I use K6 JRFs air conditioning diagnostic protocol to get the supposedly accurate engine temp etc ,(Number 6 in the diagnostic mode) that I use to monitor engine in my notes, and my posts.


My 60-65 mph highway temps have been around 67-71 C here in NY area when its in the 30s outside . Then when I am driving back in the city, temps get to 85c fairly steadily. Then when I am driving with the rapid starts etc trying to diagnose a flat spot I have mentioned in another post

400 SEL BREAKING UP AROUND 1500 RPM

temps get to 103C and prolly even higher if I continued.

Somone suggested that this fluctuation in temps is not normal. The gauge on instrument cluster tracks the air conditioning display but never gets as high as fast as the ( I assume ) more accurate air conditioning display shows.

My question has 5 parts

#1 Do you think that these fluctuations are abnormal?

# 2 And if so can they affect air flow mixture sensors etc .

#3 what is the normal range for this time of year with 30 degree outside temps, and climate control on at around 69 degrees, and I assume no air conditioning being used ?

#4 What should be the highway 30 mile trip temp

#5 And what should be the city driving stop and go 45 minute trip temp

Thanks

Coolant temperature should remain essentially constant when the ambient temperature is 30 degress F, around 80 degrees C. I would suspect a thermostat, stuck partially open, is causing the coolant temperature fluctuations. Install a new Behr or Wahler thermostat in your 1993 400SEL.

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Fred Hoelzle

When car is warm , and upon rapid accelleration , could weak coolant hoses cause a loss of vacuum that causes the car to miss, stumble, almost stall and raise the engine temperature at the same time as per this thread

400 SEL BREAKING UP AROUND 1500 RPM

and then stay there,,,bouncing is no good

If it's not the thermostat, it could be the temp sensor. When I replaced my sensor it fixed the strange fluctuations on my temp gauge.

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1984 300D, 228k, Light Ivory, Java MB-Tex

if your temp gauge is bouncing then you need to bleed the coolant loop....a thermostat would not cause bouncing...it would be consistant...only thing to cause bouncing would be air pockets in the coolant loop...air cools and heats much quicker than coolant...if you have air pockets in the loop that could cause this....try bleeding the coolant loop....best of luck

How do I bleed the coolant loop. The coolant was replaced at the end of the summer with 2 gallons of MB coolant.

Would the rapid accelerations under load cause the engine temperature to raise quickly because
A: I am putting car under load and mashing throttle every time I pull away,

B: creating a miss/stumble/hesitation by accelerating rapidly or attempting to, that might raise engine temperature by itself ? If there was any type of preigntion or bad ignition would that not cause engine temp to rise dramatically ?

First, make sure the aux electric fans are functioning. When the temps are hitting 103C are both fans howling?

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Gregg

'92 500E (Sold)
'07 E63 AMG (Sold)
'10 S550

I have noticed that the aux fans come on every 30 seconds or so when the climate control is on set at 70 F , outside temperature is 34 degrees and engine temp is 85C or more .


I will see if they are constantly running when engine temp gets higher.

I do not think engine temp gets higher than 83C even in traffic under normal accelration.

If I recall correctly it goes to over 100C when I am testing the car by accelerating aggresively over and over up hills etc to test for the hesitation that I have .

I dont know if they come on because compressor comes on , but does compressor come on under those condition to remove humidity ?

I am going to go drive around normally now, highway and city and record where engine temp goes, and if the fans come on at those temps ?

Fans should be running constantly at higher temps. Might just need the blue temp sensor at front of motor replaced. There is also a ballast resistor and a couple of metal fuse links to check. The fuses are strips of metal located in little plastic black boxes under the hood near the fusebox. There's usually two of them. The metal strips get old and can crack.

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Gregg

'92 500E (Sold)
'07 E63 AMG (Sold)
'10 S550

I checked today and car got to 100C when trying hard acceleration and then fans jumped in and stayed on till temp cooled.
I will change thermostat just to be sure , but engine gets back to 65C on the highway after going to 95C or higher.

Temp never got to more than 100 on instrument cluster . Is that too high with climate control on?

Does your car have a viscous fan clutch? I used to have a 400SEL, but didn't spend any time under the hood, so I don't recall.

If it does, then that could very well be the problem. It would explain why around town you're heating up, and cooling down while on the highway.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

then you have air pockets in the coolant circuit...especially possible as you have mentioned recent cooling system work. There has to be a bleed screw somewhere (I haven't had to do this on my 300TE yet so I don't know exactly), but I have done it on BMW, SAAB and Audi. Very much like like living in an older home with radiator heat and the pipes would clank and you have bleed the radiators in the house with this goofy little key and let the steam out.....close and it's all good....same thing sort of...

I dont know where the bleeder hoses are . The guy who did the coolant flush should have bled them . I will mention this to my new mechanic

In reply to post #14 my coolant is full Thx

When the car is cool, take off the upper radiator hose where it goes into the radiator. Hold the hose slightly higher than the top of the engine and pour as much coolant as you can get into the hose. Then reconnect it (be careful, the plastic radiator elbows are brittle and have been known to break easily when clamped).

You might have to do this a few times, but it worked for me. I don't know where the bleed screw is on my car either (if there is one).

I agree with Cliffmac, if the coolant system was recently flushed by someone who didn't know better, it is possible that the heads did not get filled with coolant and there are now air pockets in the system. On American cars, the water pump circulates coolant through the heads as soon as you start the engine. Mercedes is different. Do a search on it. I think I read a post here about "burping" the system after flushing or changing hoses.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles