Tim Matar

Hi there,

Need some help with a fault I am getting. I have read a few things online but nothing really helping me.

A few weeks ago I had a 'Coolant Display Defective' message appear on the dash. It also disables auto stop/start which makes sense. The car drives normally but the fans go on high speed, the gauge needle goes to zero and the red LED on the gauge flashes. If you turn the engine off and back on the message clears and the gauge works fine. The car is not overheating. There are no codes logged.

This happens randomly, hasn't happened for over a week of driving but has happened two days in a row.

The only cause I can imagine would be responsible is a faulty coolant sensor or a faulty gauge. Much more likely a sensor is faulty versus the gauge.

From my reading I understand there is a coolant sensor in the back of the motor where the crossover is and I BELIEVE this one signals the gauge. I also understand there is a sensor in the Thermostat and I BELIEVE this one signals the ECU. It would be great to have this confirmed. My Thermostat was replaced 2 years ago.

Does anyone have any advice for me? Have you replaced the rear sensor, I have been told it is a intake manifold off job.

Cheers,

Tim

mk85911

I have experienced a similar situation, but with slightly different warning messages.

I have a 2014 CS, and about ten months ago I began getting “check coolant level “ warnings, and the temperature reading would hit the maximum temperature reading, and then go back to zero. This would repeat a few times. But the temperature was actually normal.

I took it to the dealer, and they told me that the coolant level contacts in the coolant bottle were dirty, so they cleaned the contacts and topped off the coolant level, and everything was ok. They said that if the problem re occurred , they recommended replacing the coolant bottle, which has the coolant level sensor inside.

So, a couple of weeks ago, I experienced a recurrence of the same problem. This time, I looked into the coolant bottle, and I could see two prongs, which I assume are the level sensors, and I cleaned them with an old toothbrush, and topped off the coolant. This was about two weeks ago, and so far, the problem seems to have been fixed. Fingers crossed! Mike

Tim Matar

Thanks Mike but this is a different issue. Mine specifically refers to coolant display. I replaced the coolant bottle a year or two ago because it had a slight leak but at that time it was the same message you are getting.

My issue seems to occur when the car is coming up to temperature (about 10m into my morning work commute).

kon5t

Mine does the same, I have replaced the sensor at the back of the engine and the thermostat, no avail. What appears to be happening is that the thermostat opens fully and that drops the temp to around 150 (it is a thermostat with electrical override, which closes fully during warm up causing a shortened coolant circuit for quicker warm up- 3 stage instead of the usual mechanical only 2 stage). This seems to be defect, it warms up to around 167, and then drops back down to 149 as the thermostat opens fully (you can see the cycle in the ECU monitor).

The question is why is it doing it and how to fix it?

Tim Matar

Does you thermostat have a sensor with connector on top? Did you check that wiring? What model Cayenne?

i read somewhere today that the car expects the vehicle to reach operating temperature within 12m of startup. What is interesting is that this is exactly when mine throws the fault. This must mean either the thermostat is partially open and not allowing the vehicle to reach its target temp or a sensor is incorrectly informing the ECU of a lower than actual temperature (slow sensor). I am surprised you replaced both and neither fixed it. That makes me nervous about embarking on the job.

Tim Matar

OK so I did some diagnostics this morning with my iCarscan PO v2.0. Car is currently off the road until I can repair it.

Recap of issue: Car displays 'Coolant Display Defective' after approximately 10m of driving. Message displayed on dash, temp gauge drops to zero, red light flashes and fans go full speed. Turn ignition off and on and everything goes back to normal. Car is not overheating. No ECU fault codes are generated. Coolant level is fine. Theory - Car is not reaching operating temp as quickly as expected = error.

Troubleshooting: I connected the scanner to cold vehicle this morning and selected 'T140_Map-controlled thermostat activation', 'T020_Engine temperature (measured)', 'T025_Temperature: radiator outlet (measured)', 'T090_Engine temperature (sensor)' and 'T095_Temperature: radiator outlet (sensor).

To begin with and as expected both temperatures were reading around ambient temperature (~21C). Sensor voltages were within range. What was strange is the thermostat activation was 97.25% (I believe this means fully open thermostat). It is worth noting with ignition on and before starting the vehicle the thermostat activation was 2.75%.

I let the car idle for 10+ minutes watching the engine temp slowly climb and the associated sensor voltage reducing as expected. The radiator outlet temperature remained cooler eventually settling at around 10-15 degrees cooler than the engine temp which makes sense to me as the radiator cools the coolant. What didn't change was thermostat activation, it remained at a constant 97.25%. The car eventually got to around 60C engine temp and wasn't getting any higher.

I then turned the ignition off and restarted the vehicle. Suddenly the thermostat activation was reading around 47% and the engine temp rose around an additional 10-15C presumably reaching operating temp.



Does this indicate a faulty thermostat? It was replaced 3 years ago but I don't see what else it can be. Unless it isn't getting the correct signal but the fact it is saying 97.25% activation on start-up to me indicates the problem is with the thermostat. What doesn't make sense is why restarting the car would cause it to suddenly start working. I would hate to go to the trouble of replacing the thermostat only to find it was some signalling or other issue.

Tim Matar

Further discussion on this topic here:
https://rennlist.com/forums/cayenne-...l#post19137723

mikescayenne

mine used to do something similar, the car was not overheating, but the coolant gauge would go all the way to max, kick on the fans full speed, and the red light in the coolant gauge would flash My car only does it when its really really cold outside though, so maybe 2 or 3 times a year in the middle of winter, and we had a warm winter last year so it didnt do it at all last winter. I just turn the car off immediately and then back on and all is well. I just chalked it up to a german electronics thing, but seems like yours is much more frequent. would love to know why it happens.... following.

Tim Matar

Mike mine doesn’t go to max, the temp gauge goes to zero like it has lost signal and then displays the error. Hopefully we will know soon as I am planning some repairs over Christmas holidays and I really hope it fixes it. I have a,so been getting noticeably worse fuel economy which is another symptom of failed thermostat. At first I didn’t suspect the thermostat as it was only replaced in the last 3 years.

Tim Matar

Hi Doug or anyone else who is following this thread. I have been doing some reading on thermistor type sensors that are used as temperature sensors in our vehicles. A suggestion was made to check all of the starting voltages of temperature sensors because in theory they should be roughly the same voltage at the same temperature. I took the below readings which show that the two coolant temp sensors have a different voltage as compared with the non-coolant sensors at the same temperature. Is anyone able to do this simple test for me on a cold engine - first confirm the measured temperatures are within ~5 degrees of each other. I am not looking at the specific voltages themselves but more so the variances between sensors at same temp. This will confirm for me that what I am seeing below is 'normal'. I suspect it is normal as it would be unlikely two sensors would be out by a similar factor. If so the ECU would know about it.

Tim Matar

I have observed the following and I would like to know whether this is ‘normal’ to help me track down why my car is throwing this damn code.

The DME has many measurable values. I have been focusing on:

• T140_Map-controlled Thermostat Activation (I understand this to be how much activation is required to reach full open)
• T005_Engine Specified Temperature (I understand this is operating temperature target - unconfirmed)
• T010_Engine Temperature
• T020_Engine Temperature (measured)
• T090_Engine Temperature (sensor)
• Temperature: Radiator outlet
• T025_Temperature: Radiator outlet (measured)
• T095_Temperature: Radiator outlet (sensor)

I have observed:

1. Engine specified temperature will either be 93.75C or 105C and it does not vary with the exception that if the car started with this value at 93.75C and the temperature exceeds this it changes to 105C. It appears to be an operating temperature target based on some variables.
2. The car appears to maintain the 105C target (ONCE/IF reached)
3. Turning on the AC reduces engine temp (in my case yesterday from 107C to 92C (15C) as fans cool engine. Why would the car do this?
4. On my daily commute to work (~15M) the car never reaches operating temp or specified engine temp. Driving with AC on makes me less likely to reach operating temp.
5. Thermostat appears to open from around 57C as this is when I see radiator outlet temps start to climb more than ambient temp increases would cause. My experience with ‘traditional’ thermostats is they open at a specified temperature - e.g. 90C

I will add to these observations in the hope it will help others diagnosing a cooling system problem in future.

garrett376

Hey Tim - so did you finally get a new thermostat in there and did it fix your problem like I'd think it would? Let us know, thanks!

Tim Matar

@garrett376 As it turns out, life threw me some hurdles so I have had to delay repairs. I have however been driving daily with the thermostat disconnected for many months with no issue - just check engine light for disconnection.

So I have finally purchased all of the parts and will be doing this job myself. Even though the water pump was supposedly replaced 3 years ago, I am fitting a new one at the same time as a genuine Thermostat. I have also decided to do my valve cover gaskets at the same time as RHS is starting to leak very minor. I am also replacing serpentine belt, all pulleys and tensioner and just for fun rear door lock actuators. SO I should have an answer by the end of next week because I am doing the job next week.

Question: As I will be using new Pink Coolant (Nulon Long Life), I was thinking of dropping the bottom hose, reconnecting and running with 20lts of distilled water for a couple of minutes and then dropping all of that before the final fill. Hoping to get ride of any trace of the old coolant in case they are not compatible. Good idea?

Tim Matar

So finally I got around to doing this repair over a year after the issue presented. Although it may be a touch too early to tell, I am quietly confident this strange issue has been fixed by replacing the thermostat. I didn’t think the thermostat was the most likely cause as it had been replaced only 2-3 years ago by an Indy when I bought the car. In addition the coolant temp readings would show that coolant was starting to pass through the radiator earlier than expected which lead me to believe the MAP thermostat was activating early for some reason. When I disconnected the MAP sensor the car would come up to temperature faster (good) once again suggesting the MAP part of the TSTAT was the problem but it would only do this if the DME asked it to and why?

So yesterday I pulled out the old tstat and replaced it with a genuine unit (so glad I did). The old one was a Euro Parts (one of the cheapest you can buy). Well the plastic fins broke when I removed it and the edges of the plastic rim were literally breaking off in multiple places with little force. I poured water into it and it ran out as soon as I did. At first I thought the tstat had failed open but I actually now suspect a seal that is supposed to go around the valve plate has perished leaving a gap.