As a followup to my last post on how to measure temperature with a reasonable amount of certainty to the accuracy of the measurement. I thought I would follow it up with some on the car measurements.

To make things interesting I had to replace my temperature gauge sender 2 weeks ago because I broke the spade connector off it when I was installing the RH air filter. Before the replacement my gauge sat just below the N in traffic so I sure this sender was the cause.

It was warm here today 30°C ambient and I had to go across the city for work, so it was a good opportunity.

Anyway cut to the chase the first picture is obviously my temperature gauge, now if any of you saw this I am sure it would provoke immediate panic and shutting off the engine. Fear NOT this is where my temperature gauge now sits (with the new sender, correct Jaguar part number) when the car is at operating temperature 95°C about 203°F. The second picture is the same meter I used in my last post checked at 100°C reading 99°C so my temperature is between 95 and 96°C.

MOTO of the story DO NOT trust your temperature gauge measure the temperature of the engine. The best place with a thermocouple is at the rear of the water rail to the back of the engine, this is where the engine will be at its hottest.

Another thing I noticed was the tendency of the gauge to drop significantly (a few mm) when the engine was shut off and immediately re-started. My theory here is the voltage being supplied to the instrument cluster is NOT regulated. My next project is to add a 12 volt regulator to see if this solves the problem. I will also add a circuit so I can adjust the gauge to the middle of the N at 95°C.

 

Warrjon
Is there a way of telling what the difference is between the old and the new senders, that such a large gauge reading results?

Very interesting post, thanks for illustrating it so clearly. I note that your fuel gauge has about the same reading as mine usually does! If only that little problem could be solved with a regulator...

Greg

 

Warren, I'm not sure I would've drawn the same conclusions as you have....but do keep us posted.

Cheers
DD

 

Because of the 1-1/2 pass flow scheme of the stock radiator, the "B" bank tends to run hotter than the "A" bank. So, the best place to monitor the hottest coolant temps might specifically be the rear of the "B" bank coolant pipe.

 

The sensor is on the B bank. Doug I am sure my temperature is good, before changing the sender my gauge used to sit just below the N. The only thing changed is the sender. Over the weekend I will add my little circuit so I can adjust the gauge down Ill use a pan of water and a known thermometer. The problem with the gauge up there is not much movement before it tops out and this would be before the engine is over hot.

All the gauges except Oil (I broke that spade off as well) and Temp work correctly. The spade terminals must have been bent before when they were straightened they both broke.

I don't see how the 1.5 pass radiator can affect the temperature of the B bank alone, the input to the engine is the same. The only thing that can affect the B bank alone is the water rail restricting flow. It is usually A6 that fails from detonation.

When my car was overheating both banks were running within a coupe of degrees.

 

Seems to me that the old sender gave a reading that was much closer to correct than the new sender.

Rather than modifying the circuits to get a correct reading I'd be wondering if there was a *problem* with the new sender that would account for the reading going so far out of range.

Or have I misunderstood something?

Cheers
DD

 

i have my digital sensor mounted in the back of the B bank, were the old AAV was mounted,.

using an infra non contact my pre-he runs B bank is around 4-5*F higher than A bank, probably not enough to matter for a street car! all measured at the rear of cyl.heads.

interesting to note my highest temps are read on the top of the cam covers over 212*F,
while coolant runs(for me) 180F.

 

Warren, I kinda feel like Doug does, that just doesn't look right.

I'd throw it in a pan of 50/50 water and ice, see what it reads, then boiling water and see what it reads (I did just that when I had mine out but of course now I can't find the results). I would think a huge jump like that is the result of a faulty sender or some other significant change.

Just wondering...does your oil pressure always show that high?

Thanks,

John
1987 XJ-S V12
63,000 miles

 

I'm lucky I have access to an environmentally controlled temperature calibration lab at work, complete with icepoint and oven accurate to 0.01°C I am using a thermocouple and meter calibrated at work last week for my measurements. I am confident the sender is different from the original, even though the part number was the correct one. I have contacted the company but had no replay as yet. It is just easier for me to add a pot and resistor to my gauge and adjust so the gauge reads correctly.

I guess the point of my post was to highlight the fact that if the temperature gauge on your car is high it might not be an overheating issue and proper temperature measurements are needed before hitting the panic button. And in my experience cheap IR devices are notoriously inaccurate. See my other post on how to measure temperature.

The sender is just a NTC (negative temperature coefficient) resistor. In other words its resistance goes down as its temperature rises, this causes the voltage drop between the gauge and sender to change, thus changing the reading on the gauge.

So the resistance of the new sender is not the same as old one.

Oil pressure is a broken spade on the sender, same as what happened to my temperature.

 

just my opinion, does it really matter what the temps are ,(AS LONG AS ENGINE IS NOT OVERHEATING), just say engine is 180f like i run, or 195f like some do?

i know someone is gonna say emissions and long engine life, comon thats not the way we should look at JAg V12s, as an entire group, we wouldnt amount to a tinkers dam to the world air quality!

and i do know from real experience, that if we used a MUCH higher boiling point coolant, V12 could run all day long at 275-300*F, along with a modern full synthetic oil!

detonation USUALLY occurs at a localized hot spot in the system, and that usually happens at the rear of any engine, right or left side!

turbo engines are notorious for hot spots , a mod that i did on some(old hot rod trick) was drill a hole into the rear of the cylinder head(at least 3/4" hose), and run the hose forward to the thermostats and pump it back into the forward part of housings.

helped cool the rear of head, with more coolant flow, stopped blowing gaskets between two rear cylinders!

heat softens the gasket and it starts to pulsate back and forth between cyls. and jag v12 with open deck,being that it is a somewhat flexible assembly, moves the gasket even more. seems everything has a plus and minus, flexible reduces cracking, ridged holds things in place better,but thermal missmatches can crack things like block bores, and cyl. heads

some engines can hot spot so quickly that any gage or your eyes dont pickup on it till detonation has allready started to set in.

 

Some good points.

Many forget that it's lack of proper coolant flow, not just outright coolant temperature, that kills many a V12

Cheers
DD

 

if any of you guys watch Nascar, and catch driver to pit talk, some run there engines coolant into the 300F range quite often, they just have to watch things closer as narrower danger level!


i run around 180F digital in 100f ambient, no probs,, but if i run 205f in 100f ambient it gives me much less of a cushion for an overheat!

now, do you think my engine really gives a hoot if im only running 175-180F, all is well under the hood(bonnet).

4-5*f doesnt mean anything in my opinion, and to measure .01 of a degree is !!!???.

warrjon no offense, next will be fuel pressure measured to .01 of a PSI.

OK starting to rant,, over&out.

 

At sea level and @ 16psi you'll boil over at about 265ºF. If "overheat" can be defined as "boiling over", then....no worries at 205ºF

But, of course, none us would be comfortable running 265ºF

If coolant FLOW is good I don't think your V12 minds running anywhere from 180F to, oh, let's say 220ºF.

That 220ºF is arbitrary on my part. That's when I'd start getting nervous, I guess.....especially of the temp kept going up-up-up

If the cooling isn't flowing properly then, heck, you could drop at valve seat at 180ºF coolant temp

Cheers
DD

 

Really good points Doug. TWR had problems with the heads lifting and moving around.

I was talking to Norm Lutz about modifying the water rail to allow more flow in the rear of the engine. I will be doing this mod on my new engine

 

detonation increases local heat in that cylinder more than surrounding aluminum , it expands and drops a seat,especially in exhaust of HEs!

only an opinion guys!!!