M103 which thermostat?
 

It is taking my '89 300CE a lot longer to get to 80C and there is a noticeable drop in heater performance. I'm going to replace the thermostat but wondering what temperature range I need? I understand that there is a 87C and a 79C version. I can only find the 87C on our host site.

Being in Canada, go with 87C. Only those who's summer temps hit 100F+ should be considering the 79C. Anybody else is just masking other problems.

I have the 79C t-stat. What I notice is that if the ambient is over about 65F, the car will still run at 87 degrees and that's with a new behr rad and fan clutch. Seems that the core and airflow will only support so much cooling.

Ok, that makes sense and I am guessing what the car came with. I also have heard that the Behr replacements are best and to avoid cheaper options?

Seems a little high, but maybe that's the turbo?

If its anything under 70F, mine runs dead on 79C...maybe creeps up to 85C at idle traffic, but soon as its moving its smack on 79C. But then again my cooling system has had a complete refresh...no mercy out here in summers hitting close to 120F :cool:

A couple coments:

1. There is a cheap thermo and a more robust one available on the aftermarket. The cheap one will actually bend the top bracket over time, and fail in the partially open mode. This makes for pretty cold winter performance.
So, buy the more expensive one - can't remember the name right off.

2. Personally I would never want my engine to run below 80C, which is 176 F. That's not the factory intent, and borders on possible sludge formation, and possibility of piston scuff, not to mention longer warmup periods - not good.

3. Why would a cooler thermo be better in 120 degree heat? The only time it would matter would be during initial warm up, where it would heat up to operating temp just a bit slower. But once it warmed up in that kinda heat, any thermo would be wide open and the radiator capacity would control the operating temp.
Now, if it were pretty cool, the system mifght be able to keep the temp down to the opening point of a cooler thermo, but the 87 thermo runs around 83 in these circumstances, so what added good would 79 or less do - see #2 above.

DG

While those may be negatives for a stock engine, mine is no such beast. I use the cooler t-stat to help prevent detonation under boost. I therefore use thinner oil and spray it under the pistons to boot. My only complaint is that it doesn't work better for me. In warmer ambient temps its effect is negated.

I bought the Behr 87C replacement and will install it later this week. In city driving it just can't get to operating temperature and it sits just above 60C so my mileage is probably quite bad right now with it sitting so close to closed/open loop. Do I need to be concerned about loosing coolant when changing it out? It looks like a pretty straightforward replacement once the t-stat cover is removed.

I was able to run more timing on less octane in the summer with a 79c Tstat in my M103. The downside was that I had to tape over 1/3 of the radiator in order to get adaquite heat below 45f ambiant. Small price to pay when you can handle summer temps in excess of 100f without a clutch fan while the temp gauge never went over 95c.

2: If you live in cold climates, yes....but as I said above, nobody should be using the 79C if you live in cold climates, there is no reason.

3: Warm up is extremely fast in summers at 120F, less than a few mins and coolant is at 80C. The 79C version just helps keep it cooler for longer. After a long drive, its a moot point...but heatsoak is really hard to control in this heat, so anything that prevents this happening sooner is worth it.

Being in Texas you should know Summer's are pretty brutal! Unfortunately the W124 chassis does not have a gigantic radiator to dissipate enough heat at 120F and keep it near t-stat opening temp...just the way it is. Plus, I am extremely meticulous and if I see it hit the 100C mark, I begin to watch extremely carefully even though I know its fine.

Yeah, I gotta agree with that. The wife pulls into the garage in the middle of the summer and the high speed fan bangs on at 110C. I hate that, but that's the way it's behaved since new.

Yeah, the Behr thermo is the better one.
DG

ps2cho,
I too have a couple of 124's, one being a E420. I noticed a difference in radiator size from my 320CE, it being smaller. But then I noticed that the radiator frame of both cars was the same, telling me I can install the larger radiator in the CE. My E420 never gets overheated.
An old mechanic told me years ago that one should never run their engine below 200F. He said that the oil runs hotter and if there is any condensation in the crankcase it would turn to sulfuric acid at lower temps. True?, don't know but it makes sense.
Anziani

The M103 and M104 have both the inlet and outlet to the radiator on the driver's side. The M119 and diesels have the outlet on the passenger side. It would be a difficult swap.

Inline 6 radiator.
http://cdn2.autopartsnetwork.com/images/catalog/wp/full/W01331901891NSS.JPG

V8 and diesel.
http://cdn0.autopartsnetwork.com/images/catalog/brand/apdi/640/8012069.jpg

I think the V8 and diesel radiator is a much better design.

I too use the 79°C here in Texas with no ill effects.

Agree 100% on the radiator, it is flat out too small... this is even more pronounced in the W126. I think part of the issue is the non lock up torque converter and its heat exchanger within the radiator driving up temperatures. It is why the W126 powered M103s are absolute slugs in the summer.

The radiator is tiny compared to the V8s and Diesel powered cars... and something I recently noticed is that the lower radiator hose on the radiator only uses about half of the core. :confused: I may be off my dot here but that seems silly if it is only using part of the small core.