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1988 300SEL 158K;1980 450SL 31K
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630 Posts
Discussion Starter #1
I don't remember when was the last time i saw my tachometer needle is raises to 1000 rpm in cold weather and settle after that, since it was a long holiday i did some work:

1- Rebuilt the fuel distributor, adjusted the plunger screw to 0.6m according to https://cis-jetronic.com/public_doc/07.3-0997-06.pdf
2- Change the adjustment screw o-ring and cooper washer for EHA valve and adjusted it to 0.4 bar difference between the working pressure and controlled pressure while ignition off, EHA unplugged, fuel relay unplugged and pin 7 and 8 jumped.
3- Adjusted the air flow pot. to 0.7 volt (pin 1 and 2) while engine running.

the engine is much smoother right now than before, and more responsive. but it is cold for the past couple of days in Texas, so i have to crank the car twice and the rpm is not raising up. i thought the cold start valve is not working or no power coming to it or something, i removed the valve and plugged the wires and started the engine to see what will happen, on a cold engine the valve only worked for only one second or so, i replaced the valve with 2 spare valved, the same result. what i am missing here ?
i know the valve is getting power from fuel pump relay, over load relay and temp sensor, i replaced all of them in the past 2 years with OEM Mercedes. what i did wrong here and out of adjustment.i thought that valve will keep working for a couple of seconds till the water temp will reach a certain temp .
 

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Registered
1990 420 SEC
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195 Posts
Did you also at least check duty cycle? Cold start valve is supposed to work just as you have noticed. After engine is running fuel amount is controlled by the computer via EHA. To be able to control fuel amount correctly computer needs correct temp info and correctly working and adjusted EHA and fuel distributor.
 

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Registered
1988 300SEL 158K;1980 450SL 31K
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630 Posts
Discussion Starter #4
Yes, i redid the duty cycle after adjusting the EHA, and rebuilt the fuel dis. its fluctuate between 48% and 52%, am i missing something, i saw gasoline nice mist from the valve just for 1 second or less after cranking the engine, but the RPM is not raising to 1000rpm or so. is something missing or out of adjustment ?


Did you also at least check duty cycle? Cold start valve is supposed to work just as you have noticed. After engine is running fuel amount is controlled by the computer via EHA. To be able to control fuel amount correctly computer needs correct temp info and correctly working and adjusted EHA and fuel distributor.
 

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Registered
1990 420 SEC
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195 Posts
Ever checked resistance on temperature sensor? Service manual has a chart of resistance/ temp I believe. Also how is duty cycle at 2400 rpm?
 

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Moderator
1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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4,290 Posts
The cold start valve is energized only as the starter motor is engaged, and then only with the coolant temperature below a certain value.
 

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1991 560sec. 1969 280SL
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1,083 Posts
Just for shits and giggles check the fuse in the OVP relay.
 

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Registered
1988 300SEL 158K;1980 450SL 31K
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630 Posts
Discussion Starter #8
i will check today, i didn't check before as the temp sensor is a month old that i bought from the dealership, i will check the duty cycle at 2400 today after work also, you don't think its the ECU ?

Ever checked resistance on temperature sensor? Service manual has a chart of resistance/ temp I believe. Also how is duty cycle at 2400 rpm?
 

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Registered
1988 300SEL 158K;1980 450SL 31K
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630 Posts
Discussion Starter #9
i saw a nice mist for 1 second or less when i cranked the engine, but i think the engine is to lean t start

The cold start valve is energized only as the starter motor is engaged, and then only with the coolant temperature below a certain value.
 

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W201 Moderator
89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
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2,623 Posts
Faiz, there is nothing wrong with your cold start valve operations. As the name suggests it is only for a cold start to get the car started for one second. Then it shuts off.
That's it's only purpose in life. This is also why these cars start on a dime when cold.

As far as I know the 1000rpm idle at cold is controlled by the ECU and the actuator is the IACV. EHA just responds to the extra air put into the intake and increases the fuel volume as well.
More air and fuel is needed for 1000 rpm during warm-up. If you measured your IACV voltage you should measure a higher voltage when cold (for the first few minute). AS someone suggested, you should check your coolant temp sensor, it could be not functioning properly. That's where I would start.
 

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Registered
1988 300SEL 158K;1980 450SL 31K
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630 Posts
Discussion Starter #12
Thank you !! i have 2 spare ECU's i will see if they will make any difference just in case, are you talking about this sensor ? its brand new i bought it 6 months ago but i will test it also.



Faiz, there is nothing wrong with your cold start valve operations. As the name suggests it is only for a cold start to get the car started for one second. Then it shuts off.
That's it's only purpose in life. This is also why these cars start on a dime when cold.

As far as I know the 1000rpm idle at cold is controlled by the ECU and the actuator is the IACV. EHA just responds to the extra air put into the intake and increases the fuel volume as well.
More air and fuel is needed for 1000 rpm during warm-up. If you measured your IACV voltage you should measure a higher voltage when cold (for the first few minute). AS someone suggested, you should check your coolant temp sensor, it could be not functioning properly. That's where I would start.
 

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W201 Moderator
89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
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2,623 Posts
No that looks like the timing retard/advance switch. It is the black one with the 4 pin barrel connector.
 

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Registered
1990 420 SEC
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195 Posts
Temp sensor for 300 SE is different from 8 cyl. engines I believe so that could be the correct one. Component locations and how to test them is explained in service manual. Link at the stickies. 300 SE uses intake air temp for injection control too.
 

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W201 Moderator
89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
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2,623 Posts
That switch just has what appears to be vacuum lines going from intake to the EZL ignition unit.
M103 engines have an inch in diameter black sensor towards the rear end of the cylinder head that has two pairs (4 wires) that end up in the ECU (only one pair)
 

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1988 300SEL 158K;1980 450SL 31K
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630 Posts
Discussion Starter #17
my M103 engine doesn't have any sensor like that (4 pins), the green sensor is what i have at the rear end ofth e cylinder head next to the fire wall, i am wondering now what will happen if i switched the wires ?

That switch just has what appears to be vacuum lines going from intake to the EZL ignition unit.
M103 engines have an inch in diameter black sensor towards the rear end of the cylinder head that has two pairs (4 wires) that end up in the ECU (only one pair)
 

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1990 420 SEC
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195 Posts
It has two channels+ common ground at the sensor body. If both channels are ok does not matter which way it is connected. If you had a new sensor how would you identify pins on it?
 

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1988 300SEL 158K;1980 450SL 31K
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630 Posts
Discussion Starter #19
both looked the same, so i plugged the nearest one on the sensor to the fire wall to the shortest wire.
It has two channels+ common ground at the sensor body. If both channels are ok does not matter which way it is connected. If you had a new sensor how would you identify pins on it?
 

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Registered
1988 300SEL 158K;1980 450SL 31K
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630 Posts
Discussion Starter #20
today i started testing the fuel injection system, i started with 07.3-100;110;111;112;115;117;120;121;121 job #
everything withing the range, EHA valve, coolant temp sensor, replaced the ECU with a spare one, no difference. the duty cycle on idle is between 48%-54% and on 2500 RPM is 49%-55%, what am missing here :cry::cry:, EHA resistance 19.5 ohms, air volume meter sensor pin 1 ans 2 voltage 0.6V, RPM at 650,coolant temp sensor 2.6 Kohms at 20 degree and 295 Kohms at 80,no fuel pressure leaks, after 30 minutes system pressure didn't drop less than 2.6 bar. i will keep continue with the service manual, but WHAT I AM MISSING HERE ????
 
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