If the thermostat inside the oil-cooler fails, does it go to full open?

ao.frog · #1 09-17-2011, 03:56 PM

Hi gang.

I have the Stewart Warner oil cooler on the baffle behind cyl # 4 (IO-360) per Vans plans.
In front of the cooler, I have installed the oil cooler shutter from Vans.

Up until now, (210 hrs) it's been working ok, but Fall has come to Norway now, and the OAT's are dropping.

During the last few flights, I have noticed that I'm not able to keep the oil-temp to where it has been up until now; 180-200*F during cruise.

Temps are in the 150-160*F range with OAT's around 35-40*F and a powersetting at 55% (21/2100).

So far, it looks like the oil temp is dropping along with the OAT as they are getting lower.

For example: I can take off on the first flight of the day with the shutter fully closed and a oil-temp around 100*F and increasing.
Then, at for example 5000', the oiltemp is 180-190 and when reducing to cruise-setting, the oil-temp start dropping to the 150-160 range.

This is NOT the way the oil temp has been during low OAT's before, because the shutter has always been able to keep the oil-temps up, even on low powersettings.

Therefore I'm wondering if anything has happened with the oil-cooler.
It has some kind of thermostat inside, right? Vernaterm valve or something?

I think I've read somewhere that this valve in the cooler never goes to fully closed; there's always some oil flowing past the valve, even in low OAT's.

But seeing that I'm not able to keep up then temps like before, I'm wondering if that valve has failed to full open? Or is such a failure not possible?

I don't have a clue of the inside of a cooler, but hopefully there's someone out there who knows more than me about this.

What I do know however, is that something prevents the oil-temps to be in normal range. I've not had any problems keeping the oil temp up during the last three winters I've had the RV.

Any good ideas where to start looking?

David-aviator · #2 09-17-2011, 07:58 PM

Alf,

Your system is perfectly normal.

Lycoming controls oil cooling, to a point, with the device commonly known as the vernatherm. (the official name is oil cooler by-pass valve)

The valve does not cause oil to by-pass the cooler, the port to the cooler is always open. The valve sits at a Y junction where oil can flow to the cooler or back into the engine. When oil is hot, the valve closes the port to the engine and oil is forced through the cooler. If the valve fails, the port to the cooler is always open. The port to the engine may be blocked but the port to the cooler is always open.

With certified aircraft the oil cooler is covered with a plate below a certain temperature to block air flow through it. With the Cherokee 180, I believe the cover is installed at 30F and below.

Your best option is find a way of blocking air flow through the oil cooler for cold weather operations. I have an air shutoff valve but even with it full closed, on a cold, cold day, the oil temp may not come up to 140. In cold climates, I believe there also is a method of blocking air flow across the cylinders to get CHT's up which will also increase oil temperature.

Oil coolers do not have built in thermostats.

ao.frog · #3 09-18-2011, 04:22 AM

Hi David.

Thanks for the reply, but I probably didn't use the correct words, because I think you misunderstood me: I already have a system for blocking air to the cooler: the Vans oil cooler shutter. See pics below.

This shutter has performed great for three winters now.

Based on what you are writing above: maybe it's possible that the valve you are mentioning now has failed on my cooler and that the oil is always flowing through the cooler?

That could explain why I'm not getting the oiltemps up, even with the shutter fully closed (no air into the cooler)

I know that SOMETHING has changed since last winter, because back then I had no problems keeping the oil temps up....

Wayne Gillispie · #4 09-18-2011, 06:55 AM

Mine is below oil filter. When heated in an oil bath to 180-190F it should extend. If yours is stuck in the extended position closing off the port to engine then all of your oil goes through oil cooler. Even with no airflow your uninsulated cooler still makes a good heat exchanger.

Also compare the oil cooler temp to engine oil sump temp immediately after a flight. If they are the same then your getting too much flow through cooler.

You can also use two thermistor sensors, one just in front and one in rear of cooler to check temperature rise(with shutter open). If it is more than a few degrees then your getting too much flow.

David-aviator · #5 09-18-2011, 07:22 AM

You're correct, Alf, I did not completely understand the situation.

Something has changed since previous winters, that's for sure.

I've read the only sure way to control oil temp in cold weather is to have shut off valve in the line to the cooler. The guy who wrote that piece has such a valve controlled from the flight deck. It's a brass ball valve of some sort.

Could be the vernatherm is stuck in the engine port.

DanH · #6 09-18-2011, 08:28 AM

There is no valve inside the cooler body.

Oil always flows through the cooler. An extended (hot) vernatherm merely closes the cooler bypass.

Bob Ellis · #7 09-18-2011, 11:18 AM

Picture paints a thousand words Dan and that one certainly does. Do you mind if I copy it for my Pilot Manual?

Regards
Bob
www.rv-8.co.uk

ao.frog · #8 09-18-2011, 11:55 AM

Thanks for the drawing Dan.

If I understand that drawing correctly, then the vernatherm valve is mounted somewhere outside the cooler then? maybe somewhere below the oilfilter?

Maybe underneath the big silver-coloured nut just below the oilfilter on this pic? (the one that is safety-wired)

Neal@F14 · #9 09-18-2011, 12:29 PM

Quote:

Originally Posted by ao.frog

Maybe underneath the big silver-coloured nut just below the oilfilter on this pic? (the one that is safety-wired)

That's it! The vernatherm is under that nut.

On the RV-8 in my avatar, I did something a little different that most folks do to get the oil temps up in winter... I made a hinged door on the rear of the oil cooler to block off airflow, that is opened and closed by an electric servo motor (actually a Ray Allen trim servo). It's worked quite well in as low as 20F OAT. In the summertime, we have to remove the door completely to ensure enough air goes thru the cooler to keep the oil temps down...

Pics in this thread: http://www.vansairforce.com/communit...il+cooler+door

Edited to add: Vernatherms rarely ever fail in the full-extended position (which would force most of the oil to flow thru the cooler). Most often, they fail to extend fully, or seal fully, which will cause less oil to flow thru the cooler, and result in higher oil temps.

penguin · #10 09-18-2011, 01:20 PM

What Dan said (excellent picture)! The large silver nut is the top of the Vernatherm. It will extend by about 1/4" when its hot - take it out, measure it and dunk it in boiling water and re-measure. Also check your temperature gauge, the sender is just a resistor - ask the gauge manufacturer what the cold and hot values are.

Pete