Can't get your Subaru Oil Pump to Prime? What to check and a possible solution.
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Right before you start your engine for the first time, one of the last things that you typically do is crank the engine over to build a little oil pressure. On a Subaru, this normally means cranking the engine over for a couple seconds a few times until the oil pressure light turns off.
But what if that doesn't happen?
In the course of working with Allen from Subie Recycler on his Legacy Spec-B build, this is the problem that Allen ran into, and through the years we have worked with other customers that have run into this issue as well.
The first thing that we looked at were all of the possible mechanical causes for the pump not to build pressure. The oil pan was removed to make sure that the oil pick-up tube was not damaged or cracked, and the o-ring between the pick-up tube and the block was also inspected. Then, the oil pump was removed to inspect the seal at the back where oil goes into the block, and the o-ring that creates that seal as well.
Everything checked out, and at each stage, Allen tried to prime the oiling system again with the same result.
In the end, what we found was a very old article by Outfront Motorsports about an issue that can arise from filling the oil filter with oil and installing it on the block before you try to prime the system. The argument is that if the filter is full of oil, the passage in the block between the filter and the pump is then sealed, so the oil pump would have to create enough pressure to force that air through the filter before it can start pulling oil up from the pan. If the pump can't do that for some reason, then it will not prime.
The test to confirm this was easy. Allen simply removed the oil filter, and cranked the engine over. Very quickly, oil was coming out of the block because the pump was now pulling from the pan and priming. From that point, the filter was put back on, and the pump started making pressure when it was cranked over again.
The take away from this point is that when priming the oiling system, it would be ideal to crank the engine without the filter installed until you see oil coming out, and then to install the filter, to finish priming the engine.
The one other thing of note worth mentioning is that the pump was installed dry, with no oil, lube, or packing put into the rotor to help it work more efficiently. There is a possibility that what ultimately caused the issue where the oil pump would not prime was both filling the filter with oil, and at the same time, not putting any oil or packing in the oil pump to help it work more efficiently.
We may have to do a little more testing on this, but for now, there is a simple change to the best practice of priming the oiling system in your new engine that should help eliminate this issue.