1975 Cessna 172M - Lycoming O-320-E2D - Calling all A&P Types

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Jon Wilder

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Hi all.

I posted a previous thread regarding high oil temps on a 1975 Cessna 172M with Lycoming O-320-E2D. Everything I've heard from the owner/operator -

"It's always ran right up on the red line."

"This year/model had oil temp issues."

"I've been around this aircraft for many years even with the previous owner and it's ran like that even as far as back then."

We did some approaches on a hot day not too long back. Even flying the approach descent at 145 MPH, the temps were 15* into the red and NOT coming down. We parked it after the 2nd approach.

A flush was done on the cooler. This greatly improved the cooling on approach. A climb to 4,000' got the temps up to about 230*. By the time we landed, temps came down to 184* once we were on the ground.

A few flights on hot days recently got the temps into the red again. Although it did cool on descent to land, it would take its sweet time getting out of the red on level cruise flight. IDK I just feel like the temps should be quite a bit better than they are.

We've checked the air box, air box baffle to cowl seals, and the seal around the air cleaner and cowl.

Today we had the cowl off for something else. I decided to get the part number off of the vernatherm and make sure it was correct for the aircraft. However, a browse through the maintenance manual/parts catalog showed me something I wasn't expecting to see. And it may completely spell out the entire issue. This is where the A&P types come in.

According to the Service Manual for the 1969 thru 1976 Model 172 Skyhawk Series, Notes in Figure 11-3 -

A spring and plunger type oil cooler bypass valve is installed in the engine of aircraft serials 17260224, 17260226 thru 17260240, 17260242 and on and F17200845 and on instead of the Vernatherm control valve.

Beginning with engine serial number L-30414-27, a spring and plunger type oil cooler bypass valve is installed in the engine accessory case and replaces the Vernatherm thermostatic control valve.
Click to expand...

Our aircraft is 17263XXX with an engine serial number of L-42667-27A. Thus it appears that ours should have the spring and plunger cooler bypass. Yet this whole time it has had a Vernatherm in the engine accessory case.

Could this be the smoking gun? Why were these engines changed from the Vernatherm to the spring/plunger setup? Would this have anything to do with the fact that it's a small oil cooler mounted on the firewall instead of directly in the air stream?

Would love to hear from the A&P types on this, especially those with direct experience on the 172M model.
 
Here is what my O320E2D does, this is installed in a Cessna 177 which is arguably too little engine for the airplane. This is configured for 150 horse, not the 160 horse.

I'd start by confirming the oil temp gauge in your airplane is accurate and go from there.

upload_2022-9-7_6-48-23.png
 
Essentially, the only thing having a vernatherm will do is allow oil to bypass the cooler during warm up. Not having one will always have oil going through the cooler. The spring and plunger is installed on the engines without a vernatherm to allow an oil cooler bypass in the event that the cooler is plugged up for some reason. The spring and plunger are supposed to be removed if you’re running a vernatherm. Lycoming has documents regarding this.

First thing I’d do is the same as was already posted. Make sure your gauge is working right.
 
How would one check to see if the temp gauge is working right?

Could the engine be running lean?

If I remember right there was AD issued something about carb jet that needed to be changed?
I know my N model was not effected but earlier models were.
Then I did find AD 12-03-07, it was listed in my logs in years past. AKA SB18.

If a oil cooler flush helped and the carb is not the problem then I'd be tempted to try a new oil cooler.
Good luck with it.
 
Jon Wilder said:
Could this be the smoking gun?
Click to expand...
Doubtful as long as the vernatherm is operating properly. There's a link below that explains the oil flow system works between the verna and bypass setup. However, since this has been an ongoing issue from before you owned the aircraft I would start at square one as mentioned above in verifying the oil temp indicating system is accurate. Also I'd check the oil pressure indicating system and ensure the engine baffling is tight and serviceable. One reason you may have a vernatherm installed is someone may have used it to try and correct the high temps you see. How old are the engine fluid lines? Do you have an external oil filter installed? If so is it Cessna or #rd party? Also what oil cooler flush procedure did your mechanic use?

https://www.lycoming.com/content/understanding-oil-flow
 
What quantity are you running? Add an extra quart or two and see if it lowers the temp.

not an expert on this engine….but you could be on to something with the vernatherm. Your mechanic will do exactly what you are doing with matching part numbers with your serial number.
 
Most engines are set up to be deliberately too rich at full power.
If you lean slightly you should pick up about 20 RPM. If it drops right off you are lean. Little less apparent in cruise.

Mag timing too advanced?

in addition to baffle/cowling seals verify the aluminum baffles are a tight fit
around the cylinders. A loose fit will not force air between the cooling fins to carry off the heat.
 
The Vernatherm is a superior method of oil temperature control and I would not fool with it other than to test it. I did that by putting it in a can of hot water to see if it extended properly. I use a candy thermometer, clipped to the edge of the can (or saucepan), to see at what point the Vernatherm starts extending. Do this on the stove. IIRC, it should be extending by around 180°.

The oil temperature gauge could be the bulb-type or an electric sensor type. In either case, I used a can of hot water with the candy thermometer, and dropped the sensor (still connected to the airplane) into the can. If it's electric, you need a jumper wire to ground the sensor body. Turn the master on. If the water needed more heat, I held the can by its edge with a pair of vise grips and used a propane torch on the can's bottom. As the water starts into a rolling boil, the thermometer should read around 212°F and the airplane's gauge should still be well in the green. The redline is at 245°F, the TCDS max temp limit. With any flame, be real cautious around the engine. No fuel or anything else flammable nearby, and watch the wiring and stuff. I never did come up with a small electric hotplate for doing this.

Many "flushes" involve nothing more than running solvent through the cooler for an hour or so. But the cooler gets heavily varnished up, just like the inside of the engine's crankcase, and it takes strippers to get that out. The varnish insulates the oil from the cooler's metalwork. With all the bother of strippers and the difficulty of getting it all out, it's cheaper and safer to replace the cooler.
 
Gary Ward said:
Could the engine be running lean?
Click to expand...
In my anecdotal experience, a lean running engine doesn’t greatly affect oil temperature. Plus, if it’s actually running lean enough to get oil temps into the red, everything else would be toast.
 
My old Cub had the viscosity valve instead of a Vernatherm. It worked just fine. I would confirm you don't have both, but whichever it does have should be acceptable. I'd investigate where your oil temp sensor is installed. If in the top of the case you'll see oil temps around 20* higher than the same sensor installed in the oil filter adapter.
 
My 76’ E2D 172 is the same way, between 2 engines.

I’ve checked the vermatherm…

I’ve put oil on a hot plate and dunked the sensor and used a digital thermocouple to read the temp and compare to the gauge, it’s accurate within 5 degrees I’d say.

Good Oil Cooler

Baffles Good

Carb Box Good

Mixture Settings Good

Oil levels good

At the end of the day it is what it is…. The new engine already has 1700 hours on it and doesn’t seem to mind. On the next engine swap, I’m going to put a 8 row oil cooler I just to see if that makes a difference, or maybe next summer. I pretty much have given up otherwise.
 
Has the Vernatherm actually been pulled and inspected?

They did have issues with the crimped nut at one time. I believe epoxy was the fix until the roll pin retainer came along. It seems the nut would come loose and the unit would no longer function.

Dimly recall some folks having to grind the valve seat. I believe Lycoming has SBs or SL’s on both problems.
 
Skymac said:
My 76’ E2D 172 is the same way, between 2 engines.
Click to expand...

Your aircraft is covered by the same service manual. If yours has a Vernatherm, but should have a viscosity valve instead, this may help you as well. Read on.

Magman said:
Has the Vernatherm actually been pulled and inspected?
Click to expand...

Not yet. We'll get to that in a few.

But first...here's what we know -

The service manual states -

A spring and plunger type oil cooler bypass valve is installed in the engine of aircraft serial numbers 17260224, 17260226 thru 17260240, 17260242 and on and F17200845 and on instead of the Vernatherm control valve.

Beginning with engine serial L-30414-27 a spring and plunger type oil cooler bypass valve is installed in the engine accessory case and replaces the Vernatherm thermostatic control valve.
Click to expand...

Now...here's what we know about the aircraft -

This aircraft is equipped with a full flow oil filter setup (NOT the pressure screen setup).

This aircraft falls within the service manual prescribed aircraft serial number range for the "Vernatherm to viscosity valve change".

This aircraft's engine falls within the service manual prescribed engine serial number range for the "Vernatherm to viscosity valve change".

It was confirmed today that there is no viscosity valve currently installed in the accessory case. Only the plug that would hold one in place is installed.

A Vernatherm, part number 53E22144, is currently installed in the oil filter housing. There should just be a plug in this location, and a viscosity valve installed in the accessory housing. Instead, there is just the plug where the viscosity valve should be, but no viscosity valve installed.

These aircraft use a rather small 6 row remote mount oil cooler (Stewart Warner p/n 8406E) mounted on the firewall. An aluminum shroud sits atop the cooler with a scat hose which routes from the back side of the engine baffle to the cooler shroud.

Clearly, there is a reason the change was made from a Vernatherm to a viscosity valve. Perhaps they decided instead to use an oil cooler size that would yield acceptable operating temperatures with continuous duty oil flow through the cooler, thus ditched the Vernatherm for the viscosity valve as the Vernatherm would, in fact, cause the oil temp to run too hot, but I'm speculating here. But clearly there was a reason for this change, and it seems maybe going to a smaller cooler and a viscosity valve with continuous duty oil flow was a lower cost option than a larger cooler with a Vernatherm. But again, I'm speculating.

Anyone else have better ideas here?

As to pulling and inspecting the Vernatherm, I'm trying to ascertain whether it is acceptable to use a Vernatherm with the oil cooler this aircraft has installed. If not, we might as well go back to the viscosity valve setup.

Thanks in advance all. Any ideas are more than welcome.
 
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I commend your research. It seems like you have to determine whether:

1. Correct components are installed.
or
2. Correct components are installed but are malfunctioning.

Good hunting!
 
The other thing to check is how far the vernatherm extends. There is a spec, but I don't know it offhand. So if you go to the effort to pull it and do the hot water test, also check to see how far it extends (assuming it extends at all).

One last thing to check on the vernatherm is the quality of the seal it creates when it extends. There should be a smooth even witness mark around the plunger of the vernatherm showng that it seals effectively against the engine case. Look for that and also look down inside the hole where the vernatherm sits and make sure the sealing surface inside the hole is in good shape. If there is trash in there, or if something is unevenly worn, you may not be getting a good seal, allowing oil to bypass the cooler.
 
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I remember reading this as well in the manual but I could have swore, at the end of the day it had been superseded at some point, my serial is 12766669.

I tested the and checked the vernatherm, it’s seating, and all was well. Talking about 2 different engines as well. I guess the part that I do not understand is, I thought when it closed it sent oil to the oil cooler, are you suggesting that since the oil cooler is smaller, by the time the oil gets to that temperature it just can’t keep up? Because either way it should be full on at that time.
 
Jon Wilder said:
A Vernatherm, part number 53E22144, is currently installed in the oil filter housing.
Click to expand...
FYI: external oil filters are mostly optional equipment even if installed by the OEM. With Cessna oil filter kits, there usually is some sort of alteration of the standard engine oil system to incorporate the filter. Given the vernatherm is installed in the oil filter housing this could be one of those alterations. If it is a Cessna filter assy. a quick to tech support should put your speculations to rest.
 
On a Lycoming?

Once again, where is the temp sensor installed?
 
Stewartb said:
On a Lycoming?

Once again, where is the temp sensor installed?
Click to expand...

Yes...Lycoming O-320-E2D. Temp sensor is installed in the top of the oil filter housing.

Bell206 said:
FYI: external oil filters are mostly optional equipment even if installed by the OEM. With Cessna oil filter kits, there usually is some sort of alteration of the standard engine oil system to incorporate the filter. Given the vernatherm is installed in the oil filter housing this could be one of those alterations. If it is a Cessna filter assy. a quick to tech support should put your speculations to rest.
Click to expand...

The notes regarding the change to a viscosity valve are on the parts breakdown for the full flow oil filter housing.
 
Change to a viscosity valve. Why not try it? Like I said, mine worked fine. I don’t think it makes oil temp different but you may as well find out for yourself.

I’m not a fan of oil coolers fed by blast tubes. That design may be the root of your problem. One easy solution may be to add a cooling lip to lower bottom cowl pressure. That may improve flow through your cooler. It sure can’t hurt.
 
Skymac said:
I remember reading this as well in the manual but I could have swore, at the end of the day it had been superseded at some point, my serial is 12766669.

I tested the and checked the vernatherm, it’s seating, and all was well. Talking about 2 different engines as well. I guess the part that I do not understand is, I thought when it closed it sent oil to the oil cooler, are you suggesting that since the oil cooler is smaller, by the time the oil gets to that temperature it just can’t keep up? Because either way it should be full on at that time.
Click to expand...

As long as the vernatherm is functioning correctly and fully seating, the system should function the same when it is up to temp as it would if the vernatherm was not installed. The vernatherm just allows oil to bypass the cooler to allow quicker warm up.
 
mondtster said:
The vernatherm just allows oil to bypass the cooler to allow quicker warm up.
Click to expand...
It does more than that. It keeps oil temps up in cold weather. If oil temps are low, as they would be with the viscosity valve, you get more crankcase moisture accumulating, and so more engine corrosion.

Viscosity valves are old tech that were superseded by the Vernatherm and there are good reasons why Lycoming would spend the extra money on it.

I'll say it once more: oil coolers get varnished up inside and need replacing.
 
Once upon a time, working on a 1976 C-172 with an O-320H2AD, I may have accidentally pushed a little too hard and drilled straight into the oil cooler instead of just the cowling fastener. Oops.

I learned two things: airplane parts are expensive and Cessna oil coolers are as small and light as possible; cooling is secondary...

OK, three things.

Don't drill holes in expensive airplane parts.
 
It does more than that. It keeps oil temps up in cold weather. If oil temps are low, as they would be with the viscosity valve, you get more crankcase moisture accumulating, and so more engine corrosion.
Click to expand...


If Vernatherms did that we wouldn’t be taping our oil coolers!
 
My previous Tiger had an oil temp sensor fail and after replacement I was deep into the RED at exactly 19 minutes on every flight. The first A&P checked vernatherm, flushed oil cooler, checked baffles etc. and essentially gave up.

Went to another A&P at a nearby field. He told me to call prior to arrival as he wanted to check the engine temperature at multiple locations on shutdown with a digital reader. Gauge was WAY OFF - plane was WAY cooler than reading in cockpit. Was traced to the previous owner spent a TON upgrading the panel and in short, the SENSOR replaced by the A&P that couldn't figure it out was for the ORIGINAL Tiger oil gauge, but the previous owner put in all new gauges that were not Tiger original (mis-match). Changing the sensor cured all ...
 
Dan Thomas said:
Viscosity valves are old tech that were superseded by the Vernatherm and there are good reasons why Lycoming would spend the extra money on it.
Click to expand...

So why would Cessna replace the vernatherm with a viscosity valve as stated in the Cessna service manual? Why would they go backwards? This is the million dollar question.
 
Jon Wilder said:
So why would Cessna replace the vernatherm with a viscosity valve as stated in the Cessna service manual? Why would they go backwards?
Click to expand...

Maybe the tech pubs are wrong? It wouldn't be the first nor the last.
 
Engine bulletins will be where to seek info. Cessna’s interest in that engine faded long ago.
 
Stewartb said:
If Vernatherms did that we wouldn’t be taping our oil coolers!
Click to expand...
The problem there is that the Vernatherm or visosity valve only closes a cooler bypass. It doesn't close off the oil cooler flow. Oil can still flow through the cooler when the valve is shut. When the oil is colder, the Vernatherm opens the bypass, limiting cooler flow. The viscosity valve will not do that.
 
Meh, to the guy driving they’re indistinguishable. And both require taping the cooler in cool weather. But take solace, Continental’s thermostatic oil temp valve isn’t any better.
 
Jon Wilder said:
Hi all.

I posted a previous thread regarding high oil temps on a 1975 Cessna 172M with Lycoming O-320-E2D. Everything I've heard from the owner/operator -

"It's always ran right up on the red line."

"This year/model had oil temp issues."

"I've been around this aircraft for many years even with the previous owner and it's ran like that even as far as back then."

We did some approaches on a hot day not too long back. Even flying the approach descent at 145 MPH, the temps were 15* into the red and NOT coming down. We parked it after the 2nd approach.

A flush was done on the cooler. This greatly improved the cooling on approach. A climb to 4,000' got the temps up to about 230*. By the time we landed, temps came down to 184* once we were on the ground.

A few flights on hot days recently got the temps into the red again. Although it did cool on descent to land, it would take its sweet time getting out of the red on level cruise flight. IDK I just feel like the temps should be quite a bit better than they are.

We've checked the air box, air box baffle to cowl seals, and the seal around the air cleaner and cowl.

Today we had the cowl off for something else. I decided to get the part number off of the vernatherm and make sure it was correct for the aircraft. However, a browse through the maintenance manual/parts catalog showed me something I wasn't expecting to see. And it may completely spell out the entire issue. This is where the A&P types come in.

According to the Service Manual for the 1969 thru 1976 Model 172 Skyhawk Series, Notes in Figure 11-3 -



Our aircraft is 17263XXX with an engine serial number of L-42667-27A. Thus it appears that ours should have the spring and plunger cooler bypass. Yet this whole time it has had a Vernatherm in the engine accessory case.

Could this be the smoking gun? Why were these engines changed from the Vernatherm to the spring/plunger setup? Would this have anything to do with the fact that it's a small oil cooler mounted on the firewall instead of directly in the air stream?

Would love to hear from the A&P types on this, especially those with direct experience on the 172M model.
Click to expand...
we have a '75 172M which was having oil temperature issues. we removed our cooler #8000075 and put in #8001733 which is supposedly way more efficient and the issue was resolved.

https://www.mcfarlaneaviation.com/products/product/8001733/
 
That’s the cooler I have. I’m thinking about switching to the 8 row. I have a 79’ with a O360 and it’s had a 8row added to it somewhere along the line and it behaves wonderfully. I understand it’s a totally different setup, but it still does good and uses the original shroud with a little piece of metal to extend the mounting holes.
 
So as I've downloaded all of our engine data (we have the Garmin G3X Touch w/Engine Analyzer)...and yes the temp reading is accurate as (1) it's a new temperature sensor; (2) when the engine is off and at ambient temperature, it indicates that correctly; (3) we're getting the same indications (actually, even greater resolution) as we were on the old mechanical temperature gauge. Nothing has changed indication-wise since we ditched the factory engine instruments for the Garmin Engine Analyzer.

On a particular flight -

Tach: 2430 RPM
Altitude: 4,388 MSL
Carb Temp: 128°
OAT at altitude: 30.6°C (87°F)

CHT Cylinder 1: 374°
CHT Cylinder 2: 335°
CHT Cylinder 3: 386°
CHT Cylinder 4: 381°

EGT Cylinder 1: 1365°
EGT Cylinder 2: 1340°
EGT Cylinder 3: 1400°
EGT Cylinder 4: 1405°

Oil Pressure: 60 PSI
Oil Temperature: 245°

Oil got up to red line on a full power climb to 4,500'. Upon leveling off, it cooled down about 5 degrees, then rose 3 degrees. Power was pulled back to about 2400 RPM upon leveling off (at the 5 degree cool down), then was changed to 2500 RPM (rose 3 degrees). It remained at 245 all the way until the descent to land. Upon touchdown, oil temp had dropped to about 199*F and was continuing to reduce until shutdown.
 
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Jon Wilder said:
So as I've downloaded all of our engine data (we have the Garmin G3X Touch w/Engine Analyzer)...and yes the temp reading is accurate as (1) it's a new temperature sensor; (2) when the engine is off and at ambient temperature, it indicates that correctly; (3) we're getting the same indications (actually, even greater resolution) as we were on the old mechanical temperature gauge. Nothing has changed indication-wise since we ditched the factory engine instruments for the Garmin Engine Analyzer.

On a particular flight -

Tach: 2430 RPM
Altitude: 4,388 MSL
Carb Temp: 128*
OAT at altitude: 30.6*C (87*F)

CHT Cylinder 1: 374*
CHT Cylinder 2: 335*
CHT Cylinder 3: 386*
CHT Cylinder 4: 381*

EGT Cylinder 1: 1365*
EGT Cylinder 2: 1340*
EGT Cylinder 3: 1400*
EGT Cylinder 4: 1405*

Oil Pressure: 60 PSI
Oil Temperature: 245*

Oil got up to red line on a full power climb to 4,500'. Upon leveling off, it cooled down about 5 degrees, then rose 3 degrees. Power was pulled back to about 2400 RPM upon leveling off (at the 5 degree cool down), then was changed to 2500 RPM (rose 3 degrees). It remained at 245 all the way until the descent to land. Upon touchdown, oil temp had dropped to about 199*F and was continuing to reduce until shutdown.
Click to expand...
which oil cooler do you have? see my post a couple up. your issue sounds exactly like ours did before the new cooler...on a warm day you basically couldn't even climb without overheating. no problems now
 
readytocopy995 said:
which oil cooler do you have? see my post a couple up. your issue sounds exactly like ours did before the new cooler...on a warm day you basically couldn't even climb without overheating. no problems now
Click to expand...

Stewart Warner 8406E. I think it's the original cooler that came with the aircraft. Would have to look in the maintenance records to confirm.
 
Dan Thomas said:
So how did you manage that carb temp? Carb heat on the whole time?
Click to expand...

After reviewing previous log data and realizing we may have set the carb temp sensor to the wrong type, we programmed it to the type we thought it was supposed to be, only to realize we had it right the first time. Thus the carb temp on this particular flight is invalid. We've since corrected that. Will do some more flights now that it's back to its original setting.
 
Jon Wilder said:
Stewart Warner 8406E. I think it's the original cooler that came with the aircraft. Would have to look in the maintenance records to confirm.
Click to expand...

I have the HE cooler he is referring too but it made zero change in mind.
 
The thing I find interesting about this is that all of the other temps and engine running parameters appear to be in check. I'm honestly impressed with the even balance I see between cylinders on the CHT and EGT since this engine is carbureted. Nothing seems to jump out as a smoking gun. Only the oil temps appear to be out of order, and perhaps this is just part of this cooler design. The newer aircraft have the cooler direct mounted to the backside of the engine box. Some 182s even have it in front of cylinder right in the cowl nostril.

I'm honestly at a loss here. Maybe Cessna did, in fact, ditch the Vernatherm in favor of a viscosity bypass on these engines BECAUSE of the remote mount design and small cooler size. It would be interesting to hear from someone who has retained the viscosity bypass valve setup on a 172M.
 
There's another factor at play in some high oil or CHT readings: Poor engine grounding.

It only takes the smallest fraction of an ohm between the engine and airframe to cause erroneous readings. The electrical oil temp gauge, for instance, uses a circuit that looks like this:

upload_2022-9-9_18-32-19.png

Electron flow is from the engine case into the sensor (5),to the oil temp gauge, then to the intrument breaker on the bus ("inst"), then from there to the battery.

Now, this is a small current, a few milliamps. If that engine case isn't VERY well grounded to the airframe, the alternator's ground electron flow looks for other ways to get to the airframe, and it finds ways such as via throttle, prop and carb heat controls, primer lines, and via that oil temp sensor. The electron flow through the sensor is thereby boosted, and the gauge will read high.

Cessna has service kits to convert the old mechanical oil temp gauges to electric, and they include a wire to run between an engine case bolt near the sensor and the gauge case, to eliminate any voltage differential that causes indication errors.

upload_2022-9-9_18-41-46.png

"Added ground wire" is the one.
 
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