Low Oil Pressure 1994 GMC

[GASoline71]

I have a 1994 GMC 1/2 ton 4X4 that I have owned for about a year now. Great truck... I just turned 175,000 miles on it over the weekend. The 350 TBI runs fantastic. I just replaced the cap and rotor, new plugs and wires too.

Anyways... Since I have owned the truck, the oil pressure will drop pretty low at idle after the truck is completely warmed up. I know a little lower pressure at warm idle is normal, but this is just above the "red" area of the gague in the dash. A little too low if you ask me.

When the truck is cold and I start it, the pressure goes up above 40psi and hangs there for a bit until warm. Then during normal driving, pressure seems normal. Then I come to a complete stop, and the oil pressore drops down by the "red" area. Whether the oil is fresh 10W30 or has 3,000 miles on it... it doesnt matter.

Anybody else's 350 TBI's do this... I'm thinking to replace my oil pump... but if this is "normal"... I'll just carry on...

Thanks,

Gary

[Restrorob]

Gary,

The best thing to do is install a manual gauge to get a accurate pressure reading, This would eliminate any possible issues with the factory gauge set-up.

Also, You may try stepping up to a 20W50 oil and see how it reacts.

[GASoline71]

I never even gave it a thought that the stock gauge is electric... and to check it with a mechanical. Thanks for the reminder...

Gary

[ChevLoRay]

One more guideline when you deal with oil pressure is that you need 10lbs of pressure for each 1000RPM's. So, at 2500rpm, you should have 25lbs of pressure.

My school bus has a high volume, low pressure oiling system. It freaked me out when the idle (800rpm) oil pressure looked like it was zero. I have been assured that it is okay. I guess so, it's got 69K miles on it.

PowerUP is an additive that we used in an industrial environment in all sorts of gearboxes in high and low temp aps. Ditto for the bus engines. It might be something that you would want to consider.

[ToughBowtieTruck]

Meh. My 1st engine and my 2nd engine do the same thing, only the 2nd engine doesn't burn oil.

75 psi cold. 30-40 psi hot at 2,000 rpms. At 400 rpms in idle in drive, I get anywhere from 4-10 psi. Lots of times the "Check Gauges" warning light comes on (95 year). Been that way for the past 60,000 miles. No issues with it.

It has a high volume oil pump in it but didn't make a difference. Switched sending units-didn't make a difference. The only thing I didn't do the engine was the rods and mains, I'm sure one of those is the bandit. But like I said, 60,000 miles and still ticking.

Sure got my $800 worth on the engine I put in.



You have to remember these things are barely running at an idle so barely any pressure is needed-if it comes back up when you idle up, don't worry if there aren't any noises.

[GASoline71]

Originally Posted by ToughBowtieTruck

You have to remember these things are barely running at an idle so barely any pressure is needed-if it comes back up when you idle up, don't worry if there aren't any noises.

That's what I was thinkin'... My 1972 GMC hot rod has a high volume pump in it, and it has gobs of PSI. Prolly more than I actually need. So did my 1979 GMC.

Thanks for the replies you'se guys... just wanted to makes sure I wasn't crazy.

Gary

[jb1016]

Originally Posted by ToughBowtieTruck

Meh. My 1st engine and my 2nd engine do the same thing.

Mine too.

[BigBlocksRule]

You'll be wasting your time if you swap the oil pump. They either work or they don't. Even a badly worn one makes enough pressure to hit the bypass at low revs. The culprit is the bearings. When they get worn and clearances open up, the engine naturally hemorrhages more oil when it's running. You're pumping close to the same amount of oil, there's just no resistance pushing back against it from correct bearing clearances.

[ChevLoRay]

When I was just out of school, I had a '56 Mercury that had oil issues. My unlcle was a mechanic and we dropped the pan to replace the rear main oil seal. While there, we miked the rear main and put in a .001 undersize bearing. Sure helped the oil pressure and the leakage.

Q. Are we able to buy bearings that are .001 undersize? Would that be anything that we'd even want to consider? You gotta know how your mains/rods mike before you could even think about replacing the stockers with undersize. To me, it's a last-ditch effort that you might want to do right before putting a FOR SALE sign in the window.

[BigBlocksRule]

Undersize are definitely available, cost a few bucks more than a .010, etc., but sure may be worth it, especially if the top end is still in good shape.

[loudnlifted88]

maybe at one point in time the motor lost the intake gaskets and all the water dumped in the base because my dads 94 did that and his good oil pressure is gone at idle now its good now.

[BigBlocksRule]

A lot of those that lost the intake gaskets ended up spinning rod bearings toward the back of the block.
If it's not knocking and you have access to a hoist, I'd pull it and drop the pan. You can replace bearings while it's in the truck, but you can't get a really good look at the crank, rods and bearings without getting it to a place where you have unobstructed visibility. If the bearings are worn but the crank is smooth, chances are you can get by with either new or .001 bearings. That would be a whole lot cheaper than a shortblock or rebuild, it's worth a shot. The alternative is running it until it can't hold enough pressure to keep the bearings at the end of the oil galley lubed and those will go.

[scotts62]

I rebuilt the engine and trans in my 92 had the engine bored .040 over with all bells and whistles and when we got to the crank it was as clean and mic'd out perfect like the day it was ground so i had it polished a bit and put .001 undersize bearings in it and put it all together, it has about 40psi at cold and once it warms up it has around 20and all seems to be good. But yeah have them mic'd before you throw the .001 unders in it because it could need ground and while your there it makes it easier than pulling it later.

[indyrenegade4x4]

i have owned tons of chevy trucks and everyone of them had low pressure at idle. One was even my built motor and it did same thing.

[scotts62]

Yeah, have heard its just the nature of the beast..

[sdelmar]

My 95 was still pushing 25-35 lbs at idle when warm up to the day I pulled the motor at 403,000! I switched to 10w-40 at about 150,000 when I began noticing a minimal amount of oil burn due to worn valve guides. New GM crate motor is 75 lbs at idle when cold (kind of worried me at first...thought I might spring a leak)!!!

I think trying a little heavier oil might be a temporary band-aid if you want to take the easy way out like I would!

[BigBlocksRule]

20K on mine, 35 at idle and 55 at 2000 when warm. I checked all the clearances before I built it, ended up with 2 1/2 thou on rods which is your main culprit when pressure starts to go.

[collins10672]

my 94 with the 305 is 60 when cold,40-55 when warm and around 30-40 at idle like when stopping at red lights.kind of freaked me out too when i first bought it and i got on here and the good guys on here said it was a normal thing in these trucks.thank goodness for you experts.keep up the good work.

[scotts62]

Yeah main bearings and cam bearings is where your oil pressure comes from, im just going to keep an eye on mine for now and if i start to hear a deep knock ill turn it off

[shawn_lail]

Howdy iv got a 94 gmc doing the same thing. it would get very low . i drive it like that for 2 month then it lost all oil pressure. i just dont know to change the oil pump i dont want spend the money and it be the cam bearings . any ideal?

[shawn_lail]

My 94 gmc with 350 started doing the same thing then few months later it lost all oil p and started knocking. now im stuck not sure to change oul pump or rebuild the motor any ideals?

[Hoods69BadBowTie]

Yea I would just leave the oil pump alone. My '95 does the same thing but it only goes down by the 20 mark when warm at idle. It starts up at around 60 when cold. Have had no major issues with it in the last 3 years. I have heard that a lot of the TBIs do this.