Advice on an O-300 with oil pressure issues (sad story)

DE727UPS

Well-Known Member
A non-pilot friend bought a 61 172 sight unseen out of Texas. He had a pre-buy done which turned into an annual. They even did a borescope on the engine. It was a good price and there was room for some investment. The ferry flight went through Hugoton, KS for cheap gas. After fueling, the ferry pilot got no oil pressure indication on startup, and shut it down. A mechanic from Liberal (I think) came out and found the oil pump had failed but likely no damage to the engine since it was shut down immediately with no oil pressure indication after starting. Ferry pilot goes home. Mechanic orders new oil pump and also says accessory case needs to be replaced, so finds another one somewhere. Engine goes back together and still no oil pressure. What to do now?

One option presented to the owner is a major overhaul. I suggested letting the mechanic dig deeper to fix the problem. I'm thinking there must be some blockage or clogging in the oil system. Now wondering if the oil pump was even bad in the first place?

I told the owner he should consider just pulling the wings off the thing and move it home to WA so he can be directly involved with further decisions and use a trustworthily local mechanic. The owner is part of a large family business that is a local fuel distributer and they have several big trucks. I'm trying to find out if they have a flatbed trailer in the mix.

Any mechanic's on here have any ideas on what the poor owner should do now short of doing a major on an O-300? Engine has 400 hours SMOH but the overhaul was likely done many years ago. I've owned 10 small GA planes and have avoided this nightmare aircraft ownership scenario. I was going to teach the owner how to fly this summer. For me, moral of the story is a pre-purchase or annual is no guarantee that something really bad might happen after you buy a plane. Maybe I've been lucky all these years.
 
If it's a good price it's worth the investment to get the engine up to scratch. Hope they're videoing the good bits of the trip.

 
*edit* let me be transparent, I know zip about O-300s. Big bore Contis and Lycs and a bit of the 4 banger Lycs have been my jam.

Low/fluctuating oil pressure can drive you nuts, no oil pressure should be fairly straightforward to troubleshoot. I’m on board with the put it on a trailer and deal with it on home turf plan.

*second edit* before giving up I would also make sure that the easy stuff is looked at-like is the line to the oil pressure gauge clear? And does the oil pressure gauge actually work properly as verified with a calibrated gauge (those direct reading ones almost never go bad but….better to check it)?

*third edit* teh interwebz say that it’s not at all uncommon for the little contis to lose prime on the oil pump, and worn oil pump/accessory case can aggravate it. There are all sorts of tips and tricks out there for restoring the prime. So maybe the original mech was on the right track, I could see very hot weather and really runny oil (like KS in the middle of summer) exacerbating the situation.
 
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*edit* let me be transparent, I know zip about O-300s. Big bore Contis and Lycs and a bit of the 4 banger Lycs have been my jam.

Low/fluctuating oil pressure can drive you nuts, no oil pressure should be fairly straightforward to troubleshoot. I’m on board with the put it on a trailer and deal with it on home turf plan.

*second edit* before giving up I would also make sure that the easy stuff is looked at-like is the line to the oil pressure gauge clear? And does the oil pressure gauge actually work properly as verified with a calibrated gauge (those direct reading ones almost never go bad but….better to check it)?

*third edit* teh interwebz say that it’s not at all uncommon for the little contis to lose prime on the oil pump, and worn oil pump/accessory case can aggravate it. There are all sorts of tips and tricks out there for restoring the prime. So maybe the original mech was on the right track, I could see very hot weather and really runny oil (like KS in the middle of summer) exacerbating the situation.

Yeah, I would also try to get a 2nd opinion before taking the wings off. Driving another mechanic out there is going to be cheaper and easier than driving a plane long distance.

Just had a friend's Continental show low/erratic oil pressure. He immediately feared it was new engine time. I suspected a sticky oil pressure relief valve. Good guess, but nope. Looked fine, shop ruled that out quickly. Further inspection, a mag had gone bad, making the engine rough. Possibly the cause of the oil warning (still TBD).

I would agree with a calibrated gauge to rule out a bad gauge. If not, drain the oil, check the screen (if there is one), cut open the oil filter, check the oil bypass valve, and rule out that the pump hadn't lost prime. I'm not an A&P - but those are the questions I would be asking mine.
 
The oil pump is nothing but gears being driven by the accessory drive so the idea of an oil pump failing with no damage to the engine is.... unlikely. The clearance in the pump housing around the gears and the side clearance between the cover and the gears is vitally important for the proper operation of the pump, so if this mechanic just slapped new gears in it its likely no better than it was. The housing is aluminum and the gears are steel or iron.

That said I second the notion of making absolutely sure it's not the pressure line or the gauge, or anything else before going any further.

If it does turn out to be engine hard parts I would advocate the complete overhaul. Good money after bad etc.
 
Thanks again for the comments. The mechanic wants to pull the engine and take it to his shop for further analysis. There is no mechanic on the airport and I think he thinks it would be better to have the engine in his shop. I'm hearing this third hand, but the owner has agreed to further troubleshooting, up to including splitting the case, to figure out what's wrong. I know engines come on and off fairly easy and think the mechanic doesn't want to mess with it anymore at a remote airport away from his hangar. I can only hope the mechanic tries some of the more simpler trouble shootings steps before he splits the case. That's the problem with the aircraft being half way across the county and having to trust a mechanic you don't know.
 
Thanks again for the comments. The mechanic wants to pull the engine and take it to his shop for further analysis. There is no mechanic on the airport and I think he thinks it would be better to have the engine in his shop. I'm hearing this third hand, but the owner has agreed to further troubleshooting, up to including splitting the case, to figure out what's wrong. I know engines come on and off fairly easy and think the mechanic doesn't want to mess with it anymore at a remote airport away from his hangar. I can only hope the mechanic tries some of the more simpler trouble shootings steps before he splits the case. That's the problem with the aircraft being half way across the county and having to trust a mechanic you don't know.
Man, I don’t know early 172s but I always used to figure 40 man hours for an engine R&R on a recip…not sure if that qualifies as coming on and off fairly easy or not. And this was in a shop where we did them all the time and every one was more or less the same
 
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The oil pump is nothing but gears being driven by the accessory drive so the idea of an oil pump failing with no damage to the engine is.... unlikely. The clearance in the pump housing around the gears and the side clearance between the cover and the gears is vitally important for the proper operation of the pump, so if this mechanic just slapped new gears in it its likely no better than it was. The housing is aluminum and the gears are steel or iron.

Yeah, I would be really curious about the nature of the "failed" oil pump. Trying to think of how that would happen without leaving behind a good bit of metal that would be seen in the filter/oil screen/etc.
 
Yeah, I would be really curious about the nature of the "failed" oil pump. Trying to think of how that would happen without leaving behind a good bit of metal that would be seen in the filter/oil screen/etc.
My guess based on the inclusion of the accessory case is by “failed” he meant “worn to the point it won’t hold prime”. Also possible he’s not experienced with that particular oddity of the little continentals.
 
Thanks again for the comments. The mechanic wants to pull the engine and take it to his shop for further analysis. There is no mechanic on the airport and I think he thinks it would be better to have the engine in his shop. I'm hearing this third hand, but the owner has agreed to further troubleshooting, up to including splitting the case, to figure out what's wrong.

Splitting the case is way past "figuring out what is wrong" in my opinion. You are in deep enough at that point that an overhaul isn't going to be much more effort. Besides, I don't think I would want the engine after the shade tree overhaul in his shop anyway.

I believe Continentals measure oil pressure further downstream than Lycomings do at the oil pump. Takes some for oil to make its way through the bearings and journals to get there, it could just be the engine wasn't warmed up enough. Could be the bearings are so shot that no oil is making it there. At 400 hours though, I would want some more evidence before pulling the motor.
 
Splitting the case is way past "figuring out what is wrong" in my opinion. You are in deep enough at that point that an overhaul isn't going to be much more effort. Besides, I don't think I would want the engine after the shade tree overhaul in his shop anyway.

I believe Continentals measure oil pressure further downstream than Lycomings do at the oil pump. Takes some for oil to make its way through the bearings and journals to get there, it could just be the engine wasn't warmed up enough. Could be the bearings are so shot that no oil is making it there. At 400 hours though, I would want some more evidence before pulling the motor.

A spun bearing where the oil pressure line is tapped can result in zero oil pressure.
 
Can John Deakin help

 
*second edit* before giving up I would also make sure that the easy stuff is looked at-like is the line to the oil pressure gauge clear? And does the oil pressure gauge actually work properly as verified with a calibrated gauge (those direct reading ones almost never go bad but….better to check it)?

A friend of mine had a 1959 C150 with perpetually low oil pressure. It was within limits but seemed like it was on its last legs.

He had it checked with a second gauge and ruled out the gauge. He had the line cleared with compressed air. No change. Eventually he sold the airplane.

The new owner let him know later that they removed the pressure gauge line, then removed the AN fitting where the line enters the case (I recall there might have been a plate with a calibrated orifice too?) and cleared a large piece of debris out of that stack up, and then the gauge read normally again. Live and learn I guess!

Anyway I suspect the O-300 is very similar and if the line is clear it’s worth checking that fitting before concluding a spun bearing or other badness.
 
A friend of mine had a 1959 C150 with perpetually low oil pressure. It was within limits but seemed like it was on its last legs.

He had it checked with a second gauge and ruled out the gauge. He had the line cleared with compressed air. No change. Eventually he sold the airplane.

The new owner let him know later that they removed the pressure gauge line, then removed the AN fitting where the line enters the case (I recall there might have been a plate with a calibrated orifice too?) and cleared a large piece of debris out of that stack up, and then the gauge read normally again. Live and learn I guess!

Anyway I suspect the O-300 is very similar and if the line is clear it’s worth checking that fitting before concluding a spun bearing or other badness.
Yeah typically somewhere in the fitting in the case is a restricted orifice to smooth out the reading and reduce leakage if there is a break in the line. On airplanes with a long distance from the gauge to the engine and that are flown in cold climates (see: piper navajo) a common trick is to fill the line up to the cockpit with 5606 to get the gauge to read more quickly on startup.
 
I realize we're talking about peri-WWII technology here, but it blows my mind there's not an electrical sender to the gauge for an oil pressure signal instead of piping a tube of hot, pressurized oil into the cockpit..

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I realize we're talking about peri-WWII technology here, but it blows my mind there's not an electrical sender to the gauge for an oil pressure signal instead of piping a tube of hot, pressurized oil into the cockpit..

View attachment 65155
Shrug. Seen lots of wires and pressure senders go wonky, never seen a direct reading oil pressure gauge quit or even be significantly out of cal.
 
I realize we're talking about peri-WWII technology here, but it blows my mind there's not an electrical sender to the gauge for an oil pressure signal instead of piping a tube of hot, pressurized oil into the cockpit..

View attachment 65155
I have first person experience with the aircraft being discussed. It has stc Electronic Oil pressure and oil temp guages. Transducer and associated flex hose was the first thing tested. Confirmed good, not the failure….Mid America A/P IA providing excellent service, especially considering his business is hopping and he is out twice a month maintaining warbirds. Substitute guage confirmed no pressure. Fresh oil with two new oil pump gears in the overhauled accessory case, same result. Screens clear.
 
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