250SL engine start issue

[mobi1207]

Hello all
My 250SL has the following phenomenon. When starting a cold engine it takes 5 to 6 seconds before the engine fires up. The idle speed is not increased and the engine needs a few extra seconds before it runs really smoothly. From then on everything is ok and the engine turns nicely.
The following happens when starting a warm engine: the engine starts, after which the idle immediately drops and the engine stops. It will continue to run on a second start, but you will notice that it takes some effort. The idle speed with a warm engine is on the high side, about 1200min-1.
When I give a little throttle when cranking, both cold and warm, the engine starts immediately and runs neatly.
Anyone have an idea what could cause this?
Thanks in advance for your help!

[mrfatboy]

Is it an auto or manual? I would suggest checking the linkage first. Please see the linkage tour here in the forum.

[mobi1207]

Hello mrfatboy
Forgot to mention, it's a manual.
I will check the linkage and post my feedback here.
Thx

[mobi1207]

Hello again
I have done some checks today, to figure out what causes the starting issues mentioned above.
First of all, checked the throttle linkage, which seemed to be ok. Only finetuned one setting, in order to have the throttle valve fully opened at 100% throttle.
After that I have checked the Thermo Time Switch, which is also functioning correctly.
I have also checked the CSV and this one also does its job.
I have discovered a small device which function is not totally clear for me, especially when discovering the position of the bolt which should activate the microswitch.
Any ideas about this, as well as other possible options to make the engine start and run smoother?
Thanks in advance!

[mobi1207]

Hello all
Some more information on this.
The Cold Start Thermostat seems to be ok also, as I checked it with the small air muffler off, during warming up I hear the thermostat closing slowly.
I mentioned before that the idle is set quite high, at about 1200 min-1. I have now tuned it down to 800 min-1 at operating temperature and it runs really nice like that.
When shutting off the engine and trying to start it again, the engine fires up immediately but stalls directly also. When starting with a little bit of throttle everything goes fine and than it keeps running with no throttle, at about 800 min-1.
I will let it cool down now and see what it does when starting cold with this idle setting.
I am quite sure it won't fire up with no additional throttle, but we'll see what it brings.
I'm fully open to your inspiration.
Thx in advance!

[Pawel66]

I am not a mechanic, so do not treat me seriously.

I think the devise you are talking about is in the US cars and has something to do with the Emission Control System - I think I received this information when we were discussing bent gas pedal linkage rod in one of the threads.

It is a shot from the hip, but your symptoms remind me somewhat the issue I had that turned out to be not enough fuel getting to engine. Difficult warm start like you describe happens often due to a very lean condition of the engine at that moment. If fuel flow is disturbed - that is what you will have.

When you brought idle from 1200rpm to 800rpm - how did you do it? Did you re-set idle or just screw down the air screw?

I would follow the advise of the Members above, made sure that the tuning elements of the car are ok (ignition, valves, etc.) and linkage is ok. I would make sure idle mixture is set and check the spark plugs colour after driving. If the symptoms continue, I would probably do a fuel flow test (between say 0.7 and a litre pumped at the end of return fuel line at the tank in 15 seconds).

But again, someone more knowledgeable may say something completely different.

[mobi1207]

Hello Pawel
Thank you for your reply on my topic!
In the meantime I have checked many elements that can possibly influence the engine's behaviour, as described earlier. Linkage is set correctly and additional devices work properly.
In reaction to your question on the idle setting, I just needed to turn the air screw, which made the idle change immediately.
Today I have connected my air-fuel mixture tester. It is an Innovate LM-2 tester that shows the Lambda value, instead of a CO2-value.
With warm engine I am running at 0,76 at idle, which means too rich. I have now turned out the mixture screw on the pump by two clicks, resulting in 0,82.
This slightly improved the starting behaviour for now, but I have to test a bit more to be sure that this really is the appropriate solution.
I will have to let it cool down also, to check what this will do for the cold start behaviour of the engine. Normally, when starting cold it should benefit from a richer setting than the opposite...
So fingers crossed on this. I will post my feedback as soon as I know more.
Best regards
Hendrik

[ja17]

You may have a flooding problem causing your stall. Hook a test light to the CSV on the intake and also one on the cold start solenoid on the injection pump. Observe the function of both during difficult starting periods. Pay especial attention that the one on the IP de-energizes immediately after the key is released from starting. If the IP solenoid remains energized for an instant after the key is released, this could be your problem.