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Discussion Starter #1
hey guys, i was driving home from work today and i noticed that the oil pressure goes real low when i slow down and stop, but stays at normal levels when i'm driving and on the freeway.... i've noticed this before, but it wasn't that bad before, it would just go down a little bit. today, its a drastic change.... any thoughts? can bad seals do this? i know i need to do a rear main seal job, its leaking not in a bad way, but not a good way either. about a quart a week. what would low oil pressure at stops do to my engine? i'd appreciate the help, i really don't know enough to diagnose on my own. tks in advance,

travis
 

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Mine does the same. I don't think it's an issue as long as (A) you have pressure, (B) your oil is at appropriate level, (C) the oil light is off.

You'll also notice that it's only higher at the higher rpms.
 

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travis_gillum said:
hey guys, i was driving home from work today and i noticed that the oil pressure goes real low when i slow down and stop, but stays at normal levels when i'm driving and on the freeway.... i've noticed this before, but it wasn't that bad before, it would just go down a little bit. today, its a drastic change.... any thoughts? can bad seals do this? i know i need to do a rear main seal job, its leaking not in a bad way, but not a good way either. about a quart a week. what would low oil pressure at stops do to my engine? i'd appreciate the help, i really don't know enough to diagnose on my own. tks in advance,

travis
How low does it get?
 

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when i'm running on the highway, it stays in the 6 level..... sorry i don't know the actual number, i just remember 4 is in the middle six is at 3/4 and 2 is at 1/4, anyway, it stays at the 6 level on the freeway and normal driving, but when i stop it hits the 2 level.... before it only dipped down to about midway at the 4 level...... again, sorry about my *rapy explanation, i'm just not very adept at explaining things like this yet. :D
 

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travis_gillum said:
when i'm running on the highway, it stays in the 6 level..... sorry i don't know the actual number, i just remember 4 is in the middle six is at 3/4 and 2 is at 1/4, anyway, it stays at the 6 level on the freeway and normal driving, but when i stop it hits the 2 level.... before it only dipped down to about midway at the 4 level...... again, sorry about my *rapy explanation, i'm just not very adept at explaining things like this yet. :D
I wouldn't worry about it. How long has it been since you had an oil change, and are you running 5W30 or 10W30?
 

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actually i don't know, i just bought the thing about 1000 miles ago, so i guess i should change it pretty soon.... and i'm running 5w30, at least that's what i'm putting in there now to cover the oil loss out of the rear main seal..... could it be pitching a fit like this if the guy i bought it from was running heavier oil? i'm in buffalo, ny, it doesn't get that hot up here, what do you think i should be running? tks for the advice,...

travis
 

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travis_gillum said:
actually i don't know, i just bought the thing about 1000 miles ago, so i guess i should change it pretty soon.... and i'm running 5w30, at least that's what i'm putting in there now to cover the oil loss out of the rear main seal..... could it be pitching a fit like this if the guy i bought it from was running heavier oil? i'm in buffalo, ny, it doesn't get that hot up here, what do you think i should be running? tks for the advice,...

travis
I would think the opposite about the oil. Usually thicker oil gives higher pressure. I run 10W30 in mine and it is fine. I would change the oil and see what happens from there.
 

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Travis,
Change the oil, and if I were you, I'd have the
oil pressure checked with a mechanical gauge to
see what the pressure really is. My truck was doing
the same thing,and I had the pressure cchecked, and
it was the oil pump going out. After I had that fixed,
it was doing the same thing, and it was the sending
unit. I put in a new sending unit, and backed that up
bu installing a mechanical gauge under the dash.
I hope this helps.
 

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How Many Miles?

I had the same thing goin' on w/ my 88 2.8l. I had alot of pressure when it first started and it would drop as the engine warmed up. Things got worse around 173,000 when the intake gasket let go and dumped water into the oil. That was b/4 I discovered these BBS's about our trucks. I flushed it out w/ drain oil and fixed the gasket and I now know, from the bbs, that I should have torn it down as the anti freeze screws up the bearings. After I got it running, the oil pressure would go to zero when I stopped. Then it started knockin' :bash: so I'm savin' for a 3.4 crate motor. There were 6 s-trucks at work and in the past 6 months mine and three others all had the same problem.:mad:
 

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I wouldn't worry about it as long as you have enough oil in your truck. Mines been doing that for over a year and a half now. I talked to the dealer and they said it was ok because they only need about 6 PSI (or 8? I can't remember) at idle. Good luck-

~b
 

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You bought a truck with a rear main seal gone?
 

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no, it will give lower readings. but that isnt ncessarily a bad thing.
as long as the presure is sufficent...
 

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03 SS said:
A thinner oil or synthetic will give a higher oil pressure. It flows more & faster.
Exactly, this is complete backwards.

A thicker oil resists flow more which creates pressure.

Trust me, I'm a certified hydraulic specialist.
 

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Also, oil choice is effected by outside air temp! That's why it is perfectly fine to run 15W40 in an out of warantee truck in the summer, but certainly not at -10F! I would definitely switch to 10W30, possibly one of the high-mileage formulations (which are close to 40 weights). I would probably avoid 10W40, as the regular dino ones shear down pretty quickly anyway. Same goes for the 5W30, after enough miles, it isn't any thicker than 5W20 hot.

In warm weather, a 15W40 would be a great plan!
 

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My oil pressure has always gone up when I switched to synthetic? If it is flowing more at a higher rate wouldnt that increase your oil pressure?
 

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no, pressure is the resistance to flow and is the product of oil viscosity.

Thicker oil, high viscosity, higher pressure.
 

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Yea, that is strange, mine dropped after the switch to synthetic.

Remember to do an engine flush before you switch to synthetic to get all the conventional oil sludge out.

You might have a partially plugged filter due to the enhanced cleaning abilities of the synthetic clogging the filter with conventional leftover sludge.

Swap out your filter.
 

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I'm VERY willing to bet that running one of the better 15W40's would show both better pressure than any 5W30, and leak a good bit less. If you have a walmart nearby, the Penzoil LL 15W40/Supertech filter combo is hard to beat on a high mileage car that uses oil!


Exactly what syn oils are we talking about...and in what viscositys.

To give a quick example,. German Made Castrol Syntec 0W30 is nearly a 40 weight, hence a good bit thicker than any of the common dino 10W30's. (It and the 5W40 "Belgian" Castrol are the only realy synthetics that Castrol sell, BTW...all of their blending in America syntec oils are actually Group III basestocks, hillariously a much cheaper oil for Castrol to make, yet it often sells for more than Mobil 1! )


On the other hand, Mobil 1 5W30 and 10W30 are on the low end of the 30 viscosity...not a bad thing in applications that don't use oil and can benifit from the increased flow+less viscous drag.



Finally, on most S-10's the pressure sending unit is immediately after the oil pump, and doesn't necessarilly reflect the oil pressure/volume at the bearings. To give a quick example, on the 2.8 trucks, the sending unit is between the filter and the pump, such that switching to a freer flowing filter from a more restrictive model (say Fram PF-47 equivalent to a ACDelco or SuperTech Pf-52 equivalent) will show a slight drop in gauge pressure while actually providing better flow and pressure at the bearings.
 
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