Coolant temp reading via OBD (98. 2.8)

Dino D

Dino D

Zorg Guru (V)
British Zeds
Joined
Mar 28, 2014
Points
176
Location
Kent
Model of Z
2.8 Manual
From where does the measurement come from when plugging in the OBD reader?

I had my reader plugged in and looked at the live data and noticed coolant temp was around 120c. Is this normal based on where it gets it reading from (is it before or after the coolant goes through the rad?)
My viscous fan works fine and so does my aux fan.

I'm in the process of testing without the viscous fan in place and before changing the aux fan temp switch to see the effects to want to know what is the baseline OBD temp I should be getting ( as I know the gauge itself doesn't move in real time with coolant temps as the fluctuate).
 
On the 1.9 it's from the right side of the head - have a look on real.oem under engine cylinder head parts. I have seen mine reading ~112 on INPA with the cluster indicator steady at 12 o'clock position.
 
The temp needle is buffered and should sit bang in the middle for a range of approx 23 degrees IIRC. I made a chart which converted the hex codes to temp readings a while back, I post it if I find it as it may help somebody. I've never really though about where the temp is taken from, hmmmmm............
 
I'm just wondering so I know if it's running hot or not and then see the difference with viscous removed.
I'm not going to change the thermostat but will do the lower temp fan switch.

Over on Bimmerforums there is an extensive experiment with the cooling and I think they running lower than 100c on fresh radiators (no viscous fans too)
But again I just want to make sense of at what point it's being measured so I can be sure I'm comparing the same thing.

It won't suprise me if the radiator is simply no longer as effective after 17 yrs, after all everything else has beenchanged in the cooling system.
 
So I've done some testing.
Viscous fan off.
Aux fan works (new) and new 80/88c fan switch fitted.

I've got my OBD reader plugged in and taking love readings.
The middle of the gauge on the dash covers around 90-120celsius.
It moves right toward the next mark (but not at the red) when temps get close to 130c.

The temp seems to sit around 120c when moving (light throttle, ambient temp of around 18c).
After a run I stop and leave the car running and after a few minutes the temp climbs to 130c (and gauge moved to the next white line).
The aux fan kicks in when OBD shows around 132c. It runs for around 45seconds only and the switches off and shortly thereafter the temp drops back closer to 120c (and this cycle repeats).

I've looked to see where the temp sensor for threads gauge and appears to be in the head (item 14 in pic)
image.webp
http://www.realoem.com/bmw/enUS/sho..._1997_Z3_BMW_Z3_28&diagId=11_6172#13621703993

As I understand these cars have a thermostat that controls the temperature of the water as it flows into the head.

The aux fan only comes on when the coolant coming out the radiator is at 80c (since I've now fitted the 80/88c fan switch).
The corresponding temp readout from the OBD when this happens is at 130+c.
So the radiator must be cooling the coolant by some 50c.

Am I understanding this right?
Is the OBD temp reading of 120-130c normal?
 
Dino,

Below is a chart I made some time ago which from the information I could gather at the time seemed to indicate the temp range of the car.

Temp%20Chart-1.png


I had my 1.9 at the time and no INPA so reading the hex codes was not too easy. I know the cooling system is pressurised allowing for higher temps but running at 120 - 130 degrees does seem a little high. The later speedo heads give a temp reading in degrees when using the test mode, I'll run my car tomorrow and get some feedback for you as to the temp I am running at.
 
Last edited:
Thanks Brian.
It's been suggested on another forum that the scanner I have may need calibration. I've emailed the supplier about this, it's a icarsoft model.

It doesn't seem to compute that the rad is reducing the temps by 50c or more at standstill with only the aux fan (the coolant coming out the rad is 80c or under otherwise the aux fan would be on all the time).

Ps out if interest when you say 'over centre' point on the sheet for the temps in red does that mean it sits at the next mark or was that a gradual shift from centre toward the the next mark?

Pps nice work on the measurements for the spreadsheet by the way!
Many thanks.
 
@Dino D, the figures in blue represent the needle from cold to the centre position, the figures in green represent the temp range whilst the needle is sat in the middle, the figures in red represent the the needle between the middle and top of the gauge, the highlighted figures in red show when the warning light comes on.
 
Very interested in the results as I am looking at removing the viscous fan & fitting an electric. How is "dash test 6" accessed, is it a button press sequence or is this a reader function?
I used an infra red thermometer on the top & bottom hoses while I carried out an oil flush, from hot engine then15 mins at high tick over, the top hose was hovering around 97 centigrade the fan did not engage at anytime in the 15 mins, although it was freewheeling. I seem to remember about an 8 to 10 degree difference from bottom hose to top hose.
 
Thanks Jonco very helpful.

A quicky for Brian H, off thread, Sorry, do you keep to handbook tyre pressures for track use? As I only do sprints which are 1.5 miles at most I put standard pressure in.
 
MartinN said:
A quicky for Brian H, off thread, Sorry, do you keep to handbook tyre pressures for track use? As I only do sprints which are 1.5 miles at most I put standard pressure in.
Click to expand...

I generally start the day cold at 30PSI all round but after the first run quickly check my tyre pressures when the tyres are still hot, I will then drop them back down to 30PSI, the front left gets the hottest and I usually have to take 12PSI or so out of it but they all need some air out of them. Depending upon the track I will then play around with the pressures, 2 - 3 PSI to get the best feel.
 
Thanks Brian.
I think my reader is off as the gauge doesn't go above the white mark between centre and red and the red light doesn't come on (which it should according the to temps the reader is giving).
 
Dino D said:
Thanks Brian.
I think my reader is off as the gauge doesn't go above the white mark between centre and red and the red light doesn't come on (which it should according the to temps the reader is giving).
Click to expand...

I put my speedo in to test mode today Dino and the car runs between 96-98 degrees, if I stop at a junction etc it will blip up to 99degrees for a second or so then return.

It is maybe worth trying this yourself?
 
Update - almost solved.

Thanks @Brian H for the tables and thank @jonco for the link to the dash test thread.

Using Brian's table I was able to easily read the actual temps via the dash test and guess what, the icarsoft code reader is waaaay off by up to 30c!!

I had given the system a quick bleed and top of up of coolant last weekend but didn't have time to test further.
Anyhow since that the gauge barely moves from centre and doesn't go up to the 3rd mark at all now like it did last weekend.

Temps when driving gently are sitting around 105c.
When stopped and left idling it slowly rises around 118c at which point the aux fan kicks in.
According to the limits BMW set this is all within normal but I would like some reserve.

I think the reason it's just over where I want it is probably because there is still some air in the system as a quick bleed last week has improved things so:
Will do longer bleed (with front if car raised).
Will also fit a new radiator cap as the system holds 2bar pressure and the cap must be the original so worth a change.
If that doesn't bring cruising temps down to under 100c then I'll probably need to do a full flush.
The radiatior seems to be doing it's job ok so probably plenty life in that still although it's plausable that the raditior is not a clean and clear as it was 17 years ago..
 
Dino D said:
........
I think the reason it's just over where I want it is probably because there is still some air in the system as a quick bleed last week has improved things so:
Will do longer bleed (with front if car raised)...........
Click to expand...

Pleased you are getting closer - hopefully it is still some air in system.
Are the hoses rock hard and bulging when up to temp? Even under pressure there should be some give when you squeeze them. Taking the cap off and squeezing hoses can sometimes help with the bleed.
Good Luck
 
You must log in or register to reply here.
Back
Top