Hey good people,

I live in East Africa where ambient temperature averages 25-35 degrees Celsius all year round. I have had coolant leak troubles around the housing which seemingly emanate from weak connection places. First it was the hose connecting the thermostat tower to the expansion tank. Fixed it. Then it was the coolant cap. Fixed it. Now it is the hose after the thermostat with twin clips.
Engine temperatures (monitored off the OBD connection) are starting to run higher than normal (98 - 105 degrees Celsius) and fans are kicking in more often.
I suspect that the thermostat is starting to get "lazy-eyed".

Mechanic (who comes to do the hands-on stuff under my supervision) suggests thermostat not necessary in tropical weather. Says we can remove it without fearing for any adverse effects. What do you think? Need all the wisdom I can get!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

Just remove the center valve from the thermostat, you need the pressure drop that the body of the thermostat provides to slow the water down for better heat transfer. If water flows to fast it develops a water film coefficient between the block and moving water and loses some ability to transfer heat. If you were driving at highway speeds the water pump would push the water through the radiator at a high rate and not transfer heat to the fins in the air stream as efficiently.

 

Thanks RonaldP. The heat transfer explanation makes sense to me!

 

Update:
1. I have had the stat removed;
2. Will take readings over the next coupla days;

Noted:
1. Thermostat labelled 84 degrees Celsius but I was generally getting temperatures around 96 degrees Celsius.

I have only removed the valve part. I put back the round body of the stat along with the gasket.

About flow coefficients and blah blah, this may help someone in future: Water velocity vs. heat transfer coefficient

Hope to post my readings sometime. Overall I still have minor leaks on the housing so am going to buy the Al housing.

 

The "water moving too fast" is an old wives tale. And, BTW, the heat transfer equations which use the "film coefficient" mentioned seam to indicate faster flow in a turbulent flow system will cause MORE heat transfer. What am I missing?

The reason to not remove the thermostat is because the ECU is designed to control the engine assuming it's temperature is stable. The gas mileage and drivability will suffer without it.

 

+1

That myth is used by mechanics who know only half the basic physics they should. If the coolant was flowing too fast to shed the heat in the radiator, it would also fail to pick up the heat in the engine. If that was true, how could the coolant temp be too high?

A properly operating theromstat has never caused a car to overheat. Look elsewhere.

 

Bottom line: put in a new complete TST after you find and fix the leak. Quit over thinking it!

 

You have done all the work ( and a bit more, taking out the centre of the thermostat ) why not just fit a new one ???


What will it cost ? £ 10 maybe £ 20 .... ??? ( do not know the exchange rate )


So you now have to buy another seal ( gasket ) to fit a new thermostat, and refill with coolant ???


Maybe its me being daft, but it do not make sense to me.

 

Thank you all for the feedback.

1a. On heat transfer
Indeed the heat transfer coefficient is directly proportional to the flow velocity. This relationship occurs until a certain "critical velocity" is reached and then heat transfer may not be quite efficient (#I stand to be corrected). I would also put in mind that the pumps are rated at a constant rhythm.
1b. ECU rated temperature
Would this be a fixed value or an optimum range? I am not knowledgeable at all concerning this but from a design point of view, I would find it problematic to tune the ECU to one fixed temperature as there are quite a number of variables especially with different materials/scenarios involved in heat transfer as the engine is running. Would it be safe to say the ECU set temperature is a range not a FIXED value?

2. New T-stat
Yes, I think am going to buy a whole new Al housing along with the thermostat. I believe the plastic housing has been compromised by hair-line leaks whose effect is amplified by greater heat in the bay. Otherwise, they remain unnoticeable at lower temperatures.

3. On fuel mileage
Yes, my STFT/LTFT readings are already higher than am used to.

Moving forward:
Going to buy the new t-stat and Al housing.

Thank you all!

 

1. The assumption here (when speaking of the block) would be the critical velocity Ronald, right?

2. About the water pump, I thought again about it and figured that if the pump is rated at the same speed under all conditions, then perhaps there should be another reason for poorer heat transfer via the radiator?

#randomthoughts

 

Well, as long as we are in a heat transfer discussion, let's consider a fairly simple analysis, not requiring sophisticated multi variable equations! If more heat is transferred from the engine that is needed to keep the engine from failing, where does that heat ultimately come from?

 

Fascinating question Sparkenzap! Do you suppose thinking of the engine safe temperature as a range is a viable position to hold?
CIP: my thermostat was rated at 84C but my obd reported consistent 96C before I removed the stat. My guess is there is a "safe" range. Or am I trying to bury my head in the sand?

 

Obviously, the answer is the gas tank! The 96c temperature is definitly in the safe zone. They would have no need to have a pressurized system if temperatures throughout were always below 100c, would they? In fact, the "normal range" extends considerably higher. I don't have that number in front of me since I don't have the manuals on this computer, but I will look it up.

 

WTF. The purpose of the TST is to allow the coolant to quickly heat to operating temperature, it opens at 84C, allowing water to flow to the radiator for cooling. The TST does not keep it from going higher: pressurization, coolant, effective water pump and those two big a.. fans keep it in the right range once the TST opens. You should be using yellow/red DEXCOOL which raises the boiling point at 50% solution to 108.9C (228F) from 212F; pressurization raises it even higher to preclude boiling over. 96C indicates that the temperature is in a safe range. On my 01 na XJ8, the TST begins to open at 180F, fully opens at 185F; the fans kick in when the temperature hits 203 - 205F (~96C), well below boiling point -- which tells me yours was working just fine with the TST installed.

I believe (all else being equal) you have nearly 15C before you need to panic. Now, where do you stand on finding and fixing the leak (which reduces the pressurization) and changes the equation That, to my mind, is your real problem.

 

Thanks for the ideas Ross. No noticeable change in fuel mileage just yet. Will test this for a couple of days and see...

 

Thanks Jim, above well-received. Leaking has sneaked to the hose with twin clips that connects the stat housing to the outlet pipe (I dunno the technical names of these parts...). I am going to order for a new Al housing from the UK and put all these troubles behind me. Just thought I would use the opportunity to throw my thoughts around and do some learning as well. Thanks again!

 

Lots of threads on replacing the TST housing (rear bolts are a thrill). Besides the short cross hose, the plastic pipe crumbles where the outlet radiator hose attaches; worth ordering a new one; new water pump too (AC Delco, from eBay or Amazon don't leak).

 

Just to confuse the discussion:

During a recent few days of the first much below normal temps this Fall I noticed that the fans were running more than I thought normal with the AC off. Read the temperature at the OBDII data port at 96*C when I had just confirmed that there was a working 84*C thermostat installed. I stated it that way because I had just replaced a leaking crossover pipe which included a new temperature sensor. In that process the couple year old 84*C thermostat was tested repeatedly in a pan of water using an IR thermostat to determine the temperature. Worked as advertised.

Used the same IR device to measure the temperature of the brass body of the new temp sensor and found an 84*C reading while the OBDII port was still reporting 96*C. That called into question the sensor.

Solution still under evaluation.

 

Thanks Jim, am quite familiar with that. The plastic piping around the housing is alright. I just suppose the lower engine temperatures will do it some more justice. I will use the intervening period to arrive at a scientific conclusion about these temperatures. I will study them daily for some time. So far, the results are counter-intuitive!!! Am amazed after close to sitting in traffic for 2 hours this morning...

 

At <84C my OBD throws a P0128 which disappears once the temperature goes above. Temperatures fall when I sit in traffic for extended periods and continue to rise when I get back on the move.

Am wondering: are you able to read off the 84C at the OBD and follow the trajectory as the temps rise up?