Coolant Temp & MPG Display - Diagnostics

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cb123

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I have a 2.4 V6 and it's been drinking fuel like mad, seems to be running very rich all the time. Also the MPG display is very erratic when driving, keeps displaying 0 and also seems to be just flashing up random numbers, seems to have little to do with the way I'm driving, have seen it flash numbers up to 154mpg - which would be nice!, is there a sensor that may be causing this?, or anything I can look up in Vagcom for clues?

Then issues with coolant temp: if I build up heat by holding the revs at 2 or 3k while stationary I can get the gauge up to 90degrees but then as soon as I start driving it will drop down rapidly - suspect thermostat jammed open - does this sound right?

I also think there may be issues with a coolant temp sensor as well. I did two sets of tests using the VAGCOM Group 4 measuring blocks and the climate control display reading 51:

Test1: VAGCOM GRP4,Block3 @ 58c while GRP4,Block4 @ 31c & Climate Control 51 @ 78c

Test2: VAGCOM GRP4,Block3 @ 62c while GRP4,Block4 @ 33c & Climate Control 51 @ 90c

Surely the coolant temps can't vary that much? can someone explain where the different readings are coming from and which sensor(s) may be faulty...

Any clues or advice would be much appreciated.
 
I dont know what your vagcom blocks are? Climate control 51 isnt the correct channel, it always just shows 90 once the cars been running for 10 mins or so. You should use 49 instead.

The car has two coolant temperature sensors, built into one package. One feeds the instrument cluster (and also the climate control) and the other feeds the ECU.

To verify the operation of the sensor, compare the Engine ECU (controller 01) reading in VCDS with the instruments reading (controller 17). They should both be within a few degrees of each other. If they are, its likely completely fine.

Its very likely its just the thermostat though, symptoms are completely typical.
 
As for the MPG display, i'd have to see it with my own eyes to see if its normal. Instantaneous MPG does move around a lot, and its perfectly normal to see --- or very high values such as 154 when slowing down (or even crusing at motorway speeds if there happens to be a bit of downhill section) becuase in those situations the ECU is injecting very little or no fuel, yet your still travelling at high speed. Similarly when accellerating most cars will be in the low teens or single figures, as accellerating uses lots of fuel.

If you want to view your average MPG, flick the display to the MPG reading with the Ø symbol at the start. You can then reset it every time you fill the tank or go on a long journey using the button on the stalk, and it will display the average over the whole trip.
 
Thanks Aragorn, that's useful info. Well I've been out and monitored CH49 on the CC and re-run VCDS and have the following to report:

On this test set the figure CC CH49 reports seems reasonable and coincides more or less with :
[VCDS CON1, GRP4, BLOCK3] = 63c

Where as [VCDS CON17, GRP2, BLOCK4] = 18C - which is the only temp I could find in these read-blocks.

There is also another temperature in [VCDS CON1, GRP4, BLOCK4] which is at 37C - any idea what this is monitoring?

So if [CON1, GRP4, BLOCK3]=63c and [CON17, GRP2, BLOCK4]=18C at the same time does that suggest CTS failure? or am I still monitoring the wrong data?

Thanks for your help.
 
37c is probably air intake temperature or something?

Not sure what 17,2,4 is either, perhaps outside air temperature? doesnt your VCDS have labels telling you what stuff is?

If the dash guage roughly agrees with climate and ECU then the sensor is fine.
 
The coolant temperature sensors are very prone to failure, intermittent and innacurate readings. The original part will be black, newer redesigned ones are green. If you have a black one replace for a green one.
Presume CTS location is the same as this.



The thermostat definately sounds as if it is sticking open so needs replacement, With the coming colder weather your fuel economy will only get a lot worse unless you sort this.
 
Thanks to you both for the replies.

In my VCDS it just says "temperature" doesn't seem to specify the temperature of what!

Do you know the exact group/block in 17 I should be monitoring, I went through all of controller 17 readings and this was the only reading labelled temperature I could find... I did have a lot of groups in 17 that said " group is unavailable" so maybe issues there?

Guess the sensor isn't that expensive so could try changing it anyway as desertstorm suggests, don't suppose anyone has a part number for the green sensor?
 
I've NEVER had a CTS fail.

Every temperature related issue i've had has been the termostat, and every one of my cars is on its original CTS, including my mums 18 year old 1.8T with 215k miles on the clock.

Everyone thinks they're unreliable, because they dont diagnose stuff properly, mis-interpret the ECU fault codes that appear with a faulty thermostat, and generally dont perform any proper diagnostics.

Leave the CTS alone, and change the thermostat, because thats whats broken.
 
I'm fine with that, and agree the thermostat is clearly stuck open. I just wanted to rule out the CTS as a contributory factor in the car running rich.

I'd rather follow through the diagnostics process and rule the CTS in or out based on a comparison of its two outputs in VCDS, but as I've said I can't seem to track down the two correct reading to compare - so if anyone can point me in the right direction on this it'd be much appreciated.

If the CTS is faulty too then it's clearly a much easier job than the thermostat so if it'd improve economy by even a fraction until I can get the thermostat sorted it would seem worth it...
 
You have been very lucky then. I have changed several on cars that have had either the temperaure gauge going up and down like a yoyo or poor fuel economy / starting on the engine due to the ECU not getting the correct engine temperature.
I do agree that the OP's main issue is the thermostat and if I was him I would be changing that first but whilst I was at it I would change the CTS as well.
The reason the newer ones are green is because the originals were a very poor design and fail a lot.
It's easy to see if the CTS has been changed by just looking to see if you have a green one.

Karl.
 
My brother ran his S4 for 9 months with a duff thermostat, due to the costs of changing it.

What we did was get a piece of hardboard, cut it to size to cover about half the radiator, and slid it down the gap between the coolant rad and the AC condenser. This cut the airflow thru the rad down, and allowed the engines temperature to sit much higher, and returned the fuel economy to more sensible levels.

Obviously care needs to be taken to ensure the cooling capacity isnt hurt too much, but for a cold british winter you wont have too much of a problem. If you find its getting too hot, make your airflow blocker smaller, and similarly if its still not hot enough make it larger.
 
Karl: i agree, they do fail, but they dont fail anything like the amount folk think they do. I've been on this forum for quite a few years, watching fairly regular flow of people come on with issues like this one, and the vast majority of the time, its a simple thermostat failure and the CTS is fine.

The problem is two fold, everyone on the forums perpetuates that the CTS's are crap, so they always change it, often doing the CTS and the stat together and finding their problem resolved. The CTS is also far easier to swap, so folk end up "just changing it anyway".
The other part of the issue, is that the ECU will log a "coolant temperature sensor, Implausible signal" fault code, on cars with a faulty thermostat. People see this code, and instantly think "CTS is broken", when all the ECU is telling you is the signal doesnt make sense. This is becuase the ECU is programmed to know that once the engine reaches 90c, it should stay there, and on cars with faulty stat, it tends to cool down again when travelling at high speed. This results in the ECU producing the fault code because it can see the signal doesnt make any sense.
 
cb123 said:
I'd rather follow through the diagnostics process and rule the CTS in or out based on a comparison of its two outputs in VCDS, but as I've said I can't seem to track down the two correct reading to compare - so if anyone can point me in the right direction on this it'd be much appreciated
Click to expand...

I'll plug mine in tonight and figure out what the group and block numbers are. Unfortunately they might appear in different places if your car is a different age or year to mine.
 
Thanks Aragorn, that's brilliant, I'll get myself some hardboard then - well worth it for the fuel saving for even a few weeks!

Should I align the board to the left or right of the rad (facing it from the front), or centre it, thought maybe I should avoid covering the area the aircon/overheat fan covers?

Mine's a 2000/X so hopefully the VCDS groups/blocks will match, fingers crossed!
 
aragorn said:
Karl: i agree, they do fail, but they dont fail anything like the amount folk think they do. I've been on this forum for quite a few years, watching fairly regular flow of people come on with issues like this one, and the vast majority of the time, its a simple thermostat failure and the CTS is fine.

The problem is two fold, everyone on the forums perpetuates that the CTS's are ****, so they always change it, often doing the CTS and the stat together and finding their problem resolved. The CTS is also far easier to swap, so folk end up "just changing it anyway".
The other part of the issue, is that the ECU will log a "coolant temperature sensor, Implausible signal" fault code, on cars with a faulty thermostat. People see this code, and instantly think "CTS is broken", when all the ECU is telling you is the signal doesnt make sense. This is becuase the ECU is programmed to know that once the engine reaches 90c, it should stay there, and on cars with faulty stat, it tends to cool down again when travelling at high speed. This results in the ECU producing the fault code because it can see the signal doesnt make any sense.
Click to expand...


YEP, my duff stat gave an engine temp that warmed up and was fine up to 3100rpm, any more than that then the increased engine speed led to the waterpump [directionally proportional remember] to pump the water through the system at a faster rate and lead to it cooling down, which could be quite "dramatic" if you forgot.....
 
Don't worry Aragorn, I've managed to find the correct readings to compare the two outputs from the CTS and as you suggested it's fine, the two temps are constantly within a couple of degrees of each other. So guess it's the hardboard shield over the rad until I can build up strength to open the wallet and get the thermostat sorted.

Thanks for all your help.
 
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