C240: Engine knock when starting warm?
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From: Gaithersburg, MD
2002 C240 6-spd (ret)
C240: Engine knock when starting warm?
I've had my C240 for about 3 months now, and I've got a question for other C240 owners that I asked some time ago -- thought I'd try again. On occasion, when I start up, I hear what sounds to be 2-3 engine knocks. This never happens during a cold start. It can happen when the engine is already warm, and has sat (shut off) for as little as 20 minutes, or up to an hour or two. I have about 3K miles on the car, and it has done this since new. I've sort of discounted it as a problem because I can't believe it's an oil or clearance related problem since it occurs after such a short "stop" time. Has anyone else noticed this?
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From: Vancouver Island, B.C. Canada
2005 smart cabrio; 2008 Mercedes-Benz B 200
Knock knock
I once had a new (0 km) Renault Supercinq TR in Europe that did just that - it had a metallic engine knock upon start-up. Over 10,000 km the problem never went away.
I only had the car for 6 weeks (purchase-repurchase plan) so didn't bother looking into it. But it sure was not confidence-inspiring for the long haul.
It is possible that it could be indicating a problem with lack of proper oiling (oil passage blockage, oil filter drainage at shutdown, low oil level, too thin an oil for bearing clearance). If the engine bearing clearances are near the larger end of factory tolerances this would be especially noticeable. If it's a bearing it'd be one of the con rod big ends.
It is also conceivable that the noise could be a valve lifter that's failed or even the timing chain tensioner. It all depends on the character of the noise, and a con rod noise is quite distinctive if you know what you're listening for.
I would go to the dealer right away about this, tell them that you are not satisfied with the noise and want something done about it. They'll probably be reluctant to pursue it, especially if it's an internal engine problem.
I'd follow it up with a letter to both the dealer and service department if you are not taken seriously. These cars are FAR too expensive to have to put up with a noise that may be indicating long-term reliability problems.
Good luck.
I only had the car for 6 weeks (purchase-repurchase plan) so didn't bother looking into it. But it sure was not confidence-inspiring for the long haul.
It is possible that it could be indicating a problem with lack of proper oiling (oil passage blockage, oil filter drainage at shutdown, low oil level, too thin an oil for bearing clearance). If the engine bearing clearances are near the larger end of factory tolerances this would be especially noticeable. If it's a bearing it'd be one of the con rod big ends.
It is also conceivable that the noise could be a valve lifter that's failed or even the timing chain tensioner. It all depends on the character of the noise, and a con rod noise is quite distinctive if you know what you're listening for.
I would go to the dealer right away about this, tell them that you are not satisfied with the noise and want something done about it. They'll probably be reluctant to pursue it, especially if it's an internal engine problem.
I'd follow it up with a letter to both the dealer and service department if you are not taken seriously. These cars are FAR too expensive to have to put up with a noise that may be indicating long-term reliability problems.
Good luck.
Thread Starter
MBWorld Fanatic!
Joined: Nov 2001
Posts: 1,230
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From: Gaithersburg, MD
2002 C240 6-spd (ret)
Thanks for the input. I guess I've sort of been "in denial" about this noise. I should have taken it in to the dealer sooner, but I've convinced myself that hey, it's an M-B, it must be normal!
When it happens, it's a "deep" knocking sound, not a light pinging that occurs when using low octane gas. It doesn't sound like a valve train noise, but deeper in the engine. Since it never happens w/ a cold start, I've figured how can it be a lube problem since I would think that would be much more likely to occur after sitting for many more hours and the oil drains completely. I've never heard the noise while the engine is already running -- it just makes this knock sound 2-4 times after cranking, and like I said, this can happen after the engine has been shut off for only 15 minutes or so. I've even thought that it might be some accessory function, like an air pump or something. Anyway, I think I'll at least call the service guys Monday and ask if they've heard of this and then make an appointment.
When it happens, it's a "deep" knocking sound, not a light pinging that occurs when using low octane gas. It doesn't sound like a valve train noise, but deeper in the engine. Since it never happens w/ a cold start, I've figured how can it be a lube problem since I would think that would be much more likely to occur after sitting for many more hours and the oil drains completely. I've never heard the noise while the engine is already running -- it just makes this knock sound 2-4 times after cranking, and like I said, this can happen after the engine has been shut off for only 15 minutes or so. I've even thought that it might be some accessory function, like an air pump or something. Anyway, I think I'll at least call the service guys Monday and ask if they've heard of this and then make an appointment.
