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1994 C280
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11 Posts
Discussion Starter · #1 · (Edited)
}= 1994 Mercedes Benz C280 Knocking Sound ={

The auto starts and drives without problem with the excepton of a knocking sound coming from the engine. After a while something cuts off (I belive compressor) and the knocking stops. Once the unit kicks in again, the knocking starts again and continues in this cycle.The car drives fine with the exception of the knocking, which I can hearing from inside the car. The knocking starts off loud on initial start, after running the car for awhile the knocking quiets, but never stops until some unit under the hood cuts off.

I beleive it may be a valve knocking, how can I verify this and does any one know how to pinpoint and replace valve.
 

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1998 c230 170K
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1,415 Posts
Do you mean the a/c compressor?

}= 1994 Mercedes Benz C280 Knocking Sound ={

The auto starts and drives without problem with the excepton of a knocking sound coming from the engine. After a while something cuts off (I belive compressor) and the knocking stops. Once the unit kicks in again, the knocking starts again and continues in this cycle.The car drives fine with the exception of the knocking, which I can hearing from inside the car. The knocking starts off loud on initial start, after running the car for awhile the knocking quiets, but never stops until some unit under the hood cuts off.

I beleive it may be a valve knocking, how can I verify this and does any one know how to pinpoint and replace valve.
Its quite possible that it is the a/c compressor on the fritz, verify this at idle with the a/c cycling, most likely you will be able to hear the sound from that area if that's the case.
Also you can put your hand on the valve cover and feel for any abnormal knocking, if so do a compression test via the spark plug holes. If there is an issue there a valve job would be in order. B
 

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96' C220
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1,275 Posts
Knock Knock

Who's there???

Maybe some good news for you...:)

Does the sound only happen when the engine is cold and for a short time, like a couple of minutes??? If so, it may only be your Air Injector pump. If the sound is coming from inside your engine... Have you checked your oil level lately?

My guess from your description is your Air Injection Pump is on its way out. This pumps runs for a while after cold engine start to infuse the Catalytic Converter with oxygen. Then it shuts off until the car cools off again. Raise the hood and start the car from cold like you normally would. The pump is down under to the left on my '96 C-220, (facing the car) probably similar on your 280. See if you can tell that's where the noise is coming from. It's better to be the Air Pump than the Engine. It's not a cheap part but not too hard to replace if you are a DIYer. The part is available online.

After reading bmeister's writing and rereading the problem... bmeister is spot on! I misread the symtoms. If the edit screen would let me I would place a bowdown to bmeister!

The a/c system may be low on lubricant, but more likely the compressor is dying... Stop using the a/c by hitting the EC button until you can have an a/c tech check it out. If you hit that button while you are hearing the knocking and it stops... you've found your problem.

Good Luck!:thumbsup:

C as in 220
 

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1994 C280
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11 Posts
Discussion Starter · #4 ·
When I initially start the car, the engine races high and quickly returns to a normal idle. It has a little over 108,000 miles and gave little to know problems with the exception of the wiring harness.

I will take both your advice and check tommorrow morning, its late here and I did have the climate control set to 72 degress and AC was running. Hope your right and I do not need valve job, I love this car and it been very reliable.
 

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'99 2.3L Kompressor
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While it is knocking, press your EC button to turn the air-co off. If it stops, then it is likely to be the compressor (had this problem last year!). If it doesn't, then you need to check things like what c220 said or one of belt tensioners, etc.

I tried to see if I could extend the life of the compressor buy having it recharged. Unfortunately, it didn't help and I was out the cost of the recharge. Once they are knocking, they are on their way out. The longer you use it, the more chance of shrapnel getting put into the system and clogging up other areas, so I would not run it if possible. Go to an indy MB mechanic (good one) and save yourself a bundle over the stealership!
 

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96' C220
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1,275 Posts
Thank You for posting your result

ChicagoMB,

Thank you for Posting your results!:thumbsup: This will help many MB owners with the same symtoms when searching the site.

Good Luck!

C as in 220
 

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97'C280
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3,184 Posts
does the air pump work in conjunction with the compressor? because I have the exact same problem. the a/c will not get cold at all and the guage reads well over 100, which is dangerous. I think I have air in the system and not an air pump problem. can anybody verify this?
 
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