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Message: [QUOTE="TARM, post: 1007713, member: 578"] The T-stat number is when the T-Stat is suppose to first start to open +/- 2-4°F. The T-Stat should continue to open as temp rise over the next 20°F at which point it should be fully open again +/- 2-4°F. Once you are above the temp a t-stat is suppose to be fully open ( and it actually is fully open) your t-stat has no influence over the coolant temp or oil temp. Its fully open. All the flow it has is flowing. As an example 195°F starts to open at 195°F is fully open @ 215°F. If its operating correctly and you have a temp of 230°F the issue is not your T-stat range. The 7.3 oil cooler as most everyone knows used coolant to maintain/control oil temps. Ideally the temps of the two fluids should be within 5-10°F of each other. If you are seeing somewhat consistent oil temps that are significantly higher than this over what your coolant temps are I would be looking at pulling the oil cooler and flushing/rebuilding it. Think of how many miles these things see and the lives many of them have had. Many times on rebuilds they just pop new gaskets on the oil cooler and slap it back on. Oil has temp ranges that it is meant to operate in to get the specified performance. Hot temps that use to cook older oils from decades ago are now easily handled by the oils we have today. If you do some checking of ratings of temp and duration for the various certs you will see that most of the top oils would be able to go well past a 5000 mile OCI at temps of 300°F+ and still maintain its performance spec for that cert rating. Consider for a moment that the stock T444E (AKA 7.3) motors that are in HD IH commercial trucks had a stock 203°F T-stat. Ford had to go to the lower 195 for the sole reason to reduce NOX along with a CCV to intake (IH drop it right to atmosphere down the front cover.) to ensure meeting EPA specs. They also put a crap cheap alum water pump instead of the nice all steel that had a built in coolant filter and cut off. Which was done to increase profit. Think the load those 7.3 is tri axle dump trucks are dealing with compared to our by comparison toy trucks. Constant running oil temps below 195°F can cause long term damage. Ideally you want oil to actually reach 212°F /100°C. This is the general temp most all oils are developed for and most all engines are designed to run at. There is a reason for this from a number of factors but one is that 212°F is the boiling temp for water. Then there are engine tolerance at specific temps. Its a reason vehicles never allowed to fully warm up need there oil changed more often and have higher wear rates. There are number of studies done on this mainly looking at engines used up in Alaska etc. But internal engine wear was measured and as temps fell below 200 wear increased and took a nose dive around 185°F IIRC as it was awhile ago. I recall doing a post over on PSN back before PSA came into being. Wear did not increase with higher oil temps numbers until it got significantly hotter. Now of course you have the peak cylinder temps to deal with but that is a different issue than the over all engine oil temps. Many studies have shown it the warm up period where temps are below these numbers of where most all engine wear happens. Further take a look at the PCM oil temp table that effects the injection time etc.. This further supports these operating ranges. I tried to find the study as it use to be listed on the SAE site but I can not figure out where I saved it. It was on diesel engines and decent graphs etc.. My point in all of this is colder oil temps are not better. Sure you do not want super hot oil temps either but anything from 210-240 is not bad. If your oil temps are higher than this then you should look into the cause not just try to patch it by putting in a lower temp T-stat. First is the oil temp at most 15 maybe 20 degrees of the water temp once its fully warmed up and put under some decent load? The closer it is to the coolant temp the more effect the oil cooler system is working. If the temps are within the range and its both temps that are higher together then the issue could be in the coolant system side of things i.e. pump, radiator, t-Stat working correct (opening closing in the rated range), what about the fan clutch ( is it locking and spinning at full speed? Any blades missing), etc. If the oil temps are outside the range or seem to always be at the very top end even under light/cruising loads then the issue is more likely in oil cooler performance. Is proper amount of coolant making it to and thru the cooler? What about the oil side? After all consider that we have plenty of half filled blocks that still have no issues keeping oil and coolant temps in proper ranges. Before you go changing a t-stat range especially to one cooler take a moment and consider why you should need one considering there are , what, millions of 7.3s running perfectly fine with many under extreme working loads with those same ones. Better to try and figure out the real issue first because the issue is not that the t-stat range is wrong. Why? In a properly matched coolant system to engine size and load the t-stat should technically only fully open at peak load. Do not forget want temp the fan clutch locks at. 190 degrees IIRC. That engine and coolant temp would put the air temp at the clutch around that? Maybe close to the same temps where the Tstat is should be just fully opening? Anyways just some things to consider and think about. [/QUOTE]