cold start valve - Mercedes-Benz Forum

 
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post #1 of 6 (permalink) Old 09-16-2016, 02:34 PM Thread Starter
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cold start valve

I now have my 1972 280 SE 4.5 back together after 5+ years of restoration and rebuilding. I finished putting the interior pieces in place, wood, glovebox, trim, etc.
The car has always started right up and never stumbled. Of course now that I am ready to make the maiden voyage, it begins to misbehave.
It has a hard time starting and staying running without keeping my foot on the gas. I have replaced the ignition wires thinking that maybe the issue.
I have inspected the wires to the injectors and the cold start valve and they look a bit tired.

My question is what symptoms would I be having if the cold start valve would need replacing?

I plan on at least replacing the plugs for the injectors and cold start valve eventually this winer, but would like to enjoy the car prior to opening up that project.

Sherif Tewfik

70 280 SE/cab
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post #2 of 6 (permalink) Old 09-16-2016, 05:05 PM
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Originally Posted by tewfiks View Post
I now have my 1972 280 SE 4.5 back together after 5+ years of restoration and rebuilding. I finished putting the interior pieces in place, wood, glovebox, trim, etc.
The car has always started right up and never stumbled. Of course now that I am ready to make the maiden voyage, it begins to misbehave.
It has a hard time starting and staying running without keeping my foot on the gas. I have replaced the ignition wires thinking that maybe the issue.
I have inspected the wires to the injectors and the cold start valve and they look a bit tired.

My question is what symptoms would I be having if the cold start valve would need replacing?

I plan on at least replacing the plugs for the injectors and cold start valve eventually this winer, but would like to enjoy the car prior to opening up that project.
The cold start valve is controlled by the thermo time switch and doesn't open unless the coolant temperature is below 35C. You could try spraying a little fuel into the intake and see if that helps. If it does, could be your CSV is not working. But not likely. You could try opening up the idle valve a turn or two once you get the engine firing.

Perhaps first check fuel pressure (put a gauge in the line going to CSV) Turn ignition on & off a few times then make sure you have about 30 psig. Could be your pump or fuel filter is partly blocked?

Also make sure your AAV (Aux air valve) is not stuck closed. It needs to be open when starting and then gradually closes as engine warms up. If not, you won't get enough fuel at idle. Best to take it off and test it in hot water or with hot air gun. Should be open when cold and closed at about 150F.

1972 350SL 4.5L - MBGraham, near Kingston, Ontario
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post #3 of 6 (permalink) Old 09-16-2016, 08:57 PM
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I agree with MBGraham. Opening the throttle to let in more air will not compensate for the cold start injector not supplying fuel. If the squirt of some fuel makes the problem less of a problem, then yes maybe cold start valve.

AAV does make more sense to be stuck closed in the warm position.

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post #4 of 6 (permalink) Old 09-17-2016, 10:18 AM Thread Starter
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Where does one find the Aux Air control on a m117?

Sherif Tewfik

70 280 SE/cab
72 280SE 4.5
78 6.9
80 280 SL
88 300 CE
91 420SEL
94 E500
95 E420
95 E320 Cab
95 E320 Wagon
05 E55 Wagon
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post #5 of 6 (permalink) Old 09-17-2016, 10:28 AM
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post #6 of 6 (permalink) Old 10-31-2016, 03:35 AM
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Hi Sehrif,

the cold start valve only injects fuel once, only during cranking with starter and limited in time by engine temperature. Above 35°C it does not inject at all, at -20°C it would be 12 sec. This is to compensate condensing fuel on a cold manifold. Your car should start without cold start valve and run smoothly, it should just take longer.

After start AAV increases rpm by more air and ECU sees a cold engine and increases fuel amount by up to 3 times, depending on engine temperature. Both AAV and fuel enrichment go gradually down with engine temperature.

Best regards, Dr-DJet



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