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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22907
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Hi My clk 270 is reluctant to start in the winter, from cold

Customer Question

My clk 270 is reluctant to start in the winter, from cold it attempts to start ( like it's using residue fuel) then stops and will not attempt to fire after. It's almost like a switch is not open? If i squirt easy start it will start and no problems after for that day
Submitted: 3 years ago.
Category: Mercedes
Expert:  Matt replied 3 years ago.

this sounds like its incorrect fuelling due to a faulty water temp sensor. If this is the problem then the ECU will never add the 'cold start' extra fuel required if the car starts OK when warm then I 'd look into changing the sensor.

the sensor is located close to the top hose of the radiator

if you follow the hose back to the engine then look around this area you should see a 2 wire sensor
Customer: replied 3 years ago.

I assume you mean the normal water temp gauge? This works fine and read accurately.

I agree it appears to be a problem when cold but still feel there a switch that isn't functioning as today when cranking the engine and not starting I let go of the key and it continued to crank and start, like a switch has been activated?

Expert:  Matt replied 3 years ago.

OK if thats OK then I'd check for an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine
Customer: replied 3 years ago.

Hi and thanks for your reply.

I was a Formula 1 engineer , I understand race cars but not much of road cars. Partly due to lack of time and partly due to not having the electronic diagnostic equipment. right now your saying, well chuck it into the local Merc dealer, let him sort it? But I'm the sort of guy who needs to understand whats going on and this is frustrating. For me the problem is like a switch does not activate (maybe extra fuel to start) but I don't think so as the car initially starts but won't be caught (like its burnt the residual diesel left)? The other day I tried to start it and churned over for ages, when I let go of the key, it continued to turn over and start> Like a switch had been turned? OR fuel had not reached pressure?

Thanks for your input


Expert:  Matt replied 3 years ago.

I'm not saying just put it into the dealer, someone with your technical know how should be able to go a long way into the diagnosis and we'll hopefully sort it

I'd agree that measuring fuel pressure is a good idea as it could be the pump is a bit tired or the filter is clogged so its tkaing a little while to get pressure up

there should be a test point on the injector rail tha looks like a tyre valve - this is for measuring pressure ( with the engine running) as it shoudl be about 3 bar

and also check for air leaks as I've suggested
Expert:  Matt replied 3 years ago.
do you still need help with this?