Cherokee 140 High Oil Temp. Update

As you may recall, I have been troubleshooting a higher than normal oil temp
in the Cherokee. Checked the baffling, replaced the oil cooler with a new
one, replaced the Vernatherm, checked the oil cooler lines, all with no
luck. Temps stay 1-3 needle marks above 180 degrees which is 1-3 needle
marks above where it should be at 180. Temps have stayed put at 180 degrees
in the summer for the past 15 years (through 2 different engines), unless it
was 95 degrees plus outside. Last 100 hours or so they have started creeping
up. Last week on a flight test (slow flight, stalls, etc.) the bugger went
up to redline. Oil pressure was right where it always is in the middle of
the green.We made a precautionary landing to check it out. Nothing unusual.
Oil cooler, lines, same temps as they usually are. No smoke, no smell, just
normal stuff.After checking it out for a grand total of 3-4 minutes, we
started up and the temps were well below 180. Seemed to have dropped quite a
bit in the few minutes we were on the ground. Took off and the temps stayed
right at 2 needle marks above 180 as they have been for the 10 minute flight
to the home field.

I have been suspecting the gauge or sender but was unable to get swap units
to try out. So, I finally pulled the sender out and heated up some oil in an
electric fondue pot to test it. I had 2 digital and 1 bulb thermometers in
the oil bath to be sure I had a good "reference" temp. Thermometers were
within 2 degrees of each other. I suspected the sender was bad and that I
would see indicated readings on the gauge that would be 30-40 degrees above
what the thermometers reported.

Surprisingly, the readings were actually LOWER in the cockpit by about 40
degrees. That means that, if this is to be believed, this plane is not only
running hot, but VERY hot. If that was true, then the redline reading I saw
last week would actually have been reflecting a 300 degree oil temp! I
cannot see how that could be so if the pressure was right smack in the
middle of the green as it always is. I initially grounded the sender to the
block for the test, but others suspected a faulty engine ground, so I also
grounded it to the firewall and several other places. Same results -
thermometers report temps 40 degrees higher than the gauge indicated.

I'm stumped now. Even if I had a main bearing shift causing the high oil
temp, I should also be seeing low oil pressures at the temps the gauge is
reading - especially when I was at redline. In addition, what the tests
suggest is that the "real" temps are actually 40 degrees HOTTER!

Some suggest that the Vernatherm seat is deformed. The original therm had a
score mark all the way around suggesting full seated contact. And, how could
a seat deform to leak that much in normal operation? Could there be a
particle stuck in the seat or some blockage in the passages to the oil
cooler lines? Even if the oil cooler was bypassing due to a poor Vernatherm
seat, wouldn't the pressure drop if the high temp was "real"? I'm going to
pull it out again and inspect up the mounting hole to see if there is a
visible obstruction or deformity.

Any other ideas?
Thanks,
Mike

Mike Spera wrote:
As you may recall, I have been troubleshooting a higher than normal oil temp
in the Cherokee. Checked the baffling, replaced the oil cooler with a new
one, replaced the Vernatherm, checked the oil cooler lines, all with no
luck. Temps stay 1-3 needle marks above 180 degrees which is 1-3 needle
marks above where it should be at 180. Temps have stayed put at 180 degrees
in the summer for the past 15 years (through 2 different engines), unless it
was 95 degrees plus outside. Last 100 hours or so they have started creeping
up. Last week on a flight test (slow flight, stalls, etc.) the bugger went
up to redline. Oil pressure was right where it always is in the middle of
the green.We made a precautionary landing to check it out. Nothing unusual.
Oil cooler, lines, same temps as they usually are. No smoke, no smell, just
normal stuff.After checking it out for a grand total of 3-4 minutes, we
started up and the temps were well below 180. Seemed to have dropped quite a
bit in the few minutes we were on the ground. Took off and the temps stayed
right at 2 needle marks above 180 as they have been for the 10 minute flight
to the home field.

I have been suspecting the gauge or sender but was unable to get swap units
to try out. So, I finally pulled the sender out and heated up some oil in an
electric fondue pot to test it. I had 2 digital and 1 bulb thermometers in
the oil bath to be sure I had a good "reference" temp. Thermometers were
within 2 degrees of each other. I suspected the sender was bad and that I
would see indicated readings on the gauge that would be 30-40 degrees above
what the thermometers reported.

Surprisingly, the readings were actually LOWER in the cockpit by about 40
degrees. That means that, if this is to be believed, this plane is not only
running hot, but VERY hot. If that was true, then the redline reading I saw
last week would actually have been reflecting a 300 degree oil temp! I
cannot see how that could be so if the pressure was right smack in the
middle of the green as it always is. I initially grounded the sender to the
block for the test, but others suspected a faulty engine ground, so I also
grounded it to the firewall and several other places. Same results -
thermometers report temps 40 degrees higher than the gauge indicated.

I'm stumped now. Even if I had a main bearing shift causing the high oil
temp, I should also be seeing low oil pressures at the temps the gauge is
reading - especially when I was at redline. In addition, what the tests
suggest is that the "real" temps are actually 40 degrees HOTTER!

Some suggest that the Vernatherm seat is deformed. The original therm had a
score mark all the way around suggesting full seated contact. And, how could
a seat deform to leak that much in normal operation? Could there be a
particle stuck in the seat or some blockage in the passages to the oil
cooler lines? Even if the oil cooler was bypassing due to a poor Vernatherm
seat, wouldn't the pressure drop if the high temp was "real"? I'm going to
pull it out again and inspect up the mounting hole to see if there is a
visible obstruction or deformity.

Any other ideas?
Thanks,
Mike



You have an oil temperature gauge that reads hot when running,
and cools readily when stopped.
You have tested that the sender appears to be OK (If I'm reading you
right, you used the airplane gauge too?)

So now the focus is on the cooling mechanism.
This involves three items: air inlet, air flow round the cylinders, and
air exhaust.

So have you looked at the outlet? That's the one you most likely missed.
Is there a door on the cowl? Is there a change in the engine
compartment that couldn't possibly affect oil temperature? There is?
That's it!
Or are you getting a hot exhaust leak into the engine compartment?

Brian W

On Mon, 28 Jun 2010 19:17:27 -0500, "Mike Spera"
wrote:

As you may recall, I have been troubleshooting a higher than normal oil temp
in the Cherokee. Checked the baffling, replaced the oil cooler with a new

after you land and park the temp guage is still indicating if powered
up.
sooooo. why dont you pull out the dipstick and measure the temperature
of the engine oil all the way down the hole?
I dont know if one of those off contact laser temp indicators would
work in the confined space but good ones are accurate and clean.
you would be able to see what the gauge should be reading.

2 cents worth and all that.
Stealth Pilot

"Stealth Pilot" wrote in message
...
On Mon, 28 Jun 2010 19:17:27 -0500, "Mike Spera"
wrote:

As you may recall, I have been troubleshooting a higher than normal oil temp
in the Cherokee. Checked the baffling, replaced the oil cooler with a new

after you land and park the temp guage is still indicating if powered
up.
sooooo. why dont you pull out the dipstick and measure the temperature
of the engine oil all the way down the hole?

That sounds like a pretty damn good idea. You can buy relatively cheap DVM's
that come with temperature probes included.
http://cableorganizer.com/triplett/d....htm#featuresI just measured my probe. It is a 37" wire lead with the tiny probe at one endand the plug at the other. That should be plenty long enough to reach down tothe oil level in an aircraft engine.You could also tape the probe to your oil cooler inlet tube to get an idea ofyour oil temperature with the engine cowled. (I do something like that tomeasure the hot fluid temperature from my car's AC compressor so I can add theproper amount of refrigerant.)Vaughn