Question for Paul McBride, the General Aviation News engines expert: Hi Paul. I appreciate what you do for us through General Aviation News!
I have an RV-8 with about 485 hours on a factory new Lycoming O-360 engine. I have been dealing with very low oil temperatures that I can’t seem to rectify.
I purchased an in-flight adjustable air blocker and placed it behind the oil cooler (far easier application in my case) and it made, essentially, no difference when closed.
My oil temps down low are around 100°F. If I fly higher (about 7,000 feet), it goes down to about 97°F. I thought it might be a defective thermostat.
I had all the avionics changed to Garmin and the engine probes were changed as well. I get the same basic temperature results.
I changed the Vernatherm two weeks ago, no changes.
If I start the engine and let it run and shut it down prior to takeoff, oil temp climbs due to the hot engine case. As soon as I take off, the temp begins to decrease.
After my last flight, I let the engine sit about 10 minutes and the oil temp showed 157°F due to the hot engine casing. I verified this with a Snap On infrared temperature measuring instrument.
My mechanic said he can’t tell me what is wrong and I should call Lycoming. I plan on doing that, but am wondering if you have any ideas on what I could be missing.
Thank you in advance for any help you can provide!
Richard Marz
Paul’s Answer
Richard, I’ve got to admit that your situation has me scratching my head.
I need to ask you some additional questions before I dash off to respond to your situation.
First, you mentioned that this is a new factory engine with about 485 hours on it. Do you have any idea when this low oil temperature issue began? I find it hard to believe that you would have ignored this situation since the engine was installed new. Might it have changed during a routine maintenance event?
I’m a bit surprised that the adjustable air blocker made no significant changes. However, if you actually do have the low oil temperatures that you claim, these wouldn’t be hot enough to force any engine oil through the cooler. Typically, the oil temperature has to come up to about 180°F before the Vernatherm fully closes and forces the oil to pass through the cooler. I really don’t think you have an issue with the Vernatherm because it probably has no impact since your oil is so cold.
You mentioned that if you start and run the engine on the ground and then shut it down, the oil temperature will rise to around 157° according to your information. Even at that temperature range, I would think the Vernatherm wouldn’t open completely forcing the oil through the cooler. Richard, I confess that your situation has me baffled, but I’m going to move forward and offer some suggestions.
What are your cylinder head temperatures? Are they in the range of 385° to 425° during normal operation? My thought is that if they are also low, then we may have too much air coming into the engine, and the result is we’re just not getting the engine up to the normal operating temperatures. If that’s the case, then you may need to reduce the amount of incoming ram air through the bug eyes in the front of the cowl.
Not knowing when this issue showed its ugly head, I’m going to ask you to do some simple troubleshooting for me.
I find it hard to believe that the new Garmin instruments would not provide accurate readings, so let’s remove that from the equation.
I have one thought and this should be where we begin because it’s probably the easiest and least time consuming. I’d like to eliminate the airframe wiring completely from the engine. You might want to consider having your maintenance technician install a new oil temperature probe on the engine and check the temperature using a calibrated or known good gauge. I’m not predicting what the results may be, but it may surprise us. If this doesn’t provide us with any different readings than before, then let’s move to the next idea that comes to mind.
What I’d like you to do is fly the aircraft until the engine comes up to “normal” operating temperatures and, following your landing and taxiing to the ramp, I’d like you to use an old fashion laboratory glass bulb type thermometer.
NOTE: I suggest you attach a piece of safety wire or strong string to the eyelet on the thermometer and allow the bulb to slowly slide down the oil filler tube until it touches the bottom of the oil sump. At this point you should pull it back up the filler tube an inch or so, which will give us a point to actually measure the oil temperature.
Once you’ve allowed the thermometer to rest there for 30 to 45 seconds — using caution — retrieve it from the engine. Record that reading and compare it with the instrument in the cockpit. I’d be shocked if they both read the same temperature.
Assuming that the bulb thermometer is correct, now we’ve got a real challenge on our hands, which is to find out what is causing the instrument to read differently.
It might be helpful to check with other Van’s RV-8 owners to see if anyone else has experienced the same situation.
Reminds me of the O-235-L2C engines in the Cessna 152 aircraft. We had five new ones in our flight school i8n the late ’70’s, and when it became time tonl adjust the valve clearances, the inside of all rocker covers were nothing but rust. Tex Francine at Lycoming said to cover the oil coolers, which I did with some aluminum plates, but the oil temperatures were still in the basement.
Reminds me of the O-235-L2C engines in the Cessna 152 aircraft. We had five new ones in our flight school in the late ’70’s, and when it became time tonl adjust the valve clearances, the inside of all rocker covers were nothing but rust. Tex Francine at Lycoming said to cover the oil coolers, which I did with some aluminum plates, but the oil temperatures were still in the basement.
I have the same issue with oil temperature on my IO-550B Continental powered Bonanza.
I use Aluminum duct tape over the FRONT of the oil cooler. In Pa.. in the winter months.
Cut tape to fit the front of the cooler, allows for modifying the coverage, as temps change.
Words great for me.
I noted that there are 2 location to install an oil temperature probe on the O-360; at the oil filter and at the front of the crankcase.
The oil filter location measures the temp after the oil passes through the oil cooler, so will be much cooler than the front location where the oil has to travel the length of the crankcase.
It would be easy to buy a digital thermocouple thermometer [ on amazon ]., and attach a thermocouple to the oil filter case., and another to the oil pan. These are very accurate and respond quickly .
I did this on my GO-300 where the oil cooler was allowing the oil temps to only get to 108 degF. After an A&P checked the vermatherm and the oil cooler, it was determined that the oil will always go though the oil cooler.
So, I have to tape off 4 to 8 of the 10 sets of fins, depending on the oat, to get the oil temp in the 160-180 degF range.
This is fairly common on RVs, they tend to cool very efficiently. I have the same issue with my RV-8 and with a previous RV-4.
With OATs around 50 degrees and below I have to start putting tape over the front face of the oil cooler to bring the oil temp up. In the winter, with OATs around 30 or below, I have a very hard time keeping my CHTs above 300 degrees. I will usually get about 325-330 in climb, but then they drop to around 300 in cruise. The only way to fix this is to reduce the air flow through the cowl. Some people do it by making a plate to block some of the exit air at the bottom of the cowl, some do it by putting some tape over the intake openings at the front of the cowl.
Putting the plate on the back of the cooler will do nothing. Maybe 3-5 degrees at most. In order to work properly, it is the front of the cooler that needs to be blocked. If you want to use the shutter, it needs to go between the baffle and the cooler. I have one that I plan to install, someday. An easier method is to put aluminum tape over the front of the oil cooler, I use 2″ tape and cut or fold it to various sizes so I have options. As the temperature dips, more tape is needed. In the heart of winter, with OATs at or below freezing, I need to have about 80% of the cooler blocked off. Using this method, I can usually get my oil temps to be very near 180 degrees. Of course, this does nothing for CHTs. Even in the summer, I don’t remove all of the tape, I still keep a third or so at the bottom taped off. It will get pretty hot (200-205ish) climbing out in Eastern Washington, but it cools quickly in cruise at or above 8,500′.
The drawback to the tape method is that it is more difficult to make changes as the top cowl has to come off. I usually only change it a few times a year. I try to shoot for temps between about 170-200 in cruise, though I really prefer to see 180.
Of course, before starting anything like this, you want to verify that your temperature indication is accurate.