jumpman726

After driving on the highway yesterday at relatively high speed for about an hour I approached the exit ramp and had to slow down for the winding right turn exit ramp.

I slowed down to about 30mph and noticed that the needle on my engine temp gauge had dropped to the 80 mark.

I've never seen it do that before. Normal operating temp is around the 90 mark. I've never seen the needle drop while driving. Anyway, once I made it around the tight exit ramp and back on to the main road the needle went back up to 90 once I started accelerating.

Does anyone know why the engine temp would suddenly drop like that. My car is out of warranty. Do you think the thermostat needs to be replaced?

jumpman726

I also recently starting noticing a single knocking sound or "pop" coming from behind the driver side dash. Maybe from the engine area. I'm not sure if it's related to the problem I mentioned above.

The single knock happens at random times. Sometimes while stopped at a red light and sometimes while driving around town. It's hard to hear the knock with the radio on.

KEY08

You will have an active CEL if there is a problem with your thermostat causing your car to operate out of the temperature range. It is self diangnosed and the CEL pops on. I am in the middle of replacing my thermostat for this issue. My car does not come up to proper temp due to a thermostat issue. If you don't have an active CEL, you should be fine.

Arrie

I have never noticed this but it may be just normal behavior.

As you drive fast the engine works hard and generates lots of heat so the thermostat opens up a lot to circulate coolant thru the radiator to keep the engine from over heating. When you suddenly slow down the heat generation goes way down but the amount of cooling does not change that quick. Thermostat moves slowly and for a moment your engine can be "over cooled" until the thermostat closes up enough.

Like I said I have not noticed this but I will keep an eye on it. Perhaps your thermostat moves slower than it should and needs replacement soon?

ss3964spd

If the coolant level is low (which could produce a temp variance) you'll get the coolant level warning, and possibly a CEL. If the coolant level is correct but the temp swings out of spec'd range, you'll get the CEL for t-stat.

The temp on my previous '05 E500 was always rock solid, as is the temp on my pick up truck and wife's BMW 330. My current 212 E550 was rock solid up until about a year ago but just flashed the CEL recently. Scanned it myself, code came out to be the t-stat, and is corroborated by the gauge. Easy replacement but crazy expensive t-stat.

KEY08

Yes, the stat is not the same as from a '69 Ford Mustang. Lol
It is more machine than just a stat.

ss3964spd

True, but it's still just a temp sensitive spring with poppet valve. The expense is because it's integrated with the housing and temp sensor.

jumpman726

You might be right, because I just noticed it happened again yesterday. I was driving on the highway with rapid bursts of acceleration and at speeds above 100mph. I noticed the engine temp needle suddenly drop to 80.

I took my foot off the accelerator and let the car slow down to about 90mph. As soon as I gently put my foot on the accelerator to maintain my speed the engine temp needle moved back up to the normal operating range (around the 90 degree mark).

On another note, I think the knocking sound I was hearing is coming from my engine. I only hear it when I'm stopped at a light or under light acceleration around town. It's a single knock that sounds like it's coming from behind the steering column. When I shut the car off and exit the vehicle I can hear the engine knock better, and I think that's what I'm hearing from inside the car while I'm driving. Engine knocking isn't normal, right?

Zavato

"When I shut the car off and exit the vehicle I can hear the engine knock better, and I think that's what I'm hearing from inside the car while I'm driving. Engine knocking isn't normal, right?"

If you shut the car off and you still hear the knock, how can it be the engine? Car's off-

sghokie

I think thermostats on these cars should just be considered a maintenance item. Mine failed at 50k 5 years.