Hi, I just have some trouble with my XJ8. For about two months the car was sitting outside on my trailer platform (about 2 feets from the ground) as I have to change the front bearings and it took some time to find the right tools.
I started the engine a few times, just to keep the battery charged, it was ok.
Mid november all was done with the bearings, but another problem just started.
Starting the car, it was a little difficult as the starter turned the engine but without succes. A few more turns and it started. I just seen that during the first tens of miles the water temp indicator was down, but my supplementary electronic gauge showed 80-90 C degrees. As I was driving on a long way, after half hour, the temp gauge was going up to the middle of the scale as normally. I seen the P0128 on the tester, indicating the thermostat is faulty.
Today I changed the ECT (engine coolant temperature sensor) testing both the old and new one with the ohm meter and they both were ok, showing 2,1 kohms at a temp of more than 20 C (as in the manual).
Tester in the OBD, and I just switched the engine on (starting ok) seen the temp going down to -8 -9 degrees C , and then climbing up to 25 , degree by degree in a few minutes.
The pipe where the ECT is mounted was around 50-60 C maybe but on the tester it was 25 C.
As the fan is started by the ECU after receiving the temp from the sensor, I am thinking of problems as my car can overheat if the fan is not switched on by the computer (it is working, as it turned on a few seconds as I turned the key without cranking the engine).

Just disconnected the battery (the remote just opens but don't close the doors, don't know if it's connected in some way) to cure the bad gremlins till tomorrow.

Any thoughts? (Other than battery voltage...)

Why my new sensor as the old one shows a lower temp than it is normally in the system?

 

Just fully charged the battery, to be sure it's not voltage related.
The engine cranks and starts immediately, but the coolant temperature on the tester is the same: going down to -20 C (car is in the garage at +15 C) and then going up to a low temperature in a few minutes. Anybody can check what is the coolant temp on start and the few minutes after?
Thank you!

 

Clean the MAF sensor and be sure the connector is installed correctly, no bent pins.

But your engine temperature is about right 90*C or 194*F for an engine that has warmed up Depending on the air temperature, it is quite likely you don't need the fan in cold weather.

 

I will check all the connectors, including MAF as well, thank you! On JTIS all the repairs regarding the temperature sensor are connectors related (cleaning, bent pins,etc). Hope to be some corroded pins and an easy repair.

 

I just cleaned and sprayed with some WD40 all the connectors on the cooling tower area and on the ECM and the temperature is now OK. The temperature while driving goes to 91-92C or a maximum of 94C (201F) , I hope it is Ok also.
It is very important for the engine to have the right temperature sent to the ECM as it influence the idle, gear shifting, fuel mixture ans cooling fan kick-on.
The dash gauge is moving to the middle where it stays usually.

 

Flay, it seems you are reading the engine temp via OBD (ECU) - this will have stored the old sensor feedback, try a hard reset with the new sensor and see if the ECU adjusts after a 10 minute idle. If not, and the old one works refit it? Did you fit a new T-stat also?

 

New sensor only, the T-stat was changed 2-3 years ago with the new plastic pipe+sensor, also with the aluminum tower. I am sure the old sensor is ok, just changed it as cheapest part to start the checks.
The battery was disconnected 2 times for 12-14 hours, for sure the problem was with the connectors or wiring. I have to change the ECT connector, as it is braking in parts.

 

i still have trouble with the temperature on my 2002 XJ8. The dash temp gauge is down even if the engine is hot and the OBD reader shows temperatures going to 20 C or down to 0 C sometimes.

Checking the coolant sensor I found the right values, of 1,6 Kohms at 25-30 C so I think it's ok.
I found also the oil temperature, the one on the oil filter housing and found a value of 30 kohms at the same temperature (both of them at room temperature).

Anybody can give me a right value of the oil temp sensor? Is this value normal?

Any other advice? I don't run the car as the shift is very rough, the fan don't start as the ECU thinks the engine temp is somewhere around 30 C maximum.

P0196 present currently and this was the only one seen these days.

 

It looks very much that the bad coolant temp sensor connector and/or the wiring to it is causing intermittent and varied temp readings that you have. Check carefully the crimping of the two wires to the pins in the connector, they may be corroded and loosing good contact. Anyway, as you mentioned, you need to replace the connector.

The reason why you are, at times, getting very low temp readings even though the sensor is good is some additional resistance created in a bad contact in the lines to the sensor. The sensor is an NTC (negative temperature coefficient) resistor, the resistance value of which goes down with higher temperature but, with additional resistance from a bad contact in the connector, the total resistance read by the ECU is higher than the actual resistance of the sensor (higher resistance is interpreted as lower temperature) so the ECU "thinks" the engine is cold even though it is not.

 

Thank you, just changed today the connector with one from a scrap yard. I checked also the values on the wires right on the ECU connectors, they are right with the engine off. Will try tomorrow a direct wire from the oil sensor to the ECU but don't know if the sensor is ok as don't have the right values to compare with. Are they the same as for the coolant temperature?

 

There were two types of coolant temp sensors - early (first pic) and later (second pic). The oil temp sensor is the same as the later coolant sensor. There is a big difference between the two types of sensors: early ~1.8 Kohm at 25C, later ~28 Kohm at 25C. If you have the later (fat) coolant temp sensor (3/8 BSPT, ~18 mm thread), you can use the oil temp sensor for testing.

 

Just realised that you said you measured ~1.6 Kohm on your coolant temp sensor at 25-30C. This is fine for the early coolant sensor, M12 thread. However, since your car is MY2002, your coolant temp sensor should be the later type with reading of ~28 Kohm at 25C. The later type of sensor is for VIN 853936 and later. Perhaps having the incorrect coolant temp sensor is causing the problem you have. In that case, the oil temp sensor should work fine as the coolant sensor.

 

hi, thank you for the help!
the coolant sensor is the one in the first picture, I have three of them, all of them used to work ok, including the one original to the car. The oil sensor is 18 mm thread, as in the picture but with a conical head. I didn't find the right values for the oil sensor to check with mine.
If I am right, to check the harness for shorts, it must be 5 volts at the connectors?

 

Yes, the oil sensor, and the later coolant sensor, have tapered thread (3/8 BSPT) which is roughly around 18 mm, not M18, and their reading is about 28 Kohm at 25C. It is strange that you have the early type of the coolant sensor since, from about 1998-1999, the sensor was changed to the later type (except on supercharged engines). I suggest you try connecting the oil temp sensor temporarily to the wires of the coolant sensor and see what readings you get. If the readings are very low, -C, or you get error message, go back to the "slim" coolant sensor and look for any poor contacts in its wiring, the connector side and the ECU side (I don't think a short would give you the problem you describe). Otherwise, I do not know what voltage should be on the coolant sensor.

 

Hi, today I solved the problem with my coolant temperature readings.
It was the engine's ECU, the big computer found in the bulkhead box (right if your car is LHD, left if it's RHD) behind the toolbox.
I had a car electronics specialist that found that the ECU didn't see the right temperature and I order one from the ebay a cheap 20 pounds part.
The new one must be coded to the car, so he changed the EPROM from my old unit to the new one.
The car starts and drives perfectly, the temperature is right on both gauge and on OBD reader. So, good to know our engine ECU's can go wrong!
Thank you guys for help!

 

Well, took you long enough, congrats!
And thanks for letting us know, I love threads with a (successful) end.

 

Fortunately it's not my daily driver so it stayed in the garage till I fond the solution.