1984 300D with around 200,000 miles.

This past summer after doing a coolant system flush to try and reduce temps I also replaced the thermostat. The maden voyage after the work the thermostat failed to open and the coolant temp rose to 100C. That was on the way to work. I removed the thermostat for the drive home. I tested the operation of the thermostat on the bench using a heat gun and a Fluke Multi Meter with a temp probe. It opened as advertised at around 80C. I should say also that I calibrated the gauge sending unit with a Fluke dry well calibration tool. It is accurate.

I drove around until now without the thermostat. As would be expected the car takes a long time to warm up and now that it's gotten colder the heater function has suffered.

I reinstalled thermostat #1 thinking that maybe I had not purged all the air from the system. Same result. The temp gauge reads 90-100C and the lower radiator hose is cool to the touch. I have the thermostat installed with the disk facing aft as shown on the dieselgiant page. That direction also makes logical sense to me. The bulb of the thermostat is towards the hot coolant.

Still suspecting thermostat #1 I bought thermostat #2 and again made every attempt I know off to purge all air from the system. Again, same result.

I have no idea how to procede.

Phil

I do.

After installing the thermostat, remove the upper radiator hose and pour coolant into the hose until it is full. Reconnect the hose with a valiant attempt to lose no coolant from the hose while you do it.

Both thermostats are fine.

You're assuming the cluster gauge is accurate.

Check the temp of the engine near the sensor for the gauge. Your cooling system may be operating normally and within limits, but your gauge may be reading high. There are more than a few threads on operation of the thermostat, when it opens, etc.

First post after a year joining? Is that a record?

I like it.

Listen, read, and learn.

Don't run your mouth about things that are easily available in the archives if you even bothered to look.

Way too many of the above in the forum today..............

Maybe you missed in my post that I did check the gauge sending unit assembly and it is accurate.

Any other ideas?

As for when I joined/ first posted. I'm not here to rack up post count numbers. I'm hearing trying to get my car running properly.

If you could help with that it would be much appreciated.

Phil

I did not want my initial post to become longer than it was so I did not go into too much detail about the filling process. I did just what you stated.

Here's how I fill the system:

Install radiator drain plug
Intall thermostat
Fill engine block at the upper radiator hose connection.
Fill radiator from the expansion tank. (turbo)
Continue to fill the radiator from the small tank-to-radiator hose.
Start engine and allow to come up to temperature while checking to see if the thermostat opens.

The front of the car is on jack stands about 18" tall.

Any other ideas?

Phil

You've filled it properly..........no question about that.

I will relay my own issues with the same engine on the SD, also with 230K miles.

About two years ago, the gauge began climbing toward 100C. In the last two years, I've changed the radiator, thermostat, and fan clutch. I also recently changed the gauge cluster for other reasons. The temperature gauge changed with the cluster. None of these items made a damn bit of difference. The engine runs 95-100C independent of ambient temperature. It can be 15 degrees or 100 degrees..........the engine temp does not vary.

I did take a infrared gun and checked the head. It runs 90-92C. depending on the cylinder and the gauge is reading 98C.

So, like yourself, I really don't have any good answers for it other than a slight error for the sending unit..........maybe 6 degrees or so.

I will tell you that the engine doesn't really care if it runs hotter............in fact, your fuel economy will probably improve. The SD gets an average of 29 in mixed driving.

I don't think this is an indication problem because the indicated temperature was much cooler without thermostat installed. Even if it was an indication problem the lower radiator hose should not be cool after driving around for 20 minutes.

I wonder if the water pump is suspect?

Phil

My belief is that the thermostat doesn't see the coolant temperature required to open it at a lower temperature. The 'stat should start to open at 80C............but, for reasons unknown, it doesn't see this temperature until the temperature of the head (where the temp sensor is) is considerably higher.

If the water pump was suspect, the engine would overheat in high ambient temperatures............almost immediately.

I'm musing about the SD............your situation might be different...........although I don't see any mistakes in your fill technique.

There is some discussion on the board regarding the orientation of the thermostat. Some have an arrow requiring installation with the arrow facing upward. Some don't have the arrow. The requirement for a specific orientation for performance has not been verified or understood.

I misinterpreted which gauge was calibrated - I thought you meant the Fluke probe, not the cluster gauge.

Does the lower hose get warm at any time with a thermostat installed, or only when the t-stat is removed? If the t-stat(s) don't open until above the mid 90's I suggest that the main and bypass valves aren't operating correctly.

If the bypass doesn't start to close above approx 80, then coolant will continue to circulate in the block and bypass the radiator. The bypass is supposed to fully close above 94. The main is supposed to open at 80.

There is nothing wrong with the car running 90-100 degrees. Mine runs on the upper end of this spectrum and has for 13.5 years. If it's running over 100 degrees under regular conditions then something is wrong.

__________________
1984 300 SD 384,000 Miles

I've come to the same conclusion.

However, I still struggle with why the vehicle changed over the years from a steady 90C. to a steady 99C. The sending unit is partially, but not fully, to blame.

The lower hose never gets warm with a thermostat installed. I can't say that I've ever felt it without a thermostat.

From your last part of your post I don't think I fully understand this cooling system. Are you saying that there are 2 valves in the system? One being the thermostat and the other one being what? Where?

Phil

YAK

The link you PMed me is very informative. I'm looking at it now. Thank you.

One thing I did not mention is that the upper hose gets hot to the touch.

I wonder if the bypass system is plugged. That still would not explain the thermostat not opening.

Phil

The last time I drove the car I drove it hard up hill for about 1/2 mile and the temp immediately went to 100C. I turned the heater on and drove it mellow for the remaining 1/2 mile home and it stayed at 100C. I parked it with the engine running and felt the lower hose. It was cold. The upper hose was hot and the engine side of the thermostat housing was hot.

I can't believe this is right.

Phil