EXPLITIVE continued... Over 3000rpms oil pressure drops under 10LBS Bluewater 454

Pyrojodge

Pyrojodge

Well-Known Member
TECHNICAL Contributor
May 1, 2011
4,248
Lake Erie Ohio
Boat Info
1989 Sea Ray 340 DA
Engines
twin 454 Mercs
Hey Everyone,

Been busy for me lately but Aftica let me down for the first time of the season. Took her out fishing with 6 people more then normal and had to go over 3000rpms. Normally I never go higher then 2800rpms and I cruise at 25,26000 rpms. When I got above 3 grand my oil pressure drops to near nothing and bounces around 10lbs. I put a mechanical gauge on and I got the exact same readings. I am going to be tearing the back end out this weekend and pulling the port engine Monday. My hope is it is a cracked pickup tube. Anything else that I need to look for? I also plan to replace the oil pump while I have her out. This is the only thing I can think of why it would cause this. It is also not low on oil and I tried adding an extra 1/2 qt just to make sure its not going dry.

Any other ideas, or does a cracked pick up tube seem likely?

Thanks all...
 
You could pull one valve cover off and see if you have plenty oil flow on the top end if you do it's possible you have a bearing that has spun or is worn out either cam or main bearing will do this without any noise if it were a rod bearing there's usually noise. With luck it's just the pump.
 
I'm a little confused....does the oil pressure still read fine to 2800 rpms? Or does it read very low pressure now at all rpm ranges?

-John
 
Be sure to check the pump drive system. Drive shaft and Distributor Gear. The shaft should be the slotted style and not the hex style, which are more prone to spinning. Are there any noises when the pressure drops, or anytime for that matter?
Not sure how clean the engines are, but agree with Bob to check the oil drain back holes. It is possible that all the oil is pooling in the valve cover area and can't drain back fast enough. Usually this happens on a sludgy engine, and will cause leaks at the valve covers over time since they don't seal that well. Another sludge spot would be the pickup tube screen clogged up, can also be from other junk in there ( I have seen rebuilt engine with so many pieces of rag in the oil that 3/4 of the screen was blocked.
That's about all I can think of other than a failing pump or bad news inside.
 
thanks all, she reads 50lbs under 3k. over 3k loose oil pressure. the engines are clean. we removed the covers and the internals are clean. the engine has roughly 650 hours and i change the oil religiously every 50 hours. also no sounds at all. no tapping or knocking. My mechanic was all over it looking for something that made sense and he's as upset as I am. we will pull it out and go over it until we find something... ill keep you posted.
 
Ok....before you pull the motor let me suggest something simple.

What you have said:
1) is that you never take your boat over 2800 rpms and everything is fine up to that point.
2) On the one time you go over 3000 rpms the pressure drops to zero
3) A mechanical gauge shows the same thing
4) Oil pressure returns to normal below 3000 rpms.

So you are dealing with a problem that most likely has been hidden from view for some time because you never went above 2800 rpms.

There are only a few basic things that can cause what you are experiencing but the symptom is that the oil pan is going empty at 3000 rpms which is causing the drop in pressure.

1) Broken oil pickup....separated from oil pump....basically runs out of oil
2) Broken oil pump pressure spring (unlikely because pressure would not be 50 pounds)
3) Bad oil pump drive mechanism (unlikely because it would get worse and eventually start losing pressure below 3000)
4) Bad bypass on the oil filter (what kind of oil filters do you use?)

Some oil filters use bypasses which open if the filter experiences too much pressure from the oil pump. If it opens...pressure drops. Put a mercruiser filter on it to eliminate that possiblity.


To test the broken pickup theory....put two extra quarts into the engine with it running. That will get the oil higher in the pan. If it makes it past 3000 then you know where the problem is. 1/2 quart won't do the job. Drain out the extra oil after you finish the test.

-John
 
PlayDate said:
Ok....before you pull the motor let me suggest something simple.

What you have said:
1) is that you never take your boat over 2800 rpms and everything is fine up to that point.
2) On the one time you go over 3000 rpms the pressure drops to zero
3) A mechanical gauge shows the same thing
4) Oil pressure returns to normal below 3000 rpms.

So you are dealing with a problem that most likely has been hidden from view for some time because you never went above 2800 rpms.

There are only a few basic things that can cause what you are experiencing but the symptom is that the oil pan is going empty at 3000 rpms which is causing the drop in pressure.

1) Broken oil pickup....separated from oil pump....basically runs out of oil
2) Broken oil pump pressure spring (unlikely because pressure would not be 50 pounds)
3) Bad oil pump drive mechanism (unlikely because it would get worse and eventually start losing pressure below 3000)
4) Bad bypass on the oil filter (what kind of oil filters do you use?)

Some oil filters use bypasses which open if the filter experiences too much pressure from the oil pump. If it opens...pressure drops. Put a mercruiser filter on it to eliminate that possiblity.


To test the broken pickup theory....put two extra quarts into the engine with it running. That will get the oil higher in the pan. If it makes it past 3000 then you know where the problem is. 1/2 quart won't do the job. Drain out the extra oil after you finish the test.

-John
Click to expand...

All great suggestions!
 
Also, it seems strange that you are running those engines at such a low RPM normally. I would think you are barely on plane at those RPM's and not getting the optimum efficiency?
 
PlayDate said:
4) Bad bypass on the oil filter (what kind of oil filters do you use?)

Some oil filters use bypasses which open if the filter experiences too much pressure from the oil pump. If it opens...pressure drops. Put a mercruiser filter on it to eliminate that possibility.
Click to expand...

I wondered about this one - it ended up being the cause in some similar threads here, and it would be the easiest and cheapest place to start.
 
I still think it is either a pump cavitation (SUCKING AIR) from a cracked pickup tube OR partially blocked pickup screen causing higher rpm starvation
drop the pan and the story will be told and as I said last night on the phone...no matter what you find or don't find...put a pump and pickup in it while its out
 
Do as PlatDate says and change the oil filter. I have seen defective filters cause this on a couple of occasions.
 
I am pretty sure your mechanic or yourself checked this long ago but just in case - an engine that has too much oil in it will lose pressure at higher rpm due to the pork chops on the crank slapping the oil and causing the oil to foam. Have you changed/added oil prior to the issue starting to present itself? If so, try sucking a quart of oil out then try it again to make sure not an easy fix. Good luck!


Sent from my iPad using Tapatalk HD
 
I have always used Wix filters and never had an issue. That being said, if it is happening on both engines I can't see 2 bad oil filter bypass valves. If it's only the port it can't hurt to swap it out. I'm with Mopar in thinking something in your oil pump area is the issue at best, and something deeper in the oiling system at worst. I have never seen a spun bearing not cause noise and low pressure at lower RPM, and only at high RPM.
Almost has to be loss of oil to the pump, the pump, or a mechanical release of pressure to just suddenly drop off like you describe.
 
i'll change the filter today.

i am very confident in my mechanic and trust him completely... ill keep everyone posted.
 
This happend to me once when an oil cooler failed. To get home I by passed the cooler and the oil pressure was good at lower RPM but at higher RPM it dropped very quickly. The engine temperature never went up regardless of RPM.
 
No change with the filter. Dang, guess we will pull it and check out what is going on...

thanks for the help everyone.
 
Remove some of the oil. If it's its over full, it will foam up from the crank when your at an angle getting her up on plane. I have had the exact same problem before. Drain 1 quart, then go for a ride.
 
Well, today is the day. we moved her to the ICU dock and the motor will be unbolted amd pulled. im hopeful that its a stupid fix and we are up and running for the weekend. Thanks for all the help I tried everything to keep from having to pull her out. I'll keep you guys posted.
 
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