Low oil pressure in drive but good in park???

[Kapdin]

So now i got my old 350 up and runnin good but my oil pressure gauge is confusing the hell out of me. When the engine is cold its great pressure, go and drive for a while it drops low like some times down to like 5 while going 40mph. Then ill stop and put in park and boom back to 15-20 rev it up in park and goes even a little higher. But then drop back into drive and all the way back down...... park and drive should have any actual affect on the pressure should it? tell me its the sending unit lol

 

[HotRodPC]

Sounds like your main bearings are worn. The reason it's different in Park and Drive, is that the rpm is lower from P to D. If this is the actual case, then you might consider running straight 30 weight or 40 weight instead of 10w30 or 10w40. It's also normal to have higher oil pressure when the motor is cold because the oil is thicker, which is why I'm also suggesting to change to 30 or 40 weight oil. They are a bit thicker blends than Variable Weight blends.

 

[Kapdin]

so what kinda of pressure should my engine shoot for? (before i call it dead)

 

[oneluckypops]

so what kinda of pressure should my engine shoot for? (before i call it dead)

Atleast 10 psi @ Idle

 

[HotRodPC]

I wouldn't call it dead as long as it's running good. Run it !!! Normally when the oil pressure gets so low you want to take it out of service, is when the lifters aren't getting oil and you start hearing mega valve clatter, like your motor would sound if it were low on oil but it has oil. Or you could just drive it til you develop a small rod knock, and then park it and consider a rebuild. If you let the rod knock get to bad, then the crank may not be salvagealbe with a turning, and if you let it go even further than that, it's likely to hinge a rod and put it through the block or the oil pan, which if it goes thru the block, then the motor is not rebuidable at all. At this point, if you still have good oil pressure when it's cold, you've still got some time. Just drive it. And as suggested, change to a thicker oil next oil change. That should help bring the pressure up a bit. Don't expect miracles, but it will help a little bit.

 

[Kapdin]

well i get 10 at idle in park.....u mean in drive?

 

[Kapdin]

ight well ill change oil tomorow cus its bothering me lol. damn it.

 

[Kapdin]

so should i go get 30 weight or just use the 15-40 on the farm?

 

[HotRodPC]

well i get 10 at idle in park.....u mean in drive?

And keep in mind, your guage may be inaccurate. 10psi might just be 15 or 20 psi. 10 psi in Park will be fine. If you're that concerned, bump your idle up a hair or 2 so you have 10 psi in D ideling, but really, I wouldn't worry about it. As long as it still has the ability to get good pressure when cold, and as long as pressure is increasing with rpm, then this is all just normal well worn motor symptoms. So rock with it for now, keeping in mind it can take a turn for the worst in time, so be putting a little cash back every payday for another motor, or be keeping an eye out for a good used motor for cheap. I see used 350's all the time for $300-$500 depending on how many miles or how new. Seen one not long ago, a crate motor with approx 20,000 miles on it for $400 in a wrecked truck that was being parted out. My bro has a $100 used motor in his 79 Elco that smokes for a few seconds when he starts it up in the morning but the Oil pressure and compression are good and it runs damn good. He drives it back and forth from OKC to Dallas fairly often.

 

[HotRodPC]

so should i go get 30 weight or just use the 15-40 on the farm?

NO, DO NOT run a multi grade oil. Run either SAE 30 or SAE 40. Being summer time you can probably get by with SAE 40. In the winter, maybe cut it back to SAE 30.

 

[HotRodPC]

A story you'll like....

I had a 64 Pontiac GP with a 389. Had good Stewart Warner Guages. Oil Pressure dropped to ZERO. I panicked thinking, **** am I gonna make it home and how long has it been that way since I didn't check for several miles. Pulled over check the oil, it was fine. And it wasn't making any valve clatter noise from no oil to the tappets. Got home thought maybe the gauge went bad, so I put another gauge on another port under the hood. Nope, also read Zero. Called my buddy to come see it, and we decided, time to pull it. Why not, I have a perect running spare 389 right there in the garage. We decided to get on the freeway and blow up the motor that night. We throw the tow bar in the back of his truck since we don't know where it's gonna blow and chuck a rod. Didn't want to get to far from the house though. I drove about 10-12 miles at 80mph trying to blow it up. Turned around, doing 80 all the way home, and this is when speed limit was 55, so I was afraid I'd get a damn big ticket. Made it back home, still not even a knock. I couldn't blow it up. It was just low enough on pressure to not register the gauge, but it obviously had some. I told my buddy, fugg it, I'm not gonna miss work to swap motors, so I'll drive it the 20 miles to work, and 20 miles home till it blows, up til the weekend, then I'll swap the motor. Had my buddy on stand by ready to come tow me home after he got off work if my car blew the motor. He called my house every night for 3 nights to make sure I made it home. And I did. Swapped motors Saturday morning. Sold that motor for $100 to a guy who blew up his Pontiac 400, and he put a set of rod and main bearings in it and took the used oil pump off his 400 and put in it, and he drove it for another 2 or 3 years that I know of and was still driving it when I moved to OK, so who knows how long it lasted.

 

[Kapdin]

yes that does make me feel a little better. Also on that note it got me to work today lol fuggin sweet, just purrrrrs at 70 and 55 lol. And you know wat it had ok pressure the whole way untill the very end then it drop low again but it never sits on 0 so maybe ill be ok. Has anyone ever use that lucas oil additive? any good? should i try it since this oil has about 15 miles on it before i move to 40 weight?

 

[crazy4offroad]

Just use SAE 40, you dont need to waste money on additives. They wont reverse bearing wear. Put that money towards a crate engine! If it makes you feel any better I can tell my story.

Got stuck in a mud bog, left the truck running while we hooked up straps to pull me out. Front & rear main seals were bad and engine vacuum drew muddy water into the crank case. On the way off the hill I noticed it trying to quit, and kind of noisy, and had ZERO pressure. I panicked and just wanted to get it home. It would hold about 10 psi at 2000 rpms and up so I tried to keep the r's up. Changed the oil and it looked like mud instead of oil. Changed it 3 times in 1 week and that seemed to fix it. Still today it will hold 40 psi on higher revs, 20-30 at idle.

If you flooded the engine at all, the gas will make it's way into the crank case and thin the oil, making the pressure drop. So an oil change may bring it back to life.

 

[Kapdin]

Thanks again guys. I guess i can always pull that 91 motor this winter and i guess learn to rebuild. (im a pretty quick studdy) I did notice tho, i think its either my powersteering pump or front main seal but i gots a leak. I did have to top off my powersteering pump twice cus it started makin noises but i thought i just didn't fill it full the first time.

 

[89Suburban]

What's wrong with throwing the farm oil in it, 15-40?