Coolant temperature sensor accuracy
 

Still trying to get the car to start right in winter...
(2007 V50 2.0d)

How accurate should a coolant temperature sensor be?

Car was left standing for 8.5 hours outside. Whole day it was temperature around negative 5°C, right before starting car instrument panel was showing -6°C, but engine coolant temperature -1°C

Sice car was standing still for so long, it should be competely cold and I assume readings should be closer...

As ususal it started quick but was jerking and misfiring right after starting for a couple of minutes.

How much this measurement difference could impact the cold start?

If your coolant was really -1 I think you would have a really serious issue ;) The sensor is probably not accurate enough, that would be my guess.
I dont think it is related to misfiring at all, to be honest. Maybe bad quality diesel, or glow plug relay? Did you have those issues when temps were more normal?

Surely no issue with coolant being -1 or indeed -10 as it does not freeze at those temperatures.

Is it misfireing or just running like **** from cold, mine idles horribly for a few mins when cold, especially with the air con on, but then most diesels do especially older ones.

valid point actually, for some strange reason I was thinking in the water freezing treshold way...:> I am withdrawing my comment :P

[OT question from physics - so if there is actually -10 outside and you put a normal thermometer into bowl of liquid antifreeze left outside, it should show exactly -10 aswell? ]

Don't forget the sensor feeding the -6 figure to the dash cluster is mounted low down behind front bumper and is therefore picking up more the ground temperature,where the coolant temp sensor is mounted higher on the engine and is to some extent shielded from this and is more picking up the temperature of the coolant at the particular point on the engine where it's mounted.How much of a variation there is between the two I don't know.

Dippydog could be right...Try read the MAF temp without starting the engine and see what you get...the MAF it's also "inside" ...

I am keeping mine into garage and we have above 0 celcius temps for the moment outside ,but if you are not in a hurry ,I would check it on mine too(same car/engine but 2004 made)...

Though I would rather blame the injectors...Welcome to the OLD DIESEL ENGINES REALM :D

Yes.
On a stone cold engine, the coolant temperature will always match whatever the outside ambient temperature is within 1 or 2 °C.

Hey all, thanks for answers!

Your discussion about the temp sensor difference makes perfect sense! If I recall correctly then coolant temp and intake air temp had 1 degree difference.

Diesel quality should be ok, I buy it from reputable source, it is winter diesel rated to -32°C. Also when refueling in different stations it does not make any difference.

Glow plugs have been changed a year ago (battling this issue for second season :tounge_smile: ) relay is clicking and tested one glow plug for voltage (had to check all 4 I guess) it was ok. Glow plug light and a relay click is very quick tho, something like 3 seconds after turning on ignition. Keeping ignition on for longer or turning it on and off a couple of times before cranking does not make a difference.

Injectors were also on the potential problem list, since its the only car and commuting without it would be a nightmare, I purchased a set of used - tested injectors and replaced them over a weekend. It seemed that car was running a bit smoother, but no changes in cold start.

Car runs fine and starts ok when temps are not freezing. No other issues than cold start. Until -10° it does not crank longer than usual, fires up but sounds like it will throw a rod out for couple of minutes, jiggles like a paint schaker, and puffs out white smoke (would indicate unburned diesel, but not a lot of smoke).

When it was -15°C it cranked a couple of turns longer, but also tried to fire up, made a couple of bangs and then stalled. It starts at third attempt or will keep running on first one if throttle is applied to rise the rpms up. Still sounds like it is going to disintegrate...

I assume some of you will say that compression is possibly down, but wouldnt it cause excessive blow by?

As Zork perfectly said - old diesel engine realm :teeth_smile:

Egr was blocked for a shot period to test - no difference.
No visible coolant usage.
No significant oil usage.
Air flow at idle ok according to my calculation.
Fuel pressure right after cold start goes to ~40 000 kpa, I assume its ok since it tries to run ar 950 rpm.
Throttle flap not sticking (replaced Y-shaped pipe bit, and also experimented with vacuum hose removed).
Battery is a couple of years old, with a bit more CCA.
Starter also works well.

Hooked up the DICE today and I got this:
Ambient temp +14;
Coolant temp +9;
MAF temp +14;
MAP temp +13;
So...let it be :D

Thanks for info Zork!

This confirms that sensor reading discrepancies is normal (within limits of course).

Sadly Im still not closer to the reason for engine unhappiness with cold. Since I dont have unlimited funds to spend and finding a decent specialist who can solve this issue seems impossible, I guess I will have to get rid of it...