Causes of Reduced Oil Pressure

Author Message   Al Advanced Member Username: knuckle47 Post Number: 694 Registered: 09-2007 Posted on Tuesday, October 06, 2009 - 10:03 pm:    I realize as an engine ages with use, things happen. What might I begin to consider as reasons for the lower oil pressure. These are C454 1992. 720 hours ....starboard side is a rock steady 45 at any rpm. Starting and initial pressure is 45 on the gauge (25 yrs old) but reasonably confirmed and has a new sender also. I know about using a mechanical test but I am still believing there is a drop whether it is exact or not ...it is still a drop. After about 45 minutes at 2000-3000 she may drop to 25-30as it heats up. If running under 2000 after warm up, it may recover to 40. It appears the higher RPM run time 30 minutes or more will see the drop. I don't think it is the oil.. Straight 40W Pennzoil. What might be the 5 top causes considering the recovery in pressure again at lower RPM? Again...Thanks   diver dave Advanced Member Username: diver_dave Post Number: 452 Registered: 08-2008 Posted on Wednesday, October 07, 2009 - 10:53 am:    Oils do thin with temp, so an engine with large bearing clearances could show reduced pressure when hot. It means the bypass is now zero, and all the thin(er) oil is simply being pumped thru the bearings, and not as much up top. If you said this happens after 20 hours on the oil, I'd say it's clogging the filter with soot, or some contaminant. It's definately not the oil you are running. As an experiment though, try a 50 weight oil. If it results in a higher running pressure hot, that proves my theory. Or, run a 30 weight and see even more of a drop HOT. One more thing. If you read that site i linked a while ago with the ferrari club, some folkst there expect to run a thicker oil as the clearances increase with increasing age. They may start with 20 and progress to 30 then 50, even to 60 if racing. Second note: I'm not saying you have high bearing clearances, necessarily. Another fault can be the oil pump itself. Cover to gear clearance can be high, for instance. So, the pump volume can be down, especially hot.   makomark Senior Member Username: makomark Post Number: 1617 Registered: 09-2008 Posted on Wednesday, October 07, 2009 - 11:19 am:    The pressure drop when things are 'warmed up' is pretty common. As long as the pressure delivered at cruise RPM was spec compliant, I'd be inclined not to worry about it as long as it is consistent. You could also swap the leads at the gauges as a quick check that it is the NOT specific to a gauge. (you can rig up a DPDT switch to permit easy changes for use during a test run.) The mechanical gauge may be a PITA but is still a good idea just to keep the elctrical one honest.   Al Advanced Member Username: knuckle47 Post Number: 700 Registered: 09-2007 Posted on Wednesday, October 07, 2009 - 02:25 pm:    All good ideas and I will be sure to check them all, next time out. The only question I would like to solve is: if the other engine is 45 psi any time of the engine's operation, IS THE Starbord engine trying to tell me something I should be repairing as the winter down time approaches and avoid a sudden surprise in April or May?   makomark Senior Member Username: makomark Post Number: 1622 Registered: 09-2008 Posted on Wednesday, October 07, 2009 - 04:54 pm:    If the drop is real, maybe.... have the oil analyses' trends been consistent?   Al Advanced Member Username: knuckle47 Post Number: 702 Registered: 09-2007 Posted on Wednesday, October 07, 2009 - 10:38 pm:    So far yes. My last oil analysis was not significantly different from the one about the end of LAST season (November 2008). One problem is the gauges on the lower helm are as off as the tachs. I have 4 tachs and they ALL have different readings. I can only assume the same for the other gauges. Although the 4 voltmeters work well. I had seen the 4 in 1 gauges on a new Silverton at a boat show 2 years ago but they don't appeal to me and seem harder to read except for extreme swings since they are so small. I can guess that my oil pressure is about 42 when the needle is on a position by extrapolation. On the tiny gauges...seems harder. The problem with that 42 guess is...WHO KNOWS IF THE GAUGE IS ACCURATE. Time for a complete dash makeover.   Paul White Member Username: missnancy Post Number: 100 Registered: 03-2009 Posted on Thursday, October 08, 2009 - 06:52 am:    Al, When I rebuilt the engine for my boat I installed a mechanical oil pressure gauge. It is down in the engine room but I can open the hatch and look right down on it to see what reading it has.

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