Well as usual, I get a call from the wife and she says the jag over heated. She shut it off to cool down and got her home safe and sound.

Background: I replaced the Octopus/Monkey hose from hell 9 months ago.

Today I checked the coolant and it was dry. Filled it up and brought it up to temp. I found a hairline crack in the main tank. Side note: It stayed at half temp for a couple of minutes then went redline in a matter of seconds. redline. Coolant was spraying from the crack.

Replaced the main coolant tank and cap. Replaced the 5 inch coolant hose on top of the Thermostat housing as it was bulging. Replaced coolant, ran car until fans came on, tested heater., shut off. Allowed to cool, then topped off coolant (not much). (I followed the procedure outlined in 'post 7')

Test drove the car and it did the same thing but no coolant leaks. The temp gauge will swing up from mid temp to redline and back. Mostly staying at redline IE Idiot lite coming on.

Any usual suspects that I could check out would be appreciated before I send her out to the independent Tech? And sending her out would be such a defeat

Update: There are a couple of teaspoons of coolant on the floor under the expansion tank.

 

The thermostat could be sticking in closed position, but it doesn't sound exactly right for that condition.
RJ_____________
97 XK8 Conv 85K mi

 

From dead cold, does it move to overheat quickly? In that case, how is your waterpump?

 

From dead cold it rises normally (3-5 minutes) to mid temp It will stay there and the rad fans do their job. So I get cocky and drive it in the neighborhood. I get about 1 mile before its starts racing to the danger zone. Return to home base.

From there you can visually watch the temp needle go up to red and back down to about 3/4 hot in seconds. Back and forth....no rhyme or reason.

At first I was thinking air bubble because she did dump most of the coolant. But I followed the before mentioned refilling sequence to a 'T'.

Now Im thinking some sort of temp sensor got cooked? Didnt get the coolant/H2o mixture perfect? Mayan Calender? Im half tempted to go buy a thermostat/thermostat housing unit tomorrow so at least I have one on hand to tinker with on Sunday ( I work tomorrow).

So If I play Sherlock Holmes for a minute......The coolant tank ruptured on the wife, could that have caused something cooling related to die also (sensor/thermostat/flux capacitor)? It runs fine and holds coolant normally. Except for the new dribble under the expansion tank, and from what I gather, that is to be expected when topping off coolant. Correct me if Im wrong on that last one.

Hope this helps.

 

There are two tanks. One is just behind the left front bumper area and it is ok to dribble if there is too much on a fill. But, the reservoir at the firewall should not dribble if you are filling without spilling. If it is dribbling, then look for a leak on all of the connections underneath.

Deciding that the temp sensor is bad is a bad idea unless you can absolutely guarantee that you are correct. Of course, it is not expensive, so you could replace it and see. But, don't drive it until that is confirmed.

Try filling the system by running it with the reservoir cap off, heat full on with high fan, until it reaches normal temperature, refilling it as required. Then, cap it off before stopping the engine. Let it cool and check again.

 

Thanks Plums. The firewall tank is new as of today and not leaking. And thank you for confirming that the overfill tank does indeed spew out coolant from over filling. I wasnt a 100% sure on that one, but I am sure that I probably over filled the main tank (firewall).

If i can just figure out the over heat indication........

 

You've worked through the usual suspects for overheating. Have you had a scantool on it to read the actual coolant temperature cycle?

Graham

 

I dont have a scan tool. But I my buddy is bringing over his laser temp gauge in the AM. Im guessing 180 to 210 range is norm?

Anyone know where the temp sending unit for the the cockpit gauge is?

 

Laser temp gauge should be just as good as a scantool for this particular issue.

Sender is next to the thermostat housing but slightly different on Normally Aspirated or Supercharged.

Graham

 

The thing that a laser thermometer will not do for you is to do a live trace while driving. Any of the computer or phone based scan tool + software combinations will do that.

 

Well, you have covered all the usual areas and the repairs sem OK.
That leaves only two suspects.
1. The thermostat is very sensitive to an overheat condition and should as a matter of course be replaced.
2. The expansion tank (firewall mounted) has a level sensor that fits in the bottom of the tank, this sometimes gives trouble and is easily replaced.
3. The Atmospheric catch Tank is the one mounted forward of your left front wheel and will dribble to floor if your system is overfilled.
Correct level in dead cold condition is to the base of the filler neck....any more and it will be fed to the ACT.
Disconnect your battery for 30 minutes and it should reset.

 

I would be worried that the cracked expansion tank was caused by something else. Do the pipes going to the tank (top tank) cross each other ? That's just a check since you replaced the tank. Ditto on the scan tool. You cannot trust the temp gauge. A thermostat wouldn't hurt. Be careful of the overheating !

 

Just FYI, (since I got a Realgauge temp gauge) my VDP runs right between 200 and 205F in just about any conditions.

 

The level sensor would not cause the temperature gauge to shoot up. Excess out of the catch tank will also not cause the temperature gauge to shoot up.

 

Right then. I replaced the Coolant Temp Sensor. No change in the dancing gauge. Temps on the Hot hoses are 210f and the cool side is 124f. I took these readings when the needle was pegged in the red.

The Coolant tank lines are fitted correctly ( I take before pictures......learned that lesson in my youth.)

I have an aluminum thermostat housing and thermostat en-route.

Side note....Wife just told me that the Fuel needle had been intermittently failing. Is there a common ground wire for these 2 gauges?

 

Check for adequate flow as well. Apparently you can see the water movement in the reservoir. Someone else more familiar with this might chime in and tell you what it looks like.

 

With the cap off you can see return fluid being returned. Volume increases with engine RPM.

 

FIXED!!

One thing I hate is when people ask for help (and get it) but dont post what the solution was

I had to go out of town (again), so the wife took the car to an independent shop. Drum roll please.............the thermostat was sticking closed, then full open according to the mechanic. (His words....I know sounds fishy to me as well, but its fixed.)

And the cherry on the cake........I had ordered a new thermostat / thermostat housing and it arrived the morning I left for my trip. And is still sitting in the box in the garage.

So the biggest let down was I didnt get the big 'Ta-Da' moment.......and her paying $300 for a thermostat replacement.

RJ237 called it! Thanks to everyone for the help!

 

Thanks for getting back. It is a little bit of a let down - thermostat ? Oh well, all's well and all that stuff.