C172 engine overheating question

[RPJ]

What would cause a lycoming 0-320 engine to overheat under these circumstances?

-oil is filled 7qt's with 100+ oil
-oil pressure in the green always
-just had oil/filter change 14 hours ago
-engine cowling is free of obstructions


Any ideas?

-Ryan

[JHines]

- improperly installed, damaged, or worn out engine cooling baffles or baffle seals
- defective thermostatic oil valve ("Vernatherm")
- any winterization equipment installed that shouldn't be?
- has the oil temp gauge been checked against a good gauge?
- oil cooler and ducting been checked?

[DE727UPS]

Had this happen all the time when I flew the canyon. Mechanic said as long as the pressure stays up, don't worry about it. That seemed to work okay as long as I flew out of there.

[Berkut]

We need some more information. What do you mean by "overheating?" Is it the oil, cylinder head, or exhaust gas temperature that's high? How high is it? What are the conditions under which the overheating occurs? Does anything seem to help? Did the problem come on gradually or just appear one day?

[RPJ]

Originally Posted by Berkut

We need some more information. What do you mean by "overheating?" Is it the oil, cylinder head, or exhaust gas temperature that's high? How high is it? What are the conditions under which the overheating occurs? Does anything seem to help? Did the problem come on gradually or just appear one day?

The oil temp indicator was always in the middle of the green then one day late in my flight the oil temp needle started creeping into the redline. Every flight after that the needle goes into the redline on climbouts. The only way to stop the needle is to stop the climb, reduce rpm and increase the mixture to full rich.

Most of the time now after engine start-up/run-up the oil temp needle doesent move from full left deflection until I am on take-off roll. I think its time to bring this to the attention of an a&p.

[SteveC]

When are you leaning?

[RPJ]

Originally Posted by SteveC

When are you leaning?

Above 3000ft msl

[USMCmech]

Originally Posted by RPJ

Every flight after that the needle goes into the redline on climbouts. The only way to stop the needle is to stop the climb, reduce rpm and increase the mixture to full rich.

Most of the time now after engine start-up/run-up the oil temp needle doesent move from full left deflection until I am on take-off roll. I think its time to bring this to the attention of an a&p.

I agree, high oil temps can mean bad news for your engine.


Factors that affect engine cooling, in order

By far #1 Airflow through the cooling fins.

Airspeed is the main factor, also cowl flap position, and how well the baffels are sealing.

# 2 Oil flow

Oil can provide as much as 40% of total engine cooling. Also the thicker the oil, the less friction.

#3 Power placed on the engine

More power, more heat.

#4 Mixture

Operating 50 deg either side of peak EGT is BAD! Leaner OR richer is OK, but not at peak.



If anything on your airplane suddenly changes from what is "normal" then you need to find out why.

It sounds like the oil temp guage is responding normally, so I would check things in this order.

Take the top cowl off, and look for where the baffels rub. If you see a stain showing leakage, that is allowing cooling air to escape rather than flowing past the cooling fins.

Check the oil cooler for airflow blokages. If air can't flow through, it can't cool the oil.

Swap oil pressure guages, to confirm that the guage is reading properly before you start spending real money.

Have the oil cooler flow tested, it's amazing how many of these have internal blockages.

Have the "vernathem" tested. This thing does the same job as your cars thermostat. If it isn't opening up properly

[PacMan4x4]

Had a similar prob with a C-172 I was flying and maintaining at one time. You said your oil press was in the green ... low, middle, or high green? I know...some folks may argue that it doesn't matter as long as it's in the green, but it can. If it's running low in the green for pressure, you could make an adjustment to the vernatherm (temperature bypass) valve. Your mechanic can pull the cover off of it and check the spring tension to see if it appears worn...if so, it can be replaced (not likely the prob) or he/she can add a washer which in turn will cause the oil press to be a bit higher. By increasing oil pressure, the temp can be controlled a bit.

I would do this after doing what others suggested:

Baffles
Blockage of oil cooler
Proper oil
Use long thermometer to check actual oil temp after run up to verify temp indication
Output of oil pump w/ pressure gage (checking for worn pump/low press)
etc....

Let us know how it turns out.

Pac Man

[Berkut]

Originally Posted by PacMan4x4

If it's running low in the green for pressure, you could make an adjustment to the vernatherm (temperature bypass) valve. Your mechanic can pull the cover off of it and check the spring tension to see if it appears worn...if so, it can be replaced (not likely the prob) or he/she can add a washer which in turn will cause the oil press to be a bit higher.

That sounds like the oil pressure relief valve he was adjusting rather than the vernatherm. I may be misremembering, but I think the vernatherm is non-adjustable and has the temperature rating stamped on it.

[PacMan4x4]

BERK....

I was thinking one thing, typing another. Yes, the oil press relief valve is what I was referencing too. Hope I didn't cause too much confusion--duh :-) With that same prob, I had to check the vernatherm also to ensure proper seating. Again, RPJ, my reply was about the oil press regulating valve. Some are adjustable externally and others you can add specific thickness washers to adjust spring tension.

Thanks BERK for the correction <high five>

Pac Man

[mattp1803]

Originally Posted by RPJ

What would cause a lycoming 0-320 engine to overheat under these circumstances?

-oil is filled 7qt's with 100+ oil
-oil pressure in the green always
-just had oil/filter change 14 hours ago
-engine cowling is free of obstructions


Any ideas?

-Ryan

Just turn the afterburner off.....

[CDVdriver]

I forgot which one is an electrical gauge, the temp or the press. so this might be a completely useless post.
If the wires are getting old and brittle or a little chafed the resistance will change and therefore change the indications, does it sit on the low side when you turn on the master switch or does it already indicate in the green when the engine is cold?

[RPJ]

Well today after shooting images in S. Texas my tachometer went completly dead, full scale deflection to the left. Then 2-3 minutes later the oil temp needle went full right scale deflection in the red like a rocket. It stayed pegged full scale to the right regardless of me fully enriching the mixture and reducing the throttle. I was expecting the engine to quit any moment while flying back to Laredo. I was scanning forced landing sites every friggen mile. Fortunately the oil pressure held out through this event and I landed safely at Laredo. The airplane has been scheduled for maintenance tomorrow so I will keep you guys posted on the findings.

-Ryan

[Berkut]

Notice anything funny about your magneto checks since this started? Maybe a slight roughness, or a change in the drops?

[RPJ]

Originally Posted by Berkut

Notice anything funny about your magneto checks since this started? Maybe a slight roughness, or a change in the drops?

Nope, run-up's went well. Maybe these are two separate things that went wrong at same time? I will see tomorrow lol.

[Berkut]

That would be a heck of a coincidence.

I suspect something that no one in this thread has mentioned yet. We'll see what your mechanic finds.

[RPJ]

The a&p did not find squat really. Just a very frayed tachometer cable and thats pretty much it. I know the indications I saw in the cockpit and I know I'm not crazy. Well I am not a mech so whatever, on with the show.

Berkut what did you have in mind?

-Ryan

[Berkut]

I was thinking it might be a mag problem, possibly advanced timing due to wear or some other fault. Not the most likely scenario by any means, but it does happen. I would sooner suspect the vernatherm or an indication problem, and I hope he checked those.

An improperly timed (too far advanced) magneto can cause high oil temps by causing the pressure inside the cylinder to peak too early on the power stroke. This heats up the piston and cylinder, which in turn heats up the oil. It would be much worse at high power settings (more cylinder pressure) and mixture ratios close to best-power (faster burning). Lower power settings help by reducing the mean pressure, and rich mixtures help by delaying the pressure peak, effectively retarding the timing.

It sounds like you have a mechanical tachometer, which means the problems aren't related. However, some electronic tachs use a pickup housed within the magneto. I don't think any 172s were set up this way, but you never know what changes a thirty-something year old aircraft has gone through. Kind of a long shot, but I was just speculating on ways the problems could have been related.

[trafficinsight]

If, in fact, your temp gauge is going full scale deflection (and the actual temperature isn't correspondingly high) then I would suspect your temperature sender is faulty, or the associated wiring is grounding somewhere. The temperature sensor works by varying the resistance on the ground side of the gauge, if it, or the wiring, shorts to ground then the gauge will bury itself full deflection.

If it's not going full scale deflection your mechanic can test the sensor by removing it, boiling a cup of water and dipping the probe in it, the gauge should read around 212 degrees.

[ppragman]

Originally Posted by RPJ

What would cause a lycoming 0-320 engine to overheat under these circumstances?

-oil is filled 7qt's with 100+ oil
-oil pressure in the green always
-just had oil/filter change 14 hours ago
-engine cowling is free of obstructions


Any ideas?

-Ryan

Lower the nose...

But seriously, it sounds like the guage, try swapping the guage and then runthrough a routine of slowflight, and stuff to get her hot, and see what happens.

btw, what happens to the engine with power at idle

[RPJ]

Originally Posted by ppragman

Lower the nose...

But seriously, it sounds like the guage, try swapping the guage and then runthrough a routine of slowflight, and stuff to get her hot, and see what happens.

btw, what happens to the engine with power at idle

I swapped aircraft a few days ago with another Cessna and this one has zero problems. With the aircraft in question, when I would pull to idle the engine would idle smoothly with no problems and oil pressure would remain in the green.