That’s fine to take it out to replace there will be a small amount of coolant but it wont spray out.
1 Like
According to my OBD Scan the car is at OL Open Loop! Shouldn’t it be at Closed Loop with 72 degrees? At OL the O2 sensors don’t work or am I wrong?
Is the coolant sensor ok? It should be because I have no fault codes and it shows 72 degrees? Or should I order a new one anyway? Is a VEMO spare part okay? Before I got the Autel I scanned with the ELM327 on Septemper, 4 th. and the android app also showed the 72 degrees. Thats odd. Also the app stated open loop because of low engine temp. So its the sensor or the thermostat is stuck, right?
No. Thermostat cannot cool an engine, it can only help heat it up faster.
The thermostats job is to limit the amount of coolant the engine gets until it approaches its operating temperature, then it starts to open in order to maximize the coolant to maintain the desired temperature.
If the engine continues to get hotter, there is nothing the thermostat can do to bring the temperature down…ever.
That cheap coolant temp sensor is a common failure, and will prevent the car from going closed loop. The target temperature for your engine is 88C.
I don’t get it. So is 72° too low to go into closed loop operation?
If so, the question is what the real engine temperature is. If it is 88 then the sensor is broken.
But I don’t understand why you want to rule out a thermostat that is stuck open? That can cause the engine temperature to be too low, especially at idle or low load.
Cheers,
Harald
Thats what I also thought… But some people stated, that the engine enters closed loop even earlier than 72C. I can’t comment on that obviously, cause I am a Jaguar-noob 
The thermostat is incapable of cooling an engine, it can only heat an engine.
Thermostat has essentially two options, fully closed and fully open. (There is a small window where it can be partially open, but for simplicity, let’s assume one of the other).
When the car has reached operating temperature, in this case 88C, it is fully open. The thermostat has no ability to cool the engine. It cannot get the car to 87C, much less 72C. It is not possible.
A failed thermostat either fails open, delaying the cars ability to get to operating temperature, which is bad for the engine, or it has failed closed, in which case it will overheat the engine. There is no circumstance where the thermostat can cool the engine. A failed open thermostat is merely delaying the inevitable.
If you were to install a 72C thermostat, the engine would still run at 88C. It would get to 72C quickly, then eventually get to 88C.
If you install a 92C thermostat, the engine would run hot, at 92C.
Does that make sense?
If you want to fool the engine into thinking the coolant temp sensor is hot, then unplug it and jump the two contacts on the harness.
Your car should go into closed loop and the oxygen sensors will spring to life.
1 Like
No, it doesn’t make sense.
If the thermostat was either closed, or open, what would the 92° thermostat do when the engine has reached 92°? According to your theory it would fully open. So why then would the engine run at 92° when it runs at 88° with a fully open 88° thermostat?
The cooling system has to have enough cooling capacity to cool the engine under full load in hot weather. So under light load in colder weather it would cool the engine far too much with a fully open thermostat. Therefor, the thermostat HAS TO control the temperature by opening just enough to reach the correct temperature.
Cheers,
Harald
Once the engine reaches 92C, the thermostat is indeed fully open, but now the engine that wants to run at 88 is starting to get cooler, so the 92C thermostat has to close again to prevent it from getting cooler.
On a cold winter day can the engine be cooler? Perhaps on the highway, but I don’t believe the cooling system is capable of maintaining 72C, in the summertime, at idle, with the hood down. Is it plausible, I guess it could be possible, but I’d bet against that.
Basically you are discribing a hysteresis controller where as I assume a linear controller. If the hysteresis approaches zero that is no different from a linear controller with a proportional gain that is very high. Either way, this is a closed loop control. So actually we are not far apart in our ideas of how the thermostar works.
Cheers,
Harald
FWIW wouldn’t a 92 degree thermostat be constantly going open then closed as the coolant temp cycled up and down?
Yes. I was just using that as an example.
Alex,
I understand your frustration! On my 95 XJ6VP, the CEL kept coming on saying it was the O2 sensor(s)! I changed all 4. After clearing the code, the light would come back on saying the O2 sensor(s) was bad or “not within the trim range”! Long story short, it was my engine ECU. The ECU looked perfect as the car was garaged from the day I bought it new. My electronic friend rebuilt it or resoldered the board. No more problems in 7 years!
Richard
1 Like
Hi Richard,
my CEL was never on, also no fault codes. Also the car was perfect 2500 kilometres ago. Its a mystery but it will be solved.
Yes, I thought about that too. But first I would check the harness to the ECU, also the mass point in the engine bay, the fuses, the connectors. I soldered some parts myself, but if a part is blown I would search for a new ECU.
Alex
Could that harm the ECU? I mean I am shorting the harness. Some stated test your O2 with a multimeter which resulted in killed their O2 sensors right after.
If this thing can be tricked out so easy and given the low price of it; does it read the actual degrees or just “water hot” - “water cold”. Or are the exact temperatures delivered by the one for the gauge in the instrument cluster? I would like to read the original Jaguar service manual.
I good starting point for the manual is here http://www.jagrepair.com/images/Training%20Guides/801S%20-%202000.pdf
Page 51 of the PDF is where it starts talking about the AJ16
I use this for my live data
1 Like
Hello Peter,
thank you.
Going back to your initial post you say the car runs fine, with decent fuel consumption and NO error codes or check engine lights and it is only during the idle test when the CO reading is too high.
I can’t think why you are suspecting anything in the EMS if that is the case. The Lambda sensors are unlikely to be faulty, if they were failing it wouldn’t just be during idle and you would have an error code with a CEL flashing up. If the thermostat was stuck open the car would take a noticeably longer time for the needle to move up to the N position and most likely wouldn’t move much past the cold sector while driving on a motorway, something you would definitely notice.
Your description suggests the fault is downstream from the EMS, a dirty cat for instance. In your position I would fill the tank with Optimax and add a bottle of Cataclean, check the air filter is clean and then take the car for a 50 mile motorway blast to get the cats really hot and hopefully burn off some of the contamination. Cataclean will not have any effect if the cats are breaking up or already broken inside but it’s worth trying before you start replacing parts that are probably working fine.
I bought my car from Italy in 2019. It also ran fine but did not pass the emissions test (I think due to too high CO as well, but I didn’t see the test protocol). The garage that did the testing changed both O2 sensors and everything was fine. I have driven 10000km since then and will see how the next emissions test goes in march next year.
1 Like
Hi,
thanks to all your kind advices and help. Yesterday I changed the cooling temp sensor and drove on the highway to heat the engine up. To my surprise the temperature after that was only 75 degrees C. I removed the fuses F11 and F17 to shut off the fans. The temp rose to 104 degrees C or even more. I scanned the car and its still in open loop. The O2 show no sign of life. I then tried so switch the O2-connectors (see picture) but they won’t open that easy. I was about to use stronger force or a screwdriver but before I’ll attempt this and maybe damage them I was wondering about the colours. Looking from the passenger side (EU spec) the one on the left has a red, white and two blue wires. The right one three blue and a white wire.
When I look down where the O2-sensors are located and move the left connector its connected the one sitting on the right in the exhaust, and if I move the right connector its connected to the left O2-sensor. Is that mixed up or okay? Maybe someone has a wire-diagram? But even if they are mixed, they should show some movement right?
Thanks for your patience @all. 
