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Old 07-11-2020, 02:30 PM   #1
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Default Oil Pressiure Problem....

I replace the sending unit, gage.
After the engine got up to temperature (165-170) the oil pressure is ok up to 1300-1400 RPM. From there up the gage goes crazy up and down. Do I need to replace the oil pump? Does the relief valve open? Is any trick not to take the engine out?
Mike, I would appreciate your input here. Or anybody who has any suggestions. Thanks.
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Old 07-11-2020, 05:03 PM   #2
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Hey buddy, been awhile since your last post. Since you already tried replacing the sender I would go to the auto parts store and get a manual oil pressure gauge and install it to see what itís reads. The oil pump does have a pressure release as well as the oil filter fitting on the engine block. If the manual pump confirms the pressure is really low I would first replace the relief valve at the filter location since this is fairly easy to do.
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Old 07-11-2020, 07:21 PM   #3
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I will buy that manual oil pressure gauge. That will go where is the sending unit is?
The filter fitting in the engine block has one too? Where? You have to take the filter off and is where the filter get screw in?
Thanks Mike
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Old 07-11-2020, 10:06 PM   #4
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I got the manual oil pressure gauge. Not instal yet because don't know where...
I take the oil filter off. Don't see anything there like any relief valve...Make some pictures and post them. The last two are the manual oil pressure gauge. The 3rd one , you can see the metal become rubber and metal again and screw in the engine block.
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Old 07-12-2020, 12:10 AM   #5
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You have a remote oil filter, if you follow the two hoses they connect to the lower port side of the engine which is the normal GM filter location. I would install the manual gauge fitting in place of the helm gauge sender on the back of the engine near the distributor. Under Documents of this site look for Mercruiser manual #24 to get an idea of the filter adapter with relief valve.

I find it concerning that there isnít any oil at the remote filter mount with the filter removed.
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Old 07-12-2020, 03:36 AM   #6
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That make sense with the remotr filter...
After I remove the filter was oil there but I clean everything so I can see what is going on...Lol I'm a clean freak.
Thanks Mike.
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Old 07-12-2020, 09:54 AM   #7
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I was thinking... if I install the manual gage where the sending unit is, all I check is the wire from the sending unit to the board... I have a new sending unit and a new gage. Should I replace first the filter adapter with a relief valve and see if that makes any change? I have the old gage, I tested and work great...
Thanks for your time Mike.
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Old 07-12-2020, 11:30 AM   #8
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Please explain what you mean about having an old gauge that you tested and works great. Is this an electric oil pressure gauge? If so you can wire it back at the engine to the new oil pressure sender you installed to see what it reads.
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Old 07-12-2020, 11:39 AM   #9
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Yes, is the old electric oil pressure gauge what I just replace.
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Old 07-12-2020, 02:54 PM   #10
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Ok, I did what you suggested and again, low RPM is OK once I go over 1400 start to go crazy.
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Old 07-12-2020, 04:06 PM   #11
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What psi are you getting under 1400 rpm? What range psi is it swinging between above 1400 rpm?

Iím assuming you verified the oil level on the dip stick is correct, and signs of a oil leak? A leaking crank seal will cause low oil pressure as will worn main bearings, been there.
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Old 07-12-2020, 05:17 PM   #12
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No oil leak.
The oil level is 1/16 higher than should be.
Under 1400rpm the oil pressure is 47-48
Above is swinging from 48 to 10 but sometimes just stay on the 7-10 for like 4-5 sec. and start swinging again. That's what I don't like when just stay down there on the lower 7-10.
The engine has 352 hours. That why I'm surprised by this low oil pressure. Have to be something goof. This start after I change the oil. From that time I change the oil filter two more times just in case it is something wrong with the filters.
When I got the boat and change the oil first time, the oil was black. I don't know what oil was in the engine. But now I have 25W-40 4T Marine Synthetic Blend MARINE Engine Oil NMMA FC-W.
I'm trying to give you all the information I have.
Thanks for your time Mike.
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Old 07-12-2020, 08:11 PM   #13
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Checking oil pressure with the manual gauge is definitely next step here. Also see the test procedures for the gauges and oil pressure sender in section 4D of the Merc manual. From my own experience, I had some erratic gauge readings right before my ignition switch failed.
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Old 07-12-2020, 11:39 PM   #14
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Quote:
Originally Posted by Gugi47 View Post
No oil leak.
The oil level is 1/16 higher than should be.
Under 1400rpm the oil pressure is 47-48
Above is swinging from 48 to 10 but sometimes just stay on the 7-10 for like 4-5 sec. and start swinging again. That's what I don't like when just stay down there on the lower 7-10.
The engine has 352 hours. That why I'm surprised by this low oil pressure. Have to be something goof. This start after I change the oil. From that time I change the oil filter two more times just in case it is something wrong with the filters.
When I got the boat and change the oil first time, the oil was black. I don't know what oil was in the engine. But now I have 25W-40 4T Marine Synthetic Blend MARINE Engine Oil NMMA FC-W.
I'm trying to give you all the information I have.
Thanks for your time Mike.

Good information! So both the electric and manual oil pressure gauges provided the same results? When I bought my 1997 it only had 163 hours on the 5.7 and in 2016 at 540 hours I lost oil pressure (5-10 psi) and the main bearings were knocking. Best I concluded was the previous owner didn’t chasing the oil often, which will become acidic from blow by gasses. The result was the bearing surfaces get eaten away. Not saying this is you issue just saying low hours doesn’t mean trouble free. I did a quick read and I was wrong the oil filter adapter on the block has a bypass valve Incase the filter becomes clogged not a pressure relief valve. The oil pressure relief valve is on the oil pump. If the manual gauge is providing the same reading the next step would be replace the oil pump. If you do you may want to remove a main cap to inspect the crank bearing for wear since the oil pan would be off. Else run it for the rest of the season at risk and pull the engine in the fall.
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Old 07-13-2020, 12:26 AM   #15
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The good news is, my engine is nice and quiet.
Clean it very well. Take me days to clean the engine compartment and the outside of the engine.
I buy it in March 2018. I replace the oil 4 times. So the oil right now is nice and clean. This year I have 14 hours in it.
I order the filter adapter with a relief valve. I will have to remove a bunch of staff to change that baby. Not much space there and I'm 67....LOL. But I'm going for it.
Thanks Mike. I let you know.
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