82C/180F vs 88C/190F thermostats

Promised I’d add a bit once I switched from 180F thermostats (Gates) to 190F thermostats (Waxstat). For reference, I live in a mild climate (Seattle) where it’s mostly 40Fs to 70Fs year round. We will see 80Fs during summer, and a heat wave here is over 90F. (I’ve only seen 100F one day since living here the last 15 years) I also have a 1988 HE V12 USA specs.

Here are photos of before/after gauges, both in Park at 850RPM. Forgive the gauge photos, it was hard to get a perfect center without perspective, but trust me, both gauges were right on 180F or 190F. Note my oil pressure is around 42psi at 180F, and around 35psi at 190F. I am currently running 10/40 synthetic oil. (I will soon switch to 15/50 synthetic next oil change.) Both of these are after a 30 minute drive in town.

It’s only been two days, but I prefer the 190F. Background - my XJ-S idles a wee bit rough when cold and as warming up, but eventually gets to a smooth idle.

With the 180F thermostats, it would take 10 minutes to get from cold to 180F, and then about 10 minutes running at 180F until the engine finally settled down into a smooth idle.

With the 190F thermostats, it takes about 10 minutes to get from cold to 190F, but once at 190F, the car seems to settle down into the exact same smooth idle as before.

So really, I’ve just bought myself 10 minutes more of smoother idle, but for me it’s worth it because many of my drives are only 30 minutes.

If I lived in Florida, Arizona, Texas or Australia, I’d probably keep the 180F thermostats as I’ll bet the engine has no problem getting to a hot operating temperature fast :wink:

The next tests I will do will have to wait for summer. Before the temps had to go from 180F to 200F before the aux fan kicked on, so I hardly ever saw it on. My guess is it will be much easier to get to 200F this summer.


You might find the fuel economy improves a small amount with the higher operating temp.

1 Like

Ill test that soon too. I was getting 11-12mpg in city w/ 180F.

I’ve also noticed, car warms up quicker after sitting for a while. The 190F has metal jiggle pin. The 180F i had to drill hole in top.

Took it for another spin today, i think someone here pointed this out, the HE just likes to run at 190F. I concur.

Also, maybe I’m wrong, but IMO 190F is that minimum temp where an engine starts to really burn off moisture in the oil, preventing sludge build up. Sure it burns off at 180F, but maybe takes a lot longer?

Greg, hadn’t heard of such a thing about 10 degrees warmer burning off moisture. I don’t think your engine sits enough for that to happen-glad you drive it. You would have to get your engine past 210 degrees for a certain amount of time anyways if there was moisture there to burn off.
I have used those same thermostats you mentioned above and you are correct; it really depends on your geographical location.
I use the 88c in Louvered bonnet xjs, and 82c in the stock.

1 Like


Holy Sht…

You’re one of not many v12 owners that actually wants it to run hotter. Others are watching temp gauge while doing their prayers (pray during ride should be banned by the way).
Temperature is always a killer and ageing factor for every bit in your v12.
As dieselgate originates from US - you’re probably aware that higher temperature is only good for lowering emissions. That’s what Bosch was trying to do over the last 3 years - design the diesel engine that will be able to run at 110 Celsius. They failed.

190F is still running cool for an engine. Most modern engines run at 200-210F. My 90s Volvos run a 90C/194F OEM thermostat, and they are aluminum engine blocks too.

My Aux fan kicks in at 200F, I’ll confirm this summer. So my engine should only run between 190F-200F, which is not really that hot.

Now, if I had put a thermostat in at 205F on my V12, then yes, i’d be crazy.

And like I said, if i lived in a hot climate, i may have kept teh 180F thermostats.

And many here on the forum run 190F thermostats. The key isn’t the thermostat, it’s making sure your cooling system doesn’t let temps get up to 220F.

My lump’s GM set at 200 F, Some install cool stats for power.

The clime here is mild. Range from 50 F to a rare 100 F !!!


Your last sentence overs it perfectly. The cooling system cools, the thermostat regulates…

If the cooling system can not maintain a temp under 220 F a 150F thermostat will not change that…

1 Like

I’m not sure what the relation is between coolant temp and oil temp other than oil generally runs considerably hotter – well beyond 212F so it’ll burn off any moisture. I’m sure hotter thermostats help, but what is perhaps most critical is longer runs. 10-minute drives every day won’t burn off any moisture, while a single 90-minute drive will.

1 Like

That’s what I found with my old ‘S’ type after running around town for long periods a long run down to Wellington from Auckland (<>600klms) would result in the oil level dropping, on the return trip nothing of consequence neede to be added, perhaps due to rings bedding back in and burning off blow by?

I never understood all this hype about the v12 overheating.
I too have the 90°C/194°F thermostats, and even at 40°C Southern France summer the gauge remains steady at 90°.
Mine did overheat when I first got it 15 years ago, because the rad was blocked, but I have addressed all the issues and had no problems since.
It’s true that it’s capacity is marginal, but adequate when it’s in good nick.

1 Like

Agree. I think the only thing to really worry about on this car (besides a bad cooling system causing you to overheat) is shutting down at too high a temperature. I’ve recorded a +20F increase within 15 minutes of shutdown. This is why I put in a lower aux fan thermostat, so it comes on at 200F (instead of 210F). So hottest I’ve shutdown is 200F. I’m trying to get into the habit, if I pull into my garage and the aux fan is on at 200F, I wait until it cools things down before I cut power. Sure it will run after shutdown, but it’s not circulating the cooler coolant.

Mine seldom hits 90º on the gauge. Too cool for my liking most of the time. I’d at least like it to warm up faster on cold mornings so I can get some heat out of the heater.

IMO people shouldn’t be praying too diligently for lower coolant temperatures per se. They should be praying very diligently that there is good coolant flow to all areas of the engine, cylinder heads in particular. If the flow is good I don’t think the engine cares if the coolant is at 80º, 90º, or 100º.

Problem is, how do you really know that there is good flow? Well, you don’t, really. You do everything you can to keep the system at 100% and pray. If your valve seats are still in place, you have good flow. If your valve seats are dropping, you don’t have good flow. You can drop valve seats at 80ºC coolant temp if the coolant isn’t flowing thru the heads properly.

Somewhere along the line, decades ago, we became conditioned to think that 1º over thermostat rating (or one needle-width beyond “middle of the N”) represents an overheating problem that’ll wreck the engine. Personally, I don’t think that’s the case at all.


1 Like

Thermostat gives you stabile working temperature of the engine. Higher the temperature, more deteriorating for all components in assembly. If your system cannot maintain thermostatic temperature, it’s crap.

The latest thing in engines is reverse flow: The water pump pushes coolant into the heads and from there down into the water jacket and then out to the rad. I’m pretty sure the LS does this. My 2002 Mazda does it. The primary benefit is supposedly more reliable cooling of the heads, which is critical, especially in higher-performance engines.

People have done lots of nutty things to the Jaguar V12, including electric water pumps. Has anyone tried reversing the flow yet?

I’ve certainly toyed with he idea of going electric for the water pump. But the multi-way complexity of the mechanical item looked hard to reproduce without a lot of machining for a high volume pump. Would a gutted original pump and a high flow pump in the bottom hose work?

Although now you’ve piqued my interested in pair of medium volume pumps in the top hoses. Not sure if it would work correctly with the radiator’s 1/3 & 2/3 split for the two banks though.

Testing my 97 Volvo after putting in new radiator, my Jag V12 runs cooler!

My Jag with the 190F thermostat sits at 190F while driving. When idling, it can creep up to 200F, but the aux fan kicks in. If it climbs into the high 190’s without the aux fan kicking in yet, as soon as I simply drive about 5 minutes, the temp comes back down to 190F.

My Volvo, with a 180F thermostat, only has one electric fan which the ECU turns on at 215F and goes off at 205F. While driving, the temps never really get below 195F. Most of the time, 200F-215F is it’s range. I do have a water cooled turbo, so that could explain the extra heat.

Just goes to show how much the mechanical fan is doing on the V12. Also, it’s nice being able to change the aux fan temp. The Volvo, I’d have to modify ECU.

The simplest way is to remove the V-belt pulley from the OEM water pump, install a toothed belt pulley, and mount an electric motor to drive it. Then you get into the fun of controlling that electric motor. You want it always running when the engine is running, even when cold, but it can run slow for a while and then speed up as the thermal load increases.

OK, how about this: A pair of pumps in the top hoses running backwards, pushing coolant into the coolant manifolds. Thermostats removed (control of the electric pumps takes over that job), close off the bypasses to the water pump, gut the water pump. Offhand, I suspect the 1/3-2/3 radiator wouldn’t be a problem, presuming it’s in good nick. If the two water pumps are controlled independently, one might even run a little faster than the other to correct for any imbalance in cooling.