Backup Thermostat 1

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Thermostat replacement and modification

This pictorial has actually two purposes. The first is to show you how to change out your coolant thermostat, and the second is to show you how to modify your new thermostat to keep your temperatures down, if you are having that problem. The second part about modification, is really a work around fix to a more difficult set of problems.

I have been repairing automobiles for almost 24 years. I have worked on just about everything, including class 8 Semis, and 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 again as the "pictorial" mule, is the ole '79 300d. This is my daily driver and I might say is the slowest car I have ever driven. 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, NOT HOT!

Safety and security tips:

Please remember to recycle all your used fluids at an appropriate recycling center. Be mindful to not spill or splash fluids on yourself, others or the ground. Also as a safety tip please remember anytime you are working on, around or under your car, to wear safety glasses and secure the car with wheel stops and approved jack stands!

This is our work area and is called the thermostat housing. Inside the aluminum cover contains the t-stat as we will call it, to save me from typing so much.

Simple selection of tools needed are 3/8 inch drive ratchet, 2 inch extension and 10mm socket.

First remove the radiator cap. If you have a turbo diesel then your cap is on your expansion tank located on the passenger fender well.

This is a good cap. Note that the rubber is NOT cracked or missing and is still very soft. Replace your cap every time you do a coolant flush. (2 years)

Drain the radiator from the plastic plug on the bottom of the radiator. It is on the driver side and has a slot for a large slotted screwdriver.

Yuck! This is a good time to get rid of the green anti freeze, do a flush and put in some Mercedes coolant.

Be careful with the plastic plug, it will break if you are too aggressive with it. Make sure the rubber seal in in good shape. If it looks suspect, just replace the whole thing.

Remove the 3 bolts that hold on the t-stat housing cover.....

....2nd bolt coming out......

...3rd bolt coming out

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. The other o ring in the package is for the block to housing seal. Since we did not remove that part of the housing, it will not be used.

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 the 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 flys in the face of logic, but I have done this on several cars and I have ended the cooling problem. 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 get really cold here, I will put in my 3 hole t-stat and use it till late spring.

If you don't have a cooling problem then don't do this modification.

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.

Now that was not so bad was it?