Cold Start Question? - Mercedes Benz SLK Forum

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#1 Old 04-10-2011
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Question Cold Start Question?

I've noticed that many vehicles including the SLK run at slightly higher RPMs for a minute or two after a cold start, then settle at low idle.
Way back in BFI (Before Fuel Injection) times, carb equipped engines had an auto-choke that kept the idle speed up to avoid rough idle and/or stalling after cold starts. Emissions were not an issue then.

Regarding modern fuel injected engines....is this just an emissions thing or is it tied to some other engine function(s)? Is it fair to say one should wait until the idle settles down before driving away?


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#2 Old 04-10-2011
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The owner's manual for our cars actually states NOT to sit idle to warm up the car. I do as the manual says and wait until the RPMs get to below 1000 (it usually revs at about 1200 for all of about 20 seconds) then drive away. I don't do an instant gear change because I know that my tranny doesn't like it too much (I've felt a nice hard CLUNK once or twice on super fast "gotta get going I'm late for work!" days.)

With regard to cold starts... I'm not a technician and could probably be telling you wrong information but with the engine block still being cold, compression ratios are probably being thrown a little bit out of whack until the engine temperature actually warms up. That's why we have the high rev at startup. At least it's my theory.
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#3 Old 04-10-2011
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When you start a cold engine, some of the petrol injected by the injectors condenses on the cylinder walls. Less fuel stays mixed with the air, which weakens the mixture. To overcome this and ensure a rapid start, an extra supply of fuel must be provided.



In some cases, during engine cranking, extra injection pulses in each revolution can provide the extra fuel. It depends on engine temperature, and there is a time limit to prevent flooding. The cranking period is followed by an after-start enrichment. Over about 30 seconds, this slowly reduces to normal warm-up. The engine then responds steadily, immediately after releasing the starter.


More air comes from the bypass air control valve. This bypasses the throttle valve, to raise the engine’s idle speed when it is cold, and during warm-up.



As noober says, you don't want the car to sit idling whilst it warms up as the condensed fuel will either leak past the rings into the oil, or out into the exhaust, possibly damaging the cat. If you have a manual transmission, you don't get the 'clunk' when setting off whilst the engine is cold






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#4 Old 04-10-2011
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Fuel is made from a range of hydrocarbon components (about 2000 in gasoline) each of which has a different boiling range, but also different combustion properties. The proportions of these components blended into the fuel is called the formulation. The formulation varies from fuel to fuel, country to country and even in some countries from season to season. The 'lighter' components, for example butane and pentane evaporate quite easily and are usually inclused in the formulation to help with cold starting. As Woolly says, some of the fuel i.e. the heavier components don't evaporate when the engine is cold, because their boiling point is too high for the cold conditions. So as only some of the fuel is made up of the lighter components, during a cold start more fuel is injected to compensate for the fact that some of the fuel won't fully evaporate. Liquid fuel doesn't burn of course.

With drive-by-wire throttle control, not all systems have by-pass air valves today. Essentially the ECU can just open the throttle a tad to allow a faster idle speed when the engine first starts up. This helps raise the exhaust temperature which in turns helps to warm the catalyst up so that it can manage the unburned hydrocarbons and CO that result from running rich for the cold start. So your point about emissions is a good one.

About 10 years ago in Europe, during an emissions test the engine was allowed to idle for 40 seconds before the exhaust gases were collected, followed by another 11 seconds before driving away. Today the exhaust gases are sampled from the moment the engine is started. This makes quite a difference to what the engineers can do with the cold start strategy. Also the test requires the car to start moving after 11 seconds, so no need to idle for a significant period before driving away.

To comply with US regulations, the car's emissions are assessed using the FTP75 drive cycle. I'm not as familiar with this cycle as the European one which is called the NEDC. I think it's fair to say that while 'emissions cycle beating' ECU mapping is not allowed, it would be reasonable to expect MB to map the ECU to match the market into which the car is being sold. Not only are the emissions cycles different, but the fuels and oils are different across the globe too.

Anyway, blathered on enough. Hope that was helpful in some way..
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#5 Old 04-10-2011
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Fuel is made from a range of hydrocarbon components (about 2000 in gasoline) each of which has a different boiling range, but also different combustion properties. The proportions of these components blended into the fuel is called the formulation. The formulation varies from fuel to fuel, country to country and even in some countries from season to season. The 'lighter' components, for example butane and pentane evaporate quite easily and are usually inclused in the formulation to help with cold starting. As Woolly says, some of the fuel i.e. the heavier components don't evaporate when the engine is cold, because their boiling point is too high for the cold conditions. So as only some of the fuel is made up of the lighter components, during a cold start more fuel is injected to compensate for the fact that some of the fuel won't fully evaporate. Liquid fuel doesn't burn of course.

With drive-by-wire throttle control, not all systems have by-pass air valves today. Essentially the ECU can just open the throttle a tad to allow a faster idle speed when the engine first starts up. This helps raise the exhaust temperature which in turns helps to warm the catalyst up so that it can manage the unburned hydrocarbons and CO that result from running rich for the cold start. So your point about emissions is a good one.

About 10 years ago in Europe, during an emissions test the engine was alloiwed to idle for 40 seconds before the exhaust gases were collected, followed by another 11 seconds before driving away. Today the exhaust gases are sampled from the moment the engine is started. This makes quite a difference to what the engineers can do with the cold start strategy. Also the test requires the car to start moving after 11 seconds, so no need to idle for a significant period before driving away.

To comply with US regulations, the car's emissions are assessed using the FTP75 drive cycle. I'm not as familiar with this cycle as the European one which is called the NEDC. I think it's fair to say that while 'emissions cycle beating' ECU mapping is not allowed, it would be reasonable to expect MB to map the ECU to match the market into which the car is being sold. Not only are the emissions cycles different, but the fuels and oils are different across the globe too.

Anyway, blathered on enough. Hope that was helpful in some way..
What he said

I've being dying to do that for ages- just quote a load of tech info and then say 'what he said'

My tech background is with large, (as in huge(as in the size of a cathedral)) 2 stroke diesel engines on ships (the HFO that we use could be carved with a knife at room temperature).

Getting them running (either ships or SLKs) is similar - over-fuel them and increase the RPM till they're running, but the bottom line is just fire them up and go for it.






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#6 Old 04-10-2011
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Thanks guys! That explains it! I never let the car warm up sitting still....mainly wondered about that first few seconds.


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Yes, it's inherent in the nature of gasoline engines where the fuel comes to the cylinder "indirectly" that the "choke" function is required and it's universal that this function means richer mixture combined with increased throttle opening. So yes, you'll get higher than normal warm idle rpms when the engine's cold.

Were you really old like me, you'd remember the "fast idle cam" on carburetored engines.

I suspect but have read nothing specific that the gasoline engines now coming online (in a rush) that are direct fuel injected may not have such requirements. Sorry, no 1950s 300SL experience.
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#8 Old 04-11-2011
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i've always heard warming up a car in idle builds carbon on the spark plugs??
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#9 Old 04-11-2011
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Direct injection gasoline engines still require fuel to be in the gas phase in order for it to ignite. So some enrichment is still required on a cold start for the reasons mentioned above, i.e. the fuel doesn't have enough volatile components to mix fully with the aircharge. Some fuel still condenses out in a cold engine.

Direct injection is pretty useful way of making sure that all of the fuel at least makes it into the cylinder instead of being hung up in the inlet system.
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