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Oil pressure drop after engine runs for about an hour

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briang0

Regular Contributor
I have a 1983 898 305 Mercruiser. It runs well starts quick idles smooth has about 35 - 40 psi oil pressure at idle when started. The oil level is full. I don't know what weight oil is in the engine I did not put it in, the PO did. It is clean though. I noticed that after the engine has run for about and hour the oil pressure at idle will drop on the gauge very low barely above the 0 line. The first time I saw this I got a little nervous but now I'm getting use to it. The engine sounds quiet no lifter noise and engine temperature is rock solid at 140. If I bring the RPMS up the pressure comes right up. The engine is idling around 800 RPMs. How concerned should I be. The gauge is not very fine in it's numbering. Only other issue I see is there is a Fram oil filter on it and I don't really care for Fram filters. Am I just being a worry wart???
 
briang0 said:
I have a 1983 898 305 Mercruiser. It runs well starts quick idles smooth has about 35 - 40 psi oil pressure at idle when started. The oil level is full. I don't know what weight oil is in the engine I did not put it in, the PO did. It is clean though. I noticed that after the engine has run for about and hour the oil pressure at idle will drop on the gauge very low barely above the 0 line. The first time I saw this I got a little nervous but now I'm getting use to it. The engine sounds quiet no lifter noise and engine temperature is rock solid at 140. If I bring the RPMS up the pressure comes right up. The engine is idling around 800 RPMs. How concerned should I be. The gauge is not very fine in it's numbering. Only other issue I see is there is a Fram oil filter on it and I don't really care for Fram filters. Am I just being a worry wart???
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Dropping to just above "0" would concern me too. I would start by changing the oil and filter (Fram filters aren't my favorite either). Seeing how she's an '83, she has some wear on her and You could go to a 10 or 20w/40 weight and see what happens. PO could have put anything in there, for all you know, and it could be kind of light weight for that engine. There are a number of things that could cause what you are seeing. I would start w/ the oil change and see what happens. From there, you could plumb in a mechanical gauge to verify if it is actually a severe drop in pressure or an iffy electric gauge / sender problem. Pretty easy and quick to do. Try that and let us know what happens.
 
What part number oil filter should I use on this engine it is a 1983 305 mercruiser that has a carburetor. I don't want to put the Fram back on. Can I just look up in a car manual for a 1983 car with a 305 engine?
 
no oils starting with a 10 should go in that motor. 20W50 or 25W40 are Merc's recommendations in that era. Straight 30 is an option (that's what OMC called for in the same motor)
Some use 15W40 successfully, but my Mercruiser V6's oil pressure fell below spec. when I tried it.
 
The metal tag is on the engine and it is legible it says to put straight 30 weight in it so I'm thinking that's what I'm going to do. I did a little research and the mercruiser oil filter crosses over to an AC Delco PF454. If I get a chance today I'm going to go down and do an oil change and take the boat out for a ride and see what happens.
 
you may want to replace the sender with a mechanical gauge too and verify the readings. A piece of rtv in the sender can make it drop off to 0 reading at lower pressures.
 
I replaced the Fram filter with an AC Delco and changed out the oil with some straight 30 weight. With the combination of a increased RPM of an adjustment of timing to get it to 8 BTDC it was only at 4 when I started and the 30 weight there is a big change in the pressure both at cold and warm idle I have at least 10 psi at idle when warm although it rough estimate since the gauge is not that graduated it's about 1/2 way between the 0 and 20 at idle when warm. I don't know what oil was in there but the straight weight made a big difference.
 
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