Oil temperature wonky

[Getonit]

I have a 2000 Mooney ovation with the 310 HP upgrade and the ship oil temperature is starting to act up. Started a couple of days ago the Moritz ship gauge went though the red line but the JPI showed a normal temperature of 180 or so. Aborted the trip, talked with mechanic and he said most likely the sensor. There was no indication of overheat on CHT or oil pressure fluctuating, oil didn't smell burned or anything excessively hot.

Went to fly down to him today and after airplane sat all nite, with cooling temps in low 50's I pulled plane out and gauge showed about 95, JPI about 60. Gauge never went over red line today but it did show about 40 degrees hotter than ship gauge. I have noticed on previous trips the gauge would read cool and then the next trip would show much hotter, never really correlated it to anything.
General thoughts? I am guessing it is just the sensor. Any idea what one costs?

 

[James331]

So normal oil pressure and CHTs

I'd say that gauge or sensor is crap, I'd agree with the JPI

 

[Dan Thomas]

A poor engine grounding can make electric gauges tell lies.The ground strap might be dirty and offering some resistance to alternator charging current, and that current will try to find other paths. It can travel through control cables, and can also go though temperature sensors (because they're grounded to the engine case) and through the signal lead to the gauge, through the gauge (which interprets the higher current flow as a higher temperature), to the battery and from there to ground. In other words, the engine crankcase becomes more negative than the rest of the airframe.

We frequently find poor engine grounding.

 

[nrpetersen]

Only wires should carry electrical signals. Structure should carry forces. Lousy design?

 

[Dan Thomas]

Only wires should carry electrical signals. Structure should carry forces. Lousy design?

Normal design. All airframes except wood or composite are used for return paths for electrical current. All cars and trucks do, too. But everything ages and corrodes and gets dirty and oily, and clamps lose their grip, and contact suffers. Engines get changed and the installer sometimes doesn't take care to ensure that the grounds are clean and tight. Sometimes they even leave off the ground straps, so that current ends up travelling though any metal plumbing to the engine (like primer lines) or though engine controls, which is really stupid. Heavy starting currents can heat up those cables and wreck them or weaken them or seize them.

It only takes a fraction of an ohm to encourage current to start looking for easier ways. As the ground resistance gets higher, it gets worse.

Cessna sells retrofit kits for its older airplanes to replace the old mechanical oil temperature gauges. You get a new gauge, a temperature probe, and some wire. They subsequently came out with an SB, and later kits came with another wire, to install a dedicated ground wire from the gauge case to the engine case close to the probe, to catch that stray current and keep it out of the signal line. I had one 172 that would spike its oil temp once in a while, and fixed it with that wire.

The gauge, I believe, has a reference resistor in it that drives an electromagnet to move the needle downward. The temperature probe is the other resisitor and it drives an electromagnet to raise the needle. This is to keep voltage variations from affecting temp readings. The reference resistor is grounded to the gauge case, the temp probe to the engine case. Differences in ground potential caused by bad engine grounding will make the gauge read erroneously.

 

[Getonit]

I don't disagree with looking at grounding issues, as cheap and easy, but wouldn't that issue cause readout issues in all of the other resistance style gauges? CHT, EGT, etc.

 

[Dan Thomas]

I don't disagree with looking at grounding issues, as cheap and easy, but wouldn't that issue cause readout issues in all of the other resistance style gauges? CHT, EGT, etc.

It could affect CHT, but EGT is usually an independent thermocouple. A CHT sensor can be independent as well, but an electric sensor would just display a temperature higher than normal and perhaps still within range.