I have seen that happen under light load conditions. That is one reason folks say to take the vehicle on a highway run just before emissions testing. The full load gets the engine temperature up out of the 'effect' range of the t-stat.
When off roading, the t-stat has no effect if it is working right.
I have overheated my XJ twice while running for extended periods in 4 low in hot weather. (enough for the idiot light to come on, although it didn't boil over)
Once it was with the original radiator, the second time was with a new radiator. Once with a worn out fan clutch, the second time with a new one. I am thinking I need the extra electric fan the AC equipped XJ's have.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
A 160 t-stat and a 195 t-stat are physically the same size. When fully open they both allow the same flow.
When the engine internally reaches 160, the first one opens. When the engine internally hits 195, the second one opens.
Once the engine has passed this 160 or 195 degree mark, the thermostat never budges from wide open.
Take a TJ for instance. No matter what t-stat you put in it, it 'will' run at 210 degrees. It just warms up faster with the hotter t-stat.
This means in the winter when the fan can supercool the rad, you will only see 160 degree water in the heater core and will freeze your butt off with the cold t-stat. With the 195, you will have heat inside the vehicle.
Thermostats are only for the low temperature control, not for top end heat.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Mike is actually correct. I remember this issue distinctly even from twenty-something years ago when I was in class at Lincoln Technical Institute learning to become an automotive mechanic. It has always been a source of confusion. Perhaps the issue can best be explained this way:
The thermostat sets the FLOOR operating temperature. That is, the MINIMUM operating temperature. For example, a 195 degree thermostat will not fully open until that approximate temperature has been reached on the ENGINE BLOCK side of the cooling system loop. (Of course, actual coolant temps right next to the cylinder walls will be somewhat higher.) Once that temperature has been reached, the thermostat opens to allow coolant to flow into the radiator. Typically, as Mike points out, this process takes place during engine warmup of the first 10-20 minutes, depending on driving conditions and ambient temps. Once the engine is warmed up, the thermostat is typically wide open and does little except provide a restricted passageway that the engineers predetermined would be correct for optimal coolant flow. That is why taking the thermostat out can be problematic. Only in extremely cold weather would the thermostat continue to open and close in order to maintain the minimum operating temperature.
Now, this begs the question: how DO you change the coolant operating temperature if not through changing thermostats? Well, the original engineers determined what they thought to be the optimal operating temperature in order to maximize fuel milage and performance, then they designed the size of the radiator, the flow rate of the water pump, the airflow of the grill, the shroud and the fan, the passageways in the engine block, and other factors so that that temperature would be maintained under most driving conditions. Fan clutches and electric fans controlled by computer made it possible for some adjustment during driving. Anyone that has been stuck in an August afternoon on a hot highway with the A/C running can attest that the cooling system has limitations in design!
So, for the Jeeper who wants to increase cooling capacity, aftermarket radiators with more fins, water pumps with higher flow rates, and higher performance electric fans and shrouds would be the way to go. However, it would appear that most Jeeps were designed to run and be happy around 210 degrees. Unless your engine is highly modified and no longer is computer controlled, you should probably leave well enough alone. If trail conditions are forcing your temps well above 210, then probably a higher-performance radiator would be enough to bring the temps back down.
In any case, cooler thermostat will only set a lower operating floor, which would mean coolant flowing into the engine block sooner than it should. That means a confused computer, a choke that is improperly set, and probably reduced milage and performance for the 10 minutes or so it will take for the engine to fully warm up. Once the engine is warmed, the cooler thermostat will probably not change anything, assuming the orifice is the same as the stock unit and the weather conditions are in some normal range.
Given that the heat generated in the engine is significantly different at idle and at full throttle, I find it hard to believe that there is a single point that would be the correct flow rate.
I have changed thermostats in several cars and had the "steady state" temperature change to be the new termostats rated temp.
I guess I disagree with the statement that "only in extrememly cold weather would the thermostat continue to open and close in order to maintain the minimum operating tempurature".
Unless of course a regular 20-40 degree Ohio spring/fall morning is "extrememly cold" to some southern writer...
I was a little concerned for the first week... Then I realized it just never moves! (within reason, you understand... I'm sure the needle isn't broken :-) n.
: > : >>: : > : >> Bring a lower t-stat and some tools with you as well. : > : >> -Brian : > : : > : Brian, why did you say that? Mike says the thermostat only is relevant : > : during the warmup period of the engine. : > : : > : -- : >
: > I respectfully disagree with Mike. The t-stat is a metering valve just like : > adjusting the tap on your garden hose. A cooler t-stat slows the flow and : > allows your rad enough time to cool it's capacity. : >
: > Your original post didn't mention your operating temperature - just that it : > was hotter than before. My recommendation for a cooler t-stat addresses a : > faulty t-stat (sticking open) and compensating for the changes you made for : > your fuel delivery system.... all for about $5.00. : >
: > If you're running 210, you're fine. If you see it creeping above 225 keep a : > close eye on it. If you see prolonged 230+ you may have 'issues'. : >
: > -Brian : : A 160 t-stat and a 195 t-stat are physically the same size. When fully : open they both allow the same flow. : : When the engine internally reaches 160, the first one opens. When the : engine internally hits 195, the second one opens. : : Once the engine has passed this 160 or 195 degree mark, the thermostat : never budges from wide open. : : Take a TJ for instance. No matter what t-stat you put in it, it 'will' : run at 210 degrees. It just warms up faster with the hotter t-stat. : : This means in the winter when the fan can supercool the rad, you will : only see 160 degree water in the heater core and will freeze your butt : off with the cold t-stat. With the 195, you will have heat inside the : vehicle. : : Thermostats are only for the low temperature control, not for top end : heat. :
Mike,
I disagreed with your original post "The thermostat is only for the warm up time, it has no bearing once heated up." because it didn't take into consideration the modifications to the engine and possibility that it requires a lower base line for proper cooling. When you swap in a new intake and carb, you are significantly changing heat dissipation, fuel mixture and performance. From the original post there was no indication of how hot it was running, if the current t-stat was working properly, if in fact he had replaced it when he swapped the intake or when the 'hotter than normal' situation was occurring (highway/city/trail).
When I read the post "Running a couple of degrees warmer than I'd like" and "The plan is to take it out on some trails tomorrow after playing with the air/fuel ratio and see how it does", I think carrying a lower t-stat was not bad or costly advice.
The original post made no mention of the current operating temperature or the t-stat temperature. Shaggie installed a new intake and carb. I am only to assume he replaced the t-stat (as I would have) in which case he might have installed a hotter t-stat. I never suggested that the t-stat controlled the maximum operating temperature.
As the thread unfolds, we find out that Shaggie did indeed install a new t-stat.
I disagreed with Mike's original post suggesting the t-stat was only relevant during warm up. His subsequent posts were absolutely correct.
Here's a thought... Shaggie replaced his 195? t-stat with a 210 hence running a little warmer than he's used to. Was I wrong to suggest a lower t-stat?
On Fri, 30 Apr 2004 09:23:55 -0400 in rec.autos.makers.jeep+willys, Mike Romain wrote,
Slight correction: a 195 degree thermostat starts to crack open at 195 degrees F (90 C) enough to clear a .003 inch feeler gauge. It may take up to 218 degrees F (103 C) for it to be wide open and still be within spec. So at 210 the thermostat may still be doing some temp regulation.
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