Oil Pressure Question

[engine24355]

Need some advice on an oil pressure concern I have on a new (less than 500 miles) vortec 305. The engine has a mechanical Sunpro (cheap) gauage which reads between 40-45 psi on startup and idle which does not scare me. However, when the engine warms up it seems to slowly drop. Also, as with mechanical gauges, the higher the rpm's the higher the psi, but when the rpm's come back down to idle the gauge reads lower and lower....big swing from high rpm's to idle. I have checked the line and fittings...fixed a leak and still reading same. My biggest concern is that the oil pump is not working properly, but would not think it would be bad on a new engine. At the same time, I have seen some really weird stuff over the years. Been thinking of switching to an electronic gauge or at least putting a pressure tester on it. Any thoughts / advice?

 

[T-Test]

I would use another mechanial guage at the port close to the distributer on a Chevy, Ford close to the oil filter, just to make sure it's not a bad guage. Go from there.

I personaly don't like too much pressure for it can blow out at any seal that is weak or damaged.
45# max------20# min. Hot or cold. Forcing oil on a street car just isn't nessessary, unless you are running 500+ horses.

 

[RPM]

General rule is 10 psi for every 1000 rpms

 

[Screaming Metal]

OK, You've been given great advice by the others. If I were you I'd go to the local Napa and just get a old style stationary motor oil gauge, Tee off at the block ahead of your other oil gauge. The old stationarys are made to put on the old oilfield and welder motors and are highly accurate and extremely durable. This will allow you to check your gauge.
Next, crank her up, let her get warm. A Chevy smallblock only needs about 6PSI to float your crank at idle. 10 PSI like Ron said is OK, but on a new motor less than 5oo miles, as your bearings wear in and that main clearance opens up, that low pressure will get even lower.
Now then, I need to ask you a few questions:
1. What oil are you running, weight wise and if its a synth.
2. Your idling rpm.
3. Was this motor a complete long block? Did it have the oilpan on it?
4. Is it a Mr. Goodwrench from Mexico?
5. What PSI are you idling at?
6. Is your gauge holding steady at that PSI or is it fluctuating?

It should idle at about 20-28 psi with 10w30 at idle and about 55 to 65 at hi rpms. As your motor wears, those numbers will very some, especially at idle.
Is this a stock 305 or one of the performance 305's? Does it have a hi vol. pump?

 

[engine24355]

Thanks for the information! When I first noticed this issue was after driving the car on a freeway and exiting. When exiting and coming to a stop the psi actually dropped all the way to zero. Then back up when taking off. I really think it is the gauge as it is not a quality unit, but want to be sure. Need to just buy a tester...local store was out when I tried a week or so ago.

 

[engine24355]

OK, You've been given great advice by the others. If I were you I'd go to the local Napa and just get a old style stationary motor oil gauge, Tee off at the block ahead of your other oil gauge. The old stationarys are made to put on the old oilfield and welder motors and are highly accurate and extremely durable. This will allow you to check your gauge.
Next, crank her up, let her get warm. A Chevy smallblock only needs about 6PSI to float your crank at idle. 10 PSI like Ron said is OK, but on a new motor less than 5oo miles, as your bearings wear in and that main clearance opens up, that low pressure will get even lower.
Now then, I need to ask you a few questions:
1. What oil are you running, weight wise and if its a synth. - Mobil One synthetic 10w 30
2. Your idling rpm. - No tach, but guessing 5-7000 rpm's
3. Was this motor a complete long block? Did it have the oilpan on it? - Complete running motor...88-94 year based on block casting
4. Is it a Mr. Goodwrench from Mexico? - Not sure
5. What PSI are you idling at? - Approx. 40 until it warms up and drops
6. Is your gauge holding steady at that PSI or is it fluctuating? - Fluctuates some

It should idle at about 20-28 psi with 10w30 at idle and about 55 to 65 at hi rpms. As your motor wears, those numbers will very some, especially at idle.
Is this a stock 305 or one of the performance 305's? Does it have a hi vol. pump?

- Stock with a roller cam

Thanks for the help!!

 

[Screaming Metal]

What does that oil pressure drop down to at idle after it gets hot?
If its between 20 to 28 PSI, thats good, if its 18 to 12 psi, I'd switch to a good 20w50 racing oil of your choice. If its below 12, its time to raise your eyebrow and give it your best Spock imitation....

 

[Screaming Metal]

-0-at a stoplight ain't good! Don't drive it anymore until you veryify your pressure! Go to Harbor Freight, they have a decent one there for cheap....one of my trainee's bought one there a few weeks ago....even comes in a case.
Crank her up, then let it sit idling, and see what she drops down to. Hopefully its the line isn't bled of all the air. On all the motors I build, I put a small hi-quality gage right there on the motor so I can see the pressure at all times. If nothing else, get the small, stationary motor oil gauge, and tee it off like I said.
Now, if after its verified, if it still goes to -0-, pull it out and take it back and get another motor, if bought new or rebuild cause they did something wrong....either left a valley plug loose, oil pump loose, coverplate not flat. Alot of folks have got on the grooved bearing kick. Thats a NO-NO! If your gonna run groved, run grooved inserts in the uppers, not in the lowers. For stock street motors, non-grooved are best, for the bearing, and crank.
If its one that you bought from a salvage yard or individual, ask about your warranty, if any.
If not, then pull your timing cover and oilpan off. Check your plugs at the front of the block....make sure they are still in. Also look at the rear of your block, make sure there is no oil leaking back there. If they are all ok, its time to look at the pump itself. Is the whole thing loose? Check and make sure the pickup is good and tight. You should not be able to move it by hand.
If that all checks out fine, replace that pump, along with the drive shaft between the pump and the distributor.

 

[Screaming Metal]

If its a slow steady drop in the pressure as the motor warms up, and all else checks out, I'm afraid its either the main bearing clearances are gone way too wide, either the wrong bearings for the crank (probably reground). OR, the pressure spring in the pump or the pumps loose, covers loose or shaft ....either way, she'll have to be opened up to be fixed.

 

[Screaming Metal]

What -I- would do, would be to switch to Straight Valvoline 20w50, no synthetic, and try that, for a low buck approach, if the motor was used, has no warranty. If she'll idle down to 15 to 18 psi or higher, HOT, at idle, I'd run her like that awhile. I she'll idle at 10 or over, just add a quart o Lucas oil stabilizer, that should get you another 5 psi or so....
But to fix her right, you'll have to pull her down and put a rebuild kit in her. If you've paid good money for a rebuild motor, new motor, I'd take her back and get another motor.
If its a used motor you bought, and has some miles on it, doesn't have a warranty left, I'd do the 20w50 with the Lucas thing. Even in a hi milage motor running that weight oil with the lucas, if you don't have AT LEAST 15 to 20 PSI HOT IDLE PRESSURE, SOMETHING IS WRONG BIGTIME!
Thats my magic formula for running the big rats on the loose mains clearancing for 0 drag.

 

[Screaming Metal]

Sorry for the many posts....my guys keep asking me stuff and it distracts me....

 

[PotvinGuy]

Slightly off topic, but might save someone grief: my oil prs dropped to zero. I was just a block from home and went there. Checked the oil level, then pulled the pan and dropped the oil pump and inspected it. Then tried a different gauge. No help. Finally pulled the distributor and primed the pump with a drill motor and got prs! What goes? Here's what happened: the dizzy hold-down had loosened, allowing the dizzy to rise just enough to disengage the oil pump drive tang at the bottom. But the dizzy was still being driven by the cam, so the engine ran. All that work and it was just a loose nut.

 

[Dino]

When you hook it up was the line new to the gauge ? A old line may have some thing in it that may be blocking the oil to the gauge.

 

[Screaming Metal]

When you hook it up was the line new to the gauge ? A old line may have some thing in it that may be blocking the oil to the gauge.

Yep, a possibility there. If your gauge was reading -0- and you sat there more than a minute with -o- pressure, even with a good oil and the best additives, its gonna take about a minute to lock up and really screw things up. Slick 50 helps and will run a motor a while with 0 pressure....but its best not to chance it.
If you pull where the oil sending unit is, get a toothpick or a really small screwdriver and make sure that the oilhole is open. I believe that you got pressure, maybe low....but pressure. Motors don't last long without that oil pressure.
But, check it!

PotvinGuys situation could happen, usually, if it gets that loose, the distr. turns and it goes outta time. I've seen some really freaky crap happen with motors. They are capable of most anything.