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Discussion Starter · #1 ·
Hi all,

I currently have a '96 c200. lately I experienced an overheating problem.. temp was over 120 deg centigrade. as a result of a failed water pump. this was fixed and temp got to normal.
I bought the car used and with no updated car history aside servicing et al.

Now, I have tapping/knocking sound whenever I am on gas, and the sound is more when the engine has warmed up. when I start in the mornings and press the gas, I hardly hear this sound.
Also it revs excellently with no sound and idles very well.

My mechanic says its most likely to be the temp switch located just after the thermostat with four/4 pins.. (pls whats the function of this switch). I have also noticed increase in fuel consumption.

Please Advice so I don't make a mistake as I am very new to Mercedes.
 

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Discussion Starter · #4 ·
@ wichipong, its only when the I need to move with the throttle pressed. as in, when I tell the car to move...

u suggested it could be the tensioner, but I think the sound is from the engine. because when I drive in the morning with the temp just starting to rise, the sound isnt noticed or heard.. and it seem to come from the engine.... sound is similar to that from a cars that uses manual ignition when the setting is high... I just pray someone understands what I am tryn to explain...

Thanks all- pls help!
 

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@ wichipong, its only when the I need to move with the throttle pressed. as in, when I tell the car to move...

u suggested it could be the tensioner, but I think the sound is from the engine. because when I drive in the morning with the temp just starting to rise, the sound isnt noticed or heard.. and it seem to come from the engine.... sound is similar to that from a cars that uses manual ignition when the setting is high... I just pray someone understands what I am tryn to explain...

Thanks all- pls help!
had a problem that sounded like that a while back. sounded like it was a broken link in the chain. turned out to be a seized pulley on the alternator.
 

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Discussion Starter · #6 · (Edited)
thanks alliv, is your make same as mine?.... and how did u resolve this? ......

when u release ur foot from the throttle pedal a little the sound reduces or stops. this really limits my movt cos I fear it might damage the engine, and I fear most, I might not be able to move properly if the need arises...
 

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thanks alliv, is your make same as mine?.... and how did u resolve this? .
i,m driving a 1997 c250. i had one guy wanted to change the timing chain thats how much it sounded like it was an engine noise. if you slack off your belt and grab the pulley on the alternator and try move it, if its tight have it looked at. i just took out the alternator and took off the pulley and replaced it. its 80 euro over here about $60 i,d say. but thats if its the same problem. its called an alternator clutch pulley.
 

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Discussion Starter · #8 ·
Thanks again Alliv... I would do as suggested, ...........

but I would be damn surprised if that's what could make such sound as if its coming from the engine, I would let u have d feedback asap..
 

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I think that wichipong is providing a good idea. The belt tensioner has a shock absorber on it. The bolt that goes through the top eye of the shock absorber has a rubber washer that wears out with time, allowing additional flex between the upper shock mount and the engine. Remove this bolt and add a washer then reinstall. This additional washer will pick up the slack/flex that is causing that noise. This is a very common issue on cars with the belt tensioner setup that includes the shock absorber. Very bad examples can almost sound like diesel engines at idle.

It will take you 5 minutes with simple hand tools and cost you less than a dollar. It is worth a try before you start worrying about catastrophic engine damage.
 

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Discussion Starter · #10 ·
Thanks Jlomon... I would try what u said....

however, it revs perfectly at idle without load.. the noise is observed when on drive or when u attempt to move suddenly, or when gear changes from low to high gear- (say 2-3 or 3-4)..

Most rampant when engine has warmed up..
 

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Thanks Jlomon... I would try what u said....

however, it revs perfectly at idle without load.. the noise is observed when on drive or when u attempt to move suddenly, or when gear changes from low to high gear- (say 2-3 or 3-4)..

Most rampant when engine has warmed up..
really suspecting the belt tensioner... get/have that checked then fixed then you should be back to good...
 

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You don't say how long you ran it when overheating - if you drove it when badly overheated that noise could be big end bearings - used to be common in old worn cars, they would rattle under load, esp. with hot oil - (your mechanic would have picked that?)
Not so common these days with better engineering and synthetic lubricants.

Oberoi
 

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Discussion Starter · #13 ·
Oberoi, thanks for your response. although I am yet to get a mechanic to check the tension-er and alternator pulleys as suggested by great forum.. but your comment is actually what I was expecting to hear, I drove the car for like 10miles while it was overheating...(this makes me feel so bad now) cos it was a huge mistake, I wasnt expecting the water pump to go gaga just like that, It was after this overheat I could recall I started having this Tapping/knocking sound while accelerating.
 

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Zotye Auto 1.5T T600 2016
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Two possibilities here, big end bearings or pre ignition. Pre ignition will make the car use more fuel and the temperature sensor is the culprit. It is sending wrong signals to the ECU which will put more fuel into the chambers and this creates the pinking/tapping sound.
 

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Discussion Starter · #15 ·
@ eric242340.... Pre ignition will make the car use more fuel and the temperature sensor is the culprit. It is sending wrong signals to the ECU which will put more fuel into the chambers and this creates the pinking/tapping sound

- that confirms what the mechanic earlier said, also my fuel consumption is kinda poor now unlike when I can drive 153 urban miles with less than 25 litres.

pls eric242340.. would you suggest I go ahead to change the sensor as also agreed by the mechanic? the mechanic says the bearings are ok. I just called him.
 

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@ eric242340.... Pre ignition will make the car use more fuel and the temperature sensor is the culprit. It is sending wrong signals to the ECU which will put more fuel into the chambers and this creates the pinking/tapping sound

- that confirms what the mechanic earlier said, also my fuel consumption is kinda poor now unlike when I can drive 153 urban miles with less than 25 litres.

pls eric242340.. would you suggest I go ahead to change the sensor as also agreed by the mechanic? the mechanic says the bearings are ok. I just called him.
I have to agree with your mechanic, change the sensor and see.
 

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Discussion Starter · #17 ·
Thanks alot eric242340... ah! I see u r even the moderator.... hmm, I am quite hopeful, I would be meeting the mechanic in 5hrs time... and would sure relay the outcome to u all..
 

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Thanks alot eric242340... ah! I see u r even the moderator.... hmm, I am quite hopeful, I would be meeting the mechanic in 5hrs time... and would sure relay the outcome to u all..
I am the mod for the W221/251 forums but I am a workshop manager if that helps lol:D
 

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ive actually been having a similar issue. my engine was making the "valve noise" when i would accelerate. but, now it also makes a noise all the time, even when idling, kind of like a knocking or rapping. ive changed the entire tensioner assembly, changed all of the coolant sensors except the one for the gauge since it reads fine and doesnt control the air/fuel ratio. i thought my cat may have clogged the exhaust because it was bad, so i changed the entire exhaust, ive changed all the sensors in the air intake (maf and air temp). ive tried almost everything i think. the sound sounds like its coming from the engine. my next step is to remove the valve cover and inspect the timing chain. ive heard that the guides can become worn with age and heat. oh and ive also replaced the timing chain tensioner too...

any other ideas? i think i will try rotating all of the pulleys prior to removing the valve cover. i have noticed a drop in engine performance, it feels sluggish, especially at low rpms and i have increased fuel consumption. i was at 28mpg a few years ago and it has slowly become worse down to about 21 mpg now.
 

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^^Sounds mechanical to me, it is not pre ignition as this does not happen at idle speed but only on acceleration.
 
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