Mercruiser 7.4 454 CI loss of oil pressure

Stee6043 said:
I don't know if this was mentioned but I have been told previously that too much oil can reduce oil pressure. The story goes that you can get foaming in the oil pan preventing proper pickup.

My 7.4's hold 7 quarts of oil. Perhaps double check you're supposed to have 8 quarts before you start adding more?
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It can but if we stick with Mike's story......it did fine on the sea trial. So unless someone added a lot of oil to it between the sea trail and when he started experiencing the problem my money is that the pump is running out of oil. A mismatched pan and pump pickup fits the symptoms.
 
Stee6043 said:
I don't know if this was mentioned but I have been told previously that too much oil can reduce oil pressure. The story goes that you can get foaming in the oil pan preventing proper pickup.

My 7.4's hold 7 quarts of oil. Perhaps double check you're supposed to have 8 quarts before you start adding more?
Click to expand...
Thanks. I did check, and my engines require 8 quarts. I will try and add another quart to the port engine to see if the oil pressure will stabilize.
 
Mike,

One sure way to tell is if when you pulled the throttles back to idle.....the oil pressure returned to it's normal range. If it did that, it was running out of oil and the pump was cavitating at the higher rpms causing the alarm to sound. It would take about 15 seconds or so at idle to stabilize the pressure again.

I'm optimistic unless you tell me that when you pulled the throttle back the pressure did not re-stabilize.
 
PlayDate said:
Mike,

One sure way to tell is if when you pulled the throttles back to idle.....the oil pressure returned to it's normal range. If it did that, it was running out of oil and the pump was cavitating at the higher rpms causing the alarm to sound. It would take about 15 seconds or so at idle to stabilize the pressure again.

I'm optimistic unless you tell me that when you pulled the throttle back the pressure did not re-stabilize.
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You are correct, when I throttle back to idle the oil pressure goes up pretty quickly and stays between 20 and 40 psi.. I was concerned that it might be a bearing seal leaking but I don't see any smoke or oil level loss.
 
Mike,

Once upon a time I worked on race cars before boats. The SCCA event cars would easily wind 7-8K and run out of oil in the turns. Oil pressure was great until the G forces pushed the oil away from the pickup. It was scary to watch because if it didn't quickly recover....things went bad. We did all manor of tricks, bigger pans, windage trays, oil gates in pan and in the end only a dry sump system (not legal in all Classes) kept the oil in the pump.

So.....running out of oil seems like the most likely cause since the sea trial went well. Fingers crossed it is that simple.

If that is the case, the next problem is determining the nominal oil level for engine. While "adding another quart" solves the short term issue. Putting in too much (like 2 quarts) will create another set of issues.

A couple of things to check:

1) Do both dipsticks match in length?
2) Do the oil pans differ in dimensions between the engines?

Generally the problem is a racing pan is deeper and holds more oil. You need a matching pickup on the pump to reach deeper into the oil. A standard pickup would cavitate.
 
Mike what are your engine serial numbers ?
 
PlayDate said:
Mike,

Once upon a time I worked on race cars before boats. The SCCA event cars would easily wind 7-8K and run out of oil in the turns. Oil pressure was great until the G forces pushed the oil away from the pickup. It was scary to watch because if it didn't quickly recover....things went bad. We did all manor of tricks, bigger pans, windage trays, oil gates in pan and in the end only a dry sump system (not legal in all Classes) kept the oil in the pump.

So.....running out of oil seems like the most likely cause since the sea trial went well. Fingers crossed it is that simple.

If that is the case, the next problem is determining the nominal oil level for engine. While "adding another quart" solves the short term issue. Putting in too much (like 2 quarts) will create another set of issues.

A couple of things to check:

1) Do both dipsticks match in length?
2) Do the oil pans differ in dimensions between the engines?

Generally the problem is a racing pan is deeper and holds more oil. You need a matching pickup on the pump to reach deeper into the oil. A standard pickup would cavitate.
Click to expand...
Thanks for this advice, I will check the dip sticks to see if they match. The pans do match.
 
joeyleggz said:
Mike what are your engine serial numbers ?
Click to expand...

my mechanic and I looked all over the engines and can’t find a serial number anywhere. I’ve never had an engine without a serial number.
 
Mike Siner said:
my mechanic and I looked all over the engines and can’t find a serial number anywhere. I’ve never had an engine without a serial number.
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Mine doesn't have a serial number either. I'm sure it's a replacement engine because of that. This is what was where mine should have been.

IMG_20200919_114134.jpg
 
If your looking for serial number for parts ordering you can just use a generic number if you know what gen your engine is. Easiest way to determine this by looking at your motor.
Gen 4 will have provision for mechanical fuel pump
Gen 5 has a 10 bolt steel timing cover
Gen 6 has a 6 bolt plastic timing cover.
 
Mike Siner said:
my mechanic and I looked all over the engines and can’t find a serial number anywhere. I’ve never had an engine without a serial number.
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Very odd story Mike would be nice to identify what motors you have by serial number so you can see exactly what you have 1992 is a strange year a lot of different versions of the 7.4 taking 7 quarts or 8 quarts etc. oil cooler no oil cooler etc. Did you put a mechanical gauge on the other motor to verify what oil pressure it’s putting out? The serial numbers from my motor in 1997 gen six 7.4 is marked in two places on the Cowl and on The back of my motor starboard side ID plate .
D71D3ECD-0AD1-4304-9C8C-C055AC76FBA1.png
CB76745D-832E-45FE-A7F2-52A763AF5647.jpeg
I’ll post a few pics for you to see
 
joeyleggz said:
Very odd story Mike would be nice to identify what motors you have by serial number so you can see exactly what you have 1992 is a strange year a lot of different versions of the 7.4 taking 7 quarts or 8 quarts etc. oil cooler no oil cooler etc. Did you put a mechanical gauge on the other motor to verify what oil pressure it’s putting out? The serial numbers from my motor in 1997 gen six 7.4 is marked in two places on the Cowl and on The back of my motor starboard side ID plate . View attachment 97278 View attachment 97279 I’ll post a few pics for you to see
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I have the same motor as you. I have the serial number on the flame arrestor cover, but you can see what I have in the pic above where the serial number should be on the engine. I use my cover number for anything I need the serial number for. Block is actually a 98 Gen VI 454. Same exact motor as the 97 or possibly 96 cast that would have come in my early 290DA.
 
If the oil pans are not original......there is a very good chance the engines are not as well. When I worked on go-boats .....there was no such thing as a marine engine (other than specific components). A GM big block is a GM big block. GM heads are GM heads.

Not being original explains the oil pan and lack of an oil cooler. Nobody pulls an engine to just replace an oil pan.
 
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joeyleggz said:
Very odd story Mike would be nice to identify what motors you have by serial number so you can see exactly what you have 1992 is a strange year a lot of different versions of the 7.4 taking 7 quarts or 8 quarts etc. oil cooler no oil cooler etc. Did you put a mechanical gauge on the other motor to verify what oil pressure it’s putting out? The serial numbers from my motor in 1997 gen six 7.4 is marked in two places on the Cowl and on The back of my motor starboard side ID plate . View attachment 97278 View attachment 97279 I’ll post a few pics for you to see
Click to expand...
I did have a mechanical gage use at the block to check the oil pressure and posted those pictures in an earlier post. This Friday I’m taking it out come hell or high water...I’m going to add a quart to be safe.
 
PlayDate said:
If the oil pans are not original......there is a very good chance the engines are not as well. When I worked on go-boats .....there was no such thing as a marine engine (other than specific components). A GM big block is a GM big block. GM heads are GM heads.

Not being original explains the oil pan and lack of an oil cooler. Nobody pulls an engine to just replace an oil pan.
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That is my thoughts as well, I am working with the broker to see if he can put me in contact with the last owner.
 
Mike Siner said:
I did have a mechanical gage use at the block to check the oil pressure and posted those pictures in an earlier post. This Friday I’m taking it out come hell or high water...I’m going to add a quart to be safe.
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A bottle of Jack Daniel's says that it runs with no issues by adding another quart. Enjoy the boat. If that takes care of it.....don't worry where the engines came from. My guess is that they have a few more ponies than the original engines did.

One other thought....take along a few quarts with you. The other engine may experience the same issue since you changed the oil and potentially how much was actually in it.

Milodon racing marine oil pans are 10 quart and Summit's are 11 quarts so if it is not original it could be 9+ quarts. A picture would help.
 
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Mike Siner said:
I did have a mechanical gage use at the block to check the oil pressure and posted those pictures in an earlier post. This Friday I’m taking it out come hell or high water...I’m going to add a quart to be safe.
Click to expand...
I meant to check the other motor with the mechanical gauge just to be sure what that motor is reading is true . If booth motors have the same amount of oil and same oil pans , the oil theory shouldn’t be the issue
 
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When we had the oil cooler fail and drain oil out the oil pressure dropped and it took 3 quarts to top it up. For an engine to be low on oil it has to be very low.
 
joeyleggz said:
I meant to check the other motor with the mechanical gauge just to be sure what that motor is reading is true . If booth motors have the same amount of oil and same oil pans , the oil theory shouldn’t be the issue
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True but conditions have changed. The sea trail went well. Subsequent use surfaced the low oil pressure issue. Early CSR posters suggested an oil change which Mike did on both engines. If the pans are aftermarket marine racing pans, I can guarantee they hold more than 8 quarts. If for example they are 10 quart pans.....they are now 2 quarts low on both engines. If they are 11 quart pans, they are 3 quarts low on both engines. If the oil pump pickup is not matched.......that can make the cavitation problem worse.

An old boat racing trick was to use higher capacity pans and ditch the oil cooler.

Milodon 10 quart
mil-31557_vk_ml.jpg

Summit 11 quart

SUM-G3607_ml.jpg


Champ Pan 12 quart

chp-cp214lt_ls_ml.jpg


If the oil pans are not larger marine racing pans.....then we are back to square one.
 
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PlayDate said:
True but conditions have changed. The sea trail went well. Subsequent use surfaced the low oil pressure issue. Early CSR posters suggested an oil change which Mike did on both engines. If the pans are aftermarket marine racing pans, I can guarantee they hold more than 8 quarts. If for example they are 10 quart pans.....they are now 2 quarts low on both engines. If they are 11 quart pans, they are 3 quarts low on both engines. If the oil pump pickup is not matched.......that can make the cavitation problem worse.

An old boat racing trick was to use higher capacity pans and ditch the oil cooler.

Milodon 10 quart
mil-31557_vk_ml.jpg

Summit 11 quart

SUM-G3607_ml.jpg


Champ Pan 12 quart

chp-cp214lt_ls_ml.jpg
Click to expand...
why would we be chasing low oil volume if both engines have the same amount of oil in them both engines have the same oil pan ,in that theory both motors should be reading low oil pressure, no?
If the starboard motor is running good I would Verify that with the mechanical gauge
Proving that the starboard motor, gauges and oil volume are functioning properly and eliminate any possibility of faulty equipment.
70 pounds of oil pressure at the sea trial would indicate a high volume oil pump and the oil pan being larger would be standard with that set up . I get that 8 quarts may not be enough with that set up but if one motor is running perfectly fine then I’d be looking elsewhere. Sounds like something let go ,oil pump pickup tube? That would be rare though . I would be getting in touch with the previous owner and investigating as much as you can about what was done with those motors and when it was done etc. If adding more oil to that motor works I would be very confused considering everything I said above but hey you never know
 
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