Engine operating temperature too low
 

All,

My 1985 300D KaliKar has always run "too low" on the temperature gauge since I got it back in August. The highest it ever read was just below the mark between 40° and 80° (is that 60° C?)

So I got a new thermostat, noticed it had 80° stamped on it. Pulled the thermostat housing; out popped and nearly brand-new thermostat marked 80°. Was oriented correctly. So I went ahead and installed the new thermostat. Put everything back together and refilled the system.

Drove for two days with the same result; gauge never gets above what I interpret as 55° C. Heater gets plenty hot; I also recently replaced the monovalue plunger.

So what do I investigate next? I'd rather not spend a lot more $$s on this if I don't have to. Engine runs well on the highway. Low speed/idle is still problematic--rough idle when cold (starts immediately when cold). Thanks for any advice.

Sounds like your temp guage is malfunctioning. Also a bad glow plug or two.;)

OK, so I do go for the gauge or the temp sender?

Do the sending unit. It's much easier than the guage. I had to try three senders before I found one that was accurate on my '83.

Use a multimeter to measure cold ohms and hot ohms at the sender, then compare these reading to specifications. This should tell you if the sender is OK, or not, BEFORE you start replacing parts.

Sorry, I don't know what the readings should be, but someone here does... ;)

Thanks, that's exactly the type of thing I can do for cheap to help troubleshoot the problem.

What I would do is check the temp with a infra-rd temp gun and see what the engine temp actually is.

OK, but I already have a multimeter and I don't have/don't really want to buy a temp gun if I don't have to.

You might also just check to make sure the plug connector on the temp sensor is clean and fitting tightly. Probably not the issue, but good thing to check.

Yes; unfortunately I crushed the plastic connector housing during my recent glow plug reaming work. So the cylindrical metal connector is still there and connected, but there is no nice plastic housing any more. I guess the easiest way to sort-of fix this is putting heat shrinkable tubing over the end.

Everyone who entertains wrenching and maintaining an old classic, needs an infrared temp gun. They are cheap, and easy to use/obtain.

Once you have one, you'll find many uses for it, including bar-b-cue grilling.:)

:D OMG! I've read many ways to justify buying a new tool, but this one is GREAT! Fix the Mercedes and bar-b-cue!

The plastic cover piece is actually removable. Two halves clip together. If you find one at a junkyard you can replace yours. :D Fastlane might even be able to get you a new one.

a man's gotta have his priorities...

Got the temp sender ohm readings
 

OK, so I got the temp sender ohms readings:

cold: 3.4K ohms
hot: 7.2 ohms

By cold I mean not started yet first thing in the morning; by hot I mean just parked after a 30 minute commute at 60 mph. And yes, I meant 3.4K ohms vs 7.2 ohms; I didn't forget one of the "K"s. This was measured across the contact coming out of the center of the sender and the hex body of the sender. Thanks to anyone that can help with what these readings should be for a given temperature.

For you guys wanting me to check glow plugs because of my engine's cold stumbling idle, I checked each glow plug wire and installed a set of new glow plugs (yes, each of these checked). I did the testing by screwing the glow plug wire onto the glow plug and laying the plug on the engine/head/somewhere so it would make metal-to-metal contact and turned the key to illuminate the glow plug light on the dashboard. They all got hot and glowed red. All of this was to no avail; I still have stumbling idle when cold.