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post #1 of 6 (permalink) Old 05-13-2018, 09:06 AM Thread Starter
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Oil pressure drop after towing

My 09 Tundra has 118,000 miles and I have been using Castrol Synthetic 5W20 for nearly the whole time since the truck was new. During normal driving the oil pressure gauge once warm stays just under the half way mark even when I come to a stop at a light after driving on the highway. The concern is that when I pull my 7200 lb trailer and get off of the highway and sit at a red light the pressure slowly drops to below the half, sometimes all the way to 1/4. No warning lights come on or anything and when the AC compressor kicks in and the engine revs a bit the pressure jumps again but then drops when idle comes down again. I know that the oil will be hotter towing than not but per the owners manual 5W20 or even 0W20 is good in these trucks for towing as well. Also never did this before when newer or at least I never noticed when the truck was newer.

Does anyone else's Tundra do this or am I starting to see a problem? Truck doesn't use any oil between changes and always change at 5000 miles.

If I take it to the dealer I am concerned that they can't repeat it as they will not be towing.

Is this a problem with mileage getting higher or am I just noticing a perfectly normal thing on the truck due to higher towing temperatures on the oil?

Thanks for any insight,
Rob
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post #2 of 6 (permalink) Old 05-13-2018, 03:33 PM
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Originally Posted by Calbar View Post
My 09 Tundra has 118,000 miles and I have been using Castrol Synthetic 5W20 for nearly the whole time since the truck was new. During normal driving the oil pressure gauge once warm stays just under the half way mark even when I come to a stop at a light after driving on the highway. The concern is that when I pull my 7200 lb trailer and get off of the highway and sit at a red light the pressure slowly drops to below the half, sometimes all the way to 1/4. No warning lights come on or anything and when the AC compressor kicks in and the engine revs a bit the pressure jumps again but then drops when idle comes down again. I know that the oil will be hotter towing than not but per the owners manual 5W20 or even 0W20 is good in these trucks for towing as well. Also never did this before when newer or at least I never noticed when the truck was newer.

Does anyone else's Tundra do this or am I starting to see a problem? Truck doesn't use any oil between changes and always change at 5000 miles.

If I take it to the dealer I am concerned that they can't repeat it as they will not be towing.

Is this a problem with mileage getting higher or am I just noticing a perfectly normal thing on the truck due to higher towing temperatures on the oil?

Thanks for any insight,
Rob
Since the oil pressure returns to just below half gauge at idle when not towing, and no warning ever comes on, I wouldn't worry about it.

It's true that at 118K miles, your truck isn't new anymore, but oil pressure just below half gauge at idle is excellent in my opinion. I haven't been able to determine the actual range of the oil pressure gauges so far, but I suspect half gauge is about 20 psi. Typical engine oil pressures on older vehicles may be as low as 5-7 psi, and that's adequate for the no-load condition...yours is clearly higher than that if I'm right about 20 psi at half gauge.

As to what's causing the oil pressure to be lower when you tow, I haven't any idea. I doubt it's because of higher oil temperatures, bwdik. It seems to me 0W20 means it's 20 weight oil at normal operating temps, and towing is a normal operation.

What's your coolant temperature doing when you tow? I would suspect increased pressure drop somewhere in the oil system; e.g., is there an engine oil cooler that may only come into play when you're towing?


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post #3 of 6 (permalink) Old 05-14-2018, 04:20 AM
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I bet its an electrical issue.

The extra load on the alternator from the brakes and lights on the trailer is causing the gauge to read low. Try putting the truck in neutral when at a light and take your foot off the brake (as long as on level ground). I bet the pressure goes up.

The oil sending unit in our trucks is really not much more that a glorified dummy light. Be glad you don't have a 1st gen Tundra. Those oil pressure sending units were pure shit.

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post #4 of 6 (permalink) Old 05-14-2018, 08:02 AM
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Originally Posted by koditten View Post
I bet its an electrical issue.

The extra load on the alternator from the brakes and lights on the trailer is causing the gauge to read low. Try putting the truck in neutral when at a light and take your foot off the brake (as long as on level ground). I bet the pressure goes up.

The oil sending unit in our trucks is really not much more that a glorified dummy light. Be glad you don't have a 1st gen Tundra. Those oil pressure sending units were pure shit.
good point , and good test idea with the electrics.

Most modern vehicles the oil pressure gauge isn't a gauge at all nor the engine coolant gauge.

I forget the exact wording but it was something along theses lines " Temp and oil pressure is not an actual numerical reading , but a representation of customer expectation vs operating conditions"

In less words , its what the computer thinks you want to see and think is normal ,,, and indeed is an idiot light that looks like a gauge as the previous gentlemen mentioned .

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post #5 of 6 (permalink) Old 05-16-2018, 02:07 AM
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I suspect it simply has to do with the oil thinning more. The oil in your engine is actually a main component to your cooling system and dissipates much of the heat produced in the engine. While towing, you are obviously working the engine harder and producing more heat which is expected to be dissipated by not only the engine coolant and radiator, but the engine oil. The hotter the engine oil gets, the thinner it gets.

If you are game for it, try using a different brand oil in the same grade that has a different kinematic viscosity @100C and/or a higher Viscosity Index (either 5W20 or 0W20 oil should suffice - it *shouldn't* matter which as they are both 20 weight oils when warm). A higher kv @100C indicates the oil is not as thin as a lower kv; also, a higher VI shows that an oil's viscosity changes less with temperature. A quick look as some MSDS data shows that Castrol 5w20 is rated at 8.5 to 9 cst @100C but doesn't show a Viscosity Index (and it's strange that they give a range instead of a specific value for kv @ 100C) while Mobil1 5w20 is [email protected] and a VI 0f 160. Advanced Fuel Economy Mobil1 0w20 has a slightly lower kinematic viscosity of [email protected] but a better VI of 173. And to compare to Valvoline 5w20, kv of [email protected] and a VI of 147 and even Amsoil 5w20 signature which has a [email protected] of 8.8 and a VI of 153.

My truck as only seen Mobil1 0w20 for it's entire life and I've been very happy with it. Not saying M1 is the be-all end-all of oils, just that I've used in in almost all of my vehicles for years (I've tried many others, too, but always come back to M1) and have been pleased with the results. Some of my oil change intervals have been shorter than 10k (not per Blackstone, just my own judgement) but that's perfectly fine with me. While towing heavy in the summer, I've seen the oil gauge drop to maybe 1/3 at idle, but the vast majority of the time it sits just below 1/2 at idle.

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post #6 of 6 (permalink) Old 05-16-2018, 02:26 PM
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I suspect it simply has to do with the oil thinning more. The oil in your engine is actually a main component to your cooling system and dissipates much of the heat produced in the engine. While towing, you are obviously working the engine harder and producing more heat which is expected to be dissipated by not only the engine coolant and radiator, but the engine oil. The hotter the engine oil gets, the thinner it gets.
This spring I have been monitoring coolant, engine oil and ATF temperatures using an OBD scanner device. I don't tow anything heavy but even so the engine works harder when climbing hills. Oil and ATF temperatures track each other within a few degrees and under normal driving conditions run at 190 - 205F. Under extended hill climbing (such as Big Canyon Road in Idaho's Deep Creek Mountains), the coolant temperature reading stays under 200F but the oil and ATF can stabilize in the low 220's. I think this simply shows that the aluminum block (and tranny - since they bolted together in good thermal contact) gets warmer when under heavy load.

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