dolucasi

Good to hear there is some progress. Did you verify that the new throttle body and the TPS is operating correctly by measuring the resistances between the pins in various positions in your car?

DLL

Yes, I did.
The method I used was with the audible setting on my multimeter. Idle throttle closed, pins 1 and 2- audible signal, open throttle no audible signal. Open throttle pins 2 and 3 - no audible signal closed, audible signal open.

dolucasi

Good. Recall we are trying to understand why your ECU idle controller is not working/doing the job. That's why we swapped the busted TPS with a working one.
So what happens now with the car idling at operating temp and you unplug the IACV? Any change to the idle at all?

DLL

Ah, Prior to changing TB no change to idle speed when unplugging IACV. I haven’t tried that since changing TB, will do.​​​​​​​Since changing TB I tried unplugging:​​​​​​​TPS - no change, ​​​​​​​POT - no change,​​​​​​​IACV - Idle dropped from about 1200 to 500.​​​​​​​I tried starting car again today, still multiple hard tries to get running a few seconds at a time.​​​​​​​

dolucasi

Good, let me know. Remind me again, did you perform EHA current measurements before the TPS swap. And if so what were the results?

JohnLane

OP your description of hard start + misfires under heavy load suggest multiple issues.

Worth checking may be rest fuel pressure... check that injectors don’t dribble (leak) after shutdown (common), fuel pump volume & pressure and all secondary ignition parts.

This is in addition to the idle and EHA issue.

Multiple problems can be a real pain.

DLL

I've been away from the car for a bit, took a breather, took it to my very good local indy Mercedes shop.
They found injector seals were bad, replaced, car is better while driving. It still has that cold start issue - initially will rev high then stall, then runs a few seconds and stalls. The IACV is good. They found the car stops sending power to the IACV after starting, we don't know why yet. Have tried different ECU 's and OVP 's without further success.
Still trying to figure out.

dolucasi

This sure sounds from your description that the IACV is receiving a foul signal. And oddly the signal is drifting slowly from a high level to towards zero slowly causing the IACV slit to close as it is crossing the closed position causing the stall.

What happens if you just unplug the IACV connector on a cold start? Does it start and keep running if you never plug in the IACV connector?

DLL

No, if IACV is unplugged, it still runs a few seconds and dies, repeatedly

dolucasi

There goes that theory then. So the IACV is not involved in the stall at all. So this is either fuel delivery or ignition related.

Still the same, the rpms keep going down slowly an dying or is it sudden now?

DLL

Well, I think the IACV is involved, because testing shows no current coming to it, sometimes initially but cuts out. We have tried different ECU's and OVP's without success. ​​​​​​​I think the short initial fire up is from the fuel inserted by the cold start injector. ​​​​​​​That does raise a question of if enough fuel arrives after, but when warm (eventually) car starts and runs.​​​​​​​The temp sensor on the block tests OK.

dolucasi

I meant IACV is not involved in the stall if unplugging it does not change the behavior. Does it? Did I misunderstand that?

In either case perhaps you should meausure the voltage on it from fire-up till stall. Does it go up and then drift towards zero? That's what I wanted to find out

DLL

Hi, so no, unplugging doesn't change the behavior. Yes, the voltage drops off immediately on starting, sometimes isn't there at all

dolucasi

OK. So this is what I understand. The IACV after start receives some signal (these are actually square wave PWM pulses). And shortly after the square waves start to drift to a lower (average) voltage slowly. As this is happening the engine stalls because at around 2V or so the IACV slit is actually closed, but before it has a chance to open again below ~2V the engine stalls.

What happens if you press the gas pedal after you start the car an keep it there at high RPM? Can you keep the engine running? I assume you have tried this many times.

DLL

IACV sometimes receives some signal, stops abruptly, or, no signal at all at times.When engine is cold, application of gas pedal causes stall. Cannot keep engine running continuously until it eventually gets some heat in it.

dolucasi

Have you checked if the polarity of the EHA valve electrical leads are correct. When your engine was swapped maybe the connector fell apart and they connected it backwards.
You can imagine how that would cause major issues in cold starts and cold running. Not sure how to check that other than looking at wiring diagrams but you can also swap the wires and observe cause an effect. If issue gets even worse, reverse it back to original.

I diagnosed this one time with a forum member long time ago and sure enough something similar happened to him. Except he himself was the culprit.

DLL

Thank you. I’ll check that. I’m thinking there is something wrong with the wiring.

dolucasi

I am also thinking along the same lines. You will have to check them all but start with EHA as reverse hook up there would be very bad. Similarly with TPS and AFM Pot. Most other connectors are bipolar so they will not matter.

Herrickmky

Hi, I've encountered the similar idle issue. Engine starts at cold with no issue. Idle however starts at 1200 rpm; usually in 1-2 minutes, it rises to 1600 rpm and stays there. Pulling the throttle cable and release the cable , it usually ends up with 2 results. the RPM either rises accordingly then idle back at even higher idle, like 2400 or 2600 or the RPM refuses to rise and the airflow sensor plate is struggle to deflect. With and without the ECU connected, the same issue occurs. My car is 92 300CE-24V with M104.980 engine.

Some diagnoses done following the guidance here

• At 1600 rpm, unplug the EHA connector, no impact on the RPM, seems that the ECU is in limp mode
• Unplug the ICV connector, idle rpm drops to 1100. Plug the connector back in, idle rpm remains at 1100
• 5.67v is measured at the ICV connector
• TPS is checked with continuity and is fine. Both full throttle position and idle position indicates the proper signal
• Idle micro switch is checked and is fine

Have checked the continuity between the ECU side cable end and the relevant sensors, all are fine

Wondering how I can get the ECU out of limp mode?

Thanks in advance

Herrickmky

Hi, May I know if you have resolved the high idle issue?

Cheers!

dolucasi

I would suggest you start your own tread for your issue. It is very different from the OP's post. But not only that you have an M104 which has it's own set of EZL problems which can affect the behavior. My guess is we may not find commonality between your late model 300CE with an M104 and the early model M103. Hopefully your issue is simpler than the OP's multiple issues with a swapped engine.

All this being said, I do not believe your ECU is in limp mode, the is forcing a high idle for some unknown reason, the relatively high IACV voltage is an indication that the ECU is forcing a high idle.
We'll have to diagnose why it is forcing the high idle. It is not happy. Also the idle controller in your model is part of that MAF brick not the ECU, as far as I know. Be happy to attempt to help you in a separate thread.

Herrickmky

Thanks for replying me. As suggested, I created a new thread as 92 300CE-24 high idle issue