C-172 Oil problems

Windchill

Windchill

Well-Known Member
So I'm reading up on emergency situations, an area my instructor lacked in providing instruction ... basically I was taught how to land with an engine failure, but not really how to troubleshoot other problems.

So here's my q:

The POH for a 1983 172P says with low oil pressure and normal temp. a malfunction may be occuring with the oil pressure gauge or relief valve, and to land at the nearest airport.

Also, low pressure and high temp. means an imminent engine failure and to get on the ground immed. reducing the power to idle.

What about normal pressure, hight temp?
Normal pressure, low temp
High Pressure, w/ low, normal, or high temps?

Might one encounter any of those scenarios? What might be the cause and desired course of action?
 
I used to fly 172M´s out of the Grand Canyon doing tours. I was sooo hot that it wasn't unusual to see normal pressures and temps pegged out. The engine always ran great. Mechanic said not to worry as long as the pressure was okay.

I'm not giving advise, here, just telling you about my experiences.....
 
Oil is the lifeblood of your engine, take care of it.

High temps are the symptom, low oil pressure is the disease.

Oil under pressure acctually creates a buffer between the metal parts in your engine. Once the oil is flowing, metal never touches metal inside your engine. The oil must absorab the impact of the power stroke without allowing the crankshaft to touch the rod bearing it rides in.

The pump dosen't create pressure, it simply moves the oil. What creates pressure is the resistance to flow, useually regulated by the pressure relif valve. This is a small spring loaded "check valve" that is closed when the pressure is below its set value. When the pressure rises above the value set, then the pressure pushes the valve off it's seat, and is routed back to the inlet side of the pump.

Ocasionally this valve can get stuck open, which creates a large drop in oil pressure especially at lower RPMs. Also if the pump fails, or there is a leak, or an interuption of the supply of oil the pressure will drop.

Now, all of a sudden instead of getting that cushion, the oil is forced out and the rod hits the crank twice every revolution which creates friction and wears metal off of the bearings. This heat is carried away in the oil and the oil heats up.

Low oil pressure is an imeadiate emergency. I would recomend flying imeadiatly to the nearest airport at the lowest power setting practical. I would declare an emergency and constantly expect the engine to quit any second.

I would not recomend an imeadiate off airport landing because of the possibility of a bad guage. The exception would be if you saw oil spraying everywhere.


Re: oil temps

The oil in the engine acctually is responsible for about 30-50% of the engine cooling. High oil temps should tell you that your engine is really getting hot and is working hard. Also as oil heats up it becomes "thinner" and loses some of it's cushoning properties. High oil temps are a concern if they persist. You need to check your oil cooler, and/or switch to a higher viscosity.

Cold oil is another problem, especially in the frozen north. When oil is cold it is very thick. If it is too thick it won't flow properlly. Durring winter you need to use a lower viscosity oil so that it will flow better under cold temps. The seconds durring start and imeadiatly afterwards are very hard on your engine. Avoid high power settings & especially take off with cool oil.

Also when oil is heated up to it's operating temp the mositure and acids boil off, cleaning the oil. This is the main reson that engines which are flown reguarly last far longer than engines that sit for long periods of time. I have seen engines where you could look at the internal parts and see where the oil level was as it sat for over a year. There was a line of corosionon across the Cam gears.


Probably more than you were asking, but now you know.
 
As anyone who used to watch G.I. Joe knows, "Knowing is half the battle ..."

Thanks Scott. How's fatherhood?
 
The little guy is a real sleeper, we have to wake him up to eat.

Now that's lazy!
wink.gif
 
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Is there a situation where I might encounter an abnormally high oil pressure?

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If the pressure relif valve got stuck closed you would see very high oil pressure. This is dangerous because that pressure needs to be relived and will cause something to rupture very soon.

However this is extreeemly unlikely.


More likely is a partial blockage somewhere in the oil passages or the oil cooler. This would cause slightly higher oil pressure, and slightly higher temps.
 
How about no oil pressure. You know that you always check after starting the engine. Well we had no pressure period. Shut it down got the mechanic he checked it out and told me to look at my shoes. Yup covered in oil. The previous mechanic had broke off the tube leading to the pressure gauge while replacing something in the panel. Very good lesson to the student and me that its just not another thing to look at and forget on the checklist.

As for your other ?'s Normal pressure- high temp. I richen the mixture and lower the nose. Helpful on very hot days. I always take off with full oil 8qts in our 172 during the summer average 100-110 during the afternoon. And always have one or two qts. in the back
 
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The previous mechanic had broke off the tube leading to the pressure gauge while replacing something in the panel. Very good lesson to the student and me that its just not another thing to look at and forget on the checklist.

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Many older oil pressure guages are the "Bordon tube" type. This type of guage has a small curved tube which is filled with oil. As oil pressure increases it straightens the tube which moves the needle.

I personally don't like this type because you have a line of oil leading into the cockpit, which can break just like the situation Jay described. Also if this happened after the oil had warmed up you could be burned by very hot oil spraying into the cockpit while trying to land. OOUCH!

The other drawback is that oil drains out of the guage when the engine is shut down. When you start the engine it takes a few second to fill the line and the guage. Durring this time you have no idea wether or not your engine is getting oil.


Replacing these "Bordon tube" oil guages with electric types would be high on my To Do list if I owned a plane with them.
 
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High Pressure, w/ low, normal, or high temps?

[/ QUOTE ] I would always laugh when people would squawk airplanes for high oil pressure when it was cold in FL. Some would even taxi back after their runup and get a new plane. At FSI, they would use SAE 50 weight oil year round, so in the winter when it would occasionally in the low 50's in the morning, the aircraft were rather hard to start. You had to maintain around 1000 rpm or the engine would quit, until the oil warmed up. Even if it was in the low 60's, the oil pressure would often be pegged during takeoff, although it would be ok for the runup. Within a short period of time the engine would warm up and the pressure would come down once the oil thinned out.
 
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I always take off with full oil 8qts in our 172 during the summer ... And always have one or two qts. in the back

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Just an observation.....An engine is going to seek an equilibrium with its oil level, If you top it off with oil its going to do one of two things:Blow the excess oil through the breather tube/drain mast, or blow it through an engine seal somewhere in order to relieve itself. Most light engines settle in at around 6 qts (normal ambient temps). It would be a waste of money for us to keep pumping oil in our Cessna just to have it vented overboard.

Here's a good link" Lycoming oil levels

Here's a quote from that page:
"Pilots and mechanics should not confuse the models just discussed with other Lycoming four cylinder, wet sump engines that hold a maximum of 8 quarts of oil. There is no change in our recommendations when checking oil for the latter. During normal routine flying, oil levels are best maintained at the 6 to 7 quart level."

For comparison: Our Citation Xs hold 12 quarts of oil in each engine. The "Low Oil Level" message doesn't come on until it is 8 qts low! A 12 qt engine running on 4 qts...However, we usually operate in the 1-2 qts low range.
 
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