Low oil pressure at 4000 RPM ???????
02-07-2012, 12:22 PM
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On the rebuild last spring, the crank was ground .010" and the rods were replaced. The rod side clearance is about .030" - .033". Merc lists .012"- .024".
02-07-2012, 04:31 PM
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Still think there is a different reason here?
I am still not with this idea that rod side clearance is the cause in this case. Even though the rod side clearance is about .006" to .008" wider than I like to see on this application, I don't think this is the issue here. First the oil pressure drop off is not lineal at increasing rpms, but shows up only above 4000rpms. Unless this drop off only occurs at very hot oil temperatures at those rpms I doubt the rod side clearance is the root cause.
I know you posted earlier that the rod bearing clearance was supposed to be at .0028" and thats a good clearance, but have you meaured the actual clearances in the removed tear down engine?
You also mentioned in some of your earlier posts that you are using H-series bearings. Does the crankshaft have wide fillets where H-bearings are required? If the crankshaft has stock crankshaft short fillets ,then the added clearance of H-bearings and wide side clearance on the rods will contribute to more oil pressure loss, but his loss should show up a a wider rpm range.
I still think there may be another problem here like pump inlet or outlet volume restricitons, oil windage? oil foaming? defective oil pressure relief valve in the block or oil pump etc., the unusual wear on the distrbutor and cam gear definitly show the pump seems to be under excessive pumping restriction.
Going back to earlier questions. did you ever change out the filter before you pulled the engine and see if the problem stopped?
Does the other matching engine and block have the oil relief valve installed up inside the adaptor spin off for the oil adaptor thermostatassembly or is it out matching this engine?
Have you checked the Merc thermostatic adaptor for proper operation as you mentioned the thermostat spool valve appeared cocked in its bore?
The fact you state that it still takes 2-3 minutes for the oil pressure to return to 55 after dropping rpms under 3500 rpms, leads me to think this condition is still superheating the oil or the restiction on either inlet or outlet is taking some serious time to reopen.
Something only appears to be happening at a specific higher rpm rpm and not apparently alone from oil temperature or clearences unless we are not yet getting all the actual conditions inside this engine.
I am still stumped a bit here as everone contrbuting here, but there is a reason this is happening and there is an answer. Hopefully we will all learn something here!
Best Regards,
Ray @ Raylar
I know you posted earlier that the rod bearing clearance was supposed to be at .0028" and thats a good clearance, but have you meaured the actual clearances in the removed tear down engine?
You also mentioned in some of your earlier posts that you are using H-series bearings. Does the crankshaft have wide fillets where H-bearings are required? If the crankshaft has stock crankshaft short fillets ,then the added clearance of H-bearings and wide side clearance on the rods will contribute to more oil pressure loss, but his loss should show up a a wider rpm range.
I still think there may be another problem here like pump inlet or outlet volume restricitons, oil windage? oil foaming? defective oil pressure relief valve in the block or oil pump etc., the unusual wear on the distrbutor and cam gear definitly show the pump seems to be under excessive pumping restriction.
Going back to earlier questions. did you ever change out the filter before you pulled the engine and see if the problem stopped?
Does the other matching engine and block have the oil relief valve installed up inside the adaptor spin off for the oil adaptor thermostatassembly or is it out matching this engine?
Have you checked the Merc thermostatic adaptor for proper operation as you mentioned the thermostat spool valve appeared cocked in its bore?
The fact you state that it still takes 2-3 minutes for the oil pressure to return to 55 after dropping rpms under 3500 rpms, leads me to think this condition is still superheating the oil or the restiction on either inlet or outlet is taking some serious time to reopen.
Something only appears to be happening at a specific higher rpm rpm and not apparently alone from oil temperature or clearences unless we are not yet getting all the actual conditions inside this engine.
I am still stumped a bit here as everone contrbuting here, but there is a reason this is happening and there is an answer. Hopefully we will all learn something here!
Best Regards,
Ray @ Raylar
Last edited by Raylar; 02-07-2012 at 05:11 PM.
