Engine Temperature

[Yellow Rose]

Up until recently, my water temperature gauge needle always points at one mark below the half-way mark. This is after the engine is thoroughly warmed up, which usually occurs after two to three miles of city driving at 30 - 40 MPH. For simple math’s discussion purposes, assume this takes about four minutes.

What I have been noticing lately, is that it sometimes takes for the needle to reach its normal mark. For example, a few days ago driving 50 MPH for about three to four miles (again, approximately four minutes) the needle was only two marks from the bottom. Eventually it reached its normal mark, one below the half-way mark, but it took noticeably longer to do so.

The ambient temperature was relatively constant at +/- 80 °F during these episodes.

Today, after driving almost seven miles - five of which were at 70 MPH - the temperature gauge needle was only one or two marks from the bottom of the gauge.

The service manager at a local Honda dealership says that temperature sensors on Honda products rarely go bad. Without any diagnostic tests, he suspects the thermostat is sticking open. But this does not make sense to me, because as the engine cools off overnight, the thermostat is slowly going closed, right? So if the car begins the day with the thermostat closed, why would the temperature gauge needle indicate the engine is not up to normal operating temperature? Especially after a not-so-short drive at highway speeds.

Coolant level is fine, and the engine does not "smell" like it is running hot. There is a distinctive odor to an overheated engine.

What do you think - bad temperature sensor or bad thermostat? Anybody else experience this?

 

[Larry Bastanza]

I would suspect a bad thermostat (stuck open or partially open). Out hear in the East I have a set drive I take and know when the gauge should rise, based on ambient temp. This is the first test I do with any car before draining the coolant, tells me if I am putting in a thermostat or not.

HTH,
LarryB

 

[Andrie Hartanto]

Agree with Larry. Thermostat problem!

 

[hlweyl]

My thermostat was stuck in the open position. When I took it out, the rubber diaphragm was torn and the loose end of the black rubber was stuck inside the spring, preventing it from opening.

Same symptoms except when it started getting cold up here, as the car started to warm up, once I hit about 50 mph, the needle would plunge back down to the cold mark.

good luck

 

[NSX 3.0]

Andy,
My car was acting like yours and I replaced the thermostat and that seemed to take care of it.

 

[SNDSOUL]

Besides not having a very efficient heater, are there any drawbacks to not running a thermostat at all?

 

[hlweyl]

Besides not having a very efficient heater, are there any drawbacks to not running a thermostat at all?

Absolutely! The engine is designed to operate most efficiently at the proper temp. Running hard at a cold temp is a good way to starve the bearings when the oil is most viscous.

 

[SNDSOUL]

Absolutely! The engine is designed to operate most efficiently at the proper temp. Running hard at a cold temp is a good way to starve the bearings when the oil is most viscous.

I've never heard this and I have run several cars without the theromstat without having any ill affects. I have a Jeep with a Corvette motor in it that hasnt had a thermostat since shortly after I put the motor in the vehicle. Runs VERY strong(Pulls the front tires about 3ft off of the ground and I have the scar on my head from the rollover to prove it)

Anyone else heard of staving the bearings with a cold motor??? BTW, what bearings are you refering to?? Rod bearings?? I cant imagine oil being so cold that it would effect flow. Even on the coldest day(which around here might be 35 degrees) the oil would still be very fluid. If I am wrong, I would love to hear a logical explanation. thanks

 

[Tom239]

Besides not having a very efficient heater, are there any drawbacks to not running a thermostat at all?

Clearances between moving parts change when metal expands with increasing temperature. Engines are designed to have optimal clearances (neither too tight nor too loose) in a certain temperature range; thus cooler is not always better.

 

[Larry Bastanza]

As a matter of practicality I would say your oil, especially today's oil, will save the bearings, however it does not mean it is a good idea to not have a thermostat. The engine should run at it's designed temperature without fluctuation for many reasons. Better wear(not only bearings), less emissions aka better fuel mileage, etc.

Also in this particular case we have an engine with an Aluminum block, which is much more sensitive then an iron block to temp change.

My $.02,
LarryB

 

[nsxexotic]

My car has gone through at least 2 oem thermostats. They seem to only last about 35k for some reason. Maybe age plays a role.

 

[SNDSOUL]

As a matter of practicality I would say your oil, especially today's oil, will save the bearings, however it does not mean it is a good idea to not have a thermostat. The engine should run at it's designed temperature without fluctuation for many reasons. Better wear(not only bearings), less emissions aka better fuel mileage, etc.

Also in this particular case we have an engine with an Aluminum block, which is much more sensitive then an iron block to temp change.

My $.02,
LarryB

makes sense

 

[nsxexotic]

As noted earlier, if you run your car cold then the engine oil will be cold as well... which will result in lack of lubrication. Oil is thick when cold.

 

[fkong777]

Besides not having a very efficient heater, are there any drawbacks to not running a thermostat at all?

2 drawbacks

1. Engine need longer time to heat up to normal operating temperature.

2. Some radiators are design to cool water down at a certain rate. If not themostate the water flow will be too fast. Water wouldn't stay in the radiator long enough to effeciently cool down the water.

 

[NSX-Racer]

2 Some radiators are design to cool water down at a certain rate. If not themostate the water flow will be too fast. Water wouldn't stay in the radiator long enough to effeciently cool down the water.

Would that mean that also in the case of a NSX the engine would run cooler on the track if the thermostat works than if the circuit is always open? (I had it always open this year on the track)

 

[fkong777]

Would that mean that also in the case of a NSX the engine would run cooler on the track if the thermostat works than if the circuit is always open? (I had it always open this year on the track)

No that mean it may run hotter than normal flow. Water may not cool due to the short flow thru radiator.

 

[SNDSOUL]

As noted earlier, if you run your car cold then the engine oil will be cold as well... which will result in lack of lubrication. Oil is thick when cold.

IMO the NSX should only be run with a synthetic oil. Synthetic oil, IMO is not thick, no matter what temp. And also, if you are using a quality grade oil, it will coat the internals from normal use. I just dont think running without a thermostat will have a dramatic effect on how the NSX would peform or its reliability. If the" low oil temp is harmful b/c of lack of lubrication theory" were correct, that would mean the first 5 mins or so that you drive you car, you are doing damage. That simply is not accurate.

Can anyone quote specific reasons why it would make a difference or is it all just speculation that the engine is designed to operate at a certain temp and it probably doesnt work as well outside of that range.

Just being argumentative for the sake of knowledge.

 

[hlweyl]

Ok, think of it this way: when you turn your engine on, why don't you floor it right away? Because it's not warmed up yet, right? Why did the Honda engineers design our engine not to go into VTEC land when it's cold?

And I also disagree that the NSX should ONLY be run with synthetic. I run synthetic, but the fact is, the engine will run just fine with dino oil.

 

[SNDSOUL]

And I also disagree that the NSX should ONLY be run with synthetic. I run synthetic, but the fact is, the engine will run just fine with dino oil.

Thats why I put IMO(in my opinion)

Ok, think of it this way: when you turn your engine on, why don't you floor it right away? Because it's not warmed up yet, right? Why did the Honda engineers design our engine not to go into VTEC land when it's cold?

ok, I understand that everyone agrees that there is a reason you dont run the engine hard when it is cold, but what I am asking is what, specifically, would be harmed?

 

[Yellow Rose]

Oh Dear

First off, I’d like to thank those who responded – both private and public.

Secondly, for the sake of “Andy being Andy” I will debate with some of you (no, you are not allowed to rebut because as Left Lane has said before, “Andy’s threads have his unique rules where his card trumps your card.” ) regarding your comments on oil viscosity vs temperature.

Thirdly, I will redirect this thread to the original topic with a couple of water-related questions. Thanks.

You are not starving lubrication when the engine is cold…..that is the purpose of 5W and 10W oils for colder climates. However, you are introducing a micro-second of mechanical wear when the engine is first started because the oil pump is not fully putting out its designed discharge pressure. But this applies to an engine sitting overnight as well as one sitting for only five minutes, because the oil has already gravity-drained back to the pan.

The comment, “IMO the NSX should only be run with synthetic oil.” is total bullshit. Sorry, can’t word my feelings any other way. I have over 100K miles with my two NSXs (which are also tracked) and only one time did I try Mobil 1 for an experiment. With that experiment over, I reverted to organic and have never experienced an engine lubrication problem with organics. EVER. PERIOD.

No thermostat is generally bad because the water is flowing too fast such that (a) it does not have a chance to absorb heat from the engine and (b) it does not have a chance to reject heat to the radiator. The thermostat is a restriction in the water pump system that dictates how much the water circulation rate is. No restriction means that a centrifugal pump will “run out on its curve” and deliver too much flowrate, which means less than optimum heat transfer rate.

Even if the thermostat is stuck full open, it is still a restriction in the system…..only the engine takes longer to warm up.

Finally, back to the topic at hand. The service manual shows that the thermostat fits into a housing that bolts to the block. 1 – does this housing need to be removed when replacing the thermostat? 2 – the service manual shows o-rings between this housing and the block, but it shows nothing between the housing and the thermostat cover. Is there an o-ring, or is the cover sealed with HondaBond? 3 – on the housing is the temperature gauge sending unit and on the thermostat cover is a thermo sensor…..what is the latter device for?

 

[Autophile]

Re: Oh Dear

No thermostat is generally bad because the water is flowing too fast such that (a) it does not have a chance to absorb heat from the engine and (b) it does not have a chance to reject heat to the radiator.

I disagree. When water flows fast through a pipe (i.e., high Reynolds number), the boundary layer is small. When the boundary layer is small, the dependence on conductive heat transfer is reduced, and the influence of convective heat transfer, a more efficient heat transfer mechanism, is enhanced. So, when water is moving fast, i.e., turbulent, it is more efficient in heat transfer than slow moving water.

A traditional automobile water thermostat is used to keep water temperatures higher than what they would be without a thermostat. Time is used as a parameter in this system to retain heat, not to enhance cooling.

Go ahead Andy, fire away.

 

[hlweyl]

Re: Oh Dear

Finally, back to the topic at hand. The service manual shows that the thermostat fits into a housing that bolts to the block. 1 – does this housing need to be removed when replacing the thermostat? 2 – the service manual shows o-rings between this housing and the block, but it shows nothing between the housing and the thermostat cover. Is there an o-ring, or is the cover sealed with HondaBond? 3 – on the housing is the temperature gauge sending unit and on the thermostat cover is a thermo sensor…..what is the latter device for?

The only thing that has to be removed is the thermostat cover, the larger housing is NOT removed. From memory, there are 2 bolts that need to be removed. No Honda Bond is used, the thermostat should come with it's own rubber gasket. The thermostat fits inside the groove in the gasket thereby sealing both inner and outer surfaces when reinstalled. Helps to have two people when bleeding.

good luck

 

[Larry Bastanza]

Andy,

Order 19301-PR7-305 for a replacement thermostat "Kit" includes gaskets. This is applicable for 1993+, the early cars required you to order the gasket and thermostat separately.

As mentioned no sealer needed, it is a bolt on, make sure you have a big size pan

Also in regard to the operation. The thermostat controls the mix of cool water(coolant) from the radiator vs. hot water (coolant) from the block. There are two "coolant loops" in the system. So the "mix" determines the block coolant temp. I actually think the circulation amount is always constant, it is just the mix of the two loops that manages the temp in the block.

Without a thermostat the variation would be quite a bit, as ambient temp changed, for example the radiator would have much cooler coolant, if it was 20 degrees vs. 90 degrees. If the thermostat was open the mix is constant, and the engine is at a different temp for sure. Just like in the winter, you can see your temp gauge dive if your thermostat suddenly opens.

Another $.02.
LarryB

 

[Yellow Rose]

To Autophile

We both have our points; however, they are qualitatively based vs quantitatively based. Are you sitting down? I do agree with you that faster flow means greater turbulence which results in a thinner boundary layer. In heat transfer, the fluid provides less resistance than the boundary layer.

But let’s take the opposite extreme. If you are flowing so slow to be in laminar flow, the water can be heated to xxx °F in less time vs fully turbulent flow regime. Weak (?) analogy - pass your hand slowly over a candle vs fast over the candle and it will get hotter faster.

What we do not know is what the effect on turbulent flow is, if the thermostat is removed. Perhaps the flowrate delta is, say, 10 % (purely made up value) and that will have a small effect on heat transfer rate.

Closing thought - although my car takes a long time to warm up now because the thermostat is stuck open, eventually it reaches the normal needle position of one mark below the half-way mark on the gauge. I would venture to say that if I were in Canada with a thermostat stuck open in the winter, the engine would not warm up regardless of how long I drive.

 

[Joe Gliksman]

I am now experiencing my second stuck open themostat, and must add a drop in fuel economy. The engine runs much cooler and I think the choke must stay on longer using more fuel? Also, this month's Car and Driver magazine has a story about sludge in oil and its hazards which seems to support the discussion about increasing the potential to starve bearings due to cold thick oil. In addition, the NSX runs better warmed up!