Mercedes 280 se 1976 d jetronic extreme rich at idle

Hello,

I have here a mercedes 280 se with a djetronic injection system on.
The car came into my shop after several guys tried to get the engine back running or running good, so it came here with the engine not starting.

A lot of things where wrong, ignition points where new but adjusted to 0.8 mm i put them at 0.3 mm, the fuel pump relay was wrong wired, (with ignition on the fuel pump runs but not when starting or running engine).

I got it running again but the mixture is so extreme rich , black smoke comes out, my gas analyzer reads more then 9.0 co and 3000 hc. I haven't driven with it yet to see how it runs, but pushing on the throttle it reacts normal, but idle is difficult, i have put the ignition advance higher otherwise the engine doesn't keep running to test some things.

So this is what i have tested now:

- Water temperature sensor, good resistance when hot around 320 ohm, i tried putting a 265 ohm resistor to see if it helps but no. (all measured at the ecu connector to be sure

- Air temperature sensor also normal resistance

- TS has 0 ohm at idle and when you open it not anymore, full throttle contacts are also ok, the 10 for acceleration points i also measure,...

- map sensor, measure the 90 and 360 ohm for the coils like explained on this website, does anyone know what vacuum level you need to have at idle?
If i pull of the hose i see that the pulse width gets higher, so it reacts ,..

- I have measured the pulse width at the injectors as you can see in the attachements, at idle with warm engine i have around 7-8 ms when turning the co adjuster know on the ecu full clokwise (rich) it goes to 12-13 ms (i can't do that long because that's really to much) but this works, if i could adjust in the other way the same amount it was ok i think.
If you push the throttle the engine reacts not to bad but holding it steady is to rich also, i am not sure how the car drives now, but first this must be a bit less rich

- still have to measure what the pulswidth is when starting cold, if it's the same it's maybe something with the water temperature sensing inside the ecu???

-The trigger points i haven't check, but if you see the signals on the injectors i think they are ok? (could the trigger points influence the pulse width?


Is the injector signal ok? has anyone measure it to? and have pictures or movies of it

See here the signal in video

So i checked the pulse width with cold engine now, the engine started good, at pulse width 6 ms now only, and around 40-50 degrees water, only 4-5 ms , and it was still running not to bad, maybe a bit to rich but not that much.

Then i took my gasanalyzer and started the engine again, but now it's again worse, pw is up to 7-9 ms again and the engine is hotter now. Resistance of water and air temperature are ok.
Now it sometimes was better.

So it must be a bad contact somewhere? Or in the ecu.
Or is there something when a bit warmer that could get the mixture get richer.

its not normal that i have 5-6 cold, and 7-10 warm engine.

Is it not possible to measure inside the ecu with a scope on some places to see where the signals are +-normal what they should be, so that i can trace back where the fault is maybe?

Hi Nick,

well 6ms at cold engine seems reasonable. DId you measure engine temperature sensors at ECU connector?

The problem with cars that someone has tried to repair but failed is that mostly someone has desparately tried to tune or change everything. So it could well be that someone has detuned MAP sensor. Is the blcak cap askew or broken? That would be a hint.

However what is strange that your signal width jumps unpredictably. That sounds like a cable problem.

So i tested a lot more, i also measure on a 450 sl what i have here and also has d jetronic, the signals look the same but cold 5-6 ms and warm 3 ms.

I switched between Throttle sensors but without results, and also with the map sensor, here the car was better on the 450 map sensor (less pulse widht)

I noticed now also that the map sensor has been completely opened, they filed away the bolts or revets and fitted normal bolt/nut, the black cap you could see the also opened before.

I now measured with my oscilisope at the ecu at nearly every connection to see if there are strange spikes or so, or no stable voltage at ecu, but everything looks good, the only thing is that sometimes the engine runs well now at 3 ms and when its warm for some reason it could go to 8 or 11 and stay there and don't come back, it happend a lot of times.

Now to test (because they allready detuned the map sensor) i turned the adjusting nut under the black cap and the mixture got less rich, i did a drive with it but it's not perfect, sometimes it begins to idle now very bad and i have to turn the co adjuster richer to get it running better, but it could be that after doing that it gets to rich and i have to go the complete other way again.

As this system lets the injector pulse go trough the map sensor i'm still suspecting it to be bad, it could still be that there are cable problems or something in ecu but????

(i have measured every resistor value at the ecu connector always and they where never bad), the temperature values i also looked at with the scope but they are stable as wel as the trigger points.

See attached some screenshots

Could the map sensor behave so strangly?
The customer says me now that the car is not running for 2 years allready, and a lot of people worked on it, i'm not sure what the initial problem was but the pulse width has to be more stable.

Volker can you test the map sensor and maybe the ecu to see if everythings ok or not? Or what do you think now with these findings?

(idle above injector pulse width, lower primary coil signal)

Hi Nick,

oh Jesus, now they have also started to detune 0 280 100 111 MAP sensors. So far it was common in the US on 450 0 280 100 100 sensors and of course type 2 MAP sensors with broken diaphragms. I hoped not to see this on other types.

I can truly believe that a detuned MAP sensor creates the problems you describe. It is VERY SENSITIVE tuning. However I believe that the car has another problem. Not from the symptoms you describe. But I beleive tha no one detunes a MAP sensor on a 280 SE without being desperate for not finding the real problem in ignition and injection. So I wonder what else might be wrong in the car you work on. Life has taught me that a car with such a issue has not just one failure.

But bravo from my side! You are one of the few who could nail the problem down so narrowly.