Coolant temp sensor

[PeteK]

Good morning all,

I was thinking of replacing the coolant temp sensor on the Passat, this is following a comment made on a post by Martin1810 that if the reading does not reach 89C the engine runs on a less efficient map. I have logged the figures from a cold start on a warm spring day (outside temp 10C), as you can see the figures rise steadily to 81C then the graph levels out. I have also created another graph from a warm start and again the average temp seems to be 81C with an occasional increase to 84c, however it never reaches 89C. My car is a 2004 '54' Passat 130 Highline, has now done 174k miles and is on the original sensor. It is running like a dream at the moment and is averaging 48 / 49 mpg for every full tank so excessive consumption is not an issue, however if there is potential for it to achieve better results that would be great. I was also thinking along the lines of the thermostat is possibly opening early. Any thoughts appreciated please!

Thanks in advance,

Pete

[SONICtheONE]

I had the same problem on my 1.9 tdi awx engine, where the themostat was opening to soon, I first noticed this because the temp gauge was not quite reaching 90oC, so did vcds log and got the same sort of result, I didn't think it was the sender as the reading was stable, the time the sender failed on me a few years ago the gauge drops to zero and then back to normal and repeats.
Be warned, after I changed my themostat, my rad started to leak in the normal place, bottom N/S of the car, I put this down to proper water flow and new anti-freeze.

Alan

[martin1810]

Good morning all,

I was thinking of replacing the coolant temp sensor on the Passat, this is following a comment made on a post by Martin1810 that if the reading does not reach 89C the engine runs on a less efficient map. I have logged the figures from a cold start on a warm spring day (outside temp 10C), as you can see the figures rise steadily to 81C then the graph levels out. I have also created another graph from a warm start and again the average temp seems to be 81C with an occasional increase to 84c, however it never reaches 89C. My car is a 2004 '54' Passat 130 Highline, has now done 174k miles and is on the original sensor. It is running like a dream at the moment and is averaging 48 / 49 mpg for every full tank so excessive consumption is not an issue, however if there is potential for it to achieve better results that would be great. I was also thinking along the lines of the thermostat is possibly opening early. Any thoughts appreciated please!

Thanks in advance,

Pete

The temp graph looks good if a little low. I would do nothing as the mpg figures look ok. You could try doing the same log for fuel temp sender and see what that shows. The figure of 89 C is not set in stone as it varies slightly with model, engine, ecu etc. If the fuel temp sender looks very similar, I would leave everything alone.

[Peter D]

I think it is the stat. Cover the rad with a peice of cardboard blanking 80% of the rad leaving the top RH corner ( front front view ) uncovered. The temp should rise to 90/95.

What you have displayed is a typical stat problem. The fail due to coolant ingress into the was element which cause the stat to open early so are too far open at 87/89 degree C. If you want to be sure then measure the resistance on the two elements in the coolant sensor, they should be the same and 107 ohms at 90 degrees C. Regards Peter

[PeteK]

Thanks for the reply. The gauge always reads bang on 90C when the car has warmed up, although I understand that the gauge will always do so whether the temperature is anything from 80C up to about 100C and it does not display an exact figure. It's just from the figures it appears either the thermostat is opening a bit too soon or the sender unit is reading too low which makes me think the engine is not running on the most efficient map that it could be.

[Peter D]

If the temp gauge locks onto 88/90 then you do not have a problem The element in the Coolant sensor is different to the one that is fed to the guage although should have the same resisance at the same temp as the ECU utilised element It is NOT true that the gauge will read 90 if the temp is between 80 and 100 degrees. Regards Peter

[PeteK]

The temp graph looks good if a little low. I would do nothing as the mpg figures look ok. You could try doing the same log for fuel temp sender and see what that shows. The figure of 89 C is not set in stone as it varies slightly with model, engine, ecu etc. If the fuel temp sender looks very similar, I would leave everything alone.

Thanks Martin - here is a log of the engine, fuel and air intake temperature taken at the same time. The fuel temp is approx 20 C lower than the engine temp. Like you say, as it is returning good economy and performance it may be best to leave it alone rather than replace parts which may not be at fault.

[martin1810]

The graphs look pretty good. I wouldn't bother to change anything. Soetimes you fit new parts and they are worse than the old ones. "If it aint broke, don't fix it" as they say.

[PeteK]

As ever, thanks for your help Martin. Your website is fantastic by the way - I never knew how to create a graph in Excel before today!

Pete

[scotty33]

foe what it's worth the other thread was started by me I think? I changed the stat in mine last friday and it now goes up to 89-90 before the stat opens. The problem with mine was that the seal on the stat poppet was mis-shapen and no longer bonded to the poppet, hence some coolant flow at all times.
Even though my faulty stat was giving no more than 80 deg (at G62) my gauge was howing 90 at this point. When the g62 was 75 it was just shy of 90 by a mm. Slow warm up was telling me it was a stat problem.
I appreciate you think your car may be able to perform better, but mine was/is struggling to top 40mpg, if yours is doing fine why spend the £30? and beware the thermostat housing bolts are 7mm thread x 21.5mm long, not common in my garage stock, ask me how I know...

Agree Martin's site is brilliant, diagnostics and windows for dummies all in one place!