Yep, not normal.


OP,
Open it up and have a look see.

It's all analog components in there anyway. Might be something obvious.

The other guys have covered most of the bases. However, don't overlook the frequency valve itself. It may be dirty or sticking. I would put a bottle of Chevron Techron Concentrate in the tank, go drive the snot out of it for a week or so, then see if the idle stabilizes. If not, then remove and inspect the FV. Since it works like a fuel injector, you can have it cleaned by a shop that cleans fuel injectors.

My experience with old electronic components (as well as new ones) is that they tend to either work, or not. Sometimes they are intermittent (the worst kind). Check and clean electrical connections too.

Yes, that's case for components, but not for the circuit boards they're mount on.

First check the link Dave mentioned – fast easy and very close to your location as also based in CA IIRC.
And if maybe finally after checking the ECU it shows that a bigger repair is not worth it, have a look here.
https://nineelevenheaven.wordpress.com/the-911-sc-3-0-engine-with-catalyst-and-lambda-control/
https://nineelevenheaven.wordpress.com/the-lambda-control-of-the-911-sc/
https://nineelevenheaven.wordpress.com/optimized-control-unit-ecu-for-the-911-sc-with-lambda-control/

Thanks for the help, everyone. Andrew, your website is great.

Here is a photo of the lambda computer. Nothing sticks out to me.

http://forums.pelicanparts.com/uploa...1717418941.jpg

I'll check out the website above, but first I'm going to double-check the relay is functioning properly. I verified it is getting 12V at terminals 88 and 30 while running, but it occurs to me that I didn't check whether the switched power at terminal 87 or 87B remains hot. I'll report back tonight.

The typical circuit board problems occur on the bottom, where the circuit tracks are, and intermittencies occur.
The worst case problem occurs when the hybrid circuit fails, as there's no replacement.

http://forums.pelicanparts.com/uploa...1717421840.jpg

I hate those hybrid circuits and it seems they are almost everywhere in our cars. The heater blower relay and the newest version of heater automatic control are partly built with such hybrid cards and that makes them very hard to hack.

Good to know.

Confirmed that the relay is working perfectly fine. I will send out the ECU for testing/repair.

Closing out this thread, I sent out the lambda computer to Loren at systemsc and it turned out to be in good working order. The signal to the frequency valve was as expected and there were no other issues. Loren resoldered some connections anyway and took the time to discuss the issues I thought I was having. I highly recommend Loren's services. After some discussion, I updated my scope settings and fine-tuned the mixture screw to get the following at the test port:

https://www.youtube.com/watch?v=nZf-Si8OtXY

Loren confirmed some idle fluctuation is normal, and it looks like the duty cycle fluctuations are not as extreme. From here, Loren suggested I also confirm the following, which I will do soon:

1. monitor the cold start enrichment duty cycle (~ 70%), a ground (ECU pin 7) from the temp switch on the chain cover right bank
2. check the acceleration enrichment switch ECU pins 12 & 14, from the relay next to the ECU, as the acceleration pedal reaches about 1/2 way full throttle

Otherwise, she's up and running!

Yep, you nailed it right at the beginning.

In hindsight, the duty cycle oscillation between 60 and 45 should have informed us right off the bat.

Dave,

Do you have a financial interest in this company?

Heard about this company from the many Porsche dealers and independents they provide products and support for here in SoCal.
They support early 911s thru 906s, 959s, 993s, to the Porsche Cayenne. Surprised you haven't heard about them. Give them a call sometime.