Running temperature and thermostat.

[Gmg31]

re Renault engine: When I first set up my car I left the thermostat out to ensure it didn’t overheat during my running in period.  However I have noticed that my car still runs hot all the time and the radiator fan is constantly on in normal use.  I rang Richard and he said I needed to fit the thermostat in order to slow down the coolant flow through the radiator. Apparently it doesnt get the chance to cool down with the thermostat missing.  I really don’t want to appear to questioning Richards advice but is this the view of others in the forum?  We are having unusually hot weather here and I need to do the three drive to Silverstone next weekend. I cannot risk overheating problems. Thanks.

[BDA]

I would tend to trust what Richard says. I can't say that I've heard that but it could easily be true, although back when I raced my Spridget (shortly after dinosaurs became extinct), I didn't use a thermostat - of course I had a pretty big radiator in it so that could fit Richard's explanation.

I think I would test Richard's theory and convince myself he was right or wrong and proceed accordingly. I have a completely different engine. It has a thermostat and it doesn't use the fan at all on the highway. I would think you should expect to have a similar experience.

[TCS4605R]

One thing you might try - when I was autocrossing my 1958 MGA (like over a half a century ago), I removed the movable part of the thermostat and just used the non-movable outside 'ring' - it had just enough resistance to flow to slow down the water flow rate enough to give the radiator time to reject heat.  The MGA is still in my shop patiently waiting some attention.

Tom
74TCS - 4605R

[jbcollier]

Thermostats facilitate proper engine warm up, reducing the stress of cold parts working against each other.

Fit a 160F/71C thermostat if you can find one.  What is the rating of your rad fan switch?  Located in the inlet or outlet?

[EuropaTC]

Ok, firstly I've never had the renault engine so bear in mind that any comments are related to experiences with other engines.

First thoughts are "can I believe the temperature gauge & sender ?".  These things are normally very reliable and if a sender goes it's usually obvious, but a slight mis-match between sender & gauge or a gauge that isn't calibrated accurately can throw you a few degrees out one way or the other.   There are probably far better ways to resolve this problem on the bench if you know the resistances for the gauge/sender but one simple way is to wire up the sender on a couple of long leads, drop it into boiling water and see where it goes on the dial. That's fairly simple and crude but does but an accurate 100C mark on the dial. A refined version of that test where you have a thermometer in the water as it warms up will give you accurate scale calibration up to 100C so you'll know if it's 85C, 90C or 95C with some confidence.

If the gauge checks out then my next move would be top and bottom radiator hose temperatures. OK, not something you can do accurately but with the fan running I'd expect to feel some difference between top and bottom hoses. I don't know if Richard's theory of "too fast to cool down" is correct or not but I'd want to know if there's any significant reduction across the radiator. If not, it could be a slipping water pump belt, fluid speed or breakdown of an internal baffle but replacing the thermostat would be an easy way to eliminate one theory.

Personally I'm not convinced that removing the thermostat prevents overheating other than on short runs and that's mainly because the engine takes longer to reach working temperature. It was often claimed to solve problems with TC engines but my take is that if an engine is designed to operate with a thermostat then there should be one in there. How long is your car taking to reach operating temperature with the 'stat removed, and when it's there is it stable or still creeping upwards ?

[cwtech]

Coolant flow too fast vs. coolant flow too slow   .....This topic is often debated, with no definitive answer.

High-Flow water pumps and thermostats are often sold for aftermarket applications.  ....Why, if too much coolant flow is bad??

Opinions about thermostat operation also differ.  ....Some believe thermostats "cycle" at all times.  ....Others believe thermostats cycle until engine temperature stabilizes at (or exceeds) thermostat opening temperature.  .....If the coolant remains at 200*F with a 180*F thermostat, how can the thermostat possibly cycle??

Removing a thermostat is often said to be bad, but the installation of a flow restrictor is often recommended.  ....Thermal efficiency and power is said to increase at higher operating temperatures, but temperature too high causes engine damage.

It is often said that too much flow does not allow the radiator to disperse the heat.  .....If that is true, then coolant does not pick-up as much heat in the engine which needs to be dispersed, since the coolant is not in the engine as long, either.

Another theory suggests that the higher the difference between coolant temp and ambient air temp, the faster the heat exchange takes place.  ....Conversely, while the temp exchange may be faster, there is a greater amount of BTU's which need to be cooled by the same ambient air temp.

I don't have answers, only more questions!

One needs to consider several factors in regard to engine temperatures.  .....Adequate air flow through the radiator, whether vehicle is moving or stopped?  ....Properly operating water pump (loose belt, worn impeller, etc)?   .....Unrestricted coolant flow through the radiator? .....Suitable coolant composition? .....Adequate pressurization of the coolant system?  .....Accurate temperature gauges and sensors?  ...Etc., Etc., Etc. !!

[Bainford]

When I was taking auto mechanics in trade school (very) many moons ago, my instructor said to never remove the thermostat from the cooling system, as overheating can occur due to the increased speed of the coolant. It seemed horse pucky to me at the time (I was, afterall, still in my late teens and therefore still knew everything), but over the years I had encountered real world examples that suggested he was correct, including some personal experimentation. Also, before my Lotus life I was all about Fords, and had (still have) an old Mustang with a 351C engine, which I overhauled. I was interested to find that these engines have a brass insert, a metered orifice, in the cooling path of the block, just below the thermostat. I was cautioned by a couple of people in the know that this restriction insert must not be removed, or overheating will result. Food for thought.

[Lotuswins]

Heat transfer 101.....such a glorious topic. 

First off, the thermostat does 2 things:  Firstly it accelerates engine warm up, so all the moving bits get up to temp as quickly as possible helping minimize wear.  Secondly it provides a back pressure on the cylinder head so the max pressure in the system is where it is needed, near the combustion chambers.  This helps prevent incipient boiling in that location, which if it happens, reduces heat transfer between the chamber and the coolant.  This is why a restrictor is recommended if the thermostat is removed. 

So, does the coolant have to slow down to enable more heat transfer??  Well, actually the faster the coolant flows, the more heat transfer can take place.  Heat transfer mechanisms are somewhat similar to electrical current flow.  The larger the voltage (temperature) differences, the more current (heat energy) will flow.  Of course there are many mitigating circumstances that tend to get in the way of increasing the heat flow.  If there are restrictions or pump cavitation, etc. which will inject air bubbles into the flow, that will have a major effect on the capability of the fluid to conduct the energy away.  The air bubbles will have a tendency to cling to the surfaces of the radiator tubes, impeding the heat transfer. 

Generally though the major improvement in cooling can be achieved in the air flow and circulation into the radiator rather than on the liquid side.  If you increase the size/flow of the fan pushing the air through the radiator it should help.  Recoring or replacing the radiator with one with louvers on the fins, and more rows of tubes will help immensely.   Copper or aluminum cooling capacities are almost equal so a lighter aluminum one would be preferred in a Lotus. 

I hope this helps??

Jerry Rude (once an ME)
4005R

[BERNIEHUMBER]

HI:
Back in the day I had overheating issues and I found the problem to be a little baffle inside the rad that would redirect the coolant to make another pass.
I worked for a company where we designed heat ex-changers so I got a thermal camera(they had just come on scene) and through a lot of gymnastics got a pic of what was going on(or not).
About 1/3 of the flow bypassed the full effect of the multi pass design because that baffle was defective.
I got the rad re-cored and added some redirect to get a 4 pass instead of a 2 pass.
This causes a higher p.d. but the overall effect was successful.
If I just repaired the baffle in the header it might have sufficed.There is a law of diminishing return when it comes to passes through a rad(any HX) as the first pass is most effective because the difference in the fluid temp in and the air over is the greatest.   

[Steve_Lindford]

Check the mesh behind the radiator hasnt rusted as it will reduce the airflow. Check the radiator is not covered with dead flies which will do the same. I improved cooling by removing radiator and flushing with a garden hose. A fan override switch gives a bit of piece of mind. I have often wondered if the spare wheel restricts air flow. Also check the bonnet doesnt lift at speed - as that will be air escaping that should be used for cooling. Remember using the heater adds cooling. Dont forget their could be a queue to get into Silverstone...

Steve

[jbcollier]

Does increasing coolant flow increase heat transfer?  The answer is yes, but...

Modern engines have narrow cooling passages, densely rowed radiators and high-output water pumps.  Coolant speeds are high and the narrow passages and dense rows give high surface contact for excellent heat transfer.  My last Volvo had a total of 6 litres of coolant in the system.  Amazing really -- as an aside, one of the main drivers is reduced weight to help meet fuel economy requirements.

Older engines, like the Renault and the TC have large coolant passages.  This leads to flow characteristics that may deteriorate with higher coolant speeds.  Basically the coolant slows near the walls and accelerates in the middle reducing heat transfer.

How fast is too fast?  Hard to say, hard to predict.

Is Gmg31's cooling system working poorly solely due to the thermostat removal?  Doubt it.

First, put the thermostat back.  Not to necessarily solve the problem, but for the good reasons that it was there in the first place: speedy warm-up.  After that, we diagnose any other issues.

[Gmg31]

Wow, fantastic range of views and advice.  Thanks everyone really appreciate the time and thought you have all put into this.  This car has had a complete restoration over three years and pretty much everything is new. I would say that she has never overheated but the problem was that the fans were on for most of my journey.

So tonight I flushed the system through and added the thermostat. I took her for a good drive and I was very pleased. She appeared to be smoother and ran more consistently but I might have been imagining it.  The fan was on for a while but not the entire journey. A result so far but I guess that 3 1/2 hour trip to Silverstone will be the test. 

[BDA]

Good luck on your trip to Silverstone! I think Europas will be well represented by your car! Let us know how she does on on the trip.

[buzzer]

one other thing to consider if removing the thermostat. It may not be the rate of flow but where the water flows around the engine. quite possible that depending of configuration that water does not circulate into the rear of the block as it comes straight in from the rad and out the top as there is nothing to provide back pressure to force some circulation. 
And hopefully see you at Silverstone, I will be there Saturday and Sunday as I have my Range Rover on the Range Rover stand. The Lotus needs a few more miles and bedding in and to fix some oil leaks.
 

[Gmg31]

So I flushed her through added the thermostat and there was  a noticeable difference.  For most of the journey the fan remained off but if there was a slow patch it would come on and then stay on for the rest of the journey.  Still happy with the outcome. Thanks everyone for your advice.