300SE (M103) Coolant temp question - PeachParts Mercedes-Benz Forum

tinypanzer · #1 10-27-2008, 07:13 PM

I'm beginning to wonder if my car is getting all the way up to temp, perhaps a stuck open thermostat.

The coolant temp display never gets above 65-70 Celsius, even in Florida.

Is this normal, or is it supposed to have a higher minimum temperature?

Thanks!

mak · #2 10-28-2008, 01:11 AM

normally my 103 engine tends to run on the hot side ,close to 3/4 scale in summer. its most likely that the thermostat is stuck in open position.

normally on a cold engine the radiator remains cool to the touch until the time the thermostat opens on while the engine side hose area is warm.

mak

Ivanerrol · #3 10-28-2008, 06:29 AM

I have two M103 engined cars which recently suffered with this problem. On the freeway they would run right down to 40 degrees C.

I have discovered that the erzatz el cheapo Thermostats are supplied with a poxy gasket that doesn't let the thermostat sit in the housing correctly therefore causing bypass leaks .... resulting in too cold running.

If the engine runs too cold, the temp sensor will send the wrong information to the ECU which results in too rich a fuel mixture and consequential lousy economy

tinypanzer · #4 10-28-2008, 02:33 PM

Yeah, I was suspecting that might be the case especially after last night, where the outside temp got down to 45 degrees (Fahrenheit), and the engine temp wouldn't budge above 60 (Celsius). Also, the heat wouldn't get more than warm.

That, plus a constant deposition of soot in my tailpipe both point towards an engine that is too cold and running too rich.

I guess I'll pop open the T-stat later today and I'll post what I find.

Thanks!

tinypanzer · #5 10-28-2008, 05:01 PM

So I went to the dealer (gasp) and bought a new T-stat. $27. At least it was the real deal thermostat. The one I took out was a "motorad" brand thermostat and was definitely of a lesser quality. For one, the ball check was instead a rivet style check. The spring on the bottom portion of the unit was weak compared to the one I installed.

Also, as someone pointed out, the thing had some sort of weird o-ring that isn't an o-ring, and it looked like crap. I don't think it was sealing well, and I also think that the bottom spring was too weak to prevent bypass.

Now I just need to add about a quart of coolant and burp it out before I drive home from work.

I'll let you know if it solved the problem.

Thanks!

Jamie

tinypanzer · #6 10-28-2008, 06:01 PM

No difference. Put in the new T-stat, burped the system, took it for a drive after letting it idle for 10 minutes. Same story. Temp gauge never gets above 65.

Am I looking at a temp sensor now, or is there some other possibility I'm overlooking?

One other thing - Does the temp gauge get its reading from the main engine coolant temp sensor, or is it a separate sensor?

Thanks!

86560SEL · #7 10-28-2008, 10:17 PM

Yeah, it should be higher. I have a 1988 300SEL and it goes to slightly above 80°C in city driving and 80°C and slightly below on the highway.

Even that may be too low, but it seems OK. My old 380SE would run very cool in cool weather.

tinypanzer · #8 10-29-2008, 02:49 PM

So, what could be the problem? Hmmmm.....

Here are some symptoms. They may be related, or not.

1) The coolant just won't go above 60-65 degrees Celsius. (at least according to the gauge)
2) It takes a long time for the heater to heat the car up. The air coming out of the vents is just warm, never hot.
3) If I stop the engine for a few minutes, and start it back up, I get a nice hot blast of air that quickly cools back down to warm.

I just put a brand new thermostat (from the dealership!)...

What could be going on here? Unless I have two unrelated problems, I am a bit stumped.....

wbain5280 · #9 10-29-2008, 05:57 PM

There is no need to burp it. There is a bolt on the top of the head just above intake runner 1, or 2, that you can remove and let the air out.

tinypanzer · #10 10-31-2008, 07:05 PM

Anybody got any ideas?

I'm stumped. The only thing that makes sense to me is that the temp sensor and my monovalve are bad, but I dunno.

Any reason other than T-stat that could be causing overcooling?

Thanks!

tinypanzer · #11 11-10-2008, 05:19 PM

Nobody?

tinypanzer · #12 12-07-2009, 06:17 PM

This is an old thread, I am just posting to report that monovalve diaphragm and temp sensor were both bad, and everything is fine now. No need to reply.

86560SEL · #13 12-07-2009, 08:02 PM

My M103 (88 300SEL) runs cool I think... even in the summer in hot stop-go traffic, it never went above 1/2 way and in the winter, it usually stays at 80°C when stopped and sometimes slightly below that if I am going above 40mph and the outside temp is below 55°.

Not sure if this is normal or not either, but I would rather it run cooler than hotter.

86560SEL · #14 12-07-2009, 08:08 PM

Quote:

Originally Posted by tinypanzer

This is an old thread, I am just posting to report that monovalve diaphragm and temp sensor were both bad, and everything is fine now. No need to reply.

LOL.. I just replied and noticed it was an update and I had already replied last year. lol.

Glad you got it nipped in the bud!

(thats what Barney Fife just said anyway- lol)

Cal Learner · #15 12-09-2009, 09:05 AM

Quote:

Originally Posted by tinypanzer

This is an old thread, I am just posting to report that monovalve diaphragm and temp sensor were both bad, and everything is fine now. No need to reply.

Thanks for the update. Now, for us struggling DIYers, how does a bad monovalve affect the operating temp of the motor?