Oil pressure dropping

[Mullady76]

Hi all,
I'm having an oil pressure issues with one of the mercruiser 5.7's on my boat and I think its a windage problem. This is a new 5.7 long block from M.M. with about 30 hours on it. The oil pressure is great on startup (60 psi) then drops to about 30 psi at hot idle. The pressure increases to about 50 psi when I plane off and stays that way for about 5 minutes running at 3k rpm, then the oil pressure slowly starts to drop to about 20-25 psi. If I back off on the throttles the oil pressure jumps right back to 40+ psi. I also noticed that if I bury the bow with the trim tabs and run this engine slightly below 3k the oil pressure increases to about 30-35 psi and stays there.
The boat is a 28 rampage and runs bow high and I think the oil is getting whipped up by the crank when it gets hot. Right now oil level is halfway between the add oil and full marks. This weekend I'm going to drop the oil level down to the low end and see if that solves the problem.
Since this only happens when the oil get hot, maybe the oil is getting too hot? I'm running Castrol Classic 20w50 with zinc for the cams along with a K&N filter. Could I be over propped and working the engines too hard? Would an oil cooler help?

 

[Scott06]

Hi all,
I'm having an oil pressure issues with one of the mercruiser 5.7's on my boat and I think its a windage problem. This is a new 5.7 long block from M.M. with about 30 hours on it. The oil pressure is great on startup (60 psi) then drops to about 30 psi at hot idle. The pressure increases to about 50 psi when I plane off and stays that way for about 5 minutes running at 3k rpm, then the oil pressure slowly starts to drop to about 20-25 psi. If I back off on the throttles the oil pressure jumps right back to 40+ psi. I also noticed that if I bury the bow with the trim tabs and run this engine slightly below 3k the oil pressure increases to about 30-35 psi and stays there.
The boat is a 28 rampage and runs bow high and I think the oil is getting whipped up by the crank when it gets hot. Right now oil level is halfway between the add oil and full marks. This weekend I'm going to drop the oil level down to the low end and see if that solves the problem.
Since this only happens when the oil get hot, maybe the oil is getting too hot? I'm running Castrol Classic 20w50 with zinc for the cams along with a K&N filter. Could I be over propped and working the engines too hard? Would an oil cooler help?

I have had some similar instances like this. I also am running a then new 5.0 base engine from MM. I found I would get this when running higher rpm (above 4k close to WOT rpm limit) for a decent period of time like going at WOT RPM for 10 plus minutes. Normally at high rpm my oil pressure would eventually start dropping from 55-60 ish down into the 35-40 psi. I had spoken to Loren Haus at MM he said this is fine but I generally dont feel comfortable unless I have 10 psi/1000 rpm. Again this was after a good hard use like towing skiers and boarders for an hour then on way back home going WOT...

I do think a couple things influence it - oil level and oil type and how warm it is out.

I would definitely double check the level as I think I was over filling mine by a qt or so. I found this happened with both Rotella T4 15-40, Quicksilver 25-40 . I eventually went to Mobil 1 15-50 and this doesn't happen anymore.

Only thing I figured was happening was oil was foaming up either from windage or sucking the sump dry but I havent seen this since I switched to M1.

Is yours a new or reman engine ? What is WOT rpm

 

[kenny nunez]

What could be happening is a vortex forms under the pickup screen. Then the oil gets air bubbles. There may be too much clearance between the pickup and the bottom of the pan.

 

[Mullady76]

Scott,
Thanks for the reply. This is a new base engine from MM and the other side is a rebuilt 355. The rebuilt engine has lower oil pressure (larger clearances) in general, but doesn't have this problem.
I haven't run the engines WOT yet because the rebuilt engine is still being broken in. I briefly pushed them to 3800 rpm and they still had some more to go. I believe WOT for that engine is 4200-4600, so I'm probably closer to the lower end?

 

[Scott06]

Scott,
Thanks for the reply. This is a new base engine from MM and the other side is a rebuilt 355. The rebuilt engine has lower oil pressure (larger clearances) in general, but doesn't have this problem.
I haven't run the engines WOT yet because the rebuilt engine is still being broken in. I briefly pushed them to 3800 rpm and they still had some more to go. I believe WOT for that engine is 4200-4600, so I'm probably closer to the lower end?

Might be worth running a mechanical gauge while on water to verify.
I would verify the wot rpm with MM.

Forgot about break in. I didn't start running synthetic until after 100 hrs on the engine.

 

[Bondo]

I believe WOT for that engine is 4200-4600, so I'm probably closer to the lower end?

Ayuh,..... A 350 likes to run 4600 / 4800 rpms @ wot,....
Even 5000 won't hurt it,....

 

[Mullady76]

The mercruiser manual that came with the boat states Max. RPM @WOT 4000-4400. This is a 260hp pre-vortec flat tappet cam carb'd engine.
I agree a higher WOT rpm means the engine isn't working as hard.

 

[QBhoy]

You’re within spec there I’d say. But just admit…always found the manual suggestion of what’s acceptable in terms of the lowest acceptable figure of oil pressure…almost ridiculous. If I ever had an instance of single figure oil pressure…I’d panic no end ! My 5.0 mpi runs just over 40 psi at idle…little more cruising or higher rpms then settles just over 40 again if sustained. Backing off after a run cruising…she’ll drop to 30 or close for a while..then return to 40 psi usually. Your engine will be tight and settling into things. Give her a chance and change that oil as much as possible in the next wee while.

 

[Searay205]

I chased a Roush 427 for months with a similar problem. Oil pressure solid when cold 70psig. As soon as it got hot and above 4Krpm oil pressure would drop from 50 psi to 40 psi at 6k rpm's. Mellon (oil pump manufacturer said even though positive displacement it max flow is at 4K rpm's. I could go on and on, in the end the spring in the oil pump RV was incorrect. To add injury to insult Mellon sent wrong spring twice. Once correct spring installed, 6krpm and 65 psig oil pressure. At idle oil is so thick even though oil pump rv is lifting it can't lift enough, when oil is hot and thin the rv can finally control pressure.

 

[Mullady76]

Here’s an update. I changed the oil using an electric pump attached to dipstick tube (dipstick connects to the bottom of the oil pan) and got about 7 quarts out. It supposedly has a 5 qt. Pan so it was definitely overfilled. The oil came out black for only having 5 hours of use, so I think the crank was whipping up the oil which was causing it really heat up and thin out.
I also changed the old rubber oil lines with new 10an PTFE hoses and added an small oil cooler while I was at it.
I took it for a test run this morning and the oil pressure was 65 psi at 3000 rpm and didn’t drop all the way to WOT.
For some reason the dipstick is reading real low causing me to overfill this engine.

 

[tank1949]

Hi all,
I'm having an oil pressure issues with one of the mercruiser 5.7's on my boat and I think its a windage problem. This is a new 5.7 long block from M.M. with about 30 hours on it. The oil pressure is great on startup (60 psi) then drops to about 30 psi at hot idle. The pressure increases to about 50 psi when I plane off and stays that way for about 5 minutes running at 3k rpm, then the oil pressure slowly starts to drop to about 20-25 psi. If I back off on the throttles the oil pressure jumps right back to 40+ psi. I also noticed that if I bury the bow with the trim tabs and run this engine slightly below 3k the oil pressure increases to about 30-35 psi and stays there.
The boat is a 28 rampage and runs bow high and I think the oil is getting whipped up by the crank when it gets hot. Right now oil level is halfway between the add oil and full marks. This weekend I'm going to drop the oil level down to the low end and see if that solves the problem.
Since this only happens when the oil get hot, maybe the oil is getting too hot? I'm running Castrol Classic 20w50 with zinc for the cams along with a K&N filter. Could I be over propped and working the engines too hard? Would an oil cooler help?

Install a new manual oil pressure indicator and compare.

 

[Mullady76]

Install a new manual oil pressure indicator and compare.

 

[Mullady76]

A new sending unit was installed and the old one was pressure tested and checked out resistance wise. I have a pressure port fitting on order and will be adding a mechanical gauge to compare.