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Matt
Dodge Dakota
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8/07/2005
19:26:15
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 Subject: T-Stat and lower oil pressure
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Recently I replaced the Thermostat on my Dakota to a new and lower temperature stat. and noticed a decrease in oil pressure, all around. The original t-stat was set for 195 F, the newly installed is a 180 F. My oil pressure gauge doesn't read to any exact numbers other than 'High' or 'Low', and use to read about 3/4 full. Now it read almost half, and some times lower.? Any advice here or info would be nice, I am considering putting a stock rated t-stat back in.. . I don't have any modification, I think I should add. Also, I noticed with a cooler engine now that I get poorer MPGs, kinda a bummer. Is it worth is to use a 180 F thermostat.? A lower oil pressure couldn't be good, but then again the overall engine temperature has dropped.?
Thanx.
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Rick
Dodge Dakota
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8/07/2005
21:04:38
| RE: T-Stat and lower oil pressure
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Matt, Put the stock t-stat back in immediately. Your engine is running to cold to burn off damaging leftovers from less complete fuel combustion. The lower temperature de calibrates the computer. It is trying to compensate for a new part that is not designed to cooperate with it to achieve the designers best choice of various performance standards from air pollution to mileage to longevity of parts and acceleration to name a few. A 15 degree F. reduction in stat spec. can reduce exhaust temp.
many times that and oil temp. reduction to a lesser degree causing slower warmup which can increase engine wear drastically during that time.
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Matt
Dodge Dakota
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8/07/2005
21:26:17
| RE: T-Stat and lower oil pressure
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I hear what you're saying and its only a 10-dollar part. My only question is, is this advice from experience or are you a mechanic? You sound awfully sure and its almost surprising. I have mentioned this to others, and although they are not mechanics, they didn't respond so affirmatively against it, although against it. Just wondering how you know all that. up above.
-Matt
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N56629
Dodge Dakota
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8/07/2005
22:48:52
| RE: T-Stat and lower oil pressure
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It sounds like he out to lunch.
The engine will warm up just as quickly with a 180 as it will with a 195. They both stay closed until the reach the rated temperature. With the 180 the operating temperatures with be a bit lower.
As for the problem with your oil pressure the lower thermostat might cause the pressure to be minutely higher but certainly not lower. Higher operating temperatures cause lower oil pressure and that is just the opposite of what the 180 does.
If everyone followed Ricks advice there would be absolutely no modifications taking place.
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GB2000
GenIII
8/07/2005
23:07:39
| RE: T-Stat and lower oil pressure
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Another thing...even DC admitted the magnum engines run better at around 185*F but the 195*F was used due to emissions purposes.
Josh Blackshire - Owner, Ohio Dakotas .NET
Red 98 Dakota Sport - Check it out!
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Darryl
Dodge Dakota
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8/08/2005
12:12:54
| RE: T-Stat and lower oil pressure
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Man, the things that can be read on the internet! Rick, don't quit your day job.
Installing the 180* stat in NO WAY effected your oil psi. It's just something you noticed and thought, "Oh crap, now what did I do"?
The lower the engine temp is the cooler the oil is. Cooler oil is thicker oil and thicker oil maintains a higher oil psi.
The higher the engine temp is the hotter the oil is. Hotter oil is thinner and thinner oil maintains a lower oil psi.
Now that being said, are you going to see a huge change in oil psi based on the 180* vs 190* stat change? No, you won't.
You'll notice a psi change with the summer vs winter seasons, as well as changing from a "thinner" 5w-30 to a "thicker" 15w-40 motor oil.
And for the LAST TIME, a 180* stat will warm up just as fast as a 230* stat. That's because both stats are closed until they reach their opening temps......PERIOD!!!!!!
Darryl
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Matt
Dodge Dakota
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8/08/2005
17:08:24
| RE: T-Stat and lower oil pressure
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Well, thats what I thought initially about changing the t-stat out for a lower temp one: lower overall temperature. Thats normally a good thing. As for some other things mentioned in Rick post I didn't follow and it just seemed over kill on a follow up post. Anyways, I am still going to look into the oil pressure thing going on.
Thanks..
-Matt
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daddio
Dodge Dakota
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8/08/2005
17:10:47
| RE: T-Stat and lower oil pressure
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the factory gages are not real accurate. if you want to check it, install a mechanical in place of that sending unit. then you'll know for sure.
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Matt
Dodge Dakota
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8/08/2005
17:22:39
| RE: T-Stat and lower oil pressure
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Daddio *grins*
You may be right. I failed to mention about changing the oil pressure sensor during the same week I changed the t-stat. My oil pressure sensor was leaking gobs of oil out the top of it. I replaced it and noticed a different outlook on the oil pressure gauge. It had dropped some. Maybe the leaky one was not only dripping oil everywhere but it may have become faulty in the process.? Anyways, my oil gauge now reads 1/3 full to almost half full. The dakota has about 180K on it.! I am not too worried about it cuz it is old, but it runs good too! Any ideas on boosting oil pressure? I'll start searching the archives and the internet, I just wonder if it will make a difference in longevity with a PU with so many miles....?
-Matt
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daddio
Dodge Dakota
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8/08/2005
17:57:27
| RE: T-Stat and lower oil pressure
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depending on the climate where you live you could run either 15w-40 or 20w-50 oil. or put in some "motor honey",STP oil treatment as we used to call it. but, i don't recommend that. just try a heavier weight oil. Dodge FSM says oil press should be min.6psi @ idle to 30-80(i think) @ 3k rpm.
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93dak4x4
Dodge Dakota
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8/08/2005
22:40:28
| RE: T-Stat and lower oil pressure
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There are a few discrepencies in all of this I would like to clear up.
First, the 180 deg thermostat will warm up just as fast BUT if all of your temp sensors are not working correctly your truck may not enter closed loop netting you poor fuel economy and a computer code. the 3.9/5.2 are cool running motors and may not get hot enough with a 180. (this depends on your climate)
Second your oil pressure is about where it should be with 180k on the motor. oil pressure gradually drops with engine wear due to increased clearance between main/rod bearings. the only true way to fix this is to install new bearings. If the oil pressure becomes too low you will need to rebuild the motor. But as long as you maintain pressure at operating temp you are ok.
You may also want to try several other modifications besides the thermostat if you havent already. most people from what i have read have seen greater gains with further mods and many that live in a fairly seasonal climate have had to change back to a 195 during the fall and winter just to produce heat with their trucks.
Good Luck
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schmidtmike
Dodge Dakota
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8/09/2005
00:12:51
| RE: T-Stat and lower oil pressure
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Also, change your oil pressure sending unit, these vehicles have nothing but problems with leaky and malfunctioning ops's. I would change that to get more accurate oil pressure readings.
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Rick
Dodge Dakota
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8/09/2005
03:46:08
| RE: T-Stat and lower oil pressure
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Matt, 37 years as a mechanic a shop operator, 5 in racing. Retired. I'm sure.
N55669 and Daryll, your comments that the engine will "warm up" just as fast is patently false. Most mechanics and engineers agree that most engine wear occurs during warm up and that time should be kept to a minimum for the reasons I stated. Originally, smog laws drove up combustion temp design and manufactures learned that hotter stats were needed to conform. Efficiency and longevity depend on maintaining specs of the cooling system, also. A 180 could prevent the engine from ever warming up to the designed optimal temperature, like in freezing weather. Remember that the stat determines what temp the coolant is when it leaves the water outlet, not the temp when it enters the passageways around the combustion chambers. The fan and radiator do that. As for the DC comment, what is the definition of "run better"? With proper care and stock stats I've seen Magnum engines run great for well over 200,000 miles. Run almost any modern gas engine designed for 195 with a 180 and it and the exhaust system won't last as long. A 5 degree reduction in stat reduces thermal expansion in the combustion chamber plain and simple with negative performance and pollution results. A 15 degree reduction is exponentially worse. An engine with worn shell bearings needs the highest rate of oil flow that it can get. When a cold engine is accelerated, most of the oil pumped is dumped back into the pan via the pressure relief valve. It never gets to the bearings even though the pressure is reading high. Pressure measures force not flow. As the oil warms up properly flow increases (as viscosity decreases) because the relief valve releases less oil. More flow means less wear. Warming up the oil as quickly as possible is beneficial.
As for determining accurate oil pressure and choice, daddio has the best advice. Probably, your shell bearings are just showing their age. If you are not hearing knocks or other temporary noises upon cold start, don't worry about it.
Perjoratives and exaggeration are not substitutes for knowledge and experience.
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Matt
Dodge Dakota
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8/09/2005
19:35:28
| RE: T-Stat and lower oil pressure
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Rick
You hold your own in that last statement, and if it makes you happy to know .. I am going to change my t-stat back to 195 F cuz I am just not satisfied with the 180 F. I don't 'know' enough to keep changing things from stock and then verifying results. Regardless, my MPGs went down and that seems to be reason enough.
As for the oil pressure, thanks for your guys' input. I thought my engine was dying out, which it prolly is, but not cuz something is wrong with it. This dodge has been holding its own for quite a while. Its a 89' 4x4 3.9L. I have replaced somewhere between few and many parts, but thats to be expected with the amount of driving I do. If it make it over 200K in mileage, I wouldn't be surprised, and definitely a happy camper...
-Matt
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daddio
Dodge Dakota
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8/09/2005
20:49:02
| RE: T-Stat and lower oil pressure
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Matt, my '00 QC 3.9 did the same when i put a 180 stat. i live in south louisiana and it gets hot down here. i went back to the 195 stat, no clutch fan and even on the hottest days my gage marks just before the 2 where 210 is on the gage. about 1/3. when i changed back my mileage went up and noticed NO diff. in performance. every truck/3.9 has it's own personality. each will react different to different mods so yes, sometimes it's trial & error.
also, with that lower t/stat making it run richer you will dilute the oil with unburned fuel a little more than usual and that would also lower the oil pressure.
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Matt
Dodge Dakota
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8/09/2005
21:55:51
| RE: T-Stat and lower oil pressure
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Daddio
"also, with that lower t/stat making it run richer you will dilute the oil with unburned fuel a little more than usual and that would also lower the oil pressure."
Well, I let you know how it goes, but you'll probally be right. I hope to see how much it does makes a difference.
See'ya around.
-Matt
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??
Dodge Dakota
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8/10/2005
08:42:17
| RE: T-Stat and lower oil pressure
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Optimal operating temperature for a 3.9 magnum engine is 179-185 degrees, 182 degrees being that ABSOLUTE best. These numbers being confirmed with backyard wrench heads AND DC Engineers(you know, the ones that DESIGNED the engine) Depending on the age and physical properties(types of transistors/diodes) the engine will enter closed-loop operation @ 163-167 degrees, 165 degrees being the "magic" number here.
"N55669 and Daryll, your comments that the engine will "warm up" just as fast is patently false."
No, actually it will "warm up" just as fast, it just won't warm up the operating temp of the 195 degree t-stat, and if you are in a colder/milder climate this could be bad due to not entering closed loop and therefore running off of the PCM presets. On the other hand properly sized engine/radiator combination (i.e. radiator is designed for that engine size whether a 1, 2, etc core) will maintain that engine at whatever temp the t-stat opens up at. BUT all engines are not the same, so this mod(like daddio said) might not be the best for you.
"A 5 degree reduction in stat reduces thermal expansion in the combustion chamber plain and simple with negative performance and pollution results. A 15 degree reduction is exponentially worse."
The degree at which this reduces thermal expansion is NOT as high as you make it sound. The flash point of gasoline being -50 degrees fahrenheit and the ignition point being about 495 degrees fahrenheit. Inside the ignition chamber temperature are JUST under the point of auto ignition. Now put 2 and 2 together and I'll bet you get 4.
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