The International Cessna^® 170 Association

Richard, have you considered that your gauge may be reading accurately but is being affected by RF interference while in normal operation. It would check perfectly fine with engine shut down but could read low while in operation. This Is very possible with thermalcoulple or transducer type instruments. Does it have a shielded harness?What have you done to inspect and ensure proper shielding?

FredMa wrote:Richard, have you considered that your gauge may be reading accurately but is being affected by RF interference while in normal operation. It would check perfectly fine with engine shut down but could read low while in operation. This Is very possible with thermalcoulple or transducer type instruments. Does it have a shielded harness?What have you done to inspect and ensure proper shielding?

Fred, that's a really obscure ....and wonderful piece of thinking! When I had the speed slope windshield installed on my Baron (back in 1996) I didn't want it pierced by the OAT probe which had been installed previously. So we installed an electronic instrument OAT gauge so we could place the instrument in the panel but the probe thru the sidewall of the fuselage. The unit was quickly considered defective because of abnormal readings and it was returned for warranty. The replacement did the same. It drove us nuts. It would display correctly on the ground, but not in flight.
Then we noticed that it was off during engine runs, and went even wilder during transmission on certain freqs. Yep. It was RF interference. The generator regulators were causing the OAT incorrect readings. RF filters installed on them cured the problem, which was so obscure I had completely forgotten about until your post.
Good on ya!

hilltop170 wrote:Hey Richard, how much water condensation could you possibly get into the engine? That could be a problem along the coast somewhere but at Boulder City? My skin starts cracking the moment I go outside when I'm in that part of the country.

Except that one of the main by-products of combustion of gasoline in air is water. Burning a pound of fuel generates well over a pound of water. The vast majority of that goes out the exhaust pipe as steam, but there's always a small amount of blow-by. If the crankcase is cool, the water in the blow-by condenses there.

FredMa wrote:Richard, have you considered that your gauge may be reading accurately but is being affected by RF interference while in normal operation. It would check perfectly fine with engine shut down but could read low while in operation. This Is very possible with thermalcoulple or transducer type instruments. Does it have a shielded harness?What have you done to inspect and ensure proper shielding?

Interesting. I never even thought of that being a possibility. I will check it out since the wiring (as sent from EI) is NOT shielded in any way. The problem might be finding a way to determine this accurately. Since I can't read the instrument with the master switch off my first thought would be to pull all circuit breakers except that required for that instrument. Hmmm.

If you want another wrench thrown into the mix..... heregoes. When I pull on cabin (yes, cabin) heat, I see about a 5-10 degree increase in temperature. Push it back off, temp. goes down. This takes a few minutes to reveal itself and probably has nothing to do with anything but I thought it was very strange. I have played with this several times since noticing it and it definitely happens every time. It must be something with airflow being reduced over the engine since it is being re-directed to the cabin.

Perhaps you could try this. Note the temp before shutdown as you have done before. Shut the engine down, then turn your master back on to see if there is a difference between engine shut down with master on from when it was running. The generator produces RF even when selected off because as long as the magnets are spinning, electrons are flowing.

LBPilot82 wrote:.... When I pull on cabin (yes, cabin) heat, I see about a 5-10 degree increase in temperature. Push it back off, temp. goes down. This takes a few minutes to reveal itself and probably has nothing to do with anything but I thought it was very strange. I have played with this several times since noticing it and it definitely happens every time. It must be something with airflow being reduced over the engine since it is being re-directed to the cabin.

It may be (and this would be very odd for a quality, electronic/digital, gauge) that the unit is uncompensated.

There are compensated...and (usually cheaper) uncompensated instruments. Uncompensated gauges operate without regard to ambient temperature, while compensated gauges are aware of ambient and are designed to consider that ambient with regard to the actual temp of the item being sampled (in this case, oil temperature.) An uncompensated gauge will indicate higher on warm days and cooler on cold days items sampled regardless of the actual temperature of the item.
A well-known maker of inexpensive aircraft temperature-indicating instruments is Westach. Their units are uncompensated and is one reason the majority are not TSO'd or used as OEM instruments. An example of a good use for one of their gauges might be EGT gauges costing 1/3rd or less than an Alcor unit (all of which are compensated.) An uncompensated gauge in such service is useful because we are mostly interested in relative readings (as related to how much/how little we pull out the mixture-control) and it is not important to most operators that the EGT happens to be exactly 1342 degrees or 1445 degrees, etc.. Most operators are only interested in leaner/hotter or richer/cooler and "At what position of the mixture control results in the maximum EGT?"...and it changes each day...indeed, each flight.
Here's a Westach which sells for $115:

This is why Alcor EGT gauges, even tho' compensated, usually do not bother to calibrate their index units to a particular degree...even tho' their gauges are more accurate (expensive/compensated) ....operators still are mostly interested in relative temperature rather than exact temperature.
Here's a similar-looking Alcor which sells for $350:

Which one LOOKS like it should be more accurate (relative to actual temp) even-though it's actually cheaper and less so? Guess why ...

But an operator with a turbocharger with a turbine inlet temperature-limit (TIT) would require a compensated unit....and is why some Alcor (and others) will calibrate analog index units and/or mfr digital readout units that are compensated. After-all...why have a digital readout if it's not compensated? (Yet, cheaper, uncompensated units will still sell to unknowlegeble buyers.)
Another example of good use for uncompensated gauges might be CHT in an airplane that has no cowl flaps, because the cowl design accomodates a wide range of possible engine cyl. temps, therefore that gauge is not a required gauge. The operator is unlikely to ever see that engine approach it's limitations, but he might be curious in a relative sense, and wish to fill-in an unoccupied location in his instrument panel. For unrequired purposes, uncompensated gauges have a market.
But a design with adjustable cowl-flaps would call for a more accurate compensated gauge in order that the operator be enabled to properly adjust those cowl flaps.

In an oil temp gauge we are interested in more accurate readings so those gauges are most likely compensated for ambient temperature. By adding cabin heat you are increasing the ambient temperature of your new gauge....and it is behaving as if it's an uncompensated one. While I thought all EI units were compensated, perhaps not? or perhaps your unit is defective? You might inquire of them.