Last night I took my 99 xk8 on it's first long drive from Sacramento to Reno. The car had been sitting for 3 years with a bad A drum. Repaired that and now going through the car and fixing other maitenence issues. Last week I replace the shocks, coil springs, and upper bushings. The car was bottoming out every time I went over a bump. The lower bushings on the shocks were gone. Replaced them with Bilestein shocks and coil springs that we're on my 99 xk8 parts car that look like they may only be a year old.
The car rode and handled great. It might only need a front end alignment because it has a slight pull to the left.

The only issue I had that seem strange to me was after I drove 100 miles I stopped at the rest area at the summit. Elevation of about 7300 feet. On the entire drive to there the temp gauge was at the normal position. On the way down the summit it is probably a 6% grade. After I go up to speed I was coasting down the hill not giving it any gas a going about 70 mph. Slowly the temp gauge started to drop. It dropped all the way half way between normal and totally cold. Also the low coolant warning came on temporarily twice for a short time. I pulled over and checked under the hood. Everything looked ok but both fans were on. I continued down the hill and the temp gauge was then just a little below normal.

Question is: is this normal? Shouldn't the thermostat regulate the temperature better.

I think the low coolant level warning might be from a slightly low level and my going down a grade might have caused the warning.
You opinions will be appreciated. Thanks.

 

I forgot to mention that the temperature outside at the time was about 40-50 F.

 

If the coolant level drops below the level of the sensor it can result in erroneous readings. There have been a couple of instances where the engine was actually overheating and damage resulted while the gauge indicated low temp.

 

Thanks for the reply RJ. Correct me if I'm wrong but usally the engine overheats showing a low temp reading it is because there is no or very little water in system. I think the temp sensor needs water to work not hot air. Thinking this I turned the heater on and got hot air blowing out. If there was low or no water there would have been no water to blow hot air. I'm just concerned that the temp dropping that low the thermostat should of closed and shouldn't have the fans shut off?

 

If the AC was on the fans would be running (doesn't mean it was cooling depending on your interior temp setting). Since you were on a downward grade the engine wasn't driving much load, more coasting and at that alt and temp it could cool a bit. Don't worry about no water, if you lose it the temp gauge will go red as many on this forum have described. I'd keep a eye on the level though and keep it topped off.

 

The heater pump is at a lower level than the temp. sensor, but it doesn't seem reasonable that you could have coolant available there without the engine pump circulating it past the thermostat and therefor the temp. sensor.

The sensor will read either air or coolant temp., it can't tell the difference. So if the coolant were not circulating the crossover pipe could have air in it and not measure coolant temp. I now recall that was the situation that resulted in engine failure, but it was due to pump vane failure on early engines and is not relevant to this discussion. A major hose failure could also have that result but again is not relevant here.

So I am as confused as you are.

 

I'm heading back to Sacramento this morning. I'll check to make sure coolant is topped off. I'll see if it does the same thing down on the other side of the summit on the downgrade. I can understand the temp dropping a little on the downgrade but halfway back to cold I thought was too much. I thought the thermostat should closed some to keep the operating temperature at normal range. I'm concerned that it may be sticking a little and don't want it to stick in the closed position. When I get back I think I will change the thermostat to be safe.

 

Changing the thermostat is a good move. They have a history of sticking closed when they fail on the V8, which doesn't appear to be your problem. I have three thermostats which are very different. One opens early and the engine takes a long time to get to 185 F, another doesn't open far enough and results in temp. over 210F in summer.

I have the jagwrangler Realgauge which measures actual temp. and oil pressure and also Ultragauge which plugs into the OBD port and allows realtime monitoring of many functions including temp. and fuel trims.

 

I've been monitoring my coolant temp in real time and in real temperature measurements using the Torque App OBDII. I keep reading that the Jag coolant temp gauge is of a really poor design.

 

There is nothing wrong with the gauge. The information fed to it causes it to read mid scale for a wide temp range to avoid confusing uninformed owners. Now that I have the Realgauge mod it reads accurately.

 

Drove it home, what a nice ride. Before leaving I topped off the coolant and it took only about 12oz. As I was going down the summit there were 4 diffrent steep grades. On three of them the temp dropped one notch. Half of what it did on the trip to Reno. But one steeper grade at 6% for three miles it dropped back to halfway to cold again. It was almost predictable how it was dropping each time. As the downgrades leveled out the temp came right back to normal. I get the feeling that this must be normal for the temp to drop like this because it did it consistently every time. I never had a vehicle that the temperature dropped like that. But as noted there aren't any temp readings on the gauge so it is hard to tell what the temp actually is.

 

12 oz seems to bit alot, depending on since the last fill. You may want to pressure test the system to see if you have a leak, I'm surprised the low coolant light wasn't on solid, mine didn't have to go down far to trip it.

You might consider getting a ELM bluetooth obdII reader, someday the car will pop a CEL and you'll need to read them, with this and the torque app you can read the water temp in real time. While the tstat may well be the case here you might check the real temp before spending any $ on parts / labor.

 

I don't know the last time the coolant was topped off. I have checked it several times since getting the car restarted and saw coolant in resovoir but never topped it off. I do have a odbll
Code reader but didn't have it with me on this trip. That was my oversight. It might be a while before I put it under the same conditions but will keep monitoring the temperature.

 

After reading all the cooling horror stories here I bought a Wi-Fi OBD2 reader on Amazon and use Dash Command on my iPhone. I find that my car after warming up goes from 190 to 210 depending on the load. My thermostat opens at 190 and as long as there is little load it stays there. What are the chances that the PO installed the true temp option but didn't add the numbers. That would do what you describe.

 

Here's something interesting about the Jaguar 4.0 engine. I read that it has a Precision cooling system that was patented by Jaguar and is pretty unique. There's a low-volume cylinder head and block cooling passage into an the system. It flows longitudinally in this layout and has split block cooling and a high-efficiency water pump so the system achieves engine warm up in less than 4 minutes. It reduces the cold starting emissions, reduces fuel consumption and speeds up the interior Heating. This also allows for the capacity of the cooling system to be reduced. A technique used in their racing engines to control the temperatures. Pretty cool and interesting.

 

It sounds to me like your thermostat is stuck open. That's much better than it being stuck closed. Even if you were just idling as you were coasting downhill, the thermostat should close and keep the engine at the normal operating temp.

 

Sorry I haven't responded to your replys but I had to do my real job this week to earn money to take care of my babies (Jaguars).
RickL I don't believe the previous owner would have installed the true temp option. I have some of his service records that show only reg maitenence and minor repairs. He was an elderly gentleman that had about a dozen cars and don't think the xk8 was on the top of his list to do something like that.

Cyber3d interesting factoid. I have noticed that the engine does seem to wam up quickly.

Reverend Sam I was kind of going in that direction but it seems that the temp would fluctuate more often if it was staying open. Also thank you for your videos they have helped me when I work on my Jaguars