I would change the thermostats as the next course of action. If they're original then they're 35 years old and surely past their prime- also a cheap and easy job. The correct ones will have a little bleed hole near the edge that must be positioned at the 12 o'clock position to allow air to escape the system. You'll also need new gaskets.

Now, on bleeding the system- with the nose of the car up remove the plug on the top of the left radiator tank and fill with coolant until it starts coming out of that hole and then quickly replace the plug. It might also be worth checking the banjo bolt on the top of the RIGHT radiator tank as they become clogged and prevent the cooling system from properly bleeding.

100% agree with Doug. Your radiator could very well be and is *probably* at least partially clogged especially if it ever got the additive Jag was using through the 90's. But might as well cover the easy stuff first. 3/4 is definitely hot. Top of the "N" on that gauge is overheating- anything after that is quite scary with these engines' tendency to drop valve seats.

 

I had the banjo bolt out when I cleaned in between the heat exchangers. It was clean as a whistle. The coolant is also staying very clean.

 

What temperature is ideal and where should I be measuring? I get a big difference when I measure the metal crossover pipe vs. the rubber tube it is connected to. I would imagine the pipe is a more accurate space, but looking for the experience of the group...

 

I'm taking a lot of measurements with the infra red thermometer and getting a lot of variability. Maybe I need to learn how to use this thing. Mostly getting in the 90's (Celcius) on the inlet to radiator tubes. I get about a 10 degree differential to the outlet but that is where I am seeing the most variability in measurement maybe due to lighting and distance, I don't know... will keep at it. I'm curious what you would expect the delta T of the radiator to be if functioning properly. I would rather measure that before taking it out. Thermostats seem easy enough to change out.

 

Just to clarify... I don't have a "N" on my gauge. It only has a red and blue at the extremes with hash marks marking it off in 1/4 increments. I've seen reference to the half way mark being "normal", and I am slightly above that on the highway, up near the third hash (not quite there) in hot Miami standstill traffic.

 

On most any car, including yours, the center of the gauge scale (with or without an "N") represents normal temperature. This typically corresponds to thermostat rating. For example, if the car is designed to have 190ºF thermostats than the center of the gauge readout will probably represent about 190ºF.

In a dream world every car would have a cooling system that had the ability to over-cool under all circumstances...thus leaving the thermostat to truly control coolant temp by constantly opening and closing a bit so that the coolant temp hovers at thermostat rating. In my experience, Jag v12s can and will behave this way under most circumstances. In extended city traffic on a hot summer day....well....not so much. The coolant temp will rise beyond the thermostat control point. In my experience this will occur when ambient temps are in the 85-90ºF range. Personally, I don't worry if the needle ticks up a bit.so long as it doesn't keep going up-up-up with no end in sight....indicating an out-of-control situation.

Personally I think that too much worry is given to coolant temperature and not enough worry given to coolant flow. You can drop valve seats at 190ºF coolant temp if there isn't good flow to all areas of the engine.....cylinder heads specifically. This is why thorough bleeding is so important. Sometimes it takes 2 or 3 bleedings to get all the dreaded air pockets out.

I don't know, and don't care to learn, what the absolute maximum safe coolant temp is. But, if the flow is good I don't think the engine cares a bit if the coolant is 180-190-200-210ºF. At sea level with a 16 pound cap, boilover is, what, 260ºF? None of use would ever want to go anywhere near that high, of course.

Anyhow....

A Miami day of 78ºF shouldn't challenge the cooling system very much. The needle should sit in the center.....certainly at highway speed, at least. Maybe a slight tick upward if you're stuck in gridlocked traffic. From what you've described your cooling system does need attention. It should run cooler under the conditions you described and, more urgently, the higher temps could indicate a flow problem. Either air pockets or crud. Or both. my money is on the radiator.

Cheers
DD

 

Recoring etc a rad is a slightly expensive step. You can try adding a gallon of MetalRescue get the engine hot then sit for 24hrs followup with a rad flush product like LiquiMoly then violently shopvac the inlets repeatedly with a garden hose to refill. Flush flush flush until you can drink the outcoming water.

I've done this and turned my coolant jet black, sucked out a cup of silt and rubber crumbs. My full flow spin-on coolant filter still pulls alot of fine clay silt.

I suppose a rad shop could do it for you but then you wouldn't learn as much.

 

Re-coring is expensive, yes, if needed. And it often is.

But a radiator shop typically only charges $100 or so for a clean-out.

There are various ways to skin a cat. A DIY flush-out as you suggest may very well do the trick and saves the labor of removing the radiator...which is not pleasant work on a V12 Jag. That's a real 'plus'.

Personally, though, my experience has brought me to the "Just bite the bullet and be done with it" philosophy. And, if it needs re-coring, now's the time to find out. While the radiator is at the repair shop the time can be spent replacing hoses, belts, and thermostats.

IMO, the alá carte approach just adds to the aggravation. Do it all, do it once, do it right. Then you can forget about it for many years other than the periodic routine flushing.

Cheers
DD

 

I've ordered new STANT thermostats #45398 and will replace them with a bleed hole drilled as I have read here. When this is apart I will do my best to flush the radiator with a hose with it in the car. Probably should add some flush chemicals a day or two before, I suppose.

Doug, I appreciate your experience and you are likely correct. I don't have the best facilities here so I have to choose carefully. My measurements don't indicate overly high temps (have not seen much feedback on that) Not sure whether to trust the infra red or the gauge for accuracy.

Greg