zerobanger

I was driving home from reno today it was cold this morning, like 38 degrees or so. When I got on the interstate my car was fully warmed up, the needle was in its normal position (just below half way).


Anyway outside of reno you begin to descend for about 45 miles or so. I was driving along and after about 20 minutes I looked at my needle as I was going down the hill and it was almost completely to the left. I turned the heat off the car to help the car warm up.

I noticed when I started climbing elevation the needle would go back to the half way position, but as soon as started going down hill, it would go back to completly cold position again. these hills that I would be going down were like 4 or 5 miles each...btw.

Its ironic..in my rx7 I have to turn the heat on in the summer to keep it from overheating, in my rx8 I have to turn the heat off in the winter to keep the engine warm enough...

One last thing, because everytime I went up a hill the needle returned to normal I dont thing the gauge is bad. I also have never had this problem daily driving in bay area.

any ideas?

thanks.

Rotary Nut

It was probably because the car was coasting and the engine was not under load so it was generating less heat.

zerobanger

whoa dude, that was insightful!!

for real I kinda figured that, but I dont know if thats a good thing. My miata that used to have which I drove on many occasions on the same road never did that. On a side note, puting a turbo or a s/c on the car may actualy work well with the stock cooling system if its this efficient.

neit_jnf

Maybe your thermostat is staying open. If I was you I'd get it checked.

zerobanger

why would it stay open only then? i have had my car for a month or so, 1500 miles and it only happened when going down the mountain in mid 30 degree weather. when the engine went under load it warmed back up.

I dont know much about thermostats, but how could it be my thermostat? would it not have this problem all the time?

neit_jnf

Well, I don't own the car so I don't know if all of them do that but I used to have an RX-7 and that started happening when my thermostat stayed open. Warm up under load, cool off during coasting/cruising. It stayed in the normal position when I got it replaced.
Why would it stay open? I wouldn't know for sure. The thermostat is a temperature dependent valve, it has a wax filled actuator that expands at temperature and opens it.
Ask the service department about it, tell them about the temperature fluctuations. An engine is designed to work best under certein temperature range, it wears faster when too cold or too hot.

Haze

I had a similar thing the other night. I was driving home on basically flat roads in 5 degree farenheit weather. I could not get the car up to operating temperture even on the highway. I also turned the heat off trying to help the engine, but it never fully heated up. The trip was about forty minutes.

My feeling is that the mass of the renesis engine is so small that there isn't enough iron/aluminum in the engine to keep a more constant temperature, so Mazda is forced to make concessions in the cooling system. Erring on the side of over cooling instead of over heating is certainly the way to go with a Rotary from all that I have read. I think that that's what they have done, and I believe that that's just how the colling system works. Sucks, but if you wanted a car that wasn't idiosyncratic, you wouldn't have bought this one. That's what I'm thinking.

Gord96BRG

Previous threads on the slow warmup issue:
SLOOOOOW warmup in cold weather
Weak heat

Maybe some of the prior discussion will be helpful. It hasn't been really cold here lately, but my car will still take a long time to warm up if it's near 0F. Once it does warm up, it stays warmed up, and the heater has strong output - the problem is that if you get on the highway in near-zero temps before it's warmed up, it takes a loong time to warm up.

Regards,
Gordon

zerobanger

yea Im thinking that Mazda was well aware of the overheating problems of the FD and wanted to make sure they covered all grounds. Also, what is the range of the stock temp gauge? i notice when its completely cold its way below the first tick mark. When my car was cold after going down the hill it probably went to the 2 tick mark after the initial thicker tick marks on the gauge. Its possible that the gauge is measuring the temperature and while it was a drop, it still was within normal operating temps.

I'm not gonna complain, I know there is nothing wrong with my cooling system.

zerobanger

hmm..I dont know. I was on the highway for atleast 30 minutes and it was at the half way mark the entire time. when I went down the hill after about 5 or 6 miles is when the gauge started moving.

for what its worth, the heat wasn't cold even when the engine temp gauge showed it to be very far to the left.

neit_jnf

interesting observation! :p

<just teasing>

zerobanger

now THATS funny!!!

Nubo

Isn't the gauge one of those dumb ones that don't behave as a thermometer, but simply "cold, medium, and high"? Could be a calibration issue with the gauge/sender. On the other hand it could really be the engine is running too cold. If so, 30 degree temps shouldn't be enough to throw out of normal operating temperature. No matter how good the cooling system is, it don't do nothing once the thermostat closes.

The other wildcard is the oil coolers. I don't know what the flow rate is or if they could draw enough heat to take the engine below the thermostat setting. I also haven't been able to find out if they have a thermostat of their own.

I'd take it in. I wouldn't expect the needle to drop out of normal on the road once car's warmed up unless maybe it was far below zero. I've never had a car do this even in sub-zero temps with the exception of bad thermostat.

zerobanger

I really think the car is fine. Remember I was on cruisecontrol going down a 40 mile grade with 30 degree wind hitting the car. No load on the engine at all. As soon as I started to go up a hill..about 2 minutes the needle worked itsway toward themiddle. When i'd go back down after about 10 minutes of steady cruising down hill it went to the cool side again.

Haze

I agree with you that the car is working fine, and I don't believe that a car running cold for short periods like this can do any damage at all. It's just the way it is. It is definitely possible that a long hill decline can cool out the engine like that, and I agree with your analysis of the situation.

Interesting question as to what the temp range is. I have no way to answer that without a manual, and since this is my first rotary, I would say that you have a better idea than I do. Sorry that I can't be of more help.

neit_jnf

I saw on an Online Renesis Tech review that the oil coolers are thermostatically controlled. It doesn't specify how though.

93rdcurrent

I have had a similiar cooling in extremely cold weather at highway speeds when not climbing a hill. But we are talking about -10+ F and my needle didn't move all the way over to the left.

I will try to explain how the thermostat being open can affect the temp. The thermostat is pressure sensative and has a spring that keeps it closed until the coolant builds up enough pressure through heat from the engine to force the spring to contract and open the seal. When this happens it allows the fluid to travel from the engine to the radiator. Otherwise the fluid would all stay in the engine and continue to heat up and build pressure until it blew a seal out of the engine (in a piston engine it would be the head gasket). If the thermostat were to stay open it would not allow the coolant to get up to temp before it was cooled by the radiator causing it to drop below normal temps. If it is cool enough outside, you are moving at a good enough pace, and there were no strain on the engine you would potentially be able to take the temp down to the levels you are speaking of.

I would bring it up to the dealer because that shouldn't be happening. Here in 15-30 F temps my car operates at normal temp at highway speeds without a problem once it warms up.

Gord96BRG

Um... nope! Thermostats have nothing to do with pressure - radiator caps use pressure. Thermostats are entirely temperature based - it's easy to test, you can put a thermostat in a open pot of water, heat it to boiling, and use a candy thermometer to measure the temperature at which the thermostat opens. The thermostat begins to open when the coolant in the block reaches close to the designated temperature (probably 180F or 190F), and this allows some of the cold coolant in the radiator to start mixing with the hot coolant in the engine. The thermostat will be partially open (mixing cold coolant with hot coolant) until the entire volume of coolant is over the designated temperature, then it will remain open. At that point, coolant exiting the engine for the rad could be at 200F, but returns from the rad at 170F, for example. If the coolant cools down below 190F, then the thermostat will begin to close, restricting the flow back to the radiator to try to keep the temperature in the block at 190F (or whatever the designed opening temperature is).

Pressure only becomes a factor when the temperature reaches over the boiling point (at which point, the thermostat is fully open and out of the equation) - then, the radiator cap will release the pressure above a certain psi (typically around 6 or 8 psig). This lets the coolant refrain from boiling until around 230F rather than the atmospheric boiling temperature of 212F. Volume does increase with temperature, but that's what the coolant expansion tank is for.

Regards,
Gordon

grogiefrog

In my old '80 RX-7, I remember driving through Arizona in July '89 with the heater on... trying to suck out the heat. What a ride that was. The only positive thing about it was the blonde babe next me had very little on. :D

93rdcurrent

Gordon,

That is absolutely correct and I think that I get a C- for oversimplifying.

Haze

Yeah, Gordon's got it.

The "spring" of which you speak IS a spring, but it is also a bi-metallic valve. It is made out of two types of metal that expand at unequal rates when heated which causes the spring to compress and open. The spring tension of the spring is to make sure that the valve stays firmly closed during the warm-up period of the engine. They can be made to open at any number of temperatures depending on what the engineers are trying to accomplish, and at around that set point the valve will open and close keeping the car at a set temperature until other parts of the cooling system has to take over.

Actually, you weren't that far off. You had the spring contracting, which is the hard part, but it contracts due to the bi-metallic nature of the spring and therefore heat.

It is also useful to know that what Gordon has described is the way to test a thermostat to see if it is bad. Something seldom done these days since the price of thermostats has become so cheap, but it is doable.

Ahas

Zero,

If you don't diagnose a thermostat problem, you might want to try partially covering the front of the radiator the next time you drive long distance in cold weather, just to see how engine temp maintains itself under those conditions.

neit_jnf

I always thought thermostats were opened and closed by a wax filled actuator that expanded and contracted depending on the coolant's temperature. I didn't know anything about bi-metal springs in this application.

Haze

The only thing that I know of which uses wax in a cooling system is a german thermo-switch. You find them on older mercedeses, and they are used to mechanically unchoke the car once the engine is warmed up. Baically with a cold engine the wax is solid inside of its housing, as the engine warms, coolant which is travelling around the housing warms the wax and allows the switch to drop which unchokes the cold idle enrichment. I've never seen a thermostat that uses wax, but it is certainly possible that they are out there. I just haven't seen it.

neit_jnf

All thermostats on the cars I've owned or worked on had the wax actuator. The years of these cars ranged from 1984 - 2000 and all were Japanese or Korean (Mazda, Toyota, Nissan, Hyundai).

Heres some copy / paste information from the howstuffworks website:

How does the thermostat in a car's cooling system work?

Any liquid-cooled car engine has a small device called the thermostat that sits between the engine and the radiator. The thermostat in most cars is about 2 inches (5 cm) in diameter. Its job is to block the flow of coolant to the radiator until the engine has warmed up. When the engine is cold, no coolant flows through the engine. Once the engine reaches its operating temperature (generally about 200 degrees F, 95 degrees C), the thermostat opens. By letting the engine warm up as quickly as possible, the thermostat reduces engine wear, deposits and emissions.

If you ever have the chance to test one, a thermostat is an amazing thing to watch because what they do seems impossible. You can put one in a pot of boiling water on the stove. As it heats up, its valve opens about an inch, apparently by magic! If you'd like to try this yourself, go to a car parts store and buy one for a couple of bucks.

The secret of the thermostat lies in the small cylinder located on the engine-side of the device. This cylinder is filled with a wax that begins to melt at perhaps 180 degrees F (different thermostats open at different temperatures, but 180 F/82 C is a common temperature). A rod connected to the valve presses into this wax. When the wax melts, it expands significantly and pushes the rod out of the cylinder, opening the valve. If you have read How Thermometers Work and done the experiment with the bottle and the straw, you have seen the same process in action. The wax happens to expand a good bit more because it is changing from a solid to a liquid in addition to expanding from the heat.

This same technique is used in automatic openers for greenhouse vents and skylights. See this page for an example. In these devices, the wax melts at a lower temperature.