High oil pressure on startup, then comes down to normal

Friend is looking to buy his first boat and I am helping him make a decision as I've had some experience.

Here's the deal on the boat we're looking at. Show up to test the motor on muffs on the driveway, hasn't been run since winterized. Previous owner gets the thing started after a few minutes of playing with the choke and throttle plate while cranking. Then he proceeds to rev the engine in neutral to keep it going (to me this is very bad for the engine to rev right after startup, not enough lubrication at that point). Anyway, the thing runs lousy at first backfiring and the works, I'm watching the gauges and oil pressure is at max on the gauge which I think was 60-70 psi. Temperature was normal. We notice water filling the bilge so we shut it down and investigate. I notice one of the core plugs is missing.

So I find the plug in the bilge, gently hammer it back on to the block. PO admits to half-assing winterizing the engine. He fires it back up and revs it again, and oil pressure is still reading high, temp normal, this time the engine is running smoothly. I inspect the core plug and no leaks at the seal. After two minutes of running oil pressure drops down to the middle of the gauge which if I remember is 40 psi. PO let's engine run come down to idle and we listen to it run excellent for the 15 minutes. We let it sit while we talked, restarted it a few times and to my ear and eyes everything was sounding and looking great at this point but obviously I am suspicious right now about about this engine and specifically damage that might have happened to the block.

Couple questions.
How could a missing core plug cause backfiring and lousy performance? Seems unrelated.

High oil pressure on startup.. could this just be cuz the oil was cold and thick and needed warmed up? My fear is that the block cracked internally and water is mixing with oil causing high pressure but why then would the pressure return to normal after a minute or two?

PO is letting us test drive it on a lake on Friday. I want to check for milky oil when I run it and inspect the block for leaks. Can I tell if oil is milky by opening dipstick and looking in with a flashlight or by just inspecting the dipstick when I pull it out or do I need to change the oil when I pull her out the water to see if milky. PO is a friend of my friend and is letting us do whatever tinkering we need to do.

Don't buy it, you already know the owner has no idea of what he is doing!
If a core plug popped out and he says he half assed the winterizing the chance of a cracked block is about 100%. And cracked other things like cyl heads and the intake manifold.

louc said:

Don't buy it, you already know the owner has no idea of what he is doing!
If a core plug popped out and he says he half assed the winterizing the chance of a cracked block is about 100%. And cracked other things like cyl heads and the intake manifold.

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Thanks for the fast reply. I appreciate what you're saying but I'm more interested in finding out how to check for damage rather than assuming it's toast and moving on. We have a small budget but plenty of time to go over this engine and check things out. I can do a compression test to check the cylinder heads and if the intake manifold or block is cracked I should be able to either find a leak or I'll have water in my oil which is really where I'm asking for advice on how to check these things out. Ive seen previous threads where people have had core plugs come out from bad winterization and replaced em with no troubles that they came back to post about so I really don't think it 100% means the engine needs to be torn out. I will definitely update this thread though after I go over this boat again Friday and have it out on the water

OK if you want to go with it, there is a way to air pressure test the cooling system, this is pretty easy to do, and will or should at least tell you if you have a cracked block or head. I just have a healthy suspicion of people doing winterizing when they do not understand how raw water cooling works (that is, you don't use the suck the AF up the drive method because the stat won't open enough to let raw water out of the engine so it can be replaced with AF). That is the most common error in winterizing because people are lazy and don't want to drain the engine first, and just leave it dry, or back fill with good marine AF. If only people would READ factory shop manuals, they would learn that this is what the manufacturers recommend, not what uninformed and untrained people on you tube put on their channel.
Remember on a raw water cooled engine, the cooling system has excess capacity. So the thermostat acts as a gate valve to maintain engine temp at a certain level. On a cool fall day, it will not open far enough or long enough to let all the raw water escape via the exhaust system.

louc said:

OK if you want to go with it, there is a way to air pressure test the cooling system, this is pretty easy to do, and will or should at least tell you if you have a cracked block or head. I just have a healthy suspicion of people doing winterizing when they do not understand how raw water cooling works (that is, you don't use the suck the AF up the drive method because the stat won't open enough to let raw water out of the engine so it can be replaced with AF). That is the most common error in winterizing because people are lazy and don't want to drain the engine first, and just leave it dry, or back fill with good marine AF. If only people would READ factory shop manuals, they would learn that this is what the manufacturers recommend, not what uninformed and untrained people on you tube put on their channel.
Remember on a raw water cooled engine, the cooling system has excess capacity. So the thermostat acts as a gate valve to maintain engine temp at a certain level. On a cool fall day, it will not open far enough or long enough to let all the raw water escape via the exhaust system.

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Thanks! I ordered a pressure tester and begun researching how to pressure test the cooling system properly. I'll post the results this weekend

Ayuh,..... We're in the Omc forum, so I assume this is an Omc powered barge,....??......

If so,..... Listen to louc,.... Don't walk, Run away from it,....

Omc has been outa business for decades,.... parts are scarce, or impossible to buy, 'n Nobody will work on it, if yer bud gets in over his head,....

Somehow I messed up the pressure test I think.. there are 4 hoses connected to the thermostat. The transom hose to the outdrive connects on the left. I removed that hose from the thermostat and hooked up my pressure tester to the left port on the thermostat. The hose from the intake manifold comes to the top of the thermostat. I removed that hose and plugged the top of the thermostat. The next hose goes to the block, I left it connected. The last hose goes to the cylinder head and I left that conmected as well. I tried to pressurize the system and wouldn't hold Zippo. I inspected the block while pumping air in and couldn't identify any particular spot but to me it sounded like there I could hear air moving through the exhaust manifold. My buddy says it sounded like air was moving closer to the outdrive.

I'm wondering if the thermostat maybe stuck open. We had been running the engine prior but it had cooled down to 110 according to the gauge.

I'm feeling pretty frustrated cuz I thought going into it that I had got a good grasp on the system from the diagram but now I feel like I'm not understanding the flow of water at all. Any help is greatly appreciated. To make matters worse, I also planned to check compression which I've done a thousand times on outboards, well couldn't get to the last spark plug with my sockets, the riser is in the way so I needs figure that out and I broke a spark plugs wire when I removed it �� I got it back on and the engine runs fine but Im gonna replace when I go back.

I believe my fittings I used in the pressure test were not adaquate. I mistakingly thought the hoses were 1-1/4" inner diameter and when I got to the boat found out they are 7/8" inner diameter (1-1/4" OD). So the fittings I planned on using wouldn't work. Not having any fittings that were proper I was using plumbers tape and random things from my truck.

After I cooled off, realized this was probably my whole problem I did a trip to Lowe's, bought scrap heater hose (7/8 " inner diameter), size #20 hose clamps, and found some PEX 3/4" plugs. When I got home I tried various automotive adaptors on the heater hose and found that attachment #16 in my kit (Mercedes Benz, A-Klasse W168, Vito) fit very tightly onto the adaptor.



I setup my tester with hose, plugged it and tested it to 35 psi (not actually hooked up to the thermostat, just testing my rig to check the components). Did the same thing with a second length of hose that I plan to use on the upper thermostat port (where the intake manifold hose normally goes). Now I have confidence that my testing equipment will not be at fault. I will probably take a video next time I get to the boat to test it. That way if I fail hopefully someone can tell me what I'm doing wrong, and if it works hopefully it can be a guide for the next guy.

Update: hooked up as described above to the thermostat, still couldn't hold any pressure. Took the thermostat off and had an "ah-hah moment".. there's a bypass passage in the thermostat that goes to the exhaust on this 2.3 L. So I can't pressure test through the thermostat. Took it off and set up my pressure tester in-line with the two 1.5" hoses coming from the circulating pump and from the head. Held 15 psi for 20 minutes, unfortunately my time was limited so I had to run. Pretty satisfied with the result though. Going to run it on the lake Saturday