, kind of annoying. i pulled the whole exhaust system, filed the flanges square, installed new gaskets and it was gone.

thanks guys - actually the line from the decel valve is plugged. I will do the test on the exhaust see if I can find a leak. Motor is freshly gone thru with lots of good stuff so maybe the flanges need to be re-torqued. 3000 miles on it.

Mark Petry
Bainbridge Island, WA

check your timing too..

Have your control pressures and mixture setting been checked?

control pressure and mixture
 

that's on my list, I have the guage ordered and reading thru the previous threads on the topic.

Thanks for all the info !

Mark Petry
Bainbridge Island, WA