Differing oil pressue.

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Changed the oil in my 1985 twin 454ci's last fall when winterizing everything.
Changed from a 40W oil to a multi weight oil.
After de-winterizing this spring I noticed that the starboard enjine was holding at40psi while the port enjine was only showing 30psi.
In the past both engines have always run at 40/45 psi and have always performed identicaly in every respect.
They both run great, balanced perfectly throughout the power range with the rpm's on each engine matching perfectly throughout the throttle increase and engines in perfect sync.
Both engines have 480 hours on them.
Anyone have an idea why just changing from a straight viscosity to a multi viscosity oil would all of a sudden cause the two engines which have always had matched oil pressures to now be 10 psi different?
 
Perhaps it's a sender unit and there isn't anything wrong? I am assuming you have electrical oil pressure senders not mechanical gauges?
Bill
 
They are electric senders. I have the same readings from the top helm and the lower helm, hoverver in the past the lower helm for the port engine has not aleays matched the upper helm on that enjine but the uppers have always matched in the past.
Both oil filters match and are wix brand.
 
Went with a 10w30. Can't find 40 wieght in 5 quart containers and oils have really improved since 1985 so I just thought I'd try the multigrade once. The boat is not driven hard at all usualy just cruising at 2200 rpm.
Would that cause one engine to read differently than the other?
 
quote:
Originally posted by pdecat

Try a new sender

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Given that the lower helm port engine pressure has varied from the upper in the past, could be something going on between the sender and the two gauges.
 
Change it back and let us know why you changed viscosity in the first place. I had those engines for 19 years in my previous boat and used Shell Rotella 30w oil with great results. Whomever convinced you to switch older engines to a multi weight oil did you no favor. Don"t try to fix what aint broke. Also in my millions of miles I have have had the best results with Delo 400 and Rotella. I do not use synthetics.
 
quote:
Originally posted by Ironworks

Change it back and let us know why you changed viscosity in the first place. I had those engines for 19 years in my previous boat and used Shell Rotella 30w oil with great results. Whomever convinced you to switch older engines to a multi weight oil did you no favor. Don"t try to fix what aint broke. Also in my millions of miles I have have had the best results with Delo 400 and Rotella. I do not use synthetics.

Click to expand...


While on newer engines I only run synthetic oil, I am afraid to switch over to synthetic on an older engine as I'm afraid that the synthetic would cause the old engine to develope oil leaks.
As I stated above, the difficulty in finding a striaght 40 weight was the reason for the switch.
Certainly not a problem to change back though if multi weight oil is a problem on older engines, but why would changing viscosity make one engine's oil pressure read differently that the Other?
Just curious, why not synthetic oil on new engines?
 
does the pressure go to equal high reading at faster RPM?
 
quote:
Originally posted by pdecat

does the pressure go to equal high reading at faster RPM?

Click to expand...


Yes, the upper and lower helm gauges on the port engine stay in sync when increasing rpm which was not always the case before I changed the oil to multi weight.
 
I assume that you checked the oil for gas or water.
Low pressure at idle is usually main bearings or oil pump but 30pis is nothing to worry about.
The other engine may just be tighter with nothing wrong with either.
 
I had the same issue a couple years back. I ran seafoam through both engines before my fall oil change. It did bring the lengine with lower pressure back up.
 
I also wouldn’t run 10W30 in a Marine Power Plant of that vintage. Wix filters are GOOD filters, I’ve had an issue once with oscillating oil pressure at idle when I used a Fram filter. Changed it to a Motorcraft filter and the problem went away.
 
I would verify the oil pressure readings with a mechanical gauge before I started buying parts you might not need.

"Yes, the upper and lower helm gauges on the port engine stay in sync when increasing rpm which was not always the case before I changed the oil to multi weight."

So to be clear, the 30 and 40 psi readings you see are at idle? And the pressure goes up as the RPMs go up? And if so what are the psi readings once the engines are warmed up and you are at cruising RPMs?
 
quote:
Originally posted by pdecat

I assume that you checked the oil for gas or water.
Low pressure at idle is usually main bearings or oil pump but 30pis is nothing to worry about.
The other engine may just be tighter with nothing wrong with either.

Click to expand...

I have seen CAM bearings do this in GM based gas motors mostly the 4.3L. However I wouldn't get upset! You could check both with a mechanical gauge for more accurate readings. Also others have posted good advice, even 30 psi at idle is not near the low range.
Bill
 
I wonder why you switched from your "usual" straight 40W to 10W-30 instead of 15W-40 commonly available in gallons and qts.?

The thinner 10W-30 would be expected to provide relatively lower oil pressure. Is there any chance you might have filled one crankcase with 10W-30 and the other with 40W?

Since you report both port and both stbd OP gauges match up , like Capt. Bill I also would suggest checking both engines with a mechanical gauge to see if the P&S engines' pressure really varies by 10 psi at the same rpm or if they are much closer to same pressure and one of the senders needs replacing.
The gauges are probably OK since upper & lower agree. (Just note, 2 mechanical gauges may not agree. I've see 2 read 10psi off from each other on the same fully warmed engine at same rpms. So if you had them, don't be tempted to use different ones on the 2 engines . )
 
Ok, just remembered, before starting the port engine, but with ignition on, I did notice the port engine was showing 1,000 rpm. Knew something was not right but after starting the psi showed 30 psi at idle (1,000rpm) same as the starboard engine.
So, ok, now thinking something wrong with the rpm guage before even starting the port engine could cause the different psi readings between the starboard and port engines after throttling up from idle to 3400 rpm?
Sandy, when I went to buy the oil they didn't have any 40 weight by the gallon (multi viscosity or otherwise) so I just thought "jeasus I don't want to have to but 14 individual quarts of oil" so I just caved and bought the 10w30. I realy thought it would be ok for one season seeing as how I drive the boat and have only been putting on about 10/15 hours on the boat per season.
 
I'm still thinking it could be resistance in a ground connection to the engines gauge with the gauge reading lower and the pinned tachometer.
Bill
 
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