E36 S52 Weird OBD2 temp values

[kokarez]

Hey Guys,

car: 1999 M3 S52

I working on this since some time now, but I still don't really understand what happening here, and since I didn't found any similar thread let see if I can get some help here

So the problem is that with my OBD2 reader (2 different one), read always around 95C for the water temp. From multiple post/forum, the m3 is supposed to run at 88C in normal operation (?). This match the thermostat at 88C.

Since I take my car to HPDE, I try to have a car as robust as possible, so I've started to refresh the cooling system (which was really old and prone to failure).

Here what I've changed:
- I fluch 10 times (to be sure with distiled water)
- new full aluminum rad
- new water pump (perf +20% at 6500rpm)
- new aux fan / new clutch fan and fan
- replace all the hoses (from back to front)
- new coolant tank
- new coolant temp sensor (the one under the intake)
- new thermostat and housing (88C)
- new aux fan switch 87-89C (instead of 91-93C)

The next question will be "did you bleed correctly ?" yes I guess not sure but I did check again and again to be sure and no bubble. I do get hot air in the car too.

So with all these modifications I was thinking that my car will run closer to 88C, but no 95C again.

From this point I start debugging, I decide to read temps with a laser:
-> obd2 reader -> 95C
-> with laser on thermostat -> 88/90C
-> with laser on hose before rad -> 88C
-> with laser on hose after rad -> 60C
-> with laser on coolant tank -> 86/88C

I've also read with a multimeter (with resistance) and same value than the laser.

So here we are, I don't know what the problem is, also the value describe just below are the ~~same when I drive the car really hard (HPDE).

Even try to rev a 2500rpm in my garage during 2min after hard driving (so no vent, no speed) and 95C flat...

1) Could be a calibration problem from the OBD2 ?
2) Coolant inside the engine is higher than the coolant at the thermostat ?


Thanks in advance.

[MarcoZandrini]

The obd2 reader is showing you the temp as seen by the engine computer (ECU). The ECU uses a sensor that you replaced. Apparently that's the hottest part of the engine. I thought I you readings are in line with what the ECU is showing.

[kokarez]

The obd2 reader is showing you the temp as seen by the engine computer (ECU). The ECU uses a sensor that you replaced. Apparently that's the hottest part of the engine. I thought I you readings are in line with what the ECU is showing.

That what I though too. I mean I hope my engine is just running at the perfect temp, but since this temps is hotter than the other place where people are used to put their sensor (thermostat housing, coolant hoses, ...), I didn't see any right operating temp. Most of the thread, people answer 88C.

I guess I would just like to know if any other bmw m3 owner could also read this value and confirm that's right. I didn't see any thread about that and it's really confusing.

EDIT: Or if you know a thread with the inside cylinder temp will be awesome too.

[hello1]

Thread revival but did you ever receive an answer and does anyone know the delta between temp sensor with obd2 reading and actual gauge tapped into a different location than factory temp sensor location? Thanks in advance.

[bmwdirtracer]

The coolant temp sensor lives in the cylinder head. It's hotter there, than at the thermostat, or top hose, etc. There's absolutely nothing wrong with 95 C temp readings at the cylinder head. Within the next generation, this temp was typically over 100C, and two generations later, ~105C+. On track, I allow my M5 to reach ~112C before giving it a half lap at less than full tilt.

You're running a stock cooling system in perfect condition? Fan clutch in good shape? Fan shrouds intact? You could always consider RedLine Water Wetter, and a Stewart water pump.....