[QUOTE=tangofox007;4231193]When adding coolant to a cold engine, is there anything to prevent coolant from filling the engine via the built-in bleed passage? Is there anything that obstructs displaced air from being bled to the radiator and then the reservoir?

When you add coolant to the engine the water level rises from the bottom of the radiator and engine. If you look at the thermostat housing there is a short fat rubber hose. As coolant rises, any air between the short hose and the thermostat is trapped UNLESS the thermostat itself can bleed air to the front of the thermostat.

A correctly built Mercedes thermostat will provide an air bleed built into the thermostat so that when the engine is cold air can bleed to the front of the thermostat housing, then bleed to the front thermostat housing and into the cylinder head exit.

If you take part the thermostat housing you will see that the back side of the thermostat is sealed off from the front side. If air accumulates on the back side of the thermostat there is nowhere for it to go unless the thermostat is open, or unless the thermostat has a built in air bleed.

With the engine cold, what would prevent air anywhere in the engine from escaping via either the bypass passage or the head passage to the upper radiator hose? (The bypass passage is isolated from the head passage only when the engine is hot and the rear poppet of the bypass thermostat is fully deployed.)

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If you look at the bypass passage built into the thermostat you will see that it only bleeds air from the FRONT of the thermostat. Any air on the back of the thermostat is trapped unless the thermostat itself lets the air bleed through.

When the engine is cold, what is the condition of the thermostat rear poppet? Is it open or is it closed?

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When the engine is cold, the poppet is closed.

The front poppet is closed; the rear poppet is open. With the rear poppet open, any air behind the thermostat would have no difficulty escaping via the upper radiator hose.

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No, the bleed to the upper radiator hose is sealed from the rear thermostat compartment when the thermostat is closed. The wax element on the back of the thermostat will sit in an air bubble until the thermostat opens UNLESS the thermostat itself provides the air bleed passage.

It is clear that one of us doesn't understand the finer details of the dual-poppet bypass thermostat as well as he thinks he does.

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Anyway, today I tried installing a new thermostat with no success using my vacuum bleeder. You think a vacuum bleeder would solve any problem, but the car still over heated. I drilled a 1/16 inch hole where the arrow is on the thermostat and reinstalled it and pulled a vacuum on the system. Problem solved. So all you need is a 1/16 inch hole to replicate the same effect the factory put in there with their "ding". If a person has no problem getting their car bled, then there is no reason to drill any hole. However if you are in a situation like the OP or myself, then drilling a hole in the top of the thermostat can get you moving again.

Gonna play devils advocate here. I have yet to hear from anyone in this thread what exactly the devastating effects of drilling a small 1/16th inch hole would be. Anyone? Can't see how it could be any worse than running without the thermostat altogether (which has been advocated in a few threads over the years).

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Certainly a small hole in the thermostat would not have a devastating effect. At worst, it would permanently delay engine warm-up every time the engine is operated from a cold condition. But how much sense does it make to compromise the thermostat permanently to solve a very temporary problem?

Here is a direct quote from the OM617.95 Engine Manual:

The thermostat housing is suspended and has an integrated, independent venting system. The air will flow to the radiator and expansion tank through a bore which bypasses the coolant thermostat.

This venting method has the following advantages:

a) When filling the coolant, the cooling circuit will be automatically vented.
b) Better continuous venting of the cooling circuit when engine is operating.

I should add that the notion that air can be easily trapped behind the thermostat is disproven by the fact that the bypass thermostat can never be closed on both sides; by design, one side is always open.

Anyone advocating "running without a thermostat" completely fails to understand the operation of a bypass cooling system. And the same is probably true for those who recommend adding holes to the thermostat.

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In fact, the OP did not "get moving again" by adding a hole to his thermostat. He replaced a defective thermostat with one that functioned properly.

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Pic of OEM Wahler/Mercedes thermostat:
(Obviously not "Detroit Diesel"; ebay listings contain many extraneous words.)

Attached Thumbnails

 

The thermostat specified for the 617.95 engine has a number of applications, including at least seven other vehicle makes. Some of those applications may very well require the vent in the thermostat. The 617.95 engine absolutely does not. The built-in bleed passage provides exactly the same benefit as a hole in the thermostat. Any argument to the contrary suggests a flawed understanding of the operation of the dual-poppet bypass thermostat.

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There is no flawed understanding of a dual-poppet bypass thermostat. There is a flawed understanding of the formation of an air bubble in the REAR thermostat compartment when filling the system with coolant. The built in bleed passage only bleeds the FRONT thermostat compartment. JUST LOOK AT IT, where does the air go? Why are there symptoms of the thermostat not opening after filling the system with coolant? It is because the REAR thermostat compartment contains a big air bubble.