60 high CHT and power loss

·         Brad

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Hello Doug,

We have a customers aircraft (J160) here that is having a few issues with high CHT indications and a perceived loss of power over what it has had in the last 495hrs. During a ground run with the Jabiru carbon prop we are seeing 2850 static rpm but number 3 CHT is rising to 170+*C, next hottest cylinder is number 1 at around 145*C. i have swapped the probe between cylinders and the problem is with the cylinder not the probe.

The aircraft had a dynon d10 installed 44 hours ago and since this time the cht's and egt's have remained stable and consistent.

We have checked the compressions, all above 70\80, no excess oil consumption/catch can not filling any faster than normal and gapped the spark plugs to .022 thou. There appears to be no induction leaks and the EGT indications are within 20 degrees of each other.

Also the oil temperature is high at 118 on a short circuit but there is no corresponding reduction in oil pressure so we are planning on changing oil temp senders to confirm the temperature.

The owner has reported that inflight CHT's are 1/120 2/120 3/156 4/118.

Any hints and tips would be greatly appreciated

Brad

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·         Doug Smith

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Hi Brad,
High temps on #3 can often be purely a result of the cooling airflow over the engine, not a result of anything internal. It sounds like you've checked all the obvious things on the engine so - if I were happy that the engine was ok - I'd be looking at the baffles next. Cowl airflow issues could also explain your oil temps. See if any of the baffles have worn or torn or sagged away from the engine. In most places you don't want any gaps you can fit your finger into. Ultimately, being an air-cooled, carby-fed engine differences between cylinders are totally normal and your maintenance (bad leak downs, full catch cans, head bolts moving more than about a 1/4 turn during regular checks etc) will tell you if something needs to be looked at.

By the way, I'm assuming you're not using the CHT under the spark plug - it's common for those ring terminals to get a bit deformed, not sit centrally on the plug, allow a bit of gas leak and so read hot. Obviously that's not an issue if the CHT senders are attaching directly to the head between the plugs.

The only thing is that cooling baffle issues won't explain the loss of power - though your on-ground RPM sounds pretty healthy at first glance. Do you have old data to compare that RPM figure with (i.e. did the engine make 2,900 RPM previously?). If you're satisfied that the power loss is real then it might be worth popping that head and checking for carbon deposits, deteriorated valve sealing, valve guides, hydraulic lifter health etc... but you'd want to be sure it was a real issue and not due to changes in prop or airframe (removing spats can make an aircraft feel sluggish) before getting into all that. Depending what you find it would be worth a phone call to talk with the guys in the engine shop here before you start any major work.

Regards,
Doug.