Strange cooling system behavior: W115 300D
 

Hi guys, I'm hoping this set of symptoms sounds familiar to somebody so I can avoid replacing too much stuff ... The other day I noticed the temp gauge on my '76 was steadily climbing at a traffic light, heading toward the yellow zone. I slipped the car out of gear and revved the engine, hoping the extra airflow would encourage it to cool down ... and it worked, but the temp dropped much faster than it would have if it were just the fan blowing through the radiator. It seemed like something in the engine was blocking the flow of coolant at lower revs.

Today I glanced down and the temp was about 245, almost in the red. I shut down right away, waited for it to cool to about 210 -- that took about 10 minutes. Then I restarted. Revving it up again, I watched the needle plunge back to the normal zone again.

WTF?

It seems to me that in 10 minutes the temp of the engine should have evened out -- how could there have been a big old reservoir of cooler water someplace, just waiting for me to rev the car so it could come to the rescue?

I don't see how this experience squares with the laws of physics and I'm not sure where to start with testing and/or replacing parts (other than the Usual Suspects, viz. t-stat and water pump). Anybody have any thoughts?

--Finn

To me it sounds like a sticking thermostat, or air bubble trapped in the cooling system. The temp drops suddenly when the thermostat finally opens or air lock finally breaks and the cool water in the radiator suddenly flows into the engine.

You might also want to verify with an IR thermometer to rule out a gauge problem.

I believe that one has the mechanical temp gauge. The accuracy can certainly degrade, but I think the repeatability would tend to stay pretty close, unlike the electrical gauges.

I second the air bubble and sticking thermostat theories. Also, what kind of shape is the fan belt in?

No the temp gauge is electrical on the W115 300D.

-J

Well, it sure looks like it was a bubble! Now I have to figure out where a gallon of coolant went over the past few months ... or maybe it just wasn't all the way full when I bought it.

Thanks guys!

Completely agree -just went through this with my '75 -bleed, bleed, bleed and do verify that t-stat is opening -good luck!

Also, not to scare you, but if you bleed and bleed and the problem keeps coming back, you may want to check for a head gasket leak. I had one go on a Chevy V6 and it would air-lock itself repeatedly. I would start it from cold and it would go up into the red in about a mile, then suddenly make a massive gurgling noise and drop down to normal again as the air lock broke.

When installing new thermostats on my cars I drill a tiny hole in flat metal part of the thermostat to serve as an air bleed hole. Many newer cars have thermostats that come with a bleed hole or bleed valve.
http://www.jeepforum.com/forum/attac...ostat-hole.jpg