Rear Engine Oil Temp

[Paul Sharp]

Over the weekend when climbing to 17,500 for cloud clearance, noted that the rear engine oil temp reached redline. So pulled that engine back and increased the fuel flow some, opened the cowl flap - all in an attempt to get the oil temp back down. The oil pressure and cylinder temps were all normal. The cylinder temps went down to the bottom of the green but the oil temp stayed up there.

Finally, when over the mountaints (about half an hour, shut down the rear engine as a precautionary measure and went another 20 minutes to KIDA where I landed. On the ground I removed the avionics access hatch and swapped the oil temp gauge wires. Before swapping the rear was still reading at redline even though the engine had been shut down for half an hour including some 20 minutes in flight. Then after swapping the leads, the one being fed from the rear sensor was reading about normal operating temp on the front gauge. The one feeding from the front sensor (now on the rear gauge) read low, about where one would expect it to be by that point in the engine's cooling cycle.

So I left them that way and flew on home to KSLC. Both read aproximately normal during the ifnal hour's flight.

I don't know if those things are calibrated so that if you switch them around like I did you don't get good readings, but I know at least that the rear sensor was "stuck" in a high setting. So I'll replace that one.

Meanwhile, might I have cooling problems? I've read things in the past about rear cooling, and this is the first time I've had any trouble with it (assuming the sensor wasn't just going bad which is always a possibility). I know to have the mechanics check the oil cooler and fittings to it, etc. What else is there to check?

Is replacing the sensor difficult? Are they very expensive?

[Bob Cook]

Paul

Most likely it is a bad contact(s) on the sender or on the receive end. I would consider sensor the next step. They are available.
You can test them on the ground by heating up oil in a cup and putting the sensor in the hot oil along with an accurate temperature probe. this is the easiest way.

If you are running a P model, the rear should run about 190 depending on the OAT and the front runs about 20-30 deg cooler. Normally I see it run about 200 on climbe when above IAS.

What kind of "cloud clearance" were you given by ATC? I have received just about every clearance but do not recognize this one.

Bob

[Paul Sharp]

Thanks, Bob. I usually see about 20-30 degrees more on the rear, too. This is the first time I've seen it go up like that. I don't know why it even registered high, but it definitely didn't come down like I expected. Do they need to be put in with a special torgue, or thread sealant, or anything like that? (Haven't even looked in the book yet.)

The "clearance" I'm talking about was physical clearance. I didn't wan't to have to file IFR. I went up to 17.5 at one point to avoid the tops. I steered around the buildups, but the ceiling around even those was quite high. I could have contacted ATC and gotten a clearance and gone higher, but 17.5 enabled me to get by OK.

[Bob Cook]

Just joking re clearance.

Re temps. Oil temp does take some time to come down. Suggest you might want to look into jpi, ei or alternative oil temp sensors. They are not expensive. The OEM jobs are NOT that accurate compared to digital ones these days, especially using thermocouples. Typically oil temp sensors have the probe isolated from the body, therefore, no need for temp xfer compound. Just tight enough to stop leaking, being careful not to damage the insulator. The cht probes do conduct heat and measure the probe body temperature.

You will probably need to calibrate or check calibration of the probe when installing a new one or swapping them. They will no doubt vary. Heat up oil, or use boiling water for calibration (212 deg F). I believe the temp will go off scale if an open circuit occurs. You may want to try this on the ground to verify (fails safe).

The oil temp sensor resistance (ohms) varies with temperature. A current passes thru the probe and is measured by a meter. A thermocouple generates a minute voltage based on dissimiliar metals. The voltage is amplified and measured by a meter or Analog to digital converter (A/D) in the newer devices.

I use the JPI oil temp probes on the 760-C and don't bother with the factory probes anymore. In my opinion they are like wetting your finger and holding it up in the air to measure wind speed and direction. Middle finger works best.

What you had asked for was an anti-cloud clearance..........

bob

[Paul Sharp]

Bob - the parts manual says the probe and gage must be ordered as a kit, which would lead one to think they matched them up for resistance and/or etc.

But if I tested one by heating some oil and found it was reading right, couldn't I avoid the "kit" approach? Although, one would think that it could be possible to get a probe that was good and have it working against a gage that was off which was used to match the previous probe which was also off (so that the original two mateched but now the new probe won't even if it's right)?

In a wuandary what to do. Don't want to purchase a diff. system if a simple probe replacement will do, but...

Bob or anyone,

You ever had an issue with the rear oil temp running too LOW? I don't know if it's a sensor, guage, or what, but my past couple of trips this week have been reading bottom part of green or below green on rear oil temp. I was between 5000' and 8000' when it was reading this way, and seemed to be fine on the ground.

I'm one of the unfortunates who have rear cowl flaps stuck open right now, I'm waiting on the fix. But I can't believe open flaps at these altitudes would make that much of a difference.

Any thoughts, well . . . coherent thoughts that is, would be appreciated.

Keven
________
Rare Diseases Advice

[Paul Sharp]

To add a couple more thoughts: The probe was obviously stuck, since the minute I swapped wires to the front gage it read down in the cooled range (half a gage or more difference!). I.e., both gages read down in the normal range at that point - the range you'd expect after some cooling following shutdown.

Could that be something indicateive of wiring or do probes really get "stuck" and then unstack and maybe work fine after that?

And if I used the front engine gage as a guide, would this work:

1 - I warm up and run the engine to "normal" temp at idle.
2 - I see where the gage reads if it appears to be functioning OK for the present.
3 - I shutdown and install new probe.
4 - Restart and warm up briefly again.
5 - See if probe reads nominally "OK" as in matching reading of the old probe.

If it does then I assume the calibration is close enough and go on my way. If not, then the old probe may be bad and must be eliminated for comparison reasons. Then I might try seeing what it reads in the front engine using the same swap-around process and use it if it seems OK in general and reads OK at normal warmed-up temperature on the rear engine?

After all, as you say they are rather relative things anyway. What do you think?

[Bob Cook]

These probes and guages are not thermocouples, therefore, there is a potentiometer on the rear of the guage panel for calibration. They do vary but I do not know how much they do vary. They should be calibrated, however, as long as you know WHERE ON THE GUAGE a calibration point (212 def) is marked, then you can figure the rest out without recalibration.

I would recommend using secondary oil temp guage since this is a very important parameter. I rely on the JPI for feedback. The JPI is within ONE degree and/or 1%.

You could have had a bad connection at the sensor and by swapping you cleaned the contact. I would swap it back and see what happens before changing. BTW you can do the calibration without the engine running.

These probes rely on a calibrated voltage reference. They both can read incorrectly if this voltage drifts. !


Kevin
To answer your question, the only time it reads low is when I shut the engine down <G>. To be serious I only see the temps go to mid-green even with rear cowl flaps open. My guess is the sensor or guage per previous comments. IF it stays low then try the oil or boiling water test by pulling the sensor out and sticking it in the fluid. This test should take no more than 10 minutes including the removal of the cowls. I think the redline should be 210 degrees for oil, not the 240 Cessnas says. GMAS - opinion?

I would rip out all the guages if I could find a suitable replacement for the capacitive fuel level guages. They are a pain.

BTW anyone have comments on the Garmin 340, kma28 or pma7000 audio panel. I am upgrading from a kma24 for stereo audio. Been playing with XM or digital sat radio.

bob

[Gord]

Bob
I have the Garmin 340 audio along with the Garmin 530, works great
Gord

[kevin]

Keven,

What was your rear CHT when the oil was indicating so cold? If it was cold also, my non-expert opinion based on what I have tried to learn from others is that your engine actually WAS too cold. You *can* get your engine too cold (and increase wear) by running with the cowl flap open, unless the ambient it pretty warm (like 80F on the ground or so).

The cowl flaps are there to provide cooling in climb. When you start cruising, you run a LOT more air through the cowl, and you can overcool the engine.

I think.

Kevin

[Paul Sharp]

Bob - I tested the aircraft yesterday, getting a clearance and taking it to FL 200. Everything seemed normal except that at one point the rear temp actually went down about 50 degrees as if the probe "jumped" a little - anyway it seemed that it dropped faster than it should have. So I suspect the probe is just bad.

You mentioned the potentiometer. I'm wondering if my earlier model has that? Otherwise why would they have you get the gage and probe in a set if it were adjustable? You'd think they'd just include instrucitons on how to calibrate it, or at least give that as an option? Anyway I think if I got a diff. probe it would likely work. What you you think?

[Bob Cook]

Paul

The guages have a voltage regulator as a source of stable reference. I have a feeling the voltage regulator may be at fault, however, both front and rear should drop if this occurs.

The sensor itself is a thermistor or a resistor that varies with temperature. There is a possibility they could jump based on an internal fault. This can be caused by a temperature shift.

Think I would swap the sending units or the wires to the guages and try again.

fyi

Bob