My '94 E220 is a bit noise until it reaches full temperature. It's some kind of hard rattle that disappears when either the RPM is brought over 2000 RPM or the engine reaches normal temperature.

I was starting to believe that something was wrong with the camshaft bearings or some lifter machanism until I used the stetoscope. The noise most certainly seems to origin from the thermostat.

The thermostat is only 6 months old. When opened, it seems OK. Nothing loose and the spring is also OK.

Could it be the thermostat and what should I do with it?

Rgds,
Gunnar Woldmo

Hard to imagine the thermostat making noise. I would suspect the belt tentioner or timing chain.

Same here.
I think your stethiscope may be picking
up noise transfer to the thermo housing.

More noticed right at start-up?

Noise from coolant thermostat ?
 

I 100% agree with both of you. Thermostats don't make noise, but:

I've listened through a stetoscope and a long screwdriver on every piece around the top cover, front cover, belt tentioner, water pump and servo pump. The noise (rattle or hard knocking) is many times the frequency of each valve lifter. It's not timing chain rattle as this would change character on sudden increase and decrease of RPM. The noise don't change in volume level if I open the oil filler cap.

And, the noise is many, many times higher when listening to the thermostat housing as listening to any other part.

The '94 engine don't come with a traditional thermostat. The thermostat and the housing is a complete unit that's impossible to separate. I was just wondering if perhaps some kind of clogged bypass could result in pressure build-up and the membran (valve) vibrating. Or perhaps if air in the system could cause something similar.

Strange - but still not convinced :)

Gunnar

If you hear a rattle, like one or more small ball bearings or other metal objects knocking around in the area of the water and power steering pumps, immediately look for dislodged bolts near the water pump pulley.

I had this problem for the past two or three days -- a rattling noise emanating from the waterpump or thermostat that got progressively worse. After watching and listening to the running engine for a while, I noticed the bolt on what I believe is the horizontal part of the belt tensioner had worked its way out, to the point where its head was bumping against the water pump pulley, causing the rattle. It was also in an absolute perfect position to fall directly onto the belt, and lodge under the pulley. A few cranks with the 13mm ratchet, and the rattle was gone.

I estimate I was a day away from having it fall out completely. Had that occurred, there would have been two likely outcomes:

1) It would have bounced off the belt and fallen harmlessly to the engine pan.

2) It would have fallen onto the belt and lodged under the water pump pulley, causing the belt to jump off. Depending on engine RPM's at the time, there would have been minor, or major consequence -- and I would never have known what caused it, which would have translated into a big diagnostics and "repair" bill :eek: