Low Oil Pressure at Idle - 5.0 EFI

F

FiremistSLK

Active Member
Sep 10, 2020
106
Central Florida USA
Boat Info
2007 Chaparral Sunesta 214 Mercruiser 5.0L MPI Alpha 1
Engines
5.0L MPI 260hp w/Alpha I
I have the 5.0 EFI in my SR 215. When the engine is running at cruise (3500 RPM), the temp gauge is at 170 and oil pressure is reading about 80 psi. But when I pull the throttle to idle for docking the oil pressure gauge drops to maybe 5 or 10 psi. Is this normal? Could it be an oil viscosity issue? The oil was changed last fall by the PO so I do not know what viscosity was used then.
 
That seems low at idle and even a little high at higher rpm. Mine (4.3 v6) tends to run more around 40 idle and 60 at higher RPMs.

From my manual, this is what Mercruiser says:
Oil Pressure (at 2000 rpm) Minimum 30 psi (207 kPa)
Minimum Oil Pressure (at Idle) Minimum 4 psi (28 kPa)

So I don't think you have an issue, you would be getting an alarm if there was a real problem - the oil viscosity could very well be causing it. A 10w40 for example would definitely change the oil pressure from the recommended 25w40. What kind of oil filter is on it? That is usually a pretty good indication they did not use Mercruiser oil either. Another thing is check the wire connections to the gauge and sender, I loose or corroded connection would change how the gauge reads.
 
Hey thanks Bill.... I never thought to look in my owners manual (duuuh) which confirms what you wrote. Minimum oil press at idle for the 5.0 EFI is 4psi.
 
Agree, 80 is high and 10 is low. Check it with a mechanical gauge. No way I would be happy with 4 lbs.
 
scoflaw said:
Agree, 80 is high and 10 is low. Check it with a mechanical gauge. No way I would be happy with 4 lbs.
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Owners Manual says 4psi for minimum oil pressure. why don't you trust Mercruiser specs? I'm taking her out again today. I'll let you know what I see on the instrument gauge again.
 
The gauges on these boats are horribly inaccurate. I have the Mercruiser SC5000’s which give accurate readings.
 
IT's a disclaimer, Merc also says 100psi cranking compression is good enough.
No gearhead in his right mind would believe this. If you had a motor with 100 psi and 4lbs of oil pressure you have yourself a core motor at best.

So without changing anything, you expect to see something different?
 
GM used the high volume low pressure oil pumps in many applications. Take the diesel pick ups. The 2500 has a standard pump in the base model. High warm pressure is like 80psi. Low at idle is 30psi. Now the HD and 3500 pickups can have the standard or have the HVLP ones. Mine runs cold at 45psi and warm it's at 8-12psi warm idle. Why not have one in a marine application?
 
FiremistSLK said:
Owners Manual says 4psi for minimum oil pressure. why don't you trust Mercruiser specs? I'm taking her out again today. I'll let you know what I see on the instrument gauge again.
Click to expand...
Scoflaw is absolutely right. Anything less than 35 psi or so at idle and you have something wrong. I'd live with 35, but I wouldn't be happy about it. I idle at 60 psi with the motor I just built.

JHornsby3 said:
GM used the high volume low pressure oil pumps in many applications. Take the diesel pick ups. The 2500 has a standard pump in the base model. High warm pressure is like 80psi. Low at idle is 30psi. Now the HD and 3500 pickups can have the standard or have the HVLP ones. Mine runs cold at 45psi and warm it's at 8-12psi warm idle. Why not have one in a marine application?
Click to expand...
Because it just isn't necessary on a proper running and maintained motor. That if course depends on any performance mods.
 
scoflaw said:
IT's a disclaimer, Merc also says 100psi cranking compression is good enough.
No gearhead in his right mind would believe this. If you had a motor with 100 psi and 4lbs of oil pressure you have yourself a core motor at best.

So without changing anything, you expect to see something different?
Click to expand...
Just got in from a beautiful day on the bay. Slight chop but she ran great! At 3000 RPM oil pressure was around the 65/70 psi mark. At idle when hot oil pressure was around 15 psi.
 
Could be the gauge or sender . Maybe wrong oil. He's doing better than he was.
 
scoflaw said:
Could be the gauge or sender . Maybe wrong oil. He's doing better than he was.
Click to expand...
Could be. My point was it's not "fixed." I'd still be diagnosing it. Oil change if an unknown would be first. Mechanical gauge would be second.
 
HawkX66 said:
Scoflaw is absolutely right. Anything less than 35 psi or so at idle and you have something wrong. I'd live with 35, but I wouldn't be happy about it. I idle at 60 psi with the motor I just built.


This sounds like a lot of unnecessary strain on the distributor shaft and cam gear if that's a hot pressure? It would be as much a concern for me as too low.
Click to expand...
 
No. It drops to about 45-50 hot. Sorry. I wasn't clear.
 
My 502 is at 25 hot idle with VR1 20-50. Fresh rebuild 150 hours ago. Volume is more important than pressure. If there's no top end clatter his 15 is probably good enough.
 
scoflaw said:
My 502 is at 25 hot idle with VR1 20-50. Fresh rebuild 150 hours ago. Volume is more important than pressure. If there's no top end clatter his 15 is probably good enough.
Click to expand...
Agreed volume is more important, but dang, 15 psi is pretty rough to me.
 
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