erratic oil pressure
 

Just dropped a new BB Chevy 454 into a 18' jet boat and running at home on the hose, I always had 75-80 psi O/P, it would never go down. On the lake, the O/P started to fluctuate at higher RPM & after running 5 or 10 min under load- above about 3500. It would drop from 70-75 to about 40-45 and fluctuate there while the RPM was up & seemed to fluctuate with diffrent RPM. As soon as you let off the throttle, it would bounce right back to 70 psi or so, and stay there at idle??? Running 20/50 wt oil & the rebuild on the engine was fairly mild. Any Suggestions? Help!!

my guess is you are slinging the oil with the crank and foaming it... how deep of a sump do you have? Is it a wet sump? or a dry sump? How many quarts does it hold? Do you have a tray in the sump and trap doors?.. do you have an oil cooler or a remote filter set up? lots of things can contribute to your symptoms..

Are you using the same guage and sending unit that you had before???

I know that when I had a gaffrig guage matched with a VDO sending unit, it did the exact same thing. Basically the guage was running in reverse.
Also check the wiring at the guage to make sure its grounded and not reversed.

Also, make sure you check it with a mechanical guage.

How many quart pan do you have? Sounds like you are pumping it empty. Not an uncommon problem with a high volume oil pump and a stock or small pan.
At an idle the oil has time to run back and keep the pickup covered.





10

I've put a new gauge/sender in & also put a mchanical gauge in mounted on the transom so I can see it while running. I think its an 8 quart pan, wet sump, it has trap doors and a tray in it. The confusing thing is, it will never happen on the trailor, even at 3500 or 4000 rpm, it will never fluctuate. During the rebuild, I had the pan and filled it with nine quarts and it just started coming over the windage tray, I think that might be too much? Is it possible to pump 8 quarts out of a pan with a H/V oil pump?

No oil cooler or remote filter either...

While the boat is on the trailer it isn't getting bounced around like on the water... I think the oil is getting aerated and foaming.... once air is entrained into the oil... it becomes "compressible".. the tiny bubbles compress and the oil pump has a hard time pushing it down the oil passages. Where is your oil pump pick-up located ? Is it properly attached to the pump? Also... when you have been running it on the trailer... did you hold it at 3500 rpm's for five or ten minutes... if you did that and the oil pressure was still good.. even on the trailer... then I would say your sump has enough capacity. One other thing about the "erratic pressure". When you look at the gauge while it is erratic... is the needle "dancing"?. Or just reading a lower pressure.. Sometimes when there is air in the system the needle will dance violently. This is seen more with mechanical gauges than electric though. Also... do you have an oil temp gauge? And what are your oil temperatures?

Could also be the wrong pickup for the pan. Is it the same as before or is it entirely different?
The pickup should match the pan. This is exagerated but you could have a 15" deep pan and a 3'" pickup and still run out of oil. You should have about 1/4" between the pickup and the inside of the pan.
Easiest fix is to add an extra quart...maybe two and give it a try again. MY bet is you see an improvement.
EVERY time I've seen a problem like this , it has been a pickup problem or a level problem. In that case the guy was using the wrong dipstick and was actually a couple of quarts low , when he thought he was full:rolleyes: . drained the oil and found 5 quarts in a 10 qt. pan
:eek:
You DO sound like you are full though.








9

No..I didn't hold it at rpm for that long on the trailor..but will try it. The needle on both my gauges jump violently down to 40-45 psi and "jerk" in that range until you let off the throttle...even just 1000 rpm or so, then "jump" back to 70 or so. The pick up is almost right under the oil pump, in a BB Chevy, its at the rear of the pan. Its a milodon pickup and it definately is securely attached to the pump. I know that for a fact. No oil temp gauge though... If the crank is causing aerating, is the answer less oil to bring the level down? This is where it gets confusing

The pickup is a bit over a 1/4 inch from the bottom of the pan. Also I've had as much as 10 quarts in the pan and seen this problem.. I drained the oil yesterday and brought it up to 7 qts to try this time..don't know if it will help or not. Would a thinner oil like 10-40 instead of 20-50 make a difference?

I had the same oil pressure problem. Weld on the oil pick up cracked and the pick up fell off. At this point you are sucking oil from the highest level and getting air as RPMs go up

I'd try 7 qts and 10-40.. also... check your electrical connections to the gauge.. also make sure you have a good ground for the gauge and the engine block. If that doesn't fix the problem.... I'd take the engine out and remove the pan... see what is going on.. Maybe the pick-up has broken off...

That is exactly how my gages behaved when I purchased my boat. I experimented with lowering the oil level until this aereation would stop and oil pressure would hold at 5000rpm. There was a oil level that if I exceeded by even a 1/4 qt., it would start dropping pressure again at elevated rpm. When I pulled the engines I found no windage trays. Different problem for you, but same symptom. Like Reed said above, try lowering the number of qaurts. I just kept sucking a lttle more out with the vacuum pump and running it until the pressure wouldn't drop any more. Then I knew the proper level. Don't run the engines any more than absolutely necessary with dropped pressure, the engine is being starved for oil. Main bearings will not last.

Good luck

BT :cool: