M103 running problems, EHA, Duty cycle, etc..

[tinypanzer]

Hello everyone,

I've been experiencing problems with my 300SE even before I bit into the heater core job, now thankfully completed. Now I have to figure out why my car is running like doo-doo.

I'm going to split up lists between symptoms, and what I've done so far.

First the symptoms:

1) Occasional cold-start difficulty and cold-start rough idle.
2) Engine lacks power until car is completely up to temperature
3) Surging and pulsing during those weak accelerations
4) Seems to be missing at idle, with varying severity depending on engine temp
5) Engine seems to lack power in the 2K RPM range (even more than a normal M103!) and when revved in neutral, shows a lack of fuel delivery or similar type slow rise in RPM around that RPM range


Here's what I've done:


Duty cycle observations:

1) The duty cycle shows 70% when engine is off an ignition is in position II and car is dead cold. By the Steve Brotherton DIY and related links, I found a chart that tells me that this is normal for this type of ECU.

2) When engine is started, Duty cycle goes to STEADY 50% immediately. The same chart means that this indicates either 02 sensor not yet up to temp., or disconnected (or completely faulty). After the car warms up for a few minutes, the duty cycle then starts to fluctuate as it should, starting at about the 40% range. Then as the car warms up further, the duty cycle slowly drops to 0%. Then the duty cycle begins fluctuating between 0% steady for several seconds, then 100% steady for several seconds.


I have not yet obtained a proper duty cycle reading of a fluctuating, yet hovering around a certain percentage reading, indicating a satisfactory closed-loop operation.




Can anyone suggest the next logical step in troubleshooting this problem? Should I monitor EHA current, or check for something more simple first? o2 sensor is Bosch universal type ordered from Phil @ fastlane and is about 1 year or so old.



** Tested & Known Good **
Coolant temp sensor good
OVP good
Alternator good
Plugs & Wires good, cap & rotor 2 years old.

[ps2cho]

The 50% during cold start is normal as you are observing. The fact that after it enters closed-loop that it slowly drops to 0% indicates something is wrong.

With key to ignition, fuel pumps active, depress the throttle plate, what do you get? Then wide open throttle, what do you get?

From what I have observed on mine, it seems to have a lean stop at 18% and a rich stop at 93%. I can never achieve 0 or 100.
Since you are 1990, do you have a error code module you can check for codes?

Are you sure coolant/engine temp sensor is good? How did you test it? Did you test it both stone cold and normal operating temperature?

This is a good read also:
http://w124-zone.com/downloads/MB%20CD/W126/CIS-E%20diag.pdf

[tinypanzer]

Hi and thanks for replying.

The '90 model year has no diagnostic light, just basic info from the duty cycle, if I'm not mistaken.

Both elements of the temp sensor correspond to the temp/ohm chart both heated and cold.

Great link, BTW, I'll give it a read tonight!

[mak]

Hi , I had a similar sort of problem with my 300se , turned out to be the OVP relay . The top fuse was good ,inside the solder joint at the big Zener diode was bad . Re-soldered, and all was back to normal .
mak

[slk230red]

Originally Posted by tinypanzer

Hi and thanks for replying.

The '90 model year has no diagnostic light, just basic info from the duty cycle, if I'm not mistaken.

Both elements of the temp sensor correspond to the temp/ohm chart both heated and cold.

Great link, BTW, I'll give it a read tonight!

You'll have to build your own LED tester to check for DTC's, diagnostic trouble codes.

[tinypanzer]

I don't think my model year supports the trouble codes; there are only pins 1&3 in the firewall diagnostic connector and the rest are empty.

Thanks