Anyone tried one of these Jaguar Low-Temp Thermostat - Jaguar Performance Parts

Would running the car 10 degrees cooler freak out the computer by making it think it is never warmed up, or would it be just fine, or even an actual improvement??

 

My heater has been intermittent since I got the car, but if the engine temp is below 50% temp I am far less likely to get any heat... hope that helps.. unlike other cars I have had where you get a degree of heat whenever it is above stone cold my Jag seems to depend on reaching operating temp to get any heat :-) Hence I replaced a faulty thermostat on it where on other cars I wouls have left it

 

I just replaced my thermostat with a 185, and my XK8 runs at 195- 199 deg according to the Ultragauge. I don't think a lower temp thermostat would affect the running temp. Wouldn't Jaguar have made the change if there any benefit?

 

Isn't the thermostat the thing that completely regulates the running temp?? Unless the system is overloaded, like with too much boost.??

 

It sets the *minimum* temperature at which coolant is allowed to circulate in the radiator.. so in theory it sets not the running temp but the minimum temp...

 

This question pops up on just about every car site around and usually sets off a firestorm of debate. Although this sounds like a cheap and easy way of getting 'more power' out of an engine, the opposite is actually what happens, albeit to a very tiny degree.

The myth of 'more power' revolves around the idea of reducing the engine temperature to lower the incoming air temperature. Cooler air = more power is the principal involved. This part is true, as a 11*F reduction in intake air temp will result in a 1% increase in power. The problem is that reducing COOLANT temp has little effect on air inlet temps. A 10*F reduction in coolant temp might result in maybe a 1 *F change in inlet temp, or none at all.

On the other hand, a cooler engine (pistons, cylinders, heads) is not a positive move for power output as more of the heat energy produced by combustion is wasted by being absorbed by the metal surfaces rather than being used to 'push the pistons'. This valuable heat is then dumped overboard by the cooling system, a total waste and a losing proposition.

It is true that running an engine hotter than recommended can bring on other problems, primarily detonation or pre-ignition or lubricant breakdown which is of concern in racing situations but putting those aside, an ideal engine would run at full combustion temperatures with no cooling system in sight.

I'm putting my flame suit on now (just in case)

 

I am wondering if the car would run normally (computer closed loop) if I installed this lower temp thermostat...

If yes, then would that give me a small buffer of heat inertia for a brief full boost blast like a standing 1/4 mile.

If, no, why?

 

At one time thermostats were widely available in ratings of 160*F to 195*F. The 160*F thermostats were considered "summer" thermostats and the 195*F were considered "winter" thermostats. And yes, people did change them with the seasons. Delivering new cars with the highest rated thermostats did not become common until the advent of emissions control.

Debating whether a cooler thermostat will result in a cooler intake charge is somewhat pointless for 99 percent of readers. It does not matter unless someone is truly going to habitually run at WOT for extended periods. The handful of readers who will actually run at WOT for extended periods will already know what to do and why.

However, from a practical point of view for the daily driver there are other points to consider.

There are two main cooling systems as far as conventional automotive engines are concerned. There is the coolant based system. But, there is also the oiling system. There are many parts of the engine that depends solely on oil flow for cooling. These parts run at substantially higher temperatures than the opening temperature of the thermostat. For example, journal bearing surfaces see temperatures in the 350*F range. Generally, only the cylinder heads and cylinder jackets are cooled by coolant flow. There is a reason that Jaguars follow the pattern of performance cars and utilise larger than normal sumps. The SC cars add an oil cooler to further enhance oil cooling.

From a practical standpoint, where a cooler thermostat can have an effect on a daily driver is in engine bay ambient temperatures and contact temperatures. One killer of non-metal parts in the engine bay is heat. The higher the temperature, the greater the stress on the part. For example, elevated temperatures shorten hose life.

Cooler temperatures at shutdown help with heatsoak on hot restarts. It is also kinder to the paint on the hood.

The reason that engines are run hotter in current cars is that EGR and other emissions related variables requires the elevated temperatures to do the best job.

The reason that the thermostat is specified at one temperature is that unlike a car without a ECU, the ECU requires an assumption of the closed loop operating temperature. Until that temperature range is reached, it runs open loop.

Fuel milage has *always* been important to manufacturers, but high temperature thermostats became de riguer only with the advent of emissions controls and computer controlled engines. That should tell the reader something about where the balance would be except for these considerations.

 

Thank you for that info....but I wasn't trying for cooler intake temps. Just curious if it would confuse the computer.

 

What did it run at before with what thermostat?

 

I meant to say I replaced the water pump and also put in new 185 stat. Previous was also 185, and it ran at 201- 205. I don't know if the difference was due to the pump or the thermostat was not opening completely.
RJ________________
97 XK8 Conv. 85K mi

 

Right- I'm not able to answer that as I don't know what goes on if the computer sees that the coolant is 10*F below normal. Maybe someone else is familiar enough to comment. I know on older, simpler engines the computer would stay 'open loop' and deliver an unnecessarily rich mixture but certainly no extra power as per popular legend.

I was primarily addressing your point 'of an actual improvement' and some of the claims made in the Eurotoys advert. Some people prefer a short answer of 'no, it's bad science' others prefer to hear the reasoning behind it.

It's all good discussion.

 

Yes, everyone, please DO keep giving the info, even all the random info. I am one of those who likes to attempt to understand the dynamics of what and why (I am an inventor)

All of this info is very interesting to me. I'm just trying to compile a list of things I am going to change and research them before tearing my motor apart and doing stuff to it.

I will likely try this lower thermostat, since it seems like an easy thing to test and return to original spec if it acts funny after installation.

 

There is still a good deal of margin when run 10 degrees cooler for the engine to comfortably stay closed loop once warmed-up.

What the low temp thermostat will do is slightly slow warm-up (arguably not so good) and drop your casual crusing temperature by up to about 10 degrees F (arguably a slight benefit). It will still get just as hot while idling or while running at high revs or in extremely hot weather.

If for nothing else, as Plums elaborated on, I like the idea of reducing under hood temperatures a bit, especially on the hot running supercharged engines.

 

You could use an OBD reader connected to a laptop to record temperatures and long/short term fuel trims before and after the thermostat change.
I am sure a number of members would be interested in the results of your research.

 

I'de be happy to make my car available as a Guinea pig for testing, but I don't have any reader...

 

After purchasing an X308, I quickly realised that an OBD reader was essential.
I was getting random restricted performance errrors, and the reader showed that it was one coil which was defective. There is a list of obd readers which work with Jaguars in the stickys. I bought mine from OBDCOM.com, and am quite happy with it; less than $100.

 

This is an old thread, but people are still reading it with some confusion. Here's my 2 cents worth after 30+ yrs of repairing/modifying Jags and things. A 10 degree thermostat drop will have minimal effect on air intake temperature or anything else! It will not effect engine warm-up until after its opening temp has been reached......the engine is already warmed up before that point. For a performance increase, it is better to use a non polyglycol coolant with a lower thermostat, remove any engine covers, and louver the hood/bonnet. These louvers should be placed well forward on the hood/bonnet, NEVER toward the rear. (That's where the low pressure area is to help evacuate underhood heat.) If you want to go further, remove the heads, have them ported and polished and thermo barrier treat the combustion chambers, intake AND exhaust ports, and intake manifold runners (if possible). Combustion generates the heat that heats incoming air, thinning it. By keeping as much heat as possible out of the incoming it is cooler and denser. This means more oxygen per cylinder charge which allows more fuel to be burned at a proper ratio. The first steps will also reduce under hood/bonnet temps as a bonus.

 

As stated by others, a cooler thermostat sets the minimum temperature of the coolant before it is directed around the radiator for external cooling by the outside air.

Modern ECU's set the air/fuel mixture ratio according to the engine temperature, so running a cooler thermostat will cause the ECU to run a very slightly richer mixture. If the thermostat was too cool, then it the ECU might not reach the point where it goes into full 'closed loop' catalyst contol.

The theory is that the cooler thermostat will cause the ECU to provide a slightly richer mixture which may give a tiny power increase-but this will be offset by the greater viscosity & 'drag' of the engine oil at the cooler temp.

Also, if the car is sat in town traffic on a hot day for a long time, the underbonnet heat will still build up as normal until the electric fans switch on. There is no benefit at all in town traffic to using a cooler thermostat, as once the underbonnet temperatures have built up to the point where the electric fans cut in, the coolant temperature is then regulated at the point where the electric fans switch on.

If there is any benefit at all to using a cooler thermostat, it is marginal in the extreme & the only positive effect may be to keep underbonnet temperatures lower during highway cruising, which may have a slightly beneficial effect on those underbonnet components which are more susceptable to heat damage in the long term.

But the difference isn't great, and as soon as you hit that first traffic jam which causes the underbonnet temps to rise & trigger the electric fans, then any previous benefits will be instantly wiped out.

 

Ahhh ... but

... those few degrees may be just the difference between having those annoying fans come on during short stops and while making parking maneuvres. It remains true that extended crawling will trigger the fans. It works for the shorter slow downs because the system, including the reserve in the radiator is those few degrees short of the critical temperature. Until the reserve is warmed up a few degrees, the fans will not trigger.

The following factors come into play ...

-- the thermostat is a mechanical device and more subsceptible to being out of tolerance than an electrical device ... according to JTIS a "perfect" 84C thermostat starts to open at 80C-84C and is fully open at 96C

-- the on/off set point for the ECM fan control is reported to be 96C on and 91C off

-- the engine coolant temperature sensor is also subject to some tolerance wander.

So, you see that the fan on set point coded into the ECM is inconveniently close to where the engine likes to operate with the original spec thermostat. Especially once tolerance stack up is accounted for.

If however, a 80C thermostat is fitted, it should be fully open at 92C and try to keep the coolant at that temperature.

That's only a 4C difference, but it is just enough to stay away from the fan on set point even with some tolerance stack up.

Heater performance should not be greatly affected with only a 4C difference, and neither will closed loop performance or obd monitoring. Most of the critical thresholds mentioned in the DTC discussions centre around a minimum temperature of 80C. As a 80C rated thermostat is not fully open until 92C, it will be comfortably above the 80C thresholds in the ECM.