A lot of people ask questions about overheating and how to lower the engine temperature. A common belief is that a thermostat will not change the engine operating temperature, just take it longer to get to the same point. This is not true, at least not in highway driving conditions. Yesterday, I drove to Dallas in 98 degree temperature with the AC on and the engine temperature stayed at 173 degrees. The exact same trip in the same temperatures last summer with the stock thermostat was 193 degrees. Although my fans have been reprogrammed, they are set to turn off at 55 mph, so a lower fan setting could not have been the reason for the lower temperatures. In traffic where there is lower speeds and less air flow over the radiator, the fan programming will have a major effect. I am running a 50/50 antifreeze mix without water wetter.

One major thing I have noticed is that with the cooler temperatures, my gas mileage has gone down. This could be due to a less dense air charge with the higher intake air temps. This would require less gas for a proper mixture.

 

I agree about the lower temp stat. With my 160 deg stat in 70deg weather I cruise at 155deg coolant temp; in 90+ weather it will go up into the 170s.

I drove from SE NY to mid Ohio last Christmass and pulled 31mpg highway; on the trip from Pheonix, AZ to SE NY it only pulled 29mph, bone stock. The only place my low temp stat has hurt mileage is around town because I like to get on it more now with the added power.

Compare your highway mileage on a trip with before the low temp stat to see if it has dropped. If it's down check your O2 sensors. I had one out on my IROC last winter and it hurt highway mileage because it was in open loop mode.

 

I have a 160 t stat and with my ac on and 90 degree weather I was at 185.


[Modified by Raysur, 3:49 PM 6/13/2002]

 

Strange as it may seem, I run a 180 thermo and generally run 174 to 178 on the highway at 70mph in say outside temps up to 80.

I have been one of those who have told people who complain about running 200+ that a 160 thermo, or even a 180 will not help since once the coolant temp passes the thermo setting, the thermo is open.

I have always believed that a clean airflow to and through the radiator as well as lower fan settings are the first defense

:seeya Jack

 

Something sounds wrong to me. What year is your car? Either your thermostat is lower than you think or it is stuck open. Another possibility is a bad temperature sensor. Something I doubt but is a remote possibility is that the temperature sensor is located in a bad position. The only coolant that should be below the thermostat temperature is between the radiator and the water pump on the inlet side.

I agree that a clean radiator and fans will do the trick for most driving. The reason I went to a lower temperature is due to bad dyno numbers. The operator told me that the reason my numbers were so low is due to high engine temperature. This was with reprogrammed fans. The engine temp was around 215 for the second pull. Now that I have a 160 thermostat, I never see engine temps that high. I have not had a chance to have my car dynoed since. This was at MTI in Houston.

 

The 94-96 (LT1/4 and maybe earlier LT1s as well) stock tstat is 195* so what you are seeing is the tstat in action. Sounds like your radiator is working at optimum efficiency.

If you were up in the 200s, i.e. driving under higher loads (racing or chasing), it wouldn't matter what temp the tstat was fully open.



[Modified by John Row, 2:05 PM 6/14/2002]

 

Even with my fans set to go on @ 184 I see hi 180's coolant and mid 200's oil w/ my 160 stat in 80 degree temps. My problem is a combination of things. I don't have an oil cooler or tranny cooler, that's part of it. I'm installing a tranny cooler now. The radiator, which will be cleand since I'll have it out for the cam install, looks like a dryer lint catcher.
Another important part was the air dam. That was the first thing on my project list. When I was done I took it for a test drive. I noticed the coolant temps went down to 170's and oil was 188. When I'm all done I shouldn't have any problems with any temps. :)

 

The stock thermostat is 180 deg. I know this is hard to believe but if you go to pep boys and look up in the stant catalog it will list the stock
replacement as a 180 deg. and thats the only one stant makes for the lt-1/lt-4.

 

so are you guys changing your chip/reprogramming? or does the thermostat alone keep the temp down?

 

kkonen,

>> A common belief is that a thermostat will not change the engine operating temperature, just take it longer to get to the same point. This is not true, at least not in highway driving conditions

What you say is true, but misleading. The tstat does not change the engine's final operating temp as long "as that final temp would be greater than the temp range of the tstat being used".

When you used the stock 195 tstat it actively restricted fluid flow until temp reached 193 degrees, at which point the 195 tstat is almost fully open. The fact that the final temp (193) was LESS than the stock tstat (195) tells you that the stock unit was trying to achieve 195 degrees before becoming fully open. Had you used a 180 tstat it would have brought the temp to about 178 by again restricting flow to achieve the tstat temp (180).

Tstats are a restriction in the fluid flow. Contratry to belief, the goal of a tstat is to keep the engine warm. Not cool.

On the highway with plenty of air flow you topped out the temp at 173 degrees because there wasn't enough heat being created to overcome the cooling system capability available once your 160 tstat was fully open.

Since 173 is GREATER than 160 degrees, using a 160 means that the 160 tstat was wide open. The other tstats would not be fully open at this point because they are manufactured to try and make the base temp of the engine higher.

If you were in traffic and the heat generated vs heat dispersed numbers caused your vette to hit any value over the stock 195 tstat, (say 200 degrees), then having the 160 would have been of no use. The stock 195, a 180, and your 160, tstats would all be fully open and the limiting factor would be total flow of fluid through the radiator. A fully open 195 tstat allowas just as much flow as a fully open 160 or 180.

If you want your car to run as cool as possible whenever there is enough air flow then the 160 is for you. However,if you are seeing 195+ in traffic, then the limiting factor is not the tstat.

Since SAE (Society of Automotive Engineers) tests and certifies motor oil at an operating temp of 180 degrees, I prefer to use a 180 tstat so my vette warms up enough that the oil lubricates properly. it is cooler than the stock unit on the highway and keeps the engine warm enough that my oil temp never falls below 180.

 

I never used a 195 thermostat. A stock thermostat is 180 for a LT1/LT4 engine. My car dropped exactly 20 degrees when I put in a thermostat that was 20 degrees colder. In both cases the engine temperature was 13 degrees above the thermostat temperature.

You are correct in that a thermostat is too keep an engine warm. In most climates, a thermostat allows the engine to warm up. It takes an extremely efficient cooling system or cold weather to keep an engine from running with a thermostat at anything less than full open. But how warm is warm? The stock thermostat and fan setting is designed to keep the engine over 215 degrees. The fans don't come on until the engine hits 232. GM did this primarily for emissions. It also helps gas mileage from my personal tests.

 

No, the common misconception is:
"My car is running too hot at 225F so I'll replace the 195 thermostat with a 160 and that'll fix it."

If you're above 205F, every thermostat 160, 170, 180, 195 is wide open. It's the pathetic radiator, limited airflow, and high fan turn-on temps that make Corvettes run hot. A lower temp thermostat will only lower engine operating temperature if the cooling system provides enough heat dissipation to cool to that temperature in the given environment.



[Modified by gcrouse, 8:19 PM 6/15/2002]

 

Having read the above, I believe I should put in a 180* stat, install a fan switch, and clean the radiator. That should help my LT1 run cooler.

 

Your LT1 should already have a 180* stat.

 

I am somewhat surprised that a 195 degree T-stat would have been thought to be stock in the LT1. I bought a new replacement GM water pump for my LT1 and it came with a 180 degree T-stat already installed. I would have to think that 180 was stock (and that is what I recall from my shop maual). :confused:

 

I apologize for assuming the LT1 had a 195 thermostat. My Corvette Parts & Illustration Guide only goes thru 1991. From 1987 - 1991 it shows a stock thermostat to be 195. I don't put much faith in Pep Boys documentation, but if you say your GM Manual specifies a 180, then I believe you. I was wrong with respect to the stock LT1/LT4 thermostats.

kkonen, you say the temp differed by 20 degrees under the same circumstances but the time frame is a year apart with thermostats of different ages. Perhaps you err in your recollection of the conditions?
Are you absolutely sure the wind speed was the same on both days. The direction with respect to your travel the same? This is important since different conditions (headwind vs tailwind, or no wind) can affect air flow and engine load considerably.

Also you note the chip has been reprogrammed. Perhaps you were running much leaner and hotter then? The stock setting for L98s (and I believe still true for LT1/LT4s) is to have the fans automatically come on when the air conditioning is turned on. You state that you had the air conditioning on during this last test. This may have overode the setting to turn them off at 55 mph unless that was programmed into your chip also. Essentailly, you are going by recollection of an incident that was a year ago with a different programming for many possible variable like fuel mixture, fan actiivation times, spark advance, etc. You are comparing apples and oranges.

The job of a thermostat is to act as a progressive switch to restrict coolant flow until a desired operating temp is reached. That is all it does. A 160 tstat starts opening before coolant temp reaches 160 degrees and is wide open at 160 degrees. A 180 operates in the exact same way except it starts 20 degrees later. However, as someone else has already mentioned; when wide open ALL thermostats allow the same amount of coolant flow. A 160 does not allow more flow than a 180 or any other standard thermostat for your vehicle.

If you know how heat is dissipated by the automotive cooling system, you know that once the tstat (any tstat) is wide open, the amount of fluid flow and velocity through the radiator is the same for a given speed. The limiting factor then is the velocity and outside temp of the air passing across the radiator fins. If in your first test the water temp was 193, then both a 160 and 180 tstat would be wide open by the time you hit 181 and the EXACT SAME amount of coolant flow and velocity would have occurred. Neither one would be doing anything different to the cooling system and therefore neither would have caused the temp to drop below 193. The exception would be if the 180 tstat had started to go bad (quite likely) and was sticking partially closed or not opening fully, then under the same conditions you could have seen 193 because the 180 was defective. Less flow due to a defective 180 tstat would cause the 193 temp.

If the engine was able to dissipate heat fast enough that it maintained a temp of 173 with the 160 tstat, then a properly working 180 would not have caused the 193 temp you saw. You would however see a slightly higher temp than 173 with a good 180 tstat because it is built to restrict flow until temp reaches 180. The difference would be about 7 degrees (180) at most.

If you can find ANY college professor of Automotive Engineering or who will say that under the operatong conditions where a vehicle achieves 193 degrees with a standard correclty working 180 thermostst replacing it with a standard 160 will cause that vehicle temp to drop to 173 I will buy you a case of beer.

Whay are so many shade tree mechanics fond of 160 thermostats? The advantage of a 160 is that IF there is enough cool air flow to drop temp to 150 then a 160 tstat will allow you to drop to 160. The 160 bottom limit of the 160 tstat is 20 degrees less than the bottom temp of the 180. Once you hit traffic and air flow stops, it will take longer to go from 160 to 200 than it would to go from 180 to 200. The extra few minutes might be important to you. If you are stopped for 5 minutes and temp climbs 30 degrees from 160 to 190 and then you start moving again, the temp will start dropping due to incereased airflow. With a 180 tstat you might have seen the same 30 degree temp rise from 180 to 210. In this instance the 160 would have kept your final temp lower becaues you started moving before reaching max temp.

Had you been stuck for 20 minutes and the temp gone from 160 to level off at 215 degrees then the 180 tstat would have produced the same final temp. As I just described, under the right circumstances a 160 can aid in keeping temps down. But, if you are in traffic and seeing 225 for a long period of time, then the 160 will not help get that max temp down.

Mike
:)

 

The stock t-stat on my LT1 was stamped 180. Parts books that I've seen also list 180.

 

I have made many road trips with the 180 and the 160 thermostat. The car consistently ran at 193 with the 180 and 173 with the 160. There was very little variation. I have no reason to believe that the 180 was bad. A friend with a completely stock LT4 says he gets 194-197 on the highway. I know his fans have not been reprogrammed and he regularly sees 220 in traffic.

I have had my ECM reprogrammed. However, I did not adjust the fuel or spark curves. I did things like disable cags, adjust fan temps and remove emissions controls. I also turned off the knock sensor.

I admit I have never ran the same fan settings with both thermostats. I have never been over 200 in non race conditions with the 160. This includes 100* stop and go traffic. With my 180, it normally ran 205-210 in normal city driving and sometimes slightly higher in stop and go. The fans were set to come on at 210 and go off at 205 with the stock thermostat.

As far as a professor goes, they tend to be theory oriented. I place much more creedance in physical tests than theory. However, the theory does support my tests. Remember that when the thermostat opens the radiator is cooling as much as possible. The steady state temperature is the temperature that the cooling system can maintain by cooling coolant that starts out at the thermostat temperature. An engine does not keep increasing in heat once a static condition has been developed, as in the case of a 70 mph road trip. Since the 160* thermostat opens at a lower temperature, the ability of the cooling system to maintain a lower temperature is increased since it does not have to cool 180* water, only 160* water. Assuming the engine adds the same amount of heat to 160* coolant as it does to 180* coolant, the temperature increase would be about the same. This means that the radiator consistenly sees a 20* lower entry temperature. Since the raidator can remove the same amount of heat regardless of entry temperature, the temperature drop across the radiator should be the same. This is why dropping the thermostat temperature by 20* will reduce temperatures by approximately 20*.

Something to note when comparing the two temperatures is that while a radiator will remove the same amount of heat from the system, the corresponding temperature drop is a function of entry temperature. However, when using absolute temperature as the equations call for, there is only a 3% temperature difference so the corresponding temperature drop is approximately the same.

 

I went from the stock 180 to 160 w/o reprogramming. I was planning on reprogramming, but the engine temp went down. In my case, I would run as high as 226 on the highway! And the radiator was clean. After replacing the 180 stat, it dropped to 195 at the hottest. I believe my 180 stat was bad and that changing it to a new stock 180 would have given me the same results.

 

kkonen,

Your point about a 70 mph road trip is a very good one. Let's look at how the steady state you describe (173 degrees at 70 mph) is arrived at. You say the system would need extra cooling when using a 180 because it must cool from 180 instead of from 160. In no case do you start at 160 and even if you did, it wold not matter. As an example we can even start lower than 160. Assuming you jump in the car and get right on the highway and go 70 mph, the coolant temp will be less than 160 degreees. As you continue driving at a steady speed the heat produced by the engine remains constant. The size of the radiator, the amount of coolant, and water pump speed, all remain constant. At this time only total coolant flow varies and that is due to the varying tstat opening size as the tstat opens due to temperature increase. The temp increases toward 160 due to tstat restriction. At 160 degrees the tstat is wide open, restriction no longer varies and coolant flow rate is now a constant too. Above 160 the tstat acts no different than a plate with a hole int. Even at this steady state with all physical conditions constant the temp continues to increase past 160 until it reaches 173. Why? As temp climbs above 160 toward 173 the engine is creating more heat than the system is dissipating. That is causing the coolant temp to increase. If it wasn't the temp would remain in a steady state and not increase. As you observed however, coolant temp climbs only until it reaches 173 where it stops increasing. Why? We were at a steady state physically when it was 165, 168, and 170 degrees, yet it increased past those points. Why did it stabilize at 173? Steady state, nothing physically has changed. Even air flow is constant.

The key is in something you already said "Something to note when comparing the two temperatures is that while a radiator will remove the same amount of heat from the system, the corresponding temperature drop is a function of entry temperature. " The temperature of the coolant into the radiator is now higher at 173 than it was at 170. The thermal difference between the radiator and ambient air is great enough that energy transferrence from the radiator to the air now matches the amount of heat created by the engine. The difference between radiator temp of 173 and ambient air temp is now sufficient to stablize the system. The thermal difference between the radiator and ambient air is the mitigating factor, not the tstat.

Here is a test anyone can do do to see what I mean. Go out and drive at 70 mph until you get to equilibrium temp (173 in your case). Then drive at 80 mph for a distance (watch for cops). The temp will go up. Say to 178. Now drop back to 70 mph for 15 minutes. Will temp stay at 178 or drop to 173. According to your post the already hotter water (like when using a 180 tstat) will not drop back to 173 because the system at 70 mph is at a steady state when you resume cruising. I bet it drops back to 173. At 70 mph a radiator radiant temp above 173 dissapates more heat than the engine creates. 173 is the equilibrium state for your car at 70 mph with given ambient temp. It will seek that equilibrium temp for those conditions.

The same occurred with the 193 degree temp and the 180 thermostat. Temp stabalized above tstat fully open position, so if all other physical factors were the same either the ambient air temp was higher or the tstat was sticking. Your system has shown that it has the ability to stabalize at 173 in 70 mph conditions with coolant flowing through a fully open tstat . With a 180 fully open all of the physical conditions are the same. The fact that with the 160 temp stabalized at 173 shows that the engine did not produce enough heat to warm the water more than 173 by overcoming the thermal exchange rate produced by your radiator at 173 and ambient air temp. Keeping the coolant temp above 173 requires more heat than the radiator releases. We have already seen that even when starting at 193 with a 180 tstat, at 70 mph the engine only produces enough heat to maintain 173, so the additional thermal difference of 193 vs ambient air will cause temp to drop towards 173. The 180 will stop it at around 180 however.

Last but not least, you said "I did not adjust the fuel or spark curves. I did things like disable cags, adjust fan temps and remove emissions controls. I also turned off the knock sensor."

The knock sensor or ESC (Electronic Spark Control) affects final spark curve results. It signals the ECM/PCM which adjusts spark to avoid knock. Emmissions controls generally also affect fuel mixture (that's why they are called "emmissions" controls ) so changing that could also have an effect. Turning fans on sooner affects air flow rate changing the physical constants involved in cooling. Each of these could have affected the outcome individually. As a whole it means we are comparing apples and oranges.

I'm not saying you didn't see what you saw. I believe you did. I also believe it was due to a defective thermostat or one going bad and probably affected by chip changes as well.

Whether you agree or not, it has been enjoyable discussing this with you.
Mike


[Modified by Mike_88Z51, 3:20 PM 6/18/2002]