High CO on idle - MB 280SL 1976, M110 D-jetronic

Dear all,

I’m running out of ideas how to solve my high CO on idle (6-7%) 
I have done the following:
- checked compression - ok on all cylinders, lower on cyl 4 but within range.
- changed spark plugs, cables, cap and rotor, capacitor and points (adjusted to 0,4 mm)
—measured voltage over coil - ok
- i have good spark on all cylinders
- checked timing - ok, approx 10 dgr before tdc 
- vacuum adjustment on distributor functions and change timing
- Changed air filter 
- Measured air temp sensor - ok at sensor
- measured water temp sensor - ok at sensor
- Fuel Pressure - 2 bar constant and stay stable long after turning off engine
- MAP sensor (type 3) - measured resistance ok, vacuum test ok
- all fuel injectors replaced - exact same behavior with old injectors as with new
- gaskets for old injectors were leaking (spray with starter fluid increased rev) replaced all at injector change
- cannot find any further vacuum leaks with starter fluid, no change in rev.

adjusting potentiometer on ECU lower the CO but only 1% max. I bottom out the potentiometer, and it’s not enough.
AAV is not functioning properly - I close off hose into intake with clamp when engine is warm. That brings down idle to 800rpm

I have not measured the throttle body switch
not checked trigger contacts - how do you best measure their function without having a lab version of a dwell??
i have not measured on the harness connector at the ECU (next step)
I have not yet leak-tested w smoke the intake for leaks, only using starter fluid.

please, open for any suggestions and ideas where to look, what I might have missed?

best regards
Stephan 
Mercedes R107 280SL, M110 engine, D-jetronic, manual, 1976

Hi Stephan -
and Welcome to the forum.

Seems you have done almost anything,

Do what you intend to be the next step - do the measuring at the ECU-connector.

Possibly there is one situation at proceeding trigger point's wear they could flutter and produce extra pulses.
The easiest way to control and readjust the trigger-points is pulling them out und do a control using my tool: jetronic.org/en/kunena/jetronic/6-werkze...geber-gebiss?start=0
They have to be regreased from time to time - therefore this would be an advantage in every cause.

If you have a lathe, I could send you a drawing so you could produce it by yourself. Contact me by PM. I could send you the ready-produced tool, too.

Regards
Norbert

Thank you Norberg,
I’ll start with ECU connector. Next measure the trigger points. I’ll see if I can get hold of an oscilloscope and check the behavior.
I am obviously missing something, and in my mind, the only realistic cause for so much fuel into cylinders should be that the ECU believe there are more air (oxygen) coming into the engine than what it really is. The ECU only measure a few sensors, and those looks to be fine, so an air leak seems most likely. Let’s see what I can find tomorrow.
I’ll contact you separate on your offer for drawings!
Sincerely
Stephan

Dear Norbert,

Follow up questions:
1) Measuring from ECU connector to injector harness, I realize that injectors are counted from bulkhead, and not from radiator (front of car). Is this correct?
2) Does it matter in what order the injectors are triggered compared to firing order? (It should in my mind but the M110 has injector 1 & 3 and 2 & 4 opening at the same time.)
Please advise!

Hi Stephan,
1) Injectors are counted like the cylinders from the front (radiator).
It is a convention to count beginning with the cylinder opposite to the outlet of the engine.
2) It doesn't matter much if injectors are triggered even with a reversed timing. The M110 has three injectors 'paired' that fire at the same time: 1-5-3 and 6-2-4.
Because two of the injectors are connected parallel to the first and one injector to a second output of the ECU, the resistances you have to measure are different. Here ist a download for you: jetronic.org/de/forum/attachment/803?download=1

Regards
Norbert

Dear Norbert,

Many thanks for the pin layout above! Highly appreciated. Mine was slightly different likely because it’s for a 350/450.
I have changed the injector harness so it follow the ECU pin config. Still as you said, it doesn’t make any difference for my problem.
I have also measured the trigger points at the ecu connector, point 1 give continuity when point 2 doesn’t, and vice versa. (Rotating engine, disconnected ecu)
I have measured every sensor and connection on the ecu connector, all seems to be fine.
I cannot believe that the ECU would be broken, sounds highly unlikely.

Any suggestion for what to check/re-check again?

Please advise,
/Stephan