You know the temperature gauge with the "H-N-C" indicators.....

I know that overheating is quickly assured death to the V12 so I keep an eye on the temp gauge with an obsession (no surprise there).

The indicated temperature after warm-up seems to vary quite a bit. I'm comfortable when the needle is lined up with the bottom of the letter "N" but get nervous when it lines up with the TOP of the N. Today, it actually went a couple millimeters above the top of the N, on the highway doing a steady 60mph ~2,000 rpm for several miles.

Since it's winter here, the outside temperatures have been moderately cool; 40F-60F range these past few weeks.

When is it time to get worried? Halfway between the N and H, I suppose? What factors (outside of ambient air temp) can contribute to fluctuating temperature indications?

This engine runs so great now (the local Jag mechanic tells me I've got the best running XJS he's ever seen!) I'm halfway ready to rip out the radiator and have it recored or something.....

Thanks

Lou

 

The time to worry is now. If she's running at the top on the "N" now, what will happen in August?

My experience ....not shared by everyone....is that the factory cooling system will do the job IF it is 100% up-to-snuff. 95% won't do the trick. Radiator cleaned inside and out, new 180ºF thermostats, new fan clutch, and a properly bled system.

I might add that it isn't high temperature itself that is the big problem...although, obviously, it can cause damage if it goes too high. Instead, the design of the heads and the the way the coolant flows allows steam pockets to develop even if the engine isn't actually overheating. Steam pockets = no cooling in that area = dropped valve seats = engine damage.

That's why "top of the "N" is worrisome on a Jag V12 where we'd just shrug our shoulders on most any other car

Some guys have gone to electric fans and custom made radiators to give more cooling capacity

Cheers

DD

 

Yep, I've got trouble. Where are the thermostat(s) on the engine? Where the top radiator hoses meet the engine? One for each side?

Strangely, the thermostatically-controlled electric fan is not in total agreement with the dash gauge...it doesn't come on as often as I'd expect in a true overheat condition. Could the sender be giving a "false positive"? Where is the temp sender?

Thanks again, Doug. All makes good sense.

I should add that this came on all of a sudden. I checked the coolant level a week or more ago and today is the first I've noticed the overheating. Where to begin?

 

Thermostats: yes, one at the end of each radiator hose. Do not use generic replacements. The correct units have an air bleed hole and an extra disc at the rear to re-route coolant flow on a cold engine.

The aux electric fan should come on whenever the a/c compressor is operating or when when the coolant reaches about 195ºF (or about that). The temp switch is, as I recall, in the water pump.

I'd start by pulling the radiator. I'll wager you'll find the external fins 30% blocked by debris. As long as it is out of the car (not a 5 minute job on an XJS) take it to a radiator shop for a professional cleaning/boil out

Cheers
DD

 

Well today I bled the system as described by elevating the car more on the left etc. and I did get some air out of it. And the driveway is a complete mess so that part was a success.

But the problem today is worse. The temp now climbs to almost 1/8" above the N after running for around five miles.

This problem came on suddenly. I find it hard to believe the radiator would plug spontaneously from one day to the next, especially since the radiator was either brand new or recored at the time this engine was dropped in--less than 15,000 miles ago. The coolant is still totally green without traces of rust in it. Proved that during the bleed.

We checked the external radiator fins for debris. Pretty much clean as a whistle. No bugs, leaves, etc. in evidence.

The electric fan does come on with the climate control system, and today using a platinum RTD thermometer I verified that it comes on at around 88C (190F) even without the A/C on.

Here is a possible clue, the right top radiator hose is firmer than the left, the left one has more "squish" to it (after the car has run, and turned off, when hot). Maybe one of the thermostats is stuck?

That's my best guess. At this point I have beached the poor cat in the garage, as the climbing needle invoked imagery of Three Mile Island. I have shut her down until professional help can be obtained, or I can be guided through more troubleshooting on my own, thanks to you guys.

It has also been my experience that sometimes an effect can be linked to the most previous action--which was the replacement of the TPS unit. At the same time, I disconnected the A/C compressor electrical harness because the compressor is shot and needs replacement, it has a freon leak.

Am I overlooking something? Next place to look, thermostats?

Thanks again,

Lou

 

I would go with thermostats, yes.

The cooling system is a little strange on these cars. The radiator is partitioned with 1/3 flowing to one bank (left, I think) and the other 2/3 to the other bank....with the balance tube blending the results. I'm a little foggy on remembering the exact coolant flow pattern.

How's the fan clutch, by the way?



Cheers
DD

 

Thanks Doug. The fan clutch seems fine; the fan is moving enough air to take off if it had wings--or at least blow dry your hair.

Complicating matters is that I have lost my Haynes manual for this car. After an exhaustive search today the manual (last seen in the earlier part of the previous decade) was nowhere to be found. A trip to the local import parts store yielded only a wincing counterman who said i'd probably have to order it online.

 

Here's an update on the progress made so far in troubleshooting the "Top of the N" condition.

Took the advice about the cooling system needing to be 100%. Replaced both thermostats with the proper Jaguar 82C (180F) devices. Noted the old thermostats were 88C units. Also they were extremely clean - no evidence of rust, scale, or any other particulate matter in the coolant. I will test them in a hot pan of water tonight. My mechanic said he thinks one of them was sticking closed, or partially opening.

Ian (my mechanic) shot the radiator with an infrared thermometer on both sides, after letting the car idle for an extended period and noting that the needle went to the top of the "N"--but no further. The radiator yielded very consistent readings on both left and right sides of 185F.

Ian also concurs that the radiator appears "new".

I took the car out and tortured it on the highway, in no circumstance did the needle ever go past the top of the N as it was doing a few days ago.

Back to the 100% rule. The gauge sender. It too must die. We ordered a new one and will pop it in tomorrow.

This is different than the sensor that works the aux fan. That unit is a two-wire job, below the water pump. It doesn't seem to be always in "agreement" with the dash temp gauge as to when it thinks the motor is "hot". So the gauge sender is suspect.

The gauge temp sender apparently drops in resistance as the temperature increases. Our theory is that it has given up the fight after years of thermal cycling, and is giving erratic readings.

This also may be an obscure electrical gremlin. Never underestimate the power of the Lucas Dark Side. Why? Well, when I'm on the highway, with the climate control on, and thus aux fan spinning, the temp needle goes to the top of the N. Also, the voltmeter shows the electrical system in deficit (off charge) but that thing lies, too. Even when it is approaching the red zone (it never goes into it) battery voltage is still about 13 volts.

When I switch OFF the climate control, the TEMP needle DROPS to about the middle of the N, at the same time the voltmeter goes up to "13". So I cannot rule out that low voltage or a faulty ground somewhere MAY be contributing to this problem.

More news from the thermal war front as it becomes available.

 

Remove the instrument cluster (pretty easy) and use a pencil eraser to clean all the contact points on the printed circuit. Add a redundant ground wire to the cluster when you put it back in.

More details, to the extent my memory allows, on request

Cheers
DD

 

More details, please!

Gauge cluster: Been there, done that, a few weeks back.

Am I neurotically expecting too much? The needle stays between the middle and the top of the N depending on conditions...If I cruise along steadily, it pretty much stays in the middle, but if I have to idle at a light or in traffic, it goes to the top and doesn't like to come back down to the middle much.

What else is there? Am I destined to rip the radiator out? Is there anything I can add to the coolant to break things loose?

Any and all suggestions and knowledge, welcomed!!

Thanks

Lou

 

OK so here is a really stupid thought.

This "problem" began coincidental to the replacement of the defective TPS.

Mileage went up to an average of 12-13mpg with the new TPS, up from 9-10mpg with the old one, whose surging and erratic operation undoubtedly caused the engine to run rich.

Leaner mixture generally = hotter running. Fatter mixture generally = cooler running.

Not saying the fat mixture was "right"....but perhaps in returning the mixture to the correct balance, more heat is being created.

Stupid theory, right?

 

Cluster: remember, I'm going from distant memory :-)

In the upper corners of the cluster there are two little semi-square plugs or covers that can be popped out. Behind them are two cluster mounting screws to remove

Around the edge of the cluster is a flimsy, 2-piece finishing strip. One screw at the top and one or two screws at the left and right lower parts. Remove the screws and the finishing strip

I think you can now wriggle the cluster outward from the instrument panel but there just might be two more lower mounting screws, so be careful

The cluster will only come out a couple inches leaving little room to reach your hand behind to release the two wiring harness connectors. You feel the press-down-to-release tab on each one.

With the cluster out you'll see the flexible (and fragile!) printed circuit. One at a time remove the contact nuts (and bulb holders, too) to clean the contact points. Be gentle.

The cluster has a horizontal metal bar....can't remember if it's at the top or bottom. Attach a ground wire and run it to a known-good ground under the dashboard

This is a good time to clean the panel bulbs and replace any that are burned out.


Cooling: what you describe might be expected in Dallas in August, but not in the dead of winter. If it creeps up in traffic now just think what'll happen when summer hits!

The system has virtually no reserve capacity. Everything must be perfect or it won't work. Once you get it right it'll be good for years.

When you purged the system did you get a really good steady stream of coolant out of the bleeder? If not you still have air and you might have to make a mess of your driveway again. Been there, done that.

Cheers
DD

 

Update: Replaced the gauge temp sender. I didn't know this at the time, but Ian put in an aftermarket one.

Immediately, the "overheating" indicator got worse. WAY worse. Past halfway between the N and H! So we shot the temperature again with the infrared.

The gauge LIES! Actual temp on the LH side was 175 and RH side 185. This is normal operating temp, NOT overheating. We then shot the sender with the infrared beam. 185F. So it seems I have a gauge / sender issue, not an overheating issue.

Now as for this:

Performed all this, and more, the week after Christmas. Additionally, after cleaning the metal bar (it's at the top) we cleaned and used Pro-Gold G100L on all the connections, and tested every single bulb. Everything on the instrument cluster, with the exception of the proven-errant temp gauge, works fine. The only thing it lacks to your description is that extra ground wire.

I'm hesitant to remove the cluster again, because the printed trace ends have come loose, and some of them are literally waving in the breeze when the plugs are removed. It was a surgical operation to get them all straight and plugged in correctly when putting the cluster back in.

So back to the sleuthing...Ian has offered to get me a "real" Jaguar temp sender and install it FOC...so that's the next place we're going.

Less resistance = higher temp reading....More resistance = lower temp reading..... Does this not exonerate a potential grounding issue? Because if the ground was marginal, the gauge would "see" more resistance....or am I wrong?