Low oil pressure

gmc
sierra1500

#1

I recently bought a 1998 GMC Seira with over 211,000 miles on it. On the way home I noticed that the oil pressure was low when I stopped for red lights. Can this mean anything other that a bad oil pump?


#2

It more than likely means that the crankshaft bearings are worn out unless the oil pressure sending unit has gone iffy. Not meaning to rain on the parade, but the former is more likely than the latter.

You could have an oil pressure test performed but just replacing the sender and praying is often easier from a monetary sense.


#3

Depends on what you mean by “low”. Its normal for pressure to be low at idle and at full pressure at higher rpm. The first car I had with an oil pressure guage was a new 74 Olds. I was concerned because the pressure would be low at idle in traffic but it was normal. Now if its really low like below 5-7 psi, that could be too low.


#4

The pressure jumps up to 40 psi when the engine is revved up to traffic speed about 40 mph. I Thought of the sending unit being bad too but dismissed it to wishful thinking. I got the truck for $3500 so I expected it to have some problems.


#5

It’s that low. The check gauges light comes on.


#6

So assumming worn bearings, can he just put new main bearings in?


#7

Like ok4450 sais, have a new sending unit installed and hope for the best. If the oil pressure is still low, I’d recommend engine replacement. Exploratory surgery/partial repairs often end up that way anyway.


#8

If the sending unit doesn’t help I’d just keep driving it. Keep an eye on the oil level, but if the pressure is ok off idle you’re doing little harm. Especially when the solution is replacing the engine anyway.


#9

If this turns out to be a legitimate low oil pressure problem and not a faulty sending unit you might consider stepping up to a heavier weight oil; especially with summer coming on.

Maybe someone has 5W20 or something like that in there now and with the high miles the oil viscosity is simply too light.


#10

I would put in some 5W-50 and hope for the best.


#11

@williamclang hook up an oil pressure gauge and take a reading. That way your questions about the engine will be answered.

Is the engine making noises?


#12

I had a 74 pickup that did the same thing, pressure would show normal on cold startup then bottom out when hot…changed the sending unit and all was fine so this one may have had an internal leak in the sending unit…but a friend had this problem with an older Ford 302.

Hooking up a pressure gauge had 45 psi on cold start up then as the engine warmed up would drop to 4 - 5 lbs at idle and about 20 psi at 2000 RPM so this was engine bearing wear, but never made any noise top or bottom.

He got a lot more miles out of it by running straight 40 weight oil and a can of blue STP oil treatment. Idle pressure was now about 15 to 20 psi. We are in Hot FL so no problem with a cold start-up.
You are NOT to do this, have the pressure checked with a mechanical gauge and be done with it and find out what it is.


#13

I have this exact prob with '75 ford, 360cu in engine. bothered me a lot at first but after awhile I am not so worried. no tapping or other signs that engine is not lubed. so I just drive the darn thing.


#14

Thanks to all for the input.


#15

Consider going up a grade in oil, but not too high—if you’re using 5w30, use 10w30. If 10w30, use 10w40. I suspect the engine has worn bearings, like others have said. A little thicker oil might help you get some more life out of it. No reason not to just drive it until it dies, then if you want to keep it, get a junkyard engine for it.

An oil pressure test with a mechanical gauge when the engine is warm will determine whether you have an actual problem or not, but based on the mileage and behavior, it does sound like worn bearings.


#16

Assuming the oil is the recommended weight, the level shows to the fill line on the dipstick, that the dipstick and oil pan are original to the truck, and nobody has messed with the dipstick tube placement. Agree w/others, most likely is worn bearings. Still, I’d ask my mechanic to measure the oil pressure w/their shop gauge, to eliminate a faulty sender unit. And while oil pumps seldom fail due to being bathed in oil all the time, if it is fairly inexpensive procedure to drop the oil pan and do a visual inspection of the oil pump, I’d probably have that done too. Best of luck.


#17

@williamclang

Have you hooked up an oil pressure gauge?

Is the engine making bottom end noises?


#18

If I was going to drop the pan to check the pump, I would have a look at the bearings. A micrometer would tell you if the crank is too worn to use standard size bearings. A lot less work than changing engines.


#19

If you’re going to drop the pan anyway, you could try putting a set of bearings in it. As long as you’re not hearing any noises from the bottom end, the crank should not be damaged. Though it would be worth checking the diameters of the journals if the bearings look really bad.

I did exactly this with the engine in a 1974 Cadillac and drove it for another 50-60K miles before I got rid of it due to rust. The motor still sounded fine.


#20

Yeah, 15-40 or 20-50 oil might be the be the ticket…