I’ve noticed a gradual rise in engine operating temperature over the past year or so. I’ve got a pre-HE V12 with 84,000 miles and it has been properly serviced. The temp gauge is a round one with “C” at one end and “H” at the other with the word “normal” in between. Normally, the needle rests between the “r” and the “m” at 70 mph. and 3,000 rpm.

This is what I’ve done so far. I ran garden hose water under pressure all over the front of the radiator and between the radiator and oil cooler (some leaf bits and other trash between the two). No AC on this Jag. The four-bolt fan clutch checks out OK with less than half a revolution of the belt-driven fan when spun hot. I drained the coolant, ran lots of garden hose water through the radiator and out the drain and replaced with new coolant. Oh yes, and I replaced the radiator cap with the ROM-prescribed AC Delco 15 lb. cap. Took it out on a 200 mile trip with no noticeable improvement in temp. gauge reading. In order to get the temp needle back to where it should be, I had to drop the mph. down to 55-60 but that’s not much fun.

Here are some Fahrenheit temperature readings I took this morning with a laser temp gauge at approximately 3 minute intervals with the engine idling:

A Bank B Bank
107 100
142 128
150 143
150 160
156 175

When finished, the engine was fully warmed up and the temp. gauge needle was between the “r” and the “m". I believe B Bank temps run a bit higher but I'm not sure these temps are within normal limits.

I’m thinking that my next step would be to replace the original (as far as I know) factory 180 degree thermostats with new ROM-prescribed 180 degree thermostats with jiggle pins. I’m hoping there’s an easier fix but maybe not.

Any advice and counsel will be appreciated. I’ve got another road trip planned for next month but I won’t take her out of the garage if I can’t get this sorted out satisfactorily. Thanks.

 

You have done some very systematic testing, etc. This is good.

I suspect:
1) Thermostats are tired, It happens, as they do work very hard.
2) Radiator is clogging up internally. Crossflow radiators do that, it is not a Jag thing.
3) The hosing out of the external really achieves little. That external gunk really "sticks" inside the fins, and sometimes a blast from the engine side MAY release some, but I have had little success with that until the radiator is on the ground.

The readings are OK as I see them, but a tad too much variance.

When you get new stats, make sure they open far enough. The distance from the mounting rim TO the by-pass port, up inside, is 41mm, and Jaguar had issues with stats when I worked for them, that were too short when open, and if that port does NOT get closed when the stats open, it will never run cool.

 

Thanks, Grant. Based on your post I think I'll go with the thermostat replacements first since it's possible the once I'm running are the originals. I believe XKs Unlimited has the ROM-prescribed ones with jiggle pins, part# JA-EBC 3576. I'll also go with a rear radiator garden hose water blast. Tight quarters back there but it's worth a try. As for the Bank temps I posted, I consider them to be estimates as I'm not at all sure as to their specific degree reliablity. About all I can be reasonably conclude is that the B Bank temps are running higher than the A Bank. But don't Kirby Palm and others say that's to be expected because of the Crossflow radiator flow. Is that your understanding as well? Thanks again for your help. Dave

 

it is normal for B bank to run hotter than A bank, some say cross flow rad, I think it has to do with coolant pump not flowing equally to both banks.

also i have found that exhaust temps are hotter on B bank(by 20*F), supposedly factory observed it also, but said not enough to worry about!

 

I have read all and sundry on this temp variation over the years with this engine. I have had them in all forms, and am not convinced either one way or the other.

What I do know, is, if that by-pass port does not get closed by the disc on bottom of the thermostat that engine will ALWAYS run hot.

That radiator has always worked sweet for me, even in our 45c summers, so all the guff about altering it radically simply does not wash with me at the moment.

Cooling system cleanliness, and lots of other engine items, timing, proper bleeding of the cooling system, transmission condition, etc etc, affect temps more than the Jag designed system in MY opinion.

Agreed, I have altered some things. Transmission cooler removed from the radiator, twin thermo fans fitted to name a couple, but the basic system is as designed.

 

Thanks Grant for your most recent post and thanks to you as well, "ronbros". One feels alone at times when trying to sort out the inevitable challenges that come with owning and maintaining an elderly V12 Jaguar. I appreciate hearing from you both.

Thermostats (with jiggle pins) and gaskets have arrived. Surgery is scheduled for the morrow. Family and friends are hopeful for a speedy recovery and a return to normal operating temps.