Coolant Temperature - PeachParts Mercedes-Benz Forum

MikeCook · #1 03-01-2012, 09:15 PM

I am at a loss on how to fix this problem on my 85 300SD. All the temperature mentioned is based on the dash gage.

I little background first. About 30000 miles ago, after doing a citric acid flush, I replaced my water pump and thermostat. The car would run consistently in the low to mid 85s. This winter I replaced the thermostat again as the car would take a long time to reach 80 degrees or for the heater to blow hot.

Since replacing the thermostat, the car will heat up to the high 90s but never quite reach 100. The top radiator hose gets hot so I figure the new thermostat is working. The only time the temperature drops is when I have the heater on high. The temperature will drop to the low 90s when the blower is on low and to the mid 80s when the blower is on high. The temperature would drop in 30 to 60 seconds.

Today I replaced the radiator. There has been no difference in the running temperature or the temperature dropping when the heater is on. The only thing I can think of now is that the temperature sensor is bad. I was thinking of getting an IR gun to confirm the temperature at the sensor. Does anyone else have any ideas?

Thanks

MBeige · #2 03-01-2012, 09:32 PM

What thermostat was in there originally, and what did you replace it with? They have varying specs, maybe you put in a hotter one.

MikeCook · #3 03-01-2012, 09:40 PM

Replaced the thermostat with 80 degree Behr

vstech · #4 03-02-2012, 10:47 AM

80-100 is normal operating temp on a working thermostat...

mine sits in the 90 range, but rises up to 100 and falls as use needs.

zu! · #5 03-02-2012, 11:06 AM

I had this happen to me too. My understanding is that the engines run best just under 100. You are within the safe range.

Biodiesel300TD · #6 03-02-2012, 11:34 AM

I don't understand what you are try to change. All of those numbers are in normal operating temperature. Unless you are consistently running over 100 or under 80 then you are fine.

cjlipps · #7 03-02-2012, 01:57 PM

He's in Vancouver in the winter. It shouldn't run up to nearly 100*C and come down with the heater running unless something was amiss. Maybe 100 is within normal operating range in hot weather but I would think he has no headroom for when warmer weather arrives.
OP, have you considered trying another thermostat? Also it might be wise to verify your gauge readings. Although the fact that the heater is able to cool it down while under load is troubling. My bet is a wonky 'stat.

MikeCook · #8 03-02-2012, 04:10 PM

Going to try a new thermostat tomorrow. Parts are crazy expensive here in Canada. I usually order my parts from the US but I want to try and get this problem cleared up in a hurry so I'm getting the thermostat from the dealer. They want $38 for it. Called a few other places and they wanted as much if not more.

I will test the thermostat before installing in. At least if there is a problem I can take it back and get it replaced in a hurry.

Junkman · #9 03-02-2012, 10:08 PM

My 85 SD used to run 80* solid. I replaced the thermostat with a Behr from this site and changed the coolant. I didn't use citric acid or anything else because I didn't want to change a leaking heater core or radiator. it now runs 90* solid. I read where this is has recently become common.

90*C = 194*F. 100*C = 212 * F. I would want it in the 90 range. 100 doesn't leave any room. An infared thermometer will tell you when the thermostat opens. With the car cold start it and let it idle. The radiator should be constant and the engine measured by infared and dash gauge should steadily rise.

The radiator should suddenly heat when the thermostat opens. Note temp on dash gauge when radiator gets hot. This is ~ thermostat opening temp.

MikeCook · #10 03-04-2012, 11:35 AM

Replaced the thermostat again yesterday. Everything is back to normal in the mid 80s. Just had a bad thermostat.

At least this problem motivated me to finally replace the rad. I've had it sitting in the garage for 3 years. I bought it after the top hose barb broke off. My "temporary" fix of a copper pipe and JB weld was still holding strong.

Tanksowner · #11 03-04-2012, 11:36 AM

I'm actually having a similar problem. I had the water pump and thermostat replaced a few days ago. Put the car through its paces out on the highway and the temp range was around 95C or so. It would lower slightly as the engine idled, but for the most part, it stayed around there. I was going to call my mechanic about it, but if it's normal then I guess I could let it go. Would air bubbles cause the high temperature as well? Coolant was also flushed and replaced when the water pump was done.