Cooling system, thermostat questions, assumptions

[OldRoller]

Somewhat long post, bare with me. I can't count the number of sludged up engines I have worked on and found no thermostat. Owners inevitably said "took it out years ago, didn't want an overheat". Correct me if I am wrong (I'm cranky but never too old to learn), the thermo controls opening and flow to assure optimal engine temp. Opens at calibrated temp, restricts flow when temp of coolant drops, goes full flow when temp is above thermo specs... in an otherwise healthy cooling system. 

 

In 2016 I bought a basket case 2001 E38 740iL. PO had taken engine apart for timing chain guides. Among the parts he purchased was a new BMW thermostat. Part number matched application. After rebuild I noticed excessive pressure in the cooling system, and a lot of heat in the engine bay. Investigation found the spec from BMW for the engine to be a 105C (221f!) thermo. Sealed system, a LOT of plastic parts. BMW spec'd this for the US market to enhance fuel mileage requirements. No wonder a lot of 96 and up BMW's suffer from cooling system failures once miles accrue. Changed the thermo to an 88c unit and now run (for the last 50K miles) at 193f to 205f with ac on in traffic. Happy clam.

 

There's a reason for the prior paragraph. Again, bare with me. In May I bought the 74 02, dirty but no rust, warehoused under cover for 35 years. No head, rings seized but otherwise all there. All the associated parts were in the boot, sans head. Engine rebuilt, in. Radiator flushed, new hoses, water pump, etc. Cleaning the thermostat I found makings with "Behr X1 102" on it. Searches found the markings to the the opening spec. 102??? Really? That's 215f ! Further seached on FAQ and found optimal (for normal street) to be 186f, ballpark for an 80c thermo. RealOEM shows the 11531253249 thermo used until 1988 on the 3 and 5 series. RealOEM does not state the opening temps. I will verify this thermo in a water bath to verify it's opening temp. Is it possible the BMW used a higher opening rate on later years for the M10? This one is not going back in, regardless. My assumption is the replacement prior to the dying of the 74 in 1988 of the incorrect thermostat caused the dying, but is just an assumption. 

 

So, my questions for the gurus:

Is 186f prime operating temp for mostly stock M10?

Anyone seen a 102c thermo of the M10 style? 

If so, what application did it serve?

Is there experience with increased operating temps for fuel/US market/efficiency in the later M10?

 

The 74 lives in a temperate climate, 50's average winter, hot humid summers. Stock with exception of 9.5 Mahles and Weber 38. 

Thanks for reading, 

Cheers!

 

[tzei]

Yeah newer engines runs hotter (and higher pressures - remember to check that plastic cap too). I have seen 80˚C and 85˚C stats on M10s,  no higher. Turbo was 75˚C and i think factory race version was 70˚C.

[Mike Self]

I've been running an 80 degree C 'stat in my '73 (9.5 pistons, A/C) for many years with no problems.  The gauge will show about 2 o'clock on a hot day, highway speeds with the air on, but hasn't overheated yet.

 

For your NC climate (unless you're in the mountains) you could probably do with a 76 C 'stat and still have the temp needle at the "optimum" 3 o'clock.

 

mike

PS--where on the NC coast.  I have an affinity for the Outer Banks...

[OldRoller]

hi Mike... just south of Cape Lookout, on Bogue Sound. Morehead Beaufort area. Lived in Hatteras back in the day, old surfer dog.

I have an 80c unit on the way. No ac, stock fan.

Edited December 11, 2023 by OldRoller

[Einspritz]

That's the casting number.

 

The temperature is lightly stamped on the housing.

 

Shown are an 80&71 degree units.

[MurphinDC]

@OldRoller

 

I'm up in Williamsburg, VA and run a 75c and I'm around 4 on the dial under normal conditions and 3 in the summer heat and humidity.

 

Murph

[OldRoller]

About the same zone Murph. Should have the same or very similar readings. I am backing up the cluster needle with a VDO gauge with the sensor mounted next to the dash gauge sender on the water divider. A little ocd about engine functions... 😌

[OldRoller]

3 hours ago, Einspritz said:

The temperature is lightly stamped on the housing.

Thanks Einspritz, further cleaning and some dye treatment and I was able to see the "80", very faintly on my old one. Good to know, my questions have been answered well.