240D running (a little) hot
 

I'm not sure where to look first on this - up to now, my engine seemed to have been running a little above 175 (my gauge is in °F, but that is about 80°C), maybe 190° (87°C). It is hard to tell because it is only labled at 175 and 250, with one unlabeled mark in between - 200°? 225°?

Yesterday on my drive home, I noticed that the temp had jumped up to somewhere between 175 and the unlabeled mark - noticeably higher than it has been for the last week that I have been driving. It seemed to jump around a little bit - quickly, probably something strange with the sensor...

I thought it was just something with the sensor, BUT it continued to stay up there and wouldn't come down, so I turned on the heat for about 1-2minutes. It dropped down then pretty quickly to 175 and hovered around that the rest of the way home - another 10 minutes or so of freeway driving, even after I had turned the heat off.

Today on the way to work, the same thing happened - running a little hot, I turned on the heat, and it dropped down again and stayed there for the rest of the drive even after I turned off the heat.

Is this indicative of air in the cooling system? Looking through a search that is what it seemed like it could be, but I am not sure. Also not sure what to do about that if that is the case.

This isn't my instrument cluster, but it is a pic of the temp gauge for reference: http://userweb.windwireless.net/~jimc/mb240d/atidle.JPG

Could be air, low coolant level, faulty radiator cap, faulty temp sender, faulty guage, failing t-stat.

Do the simple stuff first, like checking the coolant level and the radiator cap/exp tank cap if so equipped, then on to the others.

I would check the coolant level first. Then I'd replace the thermostat and radiator cap with OEM parts. At least this way several basic things can be ruled out immediately and it can't hurt to have new ones even if they are not the problem. The corresponding numbers of the Celsius guage are 80 and 120, with the unmarked line between the two being 100, I'm assuming.

- Peter.

A little update on this...

I checked the fluid level, it was a little low, but took only maybe 1-2 cups worth to fill it to overflowing, so that doesn't really seem like enough to cause any problem. It also stayed full and didn't loose any coolant after a day of driving. But after reading some stuff in searches here, I see that green coolant is supposed to be bad... well, it has the green coolant, and it looks cloudy as well (not oily, just cloudy). Could definitely use a flush.

So, a little more about the symptoms - it seems the last two days that it has run more consistently warm - gets up into the 90-95° range and will stay there for a bit. I run the heat and the temp comes down a little, between I would guess 87-90 - still hot.

Both last this morning and this evening, however, I noticed that once again the temp will rise up to a little above 90° or a little more, sit there for a while, and then very suddenly drop. I am not talking slowly sink back down, I am talking jump - like a second hand on a watch - in a snap, from about 90 all the way down to just a hair above 80.

Then it seems to stay there, and slowly build up heat again, but not get up as hot as it was, probably sitting around 85° or 87° and holding there pretty steady.

The heat will make the temp slowly drop, but after I have had it on a while, the temp will slowly build up again even with the heat continually running.

It is like it is building up heat until it gets consistently hot enough for a while that something happens to make the temp drop in a blink, and then it will hold pretty steady at a slightly warm but not alarming temp.

Does this point to anything in particular? I am about to order a t-stat and new radiator cap, though I notice the cap is holding pressure fine.

Those symptoms are indicative of a bad sender. Simple inexpensive replacement.

Temperature sender?

Does that send the temp to the gauge, or is it what tells the fan to come on on the radiator?

Where is the temp sender located in a 240?

Gauge sender. I think it's in the head just above the glow plugs.

So if this is the case, are you suggesting that my car is (probably) not actually running hot, but just sending the wrong temp?

It did the same thing again today, by the way, appeared to climb pretty high (even higher this time, close to the 100° mark), then slowly dropped to around 90 when I turned the heat on, leveled out there for a while, and then I looked away and looked back, and it was hovering just above 80° where it stayed for the remainder of the drive - even after I turned the heat off.

Yes, I'm suggesting the gauge itself may be fluctuating, not the engine.

Ok... I will replace the temp sender and see what happens...

So I was about to order the temp sensor, but when I wrote Phil (because I couldn't find it in their catalog), he asked me which one I have - electrical or mechanical, but I don't know.

I am guessing it is electrical because I noticed that when I pressed in the parking brake today (and the light came on, correspondingly), the temp gauge dropped a little, but just want to make sure.

That actually made me wonder - is it possible that the issue is just a grounding or other electrical connection to the sensor or gauge? What would be the usual suspects to check for such a problem?

Thanks

It could also be a lazy t-stat that "overshoots" a little before it fully opens. I think I have one of those in the 300D because it always seems to heat up to about 90-95, then it eases back down to 85 and stays put. I would expect it to be more "jumpy" is it was the instrument.

Also, my 240D always ran 5-10 too hot until I finally got sick of messing around and replaced the radiator.

A bad ground will make the gauge act flaky. In my 240D, the fuel and temp gauges would both move together so I knew it was an electrical issue. I solved it by adding an extra ground wire from the instrument cluster. I assume the original ground was failing.

invest in a infared thermostat...

unfortunately I had a similar problem in my 450SLC this last weekend. I was getting readings of 215-265 on the top end any my gauge pegged at redline.. I rhought it was the sending unit (long story) but it ended up being that my radiator had failed.....

Im thankful for AAA for a free tow home (almost 75 miles)......


Seriously, get an infared thermometer (like $20 at autozoo) if you think your gauge is reading wrong, anything above 200C is too hot. An infared thermometer will help you (and us) better understand whats going on..

Readings on the heads and intake are the best, presumably the exhaust manifold are the worst...