Sometimes when I'm driving the oil temp readings is erratic going from stable at 97-98 C and then it starts blinking to 75,78,84,92 and so forth it just jumps for a good 5-10 mins of driving then back to normal


Am I suppose to be worried about this ? Or is this normal


I have changed the engine temp sensor recently ( there's 2 of them if I'm not wrong, what other possibility of things broken there might be ? )

How strange. My first thoughts were the sensors....but you've changed them. Hmm...

I have no idea why, but I see a similar thing with both the oil and water temp.. The majority of the time my water temp will be at 197 and then for no reason it will drop to 175 or so then slowly work its way up to 197... I see the same behavior with the oil temp reading.. In both cases this is after the car has been running for a long time and outside temperature does not seem to matter.. Almost completely random..

It drops because the fan kicks on...

I don't think it's due to the fan as it drops before it reached any high temp and it stopped after awhile and back to normal

I assume that the fan will stay on or comes on at a certain temp , these drop are intermittent and erratic and then it goes back to normal

For water temp, that's completely normal. That's what thermostats do. There is a wax pellet inside that begins to melt when coolant reaches a certain temperature, this opens the valve and allows coolant to flow through the radiator, then when the coolant drops below the melting point of the thermostat (190F in our case), the pellet hardens and shuts the valve, recirculating coolant without the radiator and heating it back up again. Cycle, repeat. (or roughly like that, it's been awhile since I studied the mechanics of a thermostat)

While I agree with the description, I would add that most thermostats are modulating - constantly adjusting - rather than an on-off device. While I would not be surprised to see a noticeable drop in water temp when it first starts to open (as the cold water in the radiator flows into the block), I would expect water temperature to be pretty stable once things evened out.
Here's a good cutaway demo of thermostat operation:

If people are seeing big and sudden drops in water temp, could that be a result of changes in heat load and a stuck-open thermostat?

I've been noticing this too. Started recently on my car. Oil temp goes from 95 celcius to 75, back to 95 and so on. All within 10 seconds. I'm guessing oil temp sensor needs replacing...

Anyone replaced oil temp sensor and this still persist ?

There's apparently two sensors and I have changed both but they still giving me erratic readings

Your temp readings sound normal

I didn't notice how low the OP said his oil temps were dropping.

Dropping below the "safe" 80°C after it has already reached operating temp is not normal.

That's true, I was thinking water temp and not in C either. Duh. So the OP is saying that his oil temp gauge goes back into the blue during normal driving, which is below 175F?

Curious since there's 2 sensors. Does the dash temp reading match what the ECU is looking at? Do you have a scanner to verify?

There are two ends to the system. The sensors and the gauge or readout. Away from home and I cannot recall the exact nature of the readout but could it be the readout end that is faulty?
As to temps I was going to make the same comment re thermostats. They are modulating device not an on or off situation. Usually they will fail open so the engine does not heat up properly. The fan only comes on when there is insufficient air flow through the rad to control temperature in the system e.g when at idle. At road speeds in moderate climates they don't run that much to reduce load on the engine and save fuel.

Pardon me for the noob question but thermostat is for Water temp or oil temp ?

My water temp very stable readings just the oil temp goes crazy it can be on normal operating temperature then it drops for like 2-3 seconds back to cold then goes back up really weird

Thermostat is for water (coolant) temp.

What is the temp gauge doing? Not the digit readout on the dash the actual gauge.

This is an oil temp reading issue, not coolant temp. That eco user (who I meant to quote earlier) had a completely unrelated coolant temp comment in this thread.

Hi, did you manage to fix the oil temperature issue? if yes, what was the issue and the way to fix it?
I have the same trouble today, the oil temperature is fluctuating as yours whereas it used to be very steady once hot.

My ML 550 did the same thing, but it was the water temperature, so may or may not be helpful. It would roll up to normal temp and then all of the sudden drop by ten degrees or so followed by a similar jump up. I changed the temp sensor and the thermostat to no effect. The water pump and circulator pumps were definitely working as well. Turns out it was the fan which did still run, but apparently there is some connection between the temp indicator and the motor controller electronics in the fan motor. Replaced the fan and all was good.

Has anyone with the fluctuating oil temperature issue on the dash read out solved it? Mine has been doing this for about a year now. Definitely not the normal jumping around in temps.. it could be reading 199 F to 201 F then jumps to 170 F and will blink since it is below the warmed up level and then jump back up to normal. This happens on and off. Doesn't matter whether the car is moving or not.

leaky injectors. Service or replace the injectors and the temps will go back to normal. Mine did that until I replaced my injectors

I already did that, but still have the same problem, anyone have a solution for this problem?

This is interesting I notice my temp jumping also. I am going to try and replace the injectors as well as I feel they are oddly loud in comparison to other M156 I have heard. My 63 has 106K on it and feeling they have never been replaced. I think I will also change the plugs while I am at it. I can report back if any changes probably won't happen for a week or more as I only drive the beast on Saturday Sunday.

We found the problem here, look at the wiring on the left side of the engine, (looking at the car from the front, right). My wiring was peeling and dry, we did all the new wiring on that side, which solved the problem. According to the electrical diagram, there is a part of the ground that shares with the fuel pressure sensor, so it is good to look at all the wiring there in the engine.