How many 911 oil pump failures?

Jim Sims · 01-16-2002, 10:22 PM

The topic of 911 engine oil pump failures comes up occasionally on this board especially in regards to possible failures if they are pumping against too much resistance in an external oil cooler circuit (lines and an oil to air heat exchanger). What do you know about these failures aside from the mention in BA's book? Now I don't mean failures from "junk" from broken engine parts being run through the pump but failures due to working against too much head or shock loading (from a slug of cold oil?). Does anyone know of these type of failures actually happening? Is it a sudden failure or a gradual degradation? What fails in the pump? Worn bushings? Ruptured pump rotor housing? Blown seals to the case? Sheared drive from the layshaft? Excessively worn aluminum gear on layshaft? Or is there just an overall reduction in oil flow and pressure with general negative consequences to the engine with features like cam lobes suffering first? Does anyone have any information or examples? Thanks, Jim Sims

Ted Stringer · 01-17-2002, 05:41 AM

The pump was enlarged on the 84 and later with the update to pressure fed tensioners. That said the early (SC) pumps seem to handle the updated tensioners and oil coolers.

The thing to remember it the oil pump is acutally 2 pumps. One is a pressure pump that feeds the oil into the engine. The other pump is the scavenge pump that bears the load of moving the oil to the external cooler, throught the filter and back into the oil tank. Yes they share a common drive, but timing chains are driven off of the shaft that drives the oil pump

Jim Sims · 01-17-2002, 08:22 AM

I understand that there've been a couple of revisions to these pumps. I seem to recall the first being (in 1976?) to the pumping volume ratio between the scavenge/pressure sections and then the later change in which total pumping volume was increased (the 1984 change?). Even though there are two pumps (really only two pumping chambers/gear sets) they are timed together and one's speed has to match the other. Since there's no mechanical slip element like a clutch or viscous coupling (there are the oil system pressure relief pistons/springs though) in the drive the pumps really can't slow down under high load. Again what fails or wears? Does a high back pressure in the scavenge side slow down scavenging and cause the oil tank to run low? It appears to me that the pressure relief valve in the external oil cooler thermostat housing would just open and send the scavenged oil back to the tank instead of going through the external oil cooler circuit first. A loss of the extra cooling and perhaps the extra load (and wear) on the pump and its drive train by operating against the relief valve pressure would be the effect but not a immediate pump failure. Comments?

Jim