An Experienced Mechanics Views on Overheating, Thermostat Replacement and Modification

This pictorial is to show you how to modify your new thermostat to keep your temperatures down if you are having overheating problems. The part about modification, is really a work around fix to a more difficult set of problems.
 

I have never seen auto maker more plagued by engine heat control problems than Mercedes. In my opinion the Mercedes have cooling systems that are just enough when in "new" condition to get the job done. In all fairness most of our beloved cars are 20 plus years old, and still do a good job, but the cooling system is not transferring the heat from the engine to the air, with the best of efficiency. You can change water pumps, radiators, fan clutches, t-stats, do a coolant flush with acid, temp sensors, temp gauges, and on and on, till your are blue in the face. Even after all that, SOME of the cars are just not going to run at the proper temp. Overheating your car is a very bad thing. You can blow head gaskets, warp heads, seize bearings, and a whole host of other problems. So thermostat modification is a good way to make your car drivable again and give you peace of mind, if you have tried just about everything else.

My recommendation on t-stat modification is that it is a LAST ditch approach. Replace or repair all the items I listed a few lines above, before you do this modification. You may find that one of them was your problem. Unfortunately none of them solve my problem, so I did the t-stat modification.

I came up with this modification after racking my brain on how to get my ole '84 300d to run between 82-90 degrees Celsius. After the modification, I could NOT get the car to overheat even with the air on. The only draw back to this modification is that, the car does take longer to get up to operating temp. So if you are in a cold climate I will make some suggestions in the pictorial on what you can do.

Now understand that this modification is not a Mercedes fix, it is my fix. It keeps the car running at the proper operating temp and that is what I am after. I weighed the trade off of, taking out the engine, tearing it down, hot tanking it, rebuilding it, and then I might have proper cooling temps. Instead I took a new t-stat and made my own set of improvements which was a lot faster and MUCH cheaper. It works for me, you decide if you need to do it.

So follow along as we replace and modify a coolant t-stat........

In the shop as the "pictorial" mule, is the ole '79 300d. This car also likes to run on the hot side, like way hot even with the air off. So I will do a t-stat modification while we are changing the old one out. Please do this when the car is cold.

Here is where the rubber meets the road. Notice the proper orientation of the t-stat. The disc on the end of the t-stat is there for a purpose and always goes toward the engine. Make sure the new one goes in the same way. It is possible to install the t-stat in backwards in these cars, and then you will have a hot problem.

The new t-stat and o ring. The part of the t-stat that you are looking at is the part that will face the radiator. The disc is what it is resting on. Always use the new gasket that fits on the outside of the t-stat. Do not reuse the old one it will probably leak.

Ok here is where we will go into the modification. I have already gotten another t-stat ready that is modified.

 

If you will notice there is a  series of 3/16 inch holes that I drilled in the rim. What this does, is allow some coolant flow in all conditions. It will not affect the operation of the t-stat but, will increase the time of warm up on a cold day slightly.

Since the purpose of the t-stat is to remain closed when the engine is cold, and slowly open as it hits a predetermined temp, this will delay the opening since cold radiator fluid is trickling in.

On a hot day it helps the t-stat equalize the temp using the entire capacity of the radiator. I know that this modification fly's in the face of logic, but it's been done like this on several cars and has ended any cooling problems. I also know that 5 holes is best for summer use and 2-3 holes is better for winter use. I have experimented with many different hole and temp factors and seem to have found a good blend for Atlanta's climate.

If you live where its really cold then I would do 2 holes and start from there. In the dead of winter 1 hole may even suffice. The key it to empower you to see what works best for your car and climate. When it gets really cold here, I will put in my 3 hole t-stat and use it till late spring.

Install the t-stat back into the housing like in the earlier pic, install the 3 bolts with some anti seize if you have it. Just tighten them down snug. Remember you are in an all aluminum housing and it will not take too much torque to strip the bolts. Make sure that you get the t-stat right side up. There may be an arrow or small jiggle hole that should point to the sky when you install.

Refill with Mercedes coolant and the appropriate mix of water, run and check for leaks.

There now... your a pro thermo techy!