duty cycle lambda 911 3.0 SC 1981 USA model

[wosm]

Today I tested my ECU lambda duty cycle by using the jimbasement workshop instructions. My car is a 911 SC 3.0 1981 USA model.

The specs say it should read 50% +/- 5% when oxygen sensor is unplugged. I measure also 50% So that is OK.

The spec for oxygen sensor unplugged + 15 degrees Celcius temp switch grounded (located @ right side) says 85% +/- 5%. I measure 37%! Is this correct?

When the engine is warm, everything is connected and I raise rpm to 3200 rmp the voltmeter also shows 37%. Is this correct?

Do I might have a failing ECU?

[RDM]

No. The frequency valve is trying to adjust the amount of fuel injected so that your exhaust gas reflects the perfect air-fuel ratio. Depending on where other things in your system are set (i.e. the allen screw on the fuel distributor), more or less compensating is necessary. The duty cycle spec assumes your timing is correct and the CO percentage is at factory spec.

edit: It also assumes fuel pressures are correct, air leaks are negligible, and so on. The fact that your duty cycle varies with input from the oxygen sensor indicates the ECU is functioning properly.

[wosm]

Hi RDM/ MArtin,

Thanks for your feedback.

I need some extra support because I am already struggling for months with an issue after the complete engine revision of my 911 3.0 SC. The first 1-2 minutes after a cold start it has backfire in the airtake manifold and the acceleration in lower range RPM is bad! I already checked on compatible components, air leaks, the ignition timing, cold/warm/system pressure, checked and measured the complete lambda wiring towards/from 12PIN connector + both the thermo switches, freq valve, Co, etc. Therefore i used the CIS primer site by jim Williams. I also swapped/checked the fuel distributor + injectors + WUR on forehand which are calibrated according spec.

So I suspect(ed) the ECU is failing, but haven’t found another one to swap and test it. Accordingly, I measured the duty cycle using the data on the CIS Primer website

I actually dont know what further investigation I can do Another rootcause could be the acceleration unit is bad??? (connected to 35 degree C thermo switch and throttle switch) although I have measured conduction and Voltages on 12PIn connector coming from this unit.

[RDM]

Why are you certain that it is fuel injection? It sounds like you have been through most of that already.

[boyt911sc]

Wouter,

Do you have a set of fuel pressure gauge to check your fuel pressures? The fuel pressures are easy to check and test. The ECU and OXS are quite different. Aside from a dwell meter, I have atleast a dozen good ECU's I used as a control. Even a new FV relay could be bad from the box. Have you confirmed that the FV is vibrating or pulsating when the FP is running? The OXS sensor does not come into play until the engine is at operating temperature and everything is controlled by the ECU (open loop).

I would like to see your cold control fuel pressure values during the first 90 sec. after a cold start (first start of the day). The colder the ambient temperature the better for this test. The numbers on the rebuild sheet for the WUR you sent me are all correct. But there was a missing data for the calibration which could be a factor to your problem/s.

Tony

[RDM]

Tony,

For back-story, go here:
CIS K-jetronic 3.0 SC (USA model) rpm issue

You posted on this problem last December through April. Like here:
CIS K-jetronic 3.0 SC (USA model) rpm issue

That may save you some research.

Dru