Oil Pump - Loss of Prime After Extended Period of Inactivity


This is a very common problem with the Continental A- and C-series engines. The problem is that the clearances in the oil pump allow the oil to drain back into the sump and the pump cannot re-prime itself. It usually gets worse over time, until the aggravation level warrants a great expenditure of cash to resolve the issue.

Folks over the years have pumped oil into just about every orifice in the engine in attempt to re-prime the pump. Sometimes it works, sometimes not. Pumping in through the oil temperature sensor is a common method, but it shouldn't work. It is common because it works on the non-electric engines found in cubs and such, but on the -12 and other cases, the screen is lower than the pump and the oil will not flow uphill to the pump, hence it should not work.

Here's a simple method I've found works every time and can be done by one person.

Get a piece of plastic or rubber hose that fits snuggly over the crankcase vent tube. If you have a "whistle slot" in the tube, the hose must cover that. Get enough hose so it will reach from the vent line up to the top of the cowl.

Remove the top plugs from the engine. Remove the oil pressure line going to the gauge from the right rear of the engine case.

Position yourself where you can operate the starter lever with one hand and watch the oil pressure port on the engine.

Blow into the plastic tube with your mouth (DO NOT USE SHOP AIR!!!) and operate the starter (mind the whirling meat cleaver at the front of the engine). Watch the pressure port for oil flow. If necessary, crank in 30 second intervals with 2 minute rests between. Oil should flow from the port within the first 2-3 cranking cycles.

Reconnect the gauge and crank again. You should see pressure indicated on the gauge. If you do, put the plugs back and go fly. If you don't, figure out why. (Most likely the gauge is bad.)

There is some anecdotal evidence to indicate that the oil filter adapter somehow aggravates the problem, but no one has come up with an explanation that makes sense to me, so I remain skeptical.

John Cooper


If you have no oil pressure upon starting the engine after the plane has been sitting for an extended period of time, then the oil pump may have lost its prime (assuming that there is oil in the sump). If your engine has an oil filter installed (oil filter adapter and filter STC kit) then you can remove the oil filter, and use a battery acid siphon bulb (for lack of a better name) to inject oil into the oil passages in the oil filter adapter. Fill the bulb with engine oil, push the tip of the bulb into each passage, and force the oil it into the passages. The bulb I'm talking about is the one that all the auto parts stores sell. It has a red rubber bulb and a black plastic tube. It's usually used for adding water to batteries. Take it apart and make sure it's perfectly clean before using it. One of the passages in the oil filter adapter leads to the oil screen cavity and then directly to the oil pump. (I don't remember which passage, but inject oil into both of them. It won't hurt.) The tip of the squeeze bulb should fit into the holes in the oil filter adapter well enough so that some pretty good pressure can be developed by squeezing the bulb.

You need to inject enough oil to fill the oil screen cavity and the oil pump cavity. If the oil screen was not removed when the oil filter kit was installed, then there may be additional resistance to the oil flowing into the passage because the oil has to flow through the oil screen to get to the pump. Forcing oil into these passages should prime the pump. In fact, it almost has to prime the pump. Then put the oil filter back on, making sure that the rubber gasket on the oil filter is intact and in good shape. It's a good idea to pre-fill the oil filter before screwing it back on the engine. Very little oil will spill out of the filter before you have it screwed on if you do it right. This avoids having to wait for the oil pump to fill the oil filter with oil before you see oil pressure upon starting. (I always do this when installing a new filter.) The filter gets torqued to 18 foot pounds (or as specified on the filter) and safety wired.

You should then have oil pressure immediately upon starting the engine. The key to this is to force oil into the pump under some pressure so that it flows in around the oil pump gears and seals up all the air gaps between the gear teeth and between the gears and the oil pump cavity and cover plate. That's what happens when the pump "loses prime". The oil runs out of the pump leaving air where oil should be, and when you start the engine the gears spin around in the air, no suction is developed, and the pump can't lift the oil from the sump to the pump. Only when there is oil taking up the clearances can the pump develop enough suction to lift the oil from the sump. There needs to be enough oil in the pump to "seal up" the air gaps so that suction is developed. Injecting oil by this method supplies oil to the pressure (discharge) side of the pump. Note that ideally you would want to prime the pump from the suction (inlet) side of the pump, but that side of the pump is connected to the oil pick-up tube in the oil sump and is not accessible for priming purposes. So push plenty of oil to the discharge side of the pump and it should prime the pump sufficiently.

Two photos show the internals of the oil pump (C90-16F - Other are similar or the same.)
By the way, all of the above applies even if you don't have an oil filter kit on your engine. The oil filter kit attaches where the cover plate for the optional oil cooler goes. (Left side of engine, aft of rear cylinder.) These passages are accessible by removing this cover plate on engines that don't have an oil filter installed. It is the upper opening (passage) that leads to the oil screen and oil pump. You will need a new gasket for the cover when you put it back together.

The more wear that the oil pump has, the more likely you will have problems with loss of prime. Obvioulsly, the greater the clearances between the parts, the easier it is for the oil to drain out when the engine is sitting. So if you have this problem when the engine hasn't been sitting too many days, you may have a worn out pump. Another thing that can cause a loss of prime is a scored oil pump cavity. If a piece of metal or dirt is caught between the tip of a tooth of an oil pump gear and the wall of the oil pump cavity, the metal or dirt can scribe a line into the aluminum oil pump cavity. The score in the pump cavity gives the oil an unintended place to flow through, both while the engine is sitting, and while it is running. A bad enough scratch can cause low oil pressure while running and loss of prime while sitting.

Of course, if you are regularly losing prime when the plane sits for only a week or so, or if you have low oil pressure, you need to have the cause of the problem properly repaired.

What do you do about a worn out oil pump? Typically replacing the gears and oil pump cover will fix the problem if those are the only parts that are significantly worn. If the pump cavity itself is worn or scratched, there are companies that can weld up the cavity and then re-machine it back to new tolerances. The pump cavity is an integral part of the rear accessory case. I haven't priced this repair, but I bet it's not cheap. Any repair to the oil pump will require that the rear accessory case be removed, which means removing mags, generator, starter, tach cable, oil sump, etc, etc. It's not an easy job. So you may want to discuss all this with your mechanic before he starts disassembling your engine. He should know all of this already, but you may want to make sure that the treatment isn't worse than the disease here.

By the way, the Continental Engine Overhaul Manual (Form X-30010) has excellent color diagrams that show all of these passages and oil paths through the engine.

As always - standard disclaimer - I'm not a licensed mechanic and I've been told before that I don't know what I'm talking about. Seek the help of a licensed professional before using any of this information.

Hope this is of some help.

Best Regards,

Wayne DelRossi
Alon Aircoupe N5618F

And from James B. Brennan

Here's the bit: Use either the side hole (at maybe 10 O'clock) on the oil filter adapter, or lacking that, the top hole in the 'case, triangular oil cooler blanking casting having been removed. I tried a pump oil can with maybe a 4" plastic tubing extension, and all I got was what you got: after a few pumps, the oil just ran back out. I had the good fortune, however, to have the help of the part-time airport manager (small airport, but actively managed by the state). He did not have the tubing I needed to get it done. He suggest a drinking straw, and, lo, I found a new one from Wendy's on the floor of my van. I stuck this over the output of my pump oil can (loaded with av. oil, of course - Exxon Elite in this case) and stuck the straw way into the oil filter adapter (F+M, from Texas - probably the only one out there). Since the manager guy (Joe) was still about, I had him turn the prop backwards as I pumped. I doubt that I pumped more than maybe a 1/2 cup (US 4 fl. oz.) of oil, with him back-turning the prop. Then we decided to test. So, I pulled out the oiler, and hit the starter (mags off, of course) and Joe observed oil being obviously pumped out of the hole. Joe kindly operated the starter for me so I could witness, and, lo, the oil was obviously being pumped out the gallery from the pump (removed screen housing, that is, the housing that used to have a screen in it) and out into my cut-up milk jug, I reinstalled the filter, started up and the oil pressure indicated promptly to maybe 30#. Yes, it really worked!

Suggestions: 1) it would be handy if the pump oiler mated well with the filter adapter gallery, better than a Wendy's straw jammed deeply into it. I will contact F+M and get the size of the hold they drilled in the thing and send it along. I expect at a good hardware store, I could find several pieces of tubing that would effectively connect my pump oil can to the feed-in gallery in the oil filter adapter, or at least something better than what I used yesterday... possibly little hose clamps might be involved. At the very least, get the in-feed really deeply into the hole. B} I think the prop. counter-rotation while the oil was being pumped in was very important to the success of the project. I would speculate that the oil pump "room" doesn't have to be filled, but full enough to seal things up so that it could, even if partially filled with air, draw oil from the sump and finish the priming, although this is theoretical speculation on my part.

Bottom line, I got this procedure from a long time veteran of Continental Motors who was one of the few left who really knows the old engines (albeit he has moved on - I'd be happy to supply contact info.) and I got it straight from the "horse's mouth" (instead of the other end of the beast). I believe he was the one who told me that lung power wasn't enough, that one would indeed need "shop air", but at about 6# - i.e. regulator turned WAY DOWN.

I should note that I was doing this monkey business in just around freezing temperatures. The routine is apt to vary with temp. (don't know if or how...) - the "old timer" from Continental, if I recall, said I might have to back-pump a 1/2 qt. of oil, but I succeeded with a much smaller amount.



Page last modified on February 14, 2011, at 04:40 PM