1982 300D here. When I changed the water pump on mine, the coolant looked fine but the temp was still a bit high. a few weeks later i did a flush and fill and when i flushed the block it was dirty / rusty. After the flush temp stays dead on at the top of the 8 of the 80 mark on the gauge. Made a loop around lake Michigan this summer after that and the temp was rock solid.. Long story short, I'd flush the block and gently flush the radiator, who knows whats in there.

I have had problems with the instrument panels on both w123 and 4 where the reading was to high. Removing cleaning contacts solved it.

I once had a similar problem and changed parts for three years. In the end it was a bad ground contact in the temperature measuring device.I have learned this is a common problem.

It is indeed a small miracle the Becker Grand Prix radio still works; a clear sign the car wants to live 🙂

My 300SD had a leaking head gasket causing high temps. But if there is no excessive pressure in the cooling system, it could be the fan clutch, refill some silicon oil or make the bimetall valve move again...

First, use a infra red thermometer to see if any cold spots in radiator, Partially clogged radiator will be fair at idle but not enough while driving. Next, running too lean will cause it to heat up too, but that's kinda rare. Fan could cause issues, but normally on affects idling temp, once car is moving forced air takes over. My 78' had aux fan for A/C.

Great video. Mine is 300D, no turbo, 1980. 1) check ventilator and it’s viscose clutch, 2) check if gauge cluster has good earth connection if the gauge does not climb to the max, 3) open valves for interior heating radiator and see if this decreases the temp reading, 4) check if the return hose from radiator is around 40-50degC and if it is, your radiator seems working.

I had a new thermostat put in but it was set on 90° Celsius, I brought it back in and they gave me the right one, no problem after that.

many greetings from germany from an old 240d-driver. A real good channel.👋👋👋

Mine was due to a partially clogged radiator. Replaced it with an aluminium one and it was fine thereafter. I would hate to think that Spence's problems would be due to a cracked head though. Maybe a good Sodium Citrate flush might clear those fine water jackets inside the block? Looking forward to seeing what the actual culprit was.

85 300TD. Having the exact same problem right now, and running at the same temp. New clutch, radiator, water pump, thermostat and cap. Gotta love throwing parts at a problem ..😞😞 Show us the way Kent and Spence!!

I've had 2 W123's with the OM617 and a W124 with the same engine. Living in Southern California heat was a constant concern. If the air bubbles are out, I would check the fan clutch - easy to do - and then that the radiator is clear and has full flow. These cars are sensitive to this. The W124 added the 2 front fans when you had AC due to the extra cooling load.

Put a new radiator in it , thinking about getting an all aluminum one from ebay for my '83

I had one of these 300D's and before I sold it the temperature was creeping up like you explained and there werent leaks and the thermostat was was 1 fine and it was the auxiliary and that wasn't kicking in the auxiliary fan needs to be running in front of the radiator for to maintain the temperature check the relay for that make sure it's not bad and you don't have a bad auxiliary fan

… drove and maintained a 617A in a 300TD Turbo Diesel for 20 years and 20 days. Hope that qualifies. Since the cooling system seems to be very clean, the next thing I’d check would be the fan clutch. The way that thing sounds it might not be an urgent problem, but I’d like to add: Having such bad injectors in hand, it’s just a slide-hammer-step away to have a look at the pre chambers, which tells a lot. Are they original, the wear at the loose side of the ball pins… well, guess you know all of that.

When I rebuild my OM617A the waterpump housing it self was plugged with a sort of goo to the motor block. I didn’t replace the pump, but I had to take it of to find the failure. I think it was the result of mixture of different coolants and sitting a long time. Very messy!

Hello Kent, Since engines disapate approximatley 40% the waste heat in the oil. I recommend using the infared thermometer to diagnois the oil cooling system. Then I would double check to be sure there is no air left in the cooling system. One thing that can help is to use the heater to be sure all air is purged from the heater core and heating system. Sometimes it can take 2-3 heat cool cycles to get all of the air out. Then the next thing wil be the fan clutch. On my 5.9L Ram with the cummins I had to replace the fan cluch at 250,000 miles.

Before you do anything further, use an external gauge to see if it's even running hot. You could be chasing your tail

I agree… Probably the radiator and certainly your heat gun will tell the tale. There’s a company in Southern California called champion that will make you an aluminum radiator for any vehicle.

I have the same car and color. Once, after installing a new thermostat, the temperature remained Hot. The problem: the New termostat did not work. It was new, but frozen. It may be a rare problem, but, Have you checked that?? I solved it with a new one, and after that, I always check them with Hot water at home. The rest of the system seems to be healthy.

I’ve had 2 W123s and currently have a W124 6 cyl turbo diesel. All ran hot then I Changed out the rad in all three. Then that all ran cooler. All were the plastic tank type.

One thing that would help, as we have always done in our Shop install the 75 Deg Thermostat from the 450 V8 which fits without modification & 5 degrees help. Both my 300 D Sedan & 207D Van have the 75 & run nice & Cool here in CA. Traffic w/ Ac on Full...

I would think it would be related to either the fan clutch or the aux water pump.

I had similar issue with my w124 om603 turbo. Replacing thermostat partially fixed the issue. What helped was replacing fan clutch so that's my guess.

1985 300TD wagon. 1. Run the combustion leak tester 2. Test and or replace water pump. 3. If you have a known good radiator temporary swap 4. Run the car with the AC on max and see if electric fans help or overheating increases. Since the overheating is occurring at speed I doubt it is the fan.

Check the injection pump timing for the correct position. This may fix the overheating. Also check the sensors at the block by the injectors: a bad resistance leads to false redings. My 2 cars 617 had one and thw other situations. Obviously I presume fan clutch is good AND the radiator cap is new. My 79 overheated until I replaced the cap.

I drove an '83 300D from 240k mi to ~600k mi, and I never knew that was considered too hot. Mine ran at a little higher than Spence the whole 19 years I used it. So, I don't know what the solution is, but I do wonder what the impact may have been from having a too-high operating temperature for so long?

M103 driver. I suspect air bubble in coolent between heater core and slightly under filled coolent level as a result. Run heater on shakedown test run and fill a little more

This is the time to do a Citric acid flush. That has helped several of Mb diesels for me. If that doesn't work out, and your certain the radiator, temp sensor, and reservoir cap are good. Then its off to drill a wee bypass hole in the thermostat. Done that on a many of w123... Other non cooling system thoughts are : Its running a bit lean. Maybe the ps pump, alternator, or water pump pulleys are hard to turn. Causing it to work a little bit more. For me after said citric acid flush, and hole in thermostat. That normally sets things right.

My 300D OM617A never ran hot during the 110K miles that I owned it. However, I have fought with cooling issues on other vehicles and found multiple new thermostats that were bad right out of the box. So that’s my guess.

A simple guess but wrong coolant? It looked green and usually our beloved MB’s need gold

I, too, think radiator. I have been driving W123 diesels since '87 and now have an '85 Cali 300 and an '81 240. I am only a shade tree mechanic. The auxiliary water pump is only to keep hot coolant to the heat exchanger under the dash. I would get the temp sensor gun out again and look for clues. Do you dare have that radiator cleaned out? Or flushing and cleaning the the whole cooling system? This car does get better every episode, so maybe it's time to spend some money.

Kent = we always run a test on any new thermostat before installation. Always surprising that some new ones fail, as if they have been manufactured years ago and during a long storage time they have seized up. Definitely avoid those Chinese ones.

Some people say to drill a small hole in the thermostat to help aid with recirculating. Have you heard of that?

Oil housing thermostat not opening to allow oil cooler to work.

Could be that fan. I do know ya don't want to over heat that sweet engine.

I own a 1991 300D. I know design was greatly improved, but I can't help to reflect upon the fan clutch. You hesitated when you commented on an earlier video. My fan clutch was a bit tired and caused the engine to run warmer some years ago. I believe that fan clutch needs some further consideration.

Could the bypass route for the coolant be clogged/restricted somehow?

Has the temp been checked with a thermometer rather than the dash gauge? May just be that... My 1979 300D usually runs very low but a slipping belt made it go higher recently. New belt solved the issue.... I think Old Spence already has new belts though so I dont think that is the problem for you...

I’m thinking it might be the temp gauge

Did you try running the heat? Did it bring the temp down?

Faulty radiator cap can cause overheating

My 300SD runs about the same temperature. Occasionally it will get up to the 100 degree area but then settle back down. It seems to run cooler with the AC running, so maybe the auxiliary fan is helping it. I would imagine my radiator is not functioning 100% and that a replacement would help. Interestingly, when I got the car it ran too cool, well below 80 degrees. I think my car was sold new in the south so maybe the original thermostat was set to a lower temperature.

Radiator

My 1985 300TD is runs similarly. It has been “fully restored” but im not too happy about the temperature creeping towards 100 especially at speed on the highway. I drove one of these cars with low coolant at 80 mph for hours- i didnt even notice until i pulled off the highway and the temp shot all the way up and it started smoking.

I think the next step would be the auxiliary water pump. Check for power and operation.

Radiatot