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1991 560sec. 1991 560sel 1969 280SL
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1,266 Posts
Discussion Starter #1
So I just bought a 1991 560SEL 199 pearl black over grey interior The damn thing is running so rich there’s black smoke spewing out the tailpipe. The car came with new plugs, MB wires, cap and rotor and a brand new air filter.
When checking the lambda it shows 99.2 percent no matter how you adjust it. HD says that the lambda reading can be interpreted as a code. I’d like to know what that is.
I unplugged the O2 and the car ran worse. I’ll get a reading from it tomorrow (forgot). There is a brand new EHA installed from the previous owner too. I filled the gas tank with premium fuel. Made no real difference. The car hesitates when accelerating and when trying to start it catches first time but if you don’t keep it running and it stalls it’s impossible to start unless you remove the fuel pump relay and then starts for a moment immediately. Once it gets going it drives well but at a light the idle is really rough. it just won’t run unless at operating temperature. Tomorrow I’m going to try my old four prong temp sensor on it and see if that makes a difference. I have the procedure to test it but it’s just quicker to just swap the sensor out.
Thoughts and suggestions please. Thanks.

Best Regards,
Gogi
 

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1990 420 SEC
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353 Posts
Didn't search for the net but doesn't 100% duty cycle just indicate that the computer isn't able to adjust mixture away from too rich? Leaking/ spraying cold start valve? Stuck control plunger or leaking plunger seal at fuel distributor?
 

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Registered
1991 560sec. 1991 560sel 1969 280SL
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1,266 Posts
Discussion Starter #3
i have installed a spare good fuel distributor and injectors and it made no difference. I thought maybe the O2 must be faulty but did not check the reading yet. Did suspect the ECM may not be up to par but that’s rare (Sold my spare). Will try a spare OVP relay I have.
 

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1990 420 SEC
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353 Posts
Does it run better EHA cable and cold start injector cable disconnected? If it does run decent then there is something wrong with electric controls. Has the lamda adjustment been touched?
 

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1989 W124 260E
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1,704 Posts
Yes if the engine temp sensor is faulty,then it will send the wrong message to the ecu that engine is cold when it is hot and it will stay on rich mixture .
 

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1989 300SE EURO
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301 Posts
Hi,

What is the procedure to test the temp sensor?

Thanks.

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1989 300SE EURO
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301 Posts
Update.
No. Anyways i replaced the faulty four prong temp sensor on front top of the intake manifold and its running fine now.
Thanks Heikkif for the help, its very much appreciated.
Isn't the front one more for checking the temp of the air that comes in and the one in the back on top of the engine the one for the ECU?

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Registered
1991 560sec. 1991 560sel 1969 280SL
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1,266 Posts
Discussion Starter #10
My 91 560sec has the one on the thermostat housing for the gauge and the one for the ecu is the four prong sensor with the black round connector the size of a quarter. I have the test procedure for that sensor in my files somewhere; I’ll get back to you on that.
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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4,506 Posts
Isn't the front one more for checking the temp of the air that comes in and the one in the back on top of the engine the one for the ECU?

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On the M103, which I assume you have, the rear sensor is for the ECU.

See manual section 073-121, pages 163 and 164 for testing.
 

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1989 300SE EURO
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301 Posts
You're right. I didn't pay attention to the engine. Will have a look at those pages! Thanks.

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1991 560sec. 1991 560sel 1969 280SL
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1,266 Posts
Discussion Starter #14
So I just bought a 1991 560SEL 199 pearl black over grey interior The damn thing is running so rich there’s black smoke spewing out the tailpipe. The car came with new plugs, MB wires, cap and rotor and a brand new air filter.
When checking the lambda it shows 99.2 percent no matter how you adjust it. HD says that the lambda reading can be interpreted as a code. I’d like to know what that is.


Best Regards,
Gogi
The reason I getting a 99.2 duty reading is because of water ingress from the passenger front water drain, it was completely plugged up with thirty years of leaves and muck. The water was entering just above the ecu drenching it and the carpet. The water continues from there into the conduit for the wiring going to the rear. The carpet has to be removed and area cleaned and dried. Ian has mentioned in several threads that those drains under the cabin air intake grill be throughly cleaned out periodically, good advice I say.
 

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1991 560sec. 1991 560sel 1969 280SL
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1,266 Posts
Discussion Starter #16
The reason I getting a 99.2 duty reading is because of water ingress from the passenger front water drain, it was completely plugged up with thirty years of leaves and muck. The water was entering just above the ecu drenching it and the carpet. The water continues from there into the conduit for the wiring going to the rear. The carpet has to be removed and area cleaned and dried. Ian has mentioned in several threads that those drains under the cabin air intake grill be throughly cleaned out periodically, good advice I say.
Well the 99.2 lambda reading was not due to the ecu. I checked and replaced the engine temp sensor and after checking all sensors and switches and in light of the fact that the eha had been replaced by PO i adjusted the lambda over a few days in increments. Then finally a few days ago I got the reading to 47-50% at idle and that’s good enough for me right now. The engine has more power now, runs smoother and is easier to start, in fact the car starts with half second crank. A faulty engine temperature sensor caused previous mechanic to adjust the lambda incorrectly. When I bought the car someone had replaced the HT leads with genuine MB, replaced air filter, plugs, distributor cap and rotor also the eha and oxygen sensor.
 
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