how to fix erratic water temp on 400e?
 

I been getting erratic air temp on my 92 400E. All other gages work except the water temp. When the car is cold, it will raise to normal temp without any issues. Once there, the temp will occassionally jump around, even higher than 120C. Then sometimes it will drop back down. I assume there is air in the system, but how do you get it out? Is there a bleed valve?

i am not familiar with your specific engine. but i have seen bleed valves located on the top of the thermostat housing. it will be at the high point of your cooling system. it will be a simple plug, like a plumbing plug, i think.

it is simple to use too. just take the plug out and add coolant til it runs over a bit then put it back in.

good luck.

tom w

i don't remember seeing a bleed plug anywhere. is there one for this car?

I don't think so
 

I don't think there's a bleed plug for the 400e. I was successful in refilling by taking out about 4-5 quarts, turning on the heater with high setting, starting the car, and filling with fairly warmed water/antifreeze. suggestion is - don't shock the engine with cold fluid.

i tried running the car with the cap off several times. the temp gage still jumps up and down whenever it get hots. is this a sensor issue?

For what it's worth, my 1994 E420 has had this problem since it was new. The previous owner said he tried everything to fix the issue but finally gave up and just lived with the erratic readings. I have owned the car for 50k miles(150k total) and it the temp guage still hops all over the place once it has warmed up to normal. Good luck in your pursuit of a fix!

kanio, replace the thermostat and see if that corrects the problem ... especially if it's still the original one.

Check the sensor connections for oil contamination and clean them with brake kleen or electrical contact cleaner(same ingredients).
Check the radiator for cold spots when hot.
The stat is probably the cause.
The fan viscous could be weak, but start with the stat first.
Alot of times the stat will cause this problem.
Also replace the filler cap on the expansion(coolant )tank.

Use MB coolant and mix it 50/50.

I basically change out the whole cooling system after i got the vehicle. I changed out the thermostat, water pump, new viscous fan clutch, radiator cap, and new 50/50 MB coolant. still getting the same problem. it reads fine when the car is not up to normal temp. however, once it is warm, it sometimes read above 120C for a while, then all of a sudden dive back to normal. it just bother me to death when i am driving at over 120C and think the car will throw a rod anytime.

Where is the coolant temp sensor for the instrument panel?

observed for erratic high speed auxillary fan function
clean terminals/connectors, fuses and fan resistors for loose contact and corrosions

sometimes we thought its working but observing closely the high fan is not working properly

check your auxiliary fans
 

I have the same car, no problems with erratic temperature. I would replace the thermostat just as cheap insurance, it is cheap, and their lifespan is not always predictable.

I also just recently replaced one of the aux cooling fans, not a pleasant experience, but my tech said it is an important part of the cooling chain, and not to be ignored on this engine. If both are not working consistently, maybe the swings correspond to a fan going in and out. Are both of yours operating? Mine was spinning, but at marginal speed. Check the connectors before you buy the fan($$$).

I might ignore a guage that twitches, but not one that swings up and down. I don't think you have a guage problem, maybe the sending unit.

Good luck.

i will check the electrical connection between the senor and wire. something could be shorting it out. does the gage goes all the way up when it is not coneccted to anything? the temp can go up and down no matter if i am cruising or idle, so i don't think it is the aux fan.

i was wondering if it could that i am low on coolant on the overflow tank. too bad the stupid overflow tank is behind the wheel well, i could easily check it if it is behind that well cover.

Remove the wire from the sensor and connect it to ground. Turn on the key and you should see full momentary deflection of the temp gauge. If so, the gauge is ok. I would change out the sensor.