Mercruiser 7.4 454 CI loss of oil pressure

[Mike Siner]

I just bought a 1992 Sea Ray 350 Express Bridge with twin 454 Mercruisers 7.4L. On the sea trial, both engines ran great with oil pressure in 70 psi range; after I bought the boat, I started it up to take it out, and the port engine low oil pressure alarm went off and the gage showed 5psi. I checked the sending unit, and it pegs out when grounded; the oil level is perfect. I can hear the valves clacking and don't see oil flowing in the valve covers. I'm not sure the last time the oil was changed, but there are Napa filters on both engines, oil is a little dark but clean looking. I found a gallon of 10w-40 on the boat but I'm not sure if that is for the gen or was used in the engines. Is there any reason the oil would pressure would drop beside a bad oil pump?

 

[scoflaw]

Start with changing the oil and filter, I like 20-50 VR1 and Napa gold filters

 

[Mike Siner]

Thanks scoflaw. I heard that the filter needs to be Quicksilver Mercury brand?

 

[Snapon24]

I run 15W40 Non-ULS diesel oil and Quiksilver filters. The higher Zinc content in the oil is better for the flat tappet cam in your engine. As for the sudden low oil pressure, I would check it with a mechanical gauge first to verify it is in fact low.

 

[scoflaw]

I like the Napa/Wix 1060

 

[Espos4]

Grounding the wire and seeing the gauge peg only tests the gauge and circuit from the sending unit wire to the gauge. It does not confirm that the sending unit is working properly.

Valve train noise is not a good sign.

You could swap sending units between motors or use a mechanical gauge in place of the sending unit to test the oil pressure.

 

[Mike Siner]

Update: I flushed the engine and replaced the filter and oil, went with AMSOIL. The pressure is now at 40 psi, but when I run the RPMs up to 2500, the pressure drops to 20 psi or a little less. The other engine goes from 40 to 60 psi when the RPMs run-up. There is no smoke, so I'm lost if it's the oil pump or main seal? The pressure was measured with a mechanical gang at the engine. Any suggestions would be appreciated, the cost to change the oil pump is around 3K here in Florida so I don't what to fix the wrong problem.

 

[northern]

My service manual says to run strait 40 weight oil. My engines have 3200 hors on them they do not use oil and oil pressure at gauges are 40 PSI on one engine and 50 on the other. An automotive trick I used once was to add a quart of ATF fluid to a car that had sticking valve lifters. If you oil pump is dirty or the screen is plugged this might help. ASK YOUR MECHANIC FIRST.

 

[scoflaw]

More than likely with an engine of that age I would suspect bearing clearances are out of spec.

 

[Steve OD]

Thanks scoflaw. I heard that the filter needs to be Quicksilver Mercury brand?

UGH!
I had the same thing happen, but lost all my oil in a matter of seconds. It turns out the adapter for the remote oil filter came loose. I immediately shut down the engine, but it was too late, I am now in the process of getting a new long block and rebuilding that engine! When the mechanic broke the engine down, he found some other issues that were problematic as well.
In retrospect, that engine was always down oil, but not much, which I considered an old engine burning oil issue. I did have some oil stains in the bilge, but they were there when I bought the boat so I didn't think much of it.
Lesson learned I guess.
Hopefully thats not your isssue, but one more thing to check out

 

[Bt Doctur]

low oil pressure can also be caused by gas intrusion into the oil,

 

[northern]

We had an oil cooler fail. The oil was forced out of the cooler and oil pressure went to 10PSI. I topped up and pressure went to 40 PSI. We were 30 miles from nearest port in 3 to 4 foot waves. Engine had 1,200 hours on it at the time. We were running at 2,000 RPM. Oil pressure dropped to 10 PSI twice more and I added oil. As soon as the pressure dropped each time I shut engine down and added oil. When we got to port I shut engine down and it sucked salt water into the oil. I changed oil 3 times and replaced the oil heat exchanger. After the first oil change I did a compression test on warm engine. All cylenders were within 5%. I do not remember the exact numbers. Engine still works well has 3,200 hours on it, does not use oil, oil pressure is 40 PSI on gauge. Check all the small things first before you look at replacing an engine.

 

[Mike Siner]

Thanks, everyone, for the replies. If I can find a mechanic who can change the oil pump while the boat is still in the water, I may do that first. I don't hear any bearing noise and am hoping like hell it's not bearings. I just want to be able to take the boat out safely and not damage the motor.

 

[PlayDate]

Thanks, everyone, for the replies. If I can find a mechanic who can change the oil pump while the boat is still in the water, I may do that first. I don't hear any bearing noise and am hoping like hell it's not bearings. I just want to be able to take the boat out safely and not damage the motor.

Mike,

Welcome to CSR!

How many hours are on each engine and are both original?

I don't think it is possible to change the oil pump on your boat without lifting the engine which then just makes the engine "disposable"......use it until it quits.

As others have suggested.....the problem of low oil pressure is most likely not the pump. A few more diagnostics would help. Put a mechanical oil pressure gauge on it and then switch to a straight 40 or 50w to see what the differences are.

 

[Mike Siner]

Mike,

Welcome to CSR!

How many hours are on each engine and are both original?

I don't think it is possible to change the oil pump on your boat without lifting the engine which then just makes the engine "disposable"......use it until it quits.

As others have suggested.....the problem of low oil pressure is most likely not the pump. A few more diagnostics would help. Put a mechanical oil pressure gauge on it and then switch to a straight 40 or 50w to see what the differences are.

The mechanical gage shows 60 psi at startup, then it drops to 40 psi once the engine warms up. When I throttle to 2500 rpm's it drops to just below 30 psi. The oil is AMSOIL 40 w. I don't think the engines are original, but they have to be from the mid-90s. I bought this boat used a few months back, everything engine wise worked fine, I was getting ready to take it out on my first trip and that's when the oil pressure went to zero, after flushing and changing the oil the pressure has come up but not stable.
Engine hours show 1100+ or - on each. No smoke at startup or when running.

 

[Mike Siner]

We had an oil cooler fail. The oil was forced out of the cooler and oil pressure went to 10PSI. I topped up and pressure went to 40 PSI. We were 30 miles from nearest port in 3 to 4 foot waves. Engine had 1,200 hours on it at the time. We were running at 2,000 RPM. Oil pressure dropped to 10 PSI twice more and I added oil. As soon as the pressure dropped each time I shut engine down and added oil. When we got to port I shut engine down and it sucked salt water into the oil. I changed oil 3 times and replaced the oil heat exchanger. After the first oil change I did a compression test on warm engine. All cylenders were within 5%. I do not remember the exact numbers. Engine still works well has 3,200 hours on it, does not use oil, oil pressure is 40 PSI on gauge. Check all the small things first before you look at replacing an engine.

The motors I have don't have oil coolers so I'm limited to where the problem could be. I know the 454s are tough engines and didn't want to mess it up anymore. As you can see in my post below yours I've flushed and changed the oil to 40 w and the pressure came up but it's not stable. I'm glad to see you are able to get so many hours out of yours...leaves me hopeful.

 

[PlayDate]

Mike,

I'm surprised you don't have an oil cooler mounted on the back of your engine. That is a common failure point that fits some of the symptoms.

It sounds like the pump is running out of oil or dramatically losing pressure as the rpms increase. The oil pump is mechanical and really has only a few components: 1) Pickup 2) the pump gears 3) a pressure spring.

Something blocking the oil pickup causing a restriction that doesn't show up until you reach a higher rpm is a potential cause. Also running out of oil can cause the same symptoms. You didn't mention how much oil you put in the engine or how much came out.

I have had several clients over the years where it turned out that their dipstick had been replaced or the new engine had a different oil pan both of which caused the engine to run out of oil as the rpms advanced. In one case the pickup came loose and the oil pump cavitated at higher rpms since the pump opening is higher than the pickup. To eliminate the potential of those situations....add another quart of oil to the engine to see if the behavior changes.

I'm optimistic because it ran fine in the sea trial.

 

[northern]

The motors I have don't have oil coolers so I'm limited to where the problem could be. I know the 454s are tough engines and didn't want to mess it up anymore. As you can see in my post below yours I've flushed and changed the oil to 40 w and the pressure came up but it's not stable. I'm glad to see you are able to get so many hours out of yours...leaves me hopeful.

When the oil cooler failed I was not able to get it replaced until I got to my home port 80 miles away. To run the engine I put in a piece of pipe to replace the oil cooler and then looped the oil cooler lines so they were a closed loop so they would not leak. The oil was no longer cooled. The engine ran at 30 to 40 PSI at 1000 to 1,200RPM. If I went to 1,800 RPM oil pressure dropped to 20 PSI. Seemed hot oil is very thin.

 

[Mike Siner]

Mike,

I'm surprised you don't have an oil cooler mounted on the back of your engine. That is a common failure point that fits some of the symptoms.

It sounds like the pump is running out of oil or dramatically losing pressure as the rpms increase. The oil pump is mechanical and really has only a few components: 1) Pickup 2) the pump gears 3) a pressure spring.

Something blocking the oil pickup causing a restriction that doesn't show up until you reach a higher rpm is a potential cause. Also running out of oil can cause the same symptoms. You didn't mention how much oil you put in the engine or how much came out.

I have had several clients over the years where it turned out that their dipstick had been replaced or the new engine had a different oil pan both of which caused the engine to run out of oil as the rpms advanced. In one case the pickup came loose and the oil pump cavitated at higher rpms since the pump opening is higher than the pickup. To eliminate the potential of those situations....add another quart of oil to the engine to see if the behavior changes.

I'm optimistic because it ran fine in the sea trial.

Each engine has 8 quarts of oil. I will try adding more to the port engine and see how it responds. I do believe the oil pan is not the marine pan but a racing pan, it is bigger and I think it is larger toward the aft of the boat / front of the engine.

 

[Stee6043]

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?