Please help diagnosing '80 SC CIS (idle) problem

[ManniB]

Originally Posted by lonewolf

Sorry .I disagree.
Your making assumptions. when at speed and you get surging.this can happen with O2 hooked up. it's common. just as an example.It can be annoying and why a lot of people disconnect them.

Your in Bavaria.what's the altitude?
Did you buy the car in the same area?
Your rich so either too much fuel or not enough air. If the car was tuned at sea level and your at 3000FT that could be it right there just as an example of how simple it could be.
You don't need an exhaust analyzer to adjust the FD

Madison, that's were the car originates from, is at an alleviation of 873 ft. Pfaffenhofen (my town) is at around 1200 ft. I assume the difference is not that significant. Can you please explain how to adjust the FD without an analyzer?

Thank you, Manfred

[Scott R]

Originally Posted by lonewolf

Sorry .I disagree.
Your making assumptions. when at speed and you get surging.this can happen with O2 hooked up. it's common. just as an example.It can be annoying and why a lot of people disconnect them.

Your in Bavaria.what's the altitude?
Did you buy the car in the same area?
Your rich so either too much fuel or not enough air. If the car was tuned at sealevel and your at 3000FT that could be it right there just as an example of how simple it could be.
You don't need an exhaust analyzer to adjust the FD

This is true, you will always get some lower RPM cruise surge on this system, it's just something you live with or you go with some other sort of induction.

I would also heed this altitude device, I'm at 6000 ft above sea level, CIS is just not as happy at my altitude.

[psalt]

This means that the FV is activated during 15% of the cycle and not during 85% as it should be.

Manfred,

You are on the right track, you will never get anything like original drivability if the FV is not pulsing at the correct default values. No amount of bodging of the mixture screw can compensate.

[Lorenfb]

"Pin 15 is 15% at ground, it is the inverse of pin 17 because the ECU activates the FV by switching the ground side of the coil."

If that's really the case, then:

1. the data obtained was incorrect, or
2. the O2 sensor input is at a plus voltage greater than ~ .50 volts, or
3. a bad ground somewhere, e.g. engine to chassis, or
4. a bad ECU (very rare).

So, based on a circuit analysis of the ECU, the signals on pins 15 & 17 are the SAME,
so #1 appears to be the issue. Pins 15 & 17 are connected by a 2K ohm resistor thru
the output series protection/current limiting resistor are thus in phase and NOT as the
quote above states. Again, mis-diagnosis doesn't help solve the problem.

Check for the FV signal waveform: Waveforms

[porwolf]

Hi Manfred,

I have asimilar situation with my US 79SC (catalytic converter without oxygen sensor). Ever since I bought the car new the cold start always resulted in the customary high RPMs, about 2000 to 2500. What I always did, and still do, is to quickly tap the acceleartor after a couple of minutes. That reduces the RPMs to about 1500, or so. Then the engine by itself would gradually slow down to the 900 RPMs, especially if quickly driven gently.

Hope it helps, and servus!
Wolfgang.

[ManniB]

Originally Posted by Lorenfb

"Pin 15 is 15% at ground, it is the inverse of pin 17 because the ECU activates the FV by switching the ground side of the coil."

If that's really the case, then:

1. the data obtained was incorrect, or
2. the O2 sensor input is at a plus voltage greater than ~ .50 volts, or
3. a bad ground somewhere, e.g. engine to chassis, or
4. a bad ECU (very rare).

So, based on a circuit analysis of the ECU, the signals on pins 15 & 17 are the SAME,
so #1 appears to be the issue. Pins 15 & 17 are connected by a 2K ohm resistor thru
the output series protection/current limiting resistor are thus in phase and NOT as the
quote above states. Again, mis-diagnosis doesn't help solve the problem.

Check for the FV signal waveform: Waveforms

After reading Lorens post above I started writing a long explanation of why my thinking was correct. When I was nearly done something struck me. When measuring dwell on a points ignition, the points are closed when active and thus ground the negative side of the coil primary side. This means that when looking at the signal with an oscilloscope, one has to divide the low (0V) time by the period of the signal to get the duty cycle. AND THIS IS EXACTLY WHAT I SEE ON MY OSCILLOSCOPE. To my excuse I must say that this is contrary to the traditional definition of duty cycle in electronics were you divide the high time of a signal by the period.

So my control unit actually works an you have saved me from sending it for a costly repair. Thanks very much, also to the others who contributed. Further, I must admit that Paul (psalt) was right to insist on measuring with a dwell meter. I should have listened to what he said.

Anyway, I dug up my old analog dwell meter this morning and gave it a try. Unfortunately it gave me a reading of 50 degrees on the four cylinder scale when the engine was still cold, and it didn't change much when the 15C switch opened. I assume the old meter, for whatever reason, is not able to read the signal correctly and I will continue to use the oscilloscope (knowing it shows the inverse duty cycle)

- Manfred

[ManniB]

OK, it looks like I'm now back to square one with the search for my idle problem. I again read all the posts an suggestions, and I think I will try the following:

- install new o rings for the injectors to get rid of vacuum leak
- adjust the CIS to 25-35% duty cycle hot closed loop as per Paul's recommendation

Regarding the "wild bouncing" of the duty cycle when warm (it increases slowly from 50% to 90% then jumps back to 50%):

- Can this be a sign that the mixture is way off and the control unit adjust it?
- Or is it possibly a bad O2 sensor (sensor has been replaced by PO 5.000 miles/4 years ago).

- Manfred

[Lorenfb]

"Regarding the "wild bouncing" of the duty cycle when warm (it increases slowly from 50% to 90% then jumps back to 50%):"

Disconnect the O2 sensor and the DC should stay at 50%. If not, you have a wiring
or ground problem. It appears the O2 sensor thinks it's too lean, so it's forcing the
ECU to richen the mixture (90% DC). Then the O2 sensor eventually 'sees' the rich
mixture and allows the ECU to return to the 50% value. Either the O2 sensor is 'lazy'
or slow, or or the system has a major air leak affecting what the O2 sensor 'sees'.