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5.9 V8 - Low Oil Pressure? What's yours?

48K views 14 replies 9 participants last post by  bobem12 
#1 ·
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The '98 Ram we recently purchased (5.9 V8; auto; 2500 4x4), seems to have low oil pressure, according to the gauge. With the engine off, the needle's pegged down below zero, which is normal. With the key in "On" position, it moves up to the zero, like it should. I fire it up, and at a cold start-up, pressure reads about 50 psi. Then, as the engine temp slowly rises to operating temp, the oil pressure slowly lowers to about 22psi. I'm glad it's not a quick drop, but still concerned that it drops at all. I'm not yet familiar with the Rams, but I thought that the pressure would increase with rpms. At least with every other vehicle that I've ever owned. But this truck, the pressure sits in one spot, and doesn't fluctuate at all. Stays steady. If I stop somewhere, and go to drive again, if the engine is still warm, the pressure reads 22psi. Only after sitting a few hours will is start out up around 50 again. And then the drop starts all over again.

I did my usual background check, as I do with all my vehicles, and checked the dealer service department visits, so that I know what work has been done in the past. Interesting enough, I found out that around '99, it was in for low oil pressure, and they finally replaced the heads to cure the oil flow problem. Our fear is that it may be acting up again, and that we may be screwed $ wise. No more warranty left. It only has 52K miles, which is why we bought it. On the DodgeForum site, I had one interesting reply:

"There is no possible way for the heads to be at fault. I consulted my repair manual for this problem and the oil pressure should read 6psi at idle and 30-80psi @3000rpm. Personally, my engine runs between 35-45psi all of the time, even at high rpms. Another possibility is the oil line plug located in the vertical passage at the rear of the block between the oil to filter and oil from filter passages. It is mechanically seated and if improper mounting depth is present a dowel drift can be used to reseat it to correct dimensions. Accordingly, this can cause low, high, or erratic oil pressure signals to the sending unit. Unfortunately, a certified mechanic is probably the best solution to have this checked. I could see this being bad and the tech's that changed the heads out missing it or believing the heads weren't allowing proper oil flow, instead of it being improper flow to the heads. I really hope you understand all of this, tended to ramble a bit. Any questions, hope I can be of further help."

Just wondering what others have for running oil pressure, so we know if we have to look at it closer. I swapped out the oil sender, and it still acts the same. It has 40+ psi at startup, and then drops down to about 20-25psi after the first mile or so of driving and just sits there for the rest of the drive. My next thought, is to install a mechanical gauge to eliminate the dash one.


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#2 ·
I don't see how the heads can limit oil pressure. Just maybe oil drainback (I still don't see how)but that is not what you talking about. I believe these engines oil the valvetrain through the pushrods. Don't they?
You need a mechanical or elect aftermarket gauge to test the oil pressure. I would only trust the OEM gauge to give me the most vague idea of what the pressure really is. In my opinion that is the first to do.
 
#3 ·
the sending units on those go bad all the time. I'd try that along with a good can of motor flush. Then refill with at least a 40w synthetic. If you still have problems, then I'd suspect a clogged pickup in the pan. They all do vary in pressure somewhat between idle and cruising, that's normal.
 
#4 ·
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Changed the sender, ran flush through it, and refilled with Mobil 1 Syn. No change.

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#5 ·
ok, the needle doesn't drop out of the "normal" range, does it ? If not, I wouldn't worry about it. If it does, I'd suspect an oil pump. Thicker oil will help too, what are you using ? My R/T stays pegged at the high side of the range, but I'm also using 15w50 Mobil one. My hunch is that you're just fine though, considering the actions you've taken already.
 
#6 ·
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Went by the stealership, and had two different techs say that our pressure is fine. Must just be a Ram thing. I'm going to get a mechanical gauge anyway, to get a better idea of what's going on.

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#7 ·
mine hits the very end of the "normal" range on cold starts and drops to the half way point when the truck warms up and is at idle, then back to the end of the "normal" range when driving

all these trucks vary
 
#8 ·
The oil pressure on my 98 will register fine (above 40) will driving but after you warm her up it will drop to 0 at stops or idling. The truck has been doing this for a year know and over 15000 miles but has never knocked or pinged during the time it has supposidly had no oil pressure I even replaced the sending unit and nothing so I tried running thicker oil,Very thick oil 2quarts of lucas and 3 of 10 30valvoline and everything reads good but we will have to see if that combo is thin enough for cold stars this winter. I dont know just my findings no one else had a suggestion except new motor and the thing runs like a champ.-BROBERG-
 
#10 ·
come to think of it, my gauge is wrong, i think the oil gauges on these trucks go out really easy, b/c my oil pressure used to never go above 40psi no matter what, then one day i had to get the truck jumped and for no reason at all it went up to the end of the normal line and has been ever since

maybe the gauges are just screwy
 
#11 ·
My oil pressure gauge never goes down to zero as long as the key is turned on. Even if it isn't running. Hows that for accuracy!
Before I went changing parts I'd pull the oil sensor sender unit and attach the tubing for a mechanical gauge or aftermarket elect gauge. Absent that you really don't know what the pressure is.
The OEM pressure gauge will alert to a problem you if it all of a sudden reads much lower than it normally does.
 
#12 ·
my '99 1500 5.9 ram the gauge drifts between 30 and 60 psi I don't worry about it because my brothers chevy suburban stays at 30 psi and my explorer I traded in for this truck stayed at 50 psi

so I figure if it ain't knocking it at broke.
 
#13 ·
mine reads high since I put the M1 on there and also a oil pressure sending unit from autozone. it will read 70 cold and 40 hot. when idling and it's hot it will drop down to 30. the coolest thing is, is that the gauge will like to dance around too. been doing this for over 10k and it runs like a champ except for the richness condition.
 
#14 ·
Mine swings around like a friggen metronome. Cold it's at like 55psi, When in warms up it's like 30-40. Certain RPM ranges produce weird oil pressures For instance, It idles at 40 psi but upon accelarating it dips to 30 or so until it gets past 1600 RPM. At 60mph (About 1600 RPM) Its just below 40 but jumps right up to just over 40 if I go 62. (about 1700 RPMS). These Dodges had mega problems with gauge cluster and body control modules so I think it's probably electrical and I ingone it.
 
#15 ·
mines right at 40 after startup then gradually moves to about 65 and stays there. :dunno:
 
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