Q: Our flying club’s airplane, a 1976 Cessna 172 Model M, has a Lycoming O-320 E2D engine that runs hot only during the summer months of June, July, and August near Grand Rapids, Michigan.
Here are the details we compiled from our JPI EDM-730-4C engine monitor on May 25, 2020, after 15 minutes of level flight:
- Outside Air Temperature at 1,800 feet MSL = 78°F
- 2500 RPM
- #3 Cylinder Exhaust Gas Temperature (EGT) = 1,470°F
- #3 Cylinder Head Temperature (CHT) = 396°F
- Oil Temperature = 215°F and slowly rising.
- Oil Pressure = 65 psi
- 7 Quarts of Oil in Engine
We have replaced the baffles, vernier-therm, oil cooler, and carburetor. We have checked the timing and tolerances on cam shaft and push rods and they are good.
Question 1: What could be causing the EGT, CHT, and oil temperature to be so high when the OAT at 1,800 MSL is only 78°F? These high values are still showing up after we did all of the items above.
Question 2: What should the maximum values be so we don’t overheat the engine?
John Kozal, President, Sparta 140 Club
A: John, the information you provided is certainly a help for me when it comes to troubleshooting, so I thank you for that.
You’ve certainly done several things already that I would normally suggest, so let’s take a step beyond what you’ve already done.
You mentioned the #3 cylinder EGT temperature will read 1,470°F during the flight profile you mentioned. Lycoming does not publish a maximum EGT temperature, but only states that the cylinder head temperature may not exceed 500°F for continuous operation. Therefore, the CHT that you’re seeing of 395°F is well within those limits.
A temperature between 385° to 435° is ideal for long term service life.
Now, let’s look at the oil temperature situation. Oil temperature maximum is 245° for continuous operation and ideally should be between 185 and 210°.
You noted the oil temperature is running 215°F and climbing, but you’ve replaced the thermostatic by-pass valve (vernatherm) which apparently didn’t help.
My question to you is when you replaced the valve did you comply with Lycoming Service Instruction 1316A and reface the corresponding seating surface?
If you only replaced the valve and didn’t comply with this Service Instruction you only completed half the job to correct the problem. So, if you haven’t reviewed SI 1316A, please have your maintenance facility check it out.
I see nothing wrong with the oil pressure of 65 PSI that you mentioned for cruise operation. It would be interesting to know the oil pressure at idle RPM around 600-700 RPM at the conclusion of the flight. Typically, the hotter the oil gets, the lower the pressure, so this may be one thing that you can check on the next flight.
John, the above information is all based on the fact that all of your instruments have been calibrated, including the tachometer. Instruments on aging aircraft are known to be out of calibration and unreliable when trying to troubleshoot an engine problem.
The best recommendation I can offer at this point is to make certain you have access to a Lycoming Operator’s Manual and the Cessna POH for all club members to review — and that all club members read the manual.
A JPI will read higher than a E.I instrument as mush as 50 to 70 degrees depending on the phase of flight. The E.I. will read a more accurate temp that is more in line with the Lycoming recommended number (JPI uses a temp probe that touches the cylinder and this will cause a faster responding probe but also a (what I believe to be) an abnormally higher reading. There have been countless discussions on this subject and friendships have been torn from the result. I believe that 400 degrees should be the absolute maximum temp during the cruise phase of flight or 430 is using a JPI unit.
I went through that issue of high temps after installing a JPI temperature monitor JPI says normal to see a 150 degree temperature split between cylinders. and so what you are suspecting Is a cylinder running lean, so you check intake pipes for leaks at the coupling and cylinder gasket, also check fuel primer lines at the cylinder head for looseness. Also pressure test the jug for any cracks this can be done by removing the valve rocker arms, bottom out the piston and use your compression tester to pressurize the jug.
After that if still running hot Enrich the fuel mixture that will cool it as well. The extra fuel burn is a lot cheaper than an engine.
Check the Standard Temp at 1800’. I believe that you will find it to be closer to 60 degrees F. This could account for nearly a 20 degree increase in the summer.
I have the same plane and engine did all the same things. You’re forgetting one main component. Check your rubber or silicon baffling make sure that engine compartment is sealed off.
It does sound like a baffling issue. Hot cylinders and hot oil indicate large air leak past the cylinders, causing a low pressure differential between the upper and lower cylinder areas. This causes little air to go through the oil cooler, so little cooling and high temps.
Check the inter-cylinder baffles to be sure they are in the correct location.
Gaps between the baffle and the case need to be sealed [ Dow 736 silicone ].
Make sure the baffles are bend up against the cylinder barrel correctly.
Use a digital camera and take pics inside, under the upper cowl, both sides and the rear baffle. I’ve found bends in the baffles around stiffener strips that cause huge gaps.
Use the 736 silicone and seal all small gaps between baffles and the case, valve cover base, side baffles.
8 qts vs 7 qts helps cooling
But doesn’t 7 quarts in that engine seem ample? Also, doesn’t that 8th quart get thrown out pretty rapidly?