Because the pressure cap can't be tested via normal pressure testers, I would replace the ten year old cap as well for piece of mind.

 

Thanks, Is the cap heat sensitive as well as pressure sensative? I'll see if I can get one locally since it says FoMoCo on the cap.

Here is the latest disappointment:
6/20/2016 133,192 (Overheated again after 100 mile ride)
Saturday we drove approximately 100 miles over four hours in 90 degree weather with four stops including lunch at a Sonic where it idled for 15 to 20 minutes with the AC on, and the coolant temperature remained at 200 degrees F plus or minus a few degrees. However, after the fourth stop to visit friends for an hour, on a 2 mile drive to Wal-mart it overheated again with the dash gage needle going to the top, the red light coming on, and the UltraGage showing 246 F. Opening the bonnet there was coolant around the filler cap and some sprayed onto the engine cover and I noticed that the upper radiator hose appeared to have backed off Ľ” from the stops on the radiator’s stub and there was coolant on the frame below it. I let it cool down enough to loosen the hose clamp (a worm drive after-market type), snugged up the hose to the stops, re-tightened the clamp, and tried running the engine at 2000 RPM and It built up to 246 F on the U-gage and lit the red light on the dash temperature gage within 2 to 3 minutes. The radiator top hose was 193 F, the T-stat housing was 142 F at the hose clamp, the right (passenger side) radiator end tank was 179 F at the top and the left (driver side) end tank was 110 F. I did not see any leaks at the hose or the pressure cap.
We called our friends for a ride home, left the jag overnight, and the next morning it got another 30 mile trailer ride back home. This morning I checked the coolant; it was down and took 12 ounces to refill the tank with the bleed screw in and a total of 44 ounces to refill it to the max mark with the bleed screw out to allow air to escape.

So….could the pressure cap and/or the top radiator hose blow off nearly 1.5 quarts in 2 miles, or might there be a leak I can’t see? Or maybe the hose was leaking all day? I had previously replaced the thermostat and done a cold pressure test at the coolant tank vent hose, and the pressure never dropped, and I had run it nose up with heater on max for 30 minutes and then checked the coolant level when cool at the bleed screw and the filler cap and it was OK. I’ll try another pressure test, but I am both disheartened and stumped that it could run fine for 100 miles and then suddenly overheat again. Perhaps I need to leave the testing T in place at the vent hose and let it get hot to see if the pressure drops then? Any suggestions will be appreciated.

 

I am curious about the fan. Are you sure it is working? On a long drive with plenty of air being forced thru the front grill maybe no problem. However on a short trip starting while the car is still hot maybe not enough air being forced thru if the fan is not working?

With the engine hot and with ignition on the fan should be blasting air through into the engine compartment and it will feel quite "warm" to say the least.

 

I forgot to mention that the electric fan was running and did so for 3 to 4 minutes after turning the engine off when it was hot. I was driving at 50 MPH when I noticed the over temp light; I had stopped watching it as closely because it had done so well all day.

The fan was also running the first time it overheated for my wife at night, and at least this time she knew what it was VS the time her van overheated and she called from out of town and said, "I can't turn the engine off! I took the key out and it keeps running! What should I do!" It took me a few more questions to figure out it was her fan and not her engine.

I was surprised at how relatively quickly this engine does cool down with the bonnet open; it took maybe half an hour to allow me to undo the hose clamp without it belching coolant. BTW, the top hose is a short elbow and it has a lot of give to it and is easy to pinch cold or warm; should it be stiffer?

 

Could it be a water pump issue after all?

 

My top hose has a fair amount of "give" in it when warm.

XJ8R mentions the water pump. When I replaced my hoses I replaced my waterpump. The original had a plastic impeller, in good shape, and the new one had metal.

If the water pump is working then temp readings either side of the radiator should show some sort of difference, logically, with the car running.

 

When you did purge the system with the ignition on it should drain the coolant reservoir fairly rapidly to make up for whatever was lost.

Did you notice this happening before?

 

I had thought it might be the water pump when it first happened but tried the T-stat on recommendation that it is an easier thing to do. The old T-stat did open, but the new one seemed to open a little farther and I was happy that it seemed to solve the problem - at first, that is. You'd think that 100 miles and 15 minutes of idling with the AC on should have shown up a bad water pump unless there is something that can happen to suddenly make it stop working and then be free again later. Right now I am thinking a leak under pressure and then overheating when it loses a quart or more of coolant?

I picked up a new coolant tank cap at O'Reilly's just now and it will not fit. It starts on but gets tight way before it is sealed and looks a bit crooked so the threads must not be the same. It also says 16 PSI instead of the 140 Kpa on the old cap which equates to 20 PSI. The old cap says FoMoCo and made in India, but it fits. Just another gift of the "global economy" and the Chinese; so much for the "premium Quality" on the box in English and Spanish.

Well, time to let this one rest and go install the alternator that came in for Momma's minivan; not a trivial task itself, but hopefully a more productive one in outcome.

 

Hi philwarner. I also found myself facing the same symptoms, including an apparently successful fix following bleeding of air from the system. I'll just share my experience and opinion, but you may not like it. However, it might save you a lot of time and money.

I also had an overheating issue that developed suddenly, in just the same manner. The signs of water you see under on the engine cover following an episode is most likely the coolant cap blowing off.

Like yourself, the car was showing overheating without seemingly being hot. Thing to bear in mind is that the temp gauge in the dash does not report temperature, rather infers it from pressure in the cooling system. When I took actual measurements using a thermocouple, there was no evidence of overheating. Furthermore, i could turn the car off and then switch the ignition back on a minute later and the temp would read normal.

Pressure test, with the bicycle pump style tester, also showed no leaks. That little pump does not replicate the conditions of a hot engine. This is a key factor.

I had many theories, thermostat, airlock, water pump, blockage etc. I inspected them all and found no problems except signs of airlock. Like yourself I also saw the odd bubble or two in the coolant tank during idling. I thought they were too small to be a head gasket leak. However, ultimately this symptom can only be explained one of two ways. Either there is air trapped in the system making its way out, or there is a head gasket leak somewhere.

The presence of air, rather than exhaust gas, in the system would also cause localised boiling of the coolant that would appear as bubbles. However, once purged the problem would disappear. For me, the problem did disappear. For a while. However, since the root cause was a blown head gasket, it came back...and each time it came back more quickly.

In my case, the gasket leak caused excessive pressure in the cooling system leading the gauge to report overheating. It also disappears when the engine isn't running, blows off or cools down just a little, or is dumping enough water out of the expansion tank to make the pressure hold steady, until it runs out altogether. The gauge then reads normal.

I hope everything is working fine now for you, but I would advise just looking at the following things, just in case.

Open the coolant tank and look at the level. Then remove the bleed plug to see if the level drops. If it drops again then you know there is more gas making its way into the system since you last purged.

check for more bubbling in the coolant tank. Try revving the engine whilst in neutral and seeing if this presents or increases the amount of bubbling in the coolant tank.

If the above show positive; open the coolant tank while the engine is running (and still warming up) and see if you can smell exhaust gases. Better yet, try a chemical sniffer test.

Hopefully I am just being a scaremongering coward. However, if not, you might be able to save yourself some hassle by taking care of the problem now.

 

It's funny, I was thinking the same thing, (HG failure).


I had similar symptoms on a Rover V8, and also a Jaguar Series 3 4.2. I had real trouble with the Rover blowing out coolant, and it was only when I took the heads off that the trouble became obvious. In those days I wasn't up-to-speed with car engines, only motorbikes. Then a few years later, it was the same with my Jaguar. This was also HGF, but with the XK engine, it was caused by the cracking between the bores syndrome. The Rover went well after I overhauled the two cylinder heads, and re-assembled. The Jaguar needed a new block or top-hat (lipped) liners inserting. I managed to get an old but uncracked block and built the engine around that and had no more trouble.

 

My initial concern was a head gasket leak so I looked for oil in the coolant or coolant in the oil and was relieved to find none, but still worried that the cooling system was getting pressure from somewhere other than heat.

So the temp gage is really a pressure gage? Does that also go for the OBDII temperature value? That might explain why the temp reported there goes up to 246 F and not above if that relates to the pressure release at the filler cap.

The one step I was not able to do was to remove the plug in the super charger cooling circuit to check level there, but I have since found a 1/2" quick-link with a long nut that is 3/4" across flats that will fit into the female recess in the plug so I can use a 3/4" socket and long breaker bar to try to loosen it.

After the last episode the level in the coolant tank did go down by a quart when the bleed screw was removed so I either did not get it completely blead or more air was introduced as you suggest.

I have let the Jag sit while I replaced the alternator in Momma's minivan so she can use it for her trip, but I will try your suggestions after I try removing the plug to do the SC purge step and I will call around to see if anyone can do a sniff test of the coolant.

One thing I also don't understand are all the dire warnings in the manual to NEVER start the engine with the pressure cap removed at the risk of catrostrophic damage to the engine. Don't drive it that way I understand, but never even start it?

Thanks again for your insight.

One other thought; if you blow a radiator hose would the temp gage never show an overheat condition with no pressure in the cooling system?

 

I think if you blow a radiator hose it will show overheating straight away. With the system no longer pressurised the water will boil, and the gauge will report the overheating. The gauge isn't quite a pressure guage i think, but it certainly isn't a temp gauge. It pretty much just shows three values: cold, normal and overheating.

I also had no coolant in the oil. The leak was from cylinder to coolant channel, so there was no mixing of oil and coolant.

Try to check for bubbling and white / steam like exhaust smoke. They are sure giveaways. The sniffer test can confirm after.

Hopefully you got it first time and you've just got a bit of air caught in the system.

 

Oh yeah, as a general rule, never start it with the cap off. However, if you're keeping an eye on it and make sure it doesn't overheat, its fine. It just that without pressure, water boils at 100°C. With pressure it boils much higher. With the cap off the water will boil away quickly, reducing the coolant level...and splash ferociously risking an injury to yourself. Once the enginer reaches temp the thermostat will open and you will get a lot of bubbling and steam. So keep water handy and make sure the level doesn't drop too far, and be carefull. You should have your information long before the thermostat opens anyway. Just use water while you're diagnosing. Once you've tracked the fault you can drain it out and use a proper coolant mixture.

Watching how the water behaves in the coolant tank can be quite informative. The owners manual is meant for drivers don't forget, not mechanics!

 

6/20/2016 133,192 (Remove Super charger fill plug, SC coolant pump does not run)
I cut the nut off of a ˝” quick link, removed the engine cover, and used the nut and a big ˝” breaker bar and ľ” deep socket to remove the super charger cooling system fill plug. It broke loose with a heavy tug and then turned easily. A little coolant came out as I removed it and the coolant level was right at the top of the filler plug casting. The 4.2 with supercharger purge instructions say to turn on the ignition with the SC plug out (but with the main pressure cap and bleed screw still in place) and allow the SC pump to run, and then to top up the coolant through the supercharger fill port and then replace the plug and tighten to 33 foot pounds.
I put paper towels around the fill plug casting, set my camera to record at the filler plug hole, turned on the ignition, and nothing happened; the SC coolant pump did not run. I haven’t yet found the SC coolant pump but while looking for it I found a two pin connector beside the air cleaner box with yellow and black wires from a device in the bottom of the air box and green and blue wires in the wiring harness, and some varmint had eaten through the black wire right at the connector. Isn’t country living wonderful. I doubt this is for the coolant pump, but I need to figure out how to repair it.
Can someone tell me where the SC coolant pump is? Left or right side? In front of or behind the main radiator? The line drawing in the manual is not clear on location.

While paging through the manual I found a test proceedure, "G549822t1 : CHECK THE ECT PID VALUE", that lists the resistance of the engine coolant temperature sensor for 20 temperature steps from -40 F to 266 F, and on Pg 3012 it says "Engine Coolant Temperature (ECT) Sensor - The engine coolant temperature (ECT) sensor detects engine coolant temperature and supplies the information to the TCM. The ECT sensor is used to control the torque converter clutch (TCC) operation."

I guess this is in the transmission section so it doesn't mention the dash temperature gage, but....is the dash temperature gage a pressure gage or an actual temperature gage?

 

The temperature gauge displays engine temperature from the ECT. It is NOT a pressure gauge. That is misinformation.

It is accurate from 125 F to 185 F. From 185 F to just under 240 F the needle remains at 185 (centered) as directed by the instrument cluster computer. Then over 240 F it rises rapidly (within about 3-4 degrees) to the top of the scale. The actual reading can be seen with an OBDII scanner.

An air pocket around the sensor can cause a false reading, since the sensor must be submerged. Because the sensor sits high in the system, low coolant on the order of a quart or more can also cause a false reading.

 

There are two electric pumps on my car. Both are on the passenger side of the radiator.

One half way down is the heater coolant electric pump. Easy to get at.

The one below it at the bottom of the radiator is the SC coolant electric pump. Difficult to get at.

You can see the bottom of the SC pump if you take out the filler "cap" next to the fog light.

It operates as soon as you switch on the ignition and before you start the engine.

If you are down say a quart and the coolant "overflow" tank is full it will rapidly lower the level of coolant in the coolant "overflow" tank.

 

No need to remove the SC full plug
Just make sure the reservoir is the highest point by raisinig the front
Park on a slope or use ramps
Fill / top up the system with the ign on (not running)
Make sure the heater is on
The Aux and SC pumps will circulate the coolant and bleed all the air back into the reservoir

After 5 mins or so replace the cap start the engine and get it up to normal temp
You can lower the front at this point
Once up to temp shut off the engine and let it cool
Top up as needed

I had a small leak in the aux pump where the body seal o ring is
I found that by looking for the crust left behind by the dry coolant
To find it I used an inspection camera going over the system bit by bit

If that failed to find the leak my next step would have been to add some AC die to the system. When the coolant leaks out it leaves the die behind. You can easily see the die with a uv lamp. If no leaks flush and replace the coolant to get rid of the die. The die wont harm the engine or parts but may react with the coolant additives over time

If you have done all that and no leak you need to look at the rest.
The valley hose will be ok because the 2 tests above would show it up
Whats left is head gaskets, water pump and thermostats and or a blocked passage

Its a good idea to fit the low temp thermostats. They are cheaper than oem as well
I did a write up on that

Cheers
34by151

 

Thanks, I'll look for the SC pump again tomorrow. Are both electric pumps supposed to run when the igniition is turned on or just the SC coolant pump? Nothing runs when I turn my ignition on, so perhps the rodent got to the SC pump wires too. That might explain why the initial overheat happened, but not so much why it would drive normally for 100 miles and then quickly overheat after a one hour stop. I may still be looking at a leak somewhere in that maize of hoses.

 

Thanks for the verification on the temp gage; interesting that it remains centered over a wide range above 185 F. I wonder what is the reasioning behind letting it stay centered and then max out over a few degrees; you'd think it would be better to give some indication of rising temps before the red light comes on.

I did see the rapid rise of the dash gage and a fairly rapid rise on my UltraGauge when it was low on coolant, but the UG temp was a more linear rise and it maxed out at 246 degrees and stayed there as I drove a mile or two further until there was a safe place to stop.

 

It is parked on an uphill slope of about 5 degrees.

So both electric pumps shoud run with the ignition on and the engine not started? Does the climate system have to be on for the Aux pump to run? I think mine was turned off today when no pumps ran with the ignition on.