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  #1  
Old 04-30-2006, 11:11 PM
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C230 detonates at idle

C 230 detonates at idle -only when hot from sitting in traffic. If I turn on the heater temp comes down 10 degrees C and the detonation stops. At highway speeds the temp comes down. Detonation does not continue during acceleration - engine controls seems to be able to compensate. Parts changeout has been as follows: 1. Timing chain tensioner (when I thought the detonation rattle was mechanical) No joy 2. New thermostat no change in operational temps and no joy with the detonation rattle. 3. Fan control unit - no change in operational temperatures and No joy with the detonation rattle. 4. EGR valve and vacumn distribution valve for egr control (An egr code came up) No carbon cloggin was evident and No joy with the problem.
At highway speeds temp gauge reads about a sixteenth of an inch above 80 so I figure the gauge is showing a little cooler than the actual temp and actually gets a little hotter than 100 C in traffic (95 - 97 on the gauge).
THIS IS MAKING ME CRAZY - should I accept the little bit of detonation at idle? My training says detonation is destructive and a no - no. Helllp!

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Old 04-30-2006, 11:45 PM
t walgamuth's Avatar
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well, probably 100 is too high.

i would look at the cooling system. radiator fins clean? all fans working? mechanical? aux.? water pump?

iif you get it down to 80 or 85 in traffic you prob will lose the problem.... and get some joy.

tom w
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Old 05-01-2006, 06:47 AM
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what year is your C230?
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Old 05-01-2006, 08:59 AM
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What type of fuel are you running? Does bumping the octane up to the next level have any effect?

Also keep in mind a lean mixture can cause pinging. Has CEL illuminated recently and/or are there any stored codes? How is timing at idle?
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Old 05-01-2006, 10:45 AM
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The fan did seem to be a little anemic but spins free - As I said I changed the control unit how would I test the fan out put? A bad water pump would overheat at highway speeds this one doesn't. The radiator seems to get hot all over so I'm betting there's no clogging - although the car is a 1997 it has low mileage - 60,000 and everything seems quite clean. Shouldn't you hear the fan spin up when the engine temp gets around 100 ? I don't hear any fan noise although you can see it spinning. There was some indication that octane makes a difference. I always burn 93 but when the oxygenate additives came in with the warm weather the problem got a little worse. I changed gas stations and that seemed to improve things a little. Hey what
happened to 97 octane I can't seem to get it here in New York City anymore. Used to be that Sunoco sold it - but no more.
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Old 05-01-2006, 11:18 AM
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Detonation at idle, but not under load is unheard of. I suspect what you think is "detonation" is caused by some mechanical issue. Big four cylinder engines, especially those that don't have balance shafts, generate vibration modes that cause all kinds of mechanical "rattles" especially as the car get older and isolation mounts age and loose their damping characteristics.

Normal operating temperature is in the range of 80-105 degrees. The thermostat should open about 87C, but they tend to "drift" to a lower opening point - about 80C- as they age, which is really no cause for concern.

On most models the viscous fan clutch tightens at about 100C, but the engine driven fan flow at idle speed in hot weather is insufficienct to maintain 100C, so the temperature will continue to climb until about 105C, when the electric fan(s) engage, which rapidly cool the engine down to about 100C where they disengage.

This cycle will continue until you get up to speed where there is sufficient dynamic flow to keep the engine under 100C.

Duke
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Old 05-01-2006, 03:34 PM
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Hello Duke and others following this particular conundrum-

This car has no viscous clutch or fan on the engine only the electric fan.
It still leaves your call that the problem is the electric fan possible. It would have to be a qualitative failure of the fan since the system stays below 100 c
and this includes my discount of the temp gauge - the visual reading on the temp guage is about three degrees lower (97 C) and could possibly be accurate. My sense is that the temp is within spec for MB. Personally I would prefer the engine to stay a few degrees cooler but I'm just the ignorant driver not the engineer. THE NOISE IS DETONATION. First, it's a really distinctive sound - I also confused it for a mechanical noise but once I entertained the thought it was detonation it clicked perfectly as matching other engines I have heard that were detonating. Now I know you may scoff that this is a subjective judgment so here's the hard evidence. It's easy to lower the engine idle temp four or five degrees by turning on the heater. As soon as the temp drops a few degrees the detonation stops and the engine sounds sweet. I had a CEL light come on during the middle of this
(the problem has existed for 5 months). The cel coded for EGR. Of course EGR
failure can cause detonation. Changeout of the EGR and Vacumn distributor switch that controls the egr did not stop the detonation at idle but stopped it when you accelerated. Of course the egr is inactive at idle so as to prevent stalling - It kicks in when you accelerate to cool combustion. The second piece of evidence that strongly suggests detonation is the noise is that it is octane sensitive. 91 octane makes more rattle than 93.
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  #8  
Old 05-01-2006, 04:16 PM
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It may be worth running a can of combustion chamber cleaner through the engine.

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