I Just put a new engine in my jag and now my fans come on after the car is turned off which didn't happen with my old engine...any idea what that means...was something out of wack with my old engines sensors or is the new engine running super hot...let me know what you think...nick

 

hey nick well i don't know if our cars work the same i have a xj8 model but when i had the same problem the car didn't show that the it overheated it ran great i ended up finding out that i just needed to changed my thermostat you might have what i hear people call a lazy thermostat i hope this helps

 

Sometimes my fans come on when the engine is turned off - depends how hard I,ve worked it and ambient temperature etc.. Why did you replace the engine, sorry if I missed that bit?.. Thanks Allan

 

The whole story behind my new engine is here
https://www.jaguarforums.com/forum/s...7156&styleid=4

 

The whole story behind my new engine is here...hydrolock

https://www.jaguarforums.com/forum/s...7156&styleid=4

I didn't think the thermostat could be a cause of this...do you how the thermostat could cause the issue? Wouldn't the car just run cold if the thermostat was stuck? Or could the thermostat have been put in backwards?

From what I understand if the fans are already on when the car is turned off they stay on until the ECT falls below a predetermined value or for 5 mins which over occurs first...

Like I mentioned I wouldn't be too concerned with this but when the car was off the fans never came on before the new engine went in so it is making me wonder what is up

 

If you want to avoid guesswork, monitor the temperature with an OBDII reader. Normal operating temperature should be about 195-205. Hard driving and long idling will raise it and trigger the fans to bring it down.

Chances are your thermostat is not fully opening.

 

It is very common for any engine to rise in temp once the car is turned off as the water pump stops circulation water and thus decreases the cooling effect.

Also, the temp sensor is mounted at a high point in the system and the hottest water will "rise" to the crossover pipe from both the front of the engine and the radiator.

I have owned other cars that the electric fans would routinely engage after the engine was stopped.

Tho it does seem to be a peculiarity with out autos. As previously stated, a sticky thermostat could be keeping the engine slightly hotter than average and then when the engine is turned off it pushes the temp above the threshold for electric fan engagement.

Does anybody know the temp at which the electric fans engage? I might have an experiment I could do on my car to see if I can duplicate pdog's symptoms.

 

I never tested it on an XJ8, but on an XK8 it is around 215 degrees. I believe it has some hysteresis, so the turn off point was lower, but I did not measure it.

The easiest way to determine this is with an OBDII scanner. Then substitute a 1K variable resistor (a 10 turn pot is ideal) for the temperature sensor. This allows you to test the function quickly without even running the engine.

 

Ok...it was the thermostat...my mechanics swapped it out and it was fine...does anyone know does an xjr and an xj8 use the same thermostat?

 

Exactly the same ;o)
BTW....the thermostat is regarded as a service item
and should be changed at the same time the coolant is replaced
which is every three years.
It's not uncommon for a thermostat to be either stuck shut....rapid heating to overheating.
Or stuck open.....which leads to long heat up and over cooling when running which leads to poor running and excessive fuel consumption.

 

Great! It goes back to the old adage: When you hear hoof beats think horses not zebras.

Your posts and replies also informed us that the fans turning on is a normal thing if the coolant temp rises too much high after engine shut down. (Or at least I think so?!?)

 

Yes it is ok if the fans come on after the engine ...but I is rare and only when the car is hot ...in Miami like a very very hot day after driving in lots of city traffic...

 

Normal to have the fan run on HOT days for a few minutes after shut down. I notice it around 100 degrees outside temperature.

 

A feature Jaguars seem to have, usually the large fan at the front of the engine block, essentially when the vehicle is turned off the cooling system (radiator processes) are turned off with the vehicle.

On warm days when the vehicle shuts off after a nice run the fan activates to help cool the engine after being parked and locked. Deactivates after cooling past a certain temperature. (Essentially to prevent engine block cracking on hot days/hard engine runs.) On easy runs and cooler days the fan wont even activate.

 

yes, it is normal for the fans to come on during very hot days, like 100 degrees like Lear45 mentions...but...if your fans are coming on high speed everytime you turn your car off in less than hot (98+ degree days) than i'd bet there is an issue like what i had...stuck thermostat or something else....remember with our non realistic temperature gauges you may never know your engine is running hotter than normali...here is the method behind the madness with the fans...and don't get me wrong the cooling approach on our cars is the best i've ever seen...

for the XJR
Fans on Slow = on at 203F Coolant Temp and off at 194F Coolant Temp
Fans on High = 221F Coolant Temp and off at 212F Coolant Temp

for the XJ8
Fans on Slow = on at 194F Coolant Temp and off at 187F Coolant Temp
Fans on High = 207.5F Coolant Temp and off at 200.5F Coolant Temp

 

My fan started running after I turned off the car on hot days (about 100 degrees). It turned out that my lower fan had failed and the upper fan had to run when the engine was off to keep the engine cool. Car never overheated, and the coolant temp gauge never went above normal. After replacing the failed fan, they no longer activate even on the hottest days (100+ degrees).

 

Well, Jaguar's very questionable definition of "normal" is 185 F to 235 F before the temperature gauge rises above center.

 

Yes ...everyone remember like WhiteXXR said...the temp gauge stays halfway until your car is in an overheat situation and then it pops up to the top and the red lights goes on...

 

I read the temps via a OBD II reader when trying to diagnose the problem. Surprisingly one fan was able to keep the car within normal temps although it had to spin its heart out to do so.

 

I had the same thing happen a couple of months ago, fans started turning on after the car was shut off. It had never happened before, so I took it to the local Jag guys and they diagnosed the faulty thermostat.

While I was being done I also took the opportunity to replace the water pump with the updated unit, as the old style have been a known weakness. Finding information on which waterpump to use was a bit of a pain... there are three of four different types. I believe the original ones are ceramic, and can break. There are aftermarket ones with metal impellors, but also genuine Jaguar ones that have ceramic (or plastic?) impellors as well. I tend to go with genuine or OEM parts, rather than risk using something aftermarket, so in the end I decided to go with a genuine Jaguar part, with a metal gasket, rather than the OEM part which looked to be the older style, and had a cheap paper gasket.

I also have a feeling that if I had a RealGuage I would have noticed an increase in engine temp whilst driving. It makes sense that if the engine is hotter than normal after you've turned it off, then it must've been hotter than normal when it was running too.

If the original waterpumps are as bad as some people say, then the RealGuage upgrade seems like a very small price to pay for piece of mind. But also, having the fans turn on when you shut the car down is a good indication that you should get it checked out as well.