Disclaimer: similar to many V12 owners, I am a bit paranoid about engine temps

Recently I flushed the coolant and replaced both thermostats. (I tested both by boiling before putting them in.)

This morning I took the car for a short drive to warm her up. (She did great -- the temp gauge stayed below N when on open road) and then I decided to leave it idling after I got back to the garage.

After some time, the temp gauge first moved to lower end of N, then to middle of N. At the very end (after about 10min idling), it was maybe a hair above the middle of N.

But as many advised here, the temp gauge may be imprecise and/or unreliable.

So, I also tried to measure temps with my infrared thermometer. I measured at the t-stat housing. What I found was interesting:LHS was running significantly hotter than RHS. RHS measured between 87C to 89C.

LHS, on the other hand, was measuring much higher: about 115C-116C at the very end (when the temp gauge was a hair above middle of N)

I remember reading that one side always tends to run hotter. But by more than 20C degrees? Isn't it excessive?

 

IIRC it is due to the radiator setup with two top hoses and an asymetrical layout...

 

Yeah, I read about the double-pass radiator setup. But should I expect the difference to be that big? 27c from right to left?

 

The temperatures should be within a few degrees of each other. Mine is usually within 1-2C. I'm willing to bet you have a stuck thermostat.

Other thing to be aware of is the IR type thermometers are very inaccurate with shiny metal surfaces, they work best on flat black. Try measuring the upper rad hoses on each side and see how much difference there is. It will be inaccurate due to the thickness of the rubber, but you're just interested in the difference, not the value.

 

I do not think it is anything to do with the radiator setup. The water pump does not quite equally send water to each bank at tickover, so that could be part of it. But like you, I think that temperature difference is excessive. Why not top up the coolant if necessary, give it a decent run, measure temps at the end of the run, then try the 10 mins tickover test again. If the same is repeated, I suspect maybe a stat is not opening fully.
Greg

 

Repeated the process. Drove around town first. The car behaved great, temps stayed below N. Got back to garage and idled for 10-15min. Gauge slowly started to rise. By the end, it again reached top of N and went slightly above it.

Measured temps with infrared thermometer at that time. (Measured at the hose this time, different areas. Results were fairly consistent)

LHS: 109C
RHS: 118C

So, not as big a difference between left and right this time.

But should the temps approach 120c when idling?

 

That's gettin' right up there...and I'm the one who is usually a bit nonchalant about coolant temperatures. And i'd expect the temp gauge needle to be nudging the "H"

Not sure where you are in Nevada but the weather channel says 104ºF today. Idling in those conditions might simply be more than the cooling system can keep up with.

But....are you sure everything in the cooling system is 100% up-to-snuff?

Cheers
DD

 

i have experianced that difference from right side VS left side, and i finally found that be sure your throttle plate set screws are adjusted properly!

i resorted to old hot rod trick of adjusting , right side higher and left side lower using RPM tach to regulate chamber temps, and final idle RPM.

yeah, i know that aint the way factory does it, but it works for me, i also have NO AAV valve!

of note my temps were 20F between banks, using a hand held digital gage!

its plenty HOT here in central Texas ,every day triple temps.

 

I'll say it again (for the umtheenth time) IR thermometers read an average temperature over the spot area. If 50% of that spot is on the exhaust manifold then the reading will be 50% thermostat housing and 50% exhaust manifold giving a very inaccurate temperature reading. Also the distance the IR is held from the surface and the surface emissivity (how the surface reflects the heat) will play a role in the reading on the IR.

The last reading you did at 109 and 118°C is ±4.5°C is most likely within the measurement uncertainty of the IR you have. I have calibrated a lot of Fluke 65's and these have a tolerance of ±2°C plus a repeatability of ±1°C this is an uncertainty of ±3°C not taking into account distance from the measuring surface, and other factors that effect accuracy.

So I would not worry too much unless the engine boils or the gauge continues toshow the engine getting hotter.

 

i have posted this before , but cant hurt to read it again.

Jag V12 are known for there overheating , 75% go down because of overheat, just gotta live with it, and be aware!

plus like Greg mentions , the water pump is NOT even in the flow of right and left banks!
many ways to correct , but all cost money ,and deviates from FACTORY setup!

 

Funny you said it. I also readjusted the throttle plates recently to 0.02" per spec

One side was off, if I remember correctly. I believe clearance on the right side was about 0.035". While left was close to spec. I wonder if previous owner applied the same trick you described?

When you say right side higher, what do you mean?make clearance wider or narrower?

 

You mention that you have flushed the coolant etc. GOOD.

When was the last time that radiator was professionally flushed, as in tanks off and rodded out??

They clog up from the bottom, and 20% can reek havoc.

OK, mostly that gives issues at higher speeds and loads, but that cooling system gets so bad so slowly, that when it is finally cleaned out, the temp gauge (as flaky as it is) barely reaches the N at any time.

Ron has NO AAV, same as all mine, and I also have NO balance pipe as such, so the throttle discs are used to set the idle by "balance", just like the OLD days of multiple carbies.

With an AAV, the throttle discs should be set at 0.002", both side, as the balance pipe will balance the vac etc (simplistic I know, but it will do).

At idle of say 650rpm, that fan is barely drawing any air, and that is also a problem, many, many threads here at the moment about fan clutches getting lazy.

I run twin thermo fans, so that issue is eliminated.

The pressure caps are simple, cheap, and the quality recently are CRAP. Waaaaay back mine had floating temps, mainly at idle and in traffic, NEW caps, no more thoughts from me on them, WRONG. Weeks later I tested the 15lb header tank cap (the actual pressure cap) and it was blowing off at 4lb, DAMN. Down to work, tester in hand, went through ALL the caps, and eventually found a 13 that blew off at 14, DONE DEAL, still on there today, and that simple "Made in China" CPC quality part fixed ALL the issues.

I need a coffee.

 

You can usually tell if a cap is any good by whether or not it snaps your hand back when removing right after a good run. Jag OEM ones feel so solid, a real bone cracker.

 

Try to measure the bottom hose temperature where the water exits the rad. At idle the water is not circulating very fast, so high top hose temps are to be expected, especially on hot days after a run. I agree with Warrjon, in view of your second set of temps, that there is nothing to worry about. As Grant says, an electric fan would do wonders, and if the rad is the original I would change it as a bit of preventative maintenance. One good idea, and relatively easy to do, would be to change the OEM electric fan for a modern one that has the highest flow and amperage that you can fit into the space.

 

Good points you guys are making about main fan idling at much lower speeds comparing to when on the road. Even though the engine is not under load, at 650 rpm the main fan is rotating at least 3 times slower than when driving. Didn't think about it from this angle. Still, I was hoping that the car should manage to keep reasonable temps when idling on her own. I guess she is asking for some help...

Will try the bottom hose test, if I can reach it with ir thermometer

 

Whilst you doing calculations:

Once the vehicle is moving at 30MPH or more, the "ram air" through the front will cool any engine.

Problem with the V12 is the air cannot get out of the engine bay, fact.

Main culprit is the very thick foam insulation pad that sits up inside the transmission tunnel. Age now has attacked the glue,and it is usually sitting ON the transmission. No bigger, other than it is blocking the exit air path.

My HE did this, but I only found out when I removed the engine/trans, and the foam came out with it, bugga.

Did not out it back, went to a local Speed Shop (Hotrods, not the stuff you inhale), and got a sheet of the new era insulation stuff they use. Very thin, better than foam as an insulator. Hi-temp adhesive and all done.

You are correct about at idle it should just sit there and do what it does without fuss, and so it should. The fan clutch comes into it once more. If it is weak, and as the air temp passing over it rises, but the weakness prevents the innards doing its thing, the fan will not rotate with any conviction, and therefore not draw air through the matrix. It is a fine line between working and not working as designed/needed. They fail slowly, just like tyres wear out slowly, and its not until new is installed that the weakness is exposed.

Mine actually locked up, at very high road speed, and spat a blade out the bonnet.

Thermo fans replaced it.

 

Took another drive this morning. This time I took her to a summit of nearby mountain pass. The car did good again on the road. (I love how it handles the curves!) By the time we reached the summit, the gauge was a hair below the bottom of N. However, on the way down, the temps decreased significantly to the mid point between L and N, and maybe even slightly lower.

After I got back I let her idle again. IR thermometer showed 95C on LHS and 87C on RHS by the time I opened up the hood I got the tool out. A little later: 104C on lhs and 94 on rhs.

I could not get close to the bottom hose, but the measurements I took varied between 85C to 88C at the bottom. What is normally expected as the difference between top and bottom?

 

I just see idling an engine as "only when necessary". Nothing is at optimum.


Yes, in heavy stop and stop and go traffic, it happens. Avoid if possible.


But, our cars have a feature that helps in that respect. Pop the forward hinged boot. Air flow much improved. At that "speed" little if any risk of hood/bonnet escape.


An old tech thing. but, still effective.


Carl

 

This is normal, my temp gauge sits just below the bottom of the N at its highest and mid way between C and N at its lowest when warm.

My Temp gauge used to sit at the middle of the N at its highest but I had to replace the sender as I broke the spade terminal off and the new one, which was out of my spare engine the gauge sat slightly lower.

 

As Warren said.

The "average" temp drop across a crossflow radiator, as in Top hose RH to Bottom LH in our set up is 15deg C at BEST.

Our market has the water pump Inlet fan switch as an 85C.

I know I have mentioned this way too many time, but did the New Stats open to more than 41mm when tested in 100C water???.

You will get this oddity of LH versus RH being more pronounced if the by pass ports on each side are not being close before the stat gets to full length. A stat at full length has lost control of the cooling system. They are constantly moving the opening of their throat port as needed and based on teh temp setting of the wax bulb.

Mine sit at about the same as Warrens, no matter what I do, uphill, fast runs, 45C in traffic, but downhill, on a trailing throttle, they go down a tad towards the C on the gauge.

V12 thermostat fiasco explained.pdf

Not sure if I sent you this in previous thread, no matter, here ya go.