69S starts to misfire when engine hits 180 degrees

[dhopkins]

My 1969 911S is doing something rather strange. After the engine reaches operating temperature of 180 degrees the engine starts to misfire. If I stop and let it cool it runs fine. Anybody got any ideas what could be causing this?

don
_______________________________
1981 911 SC
1969 911 S

[HarryD]

Don,

A possibility is that your CDI unit is being affected by the heat. My Permatune will suddenly give up the ghost on a hot day after a bit of a drive and then, after being allowed to sit for a bit started working again. I did the ice trick (see below) and volia, it fixed it self.

Try this: when it starts misfiring, put a few ice cubes on the case of the CDI unit and see if it goes away.

[roblav]

I had this issue in a 1972, and it ended being the CDI box, like HarryD says.

[dhopkins]

Is there another way to fix it instead of driving around with a tray of ice cubes?

don
______________________
1981 911 SC
1969 911 S

[HarryD]

Is there another way to fix it instead of driving around with a tray of ice cubes?

don
______________________
1981 911 SC
1969 911 S

Don,

The ice cubes are for diagnosis, the repair is to replace the unit if the cubes work.

[gruen911]

Before dropping big money on a new or rebuilt CD box, you may want to check out your spark plugs which cost only $3.00 a piece. Spark plug models are designed for peak perfoirmance at different temperature ranges. On the other hand, wheras CD boxes are not. So why would a CD Box start to fail at a certain temperature range when the CD box is in your engine but not attached to your engine. Since your plugs are screwed directly onto your cylinder heads, they would be directly exposed to your engines high heat ranges and could be significantly effected.
Also, the early 911S engines required very specific plugs due to their high compression nature. I have a 67 911S engine and had many problems with plug fouling etc until I switched to NGK B7ES plugs. What plugs are you using now?

[kenikh]

If plugs don't fix it, buy an MSD 6AL replacement for your box off of Ebay. If that doesn't work, you can always put it back up for sale.

[dhopkins]

My plugs are Bosch WR6DP. They are hotter than the recommended plug.

don
_____________________________________
1981 911 SC
1969 911 S

[pwd72s]

Blatent plug here: If you buy a rebuilt CDI unit from many sources, the odds are good that this is where it came from:
http://www.systemsc.com/

[ajbsd]

I had the same problem with my 72, replaced the CD box and the problem never came back.