High CDT

[Mikey]

Hello all. 

I was flying to Oshkosh and just west of Texas I noticed the EGT and CHT of #6 cylinder were off of what they normally are. Not enough to be alarmed and land, but just off. I went up higher to get out of some thermals and noticed the Compressor Discharge Temperature was at redline. I stopped the climb and leveled off. The CDT went down but was still reading about 25 degrees hotter than normal.  When I landed at KGPM I looked inside the cowell and saw the #6 cylinder didn’t have an exhaust gasket. Shop on the field checked it out and fixed it but said turbo was bad so they sent it to Main Turbo. Main turbo said it only needed a gasket replaced so they fixed it and bench tested it. Everything checked out.

I flew the plane back to San Diego and was still getting high discharge temps on climb and at altitude. I had the shop next to me in SD test the probe/gauge to make sure all was correct. It checked out. Took it flying and down low all is well with the CDT. At 8k and above CDT starts headed to redline.

It’s a M20K/231 with intercooler & merlin waste gate. I’ve flown the plane 600+ hours and this is the first time this has happened. Any ideas? 

[jlunseth]

Well, as far as the health of the engine itself is concerned, exceeding the CDT redline in your aircraft is not a big issue because of the intercooler.  The redline was set for the factory, non-intercooled configuration, to protect against detonation caused by induction air temperatures that are too high.  In the non-intercooled engine, CDT equals or approximately equals Induction Air Temp.  In the intercooled engine, the intercooler drops the IAT by as much as 125 degrees.  Even at 8k, and this is just a guess cause I don’t pay much attention to it, it is dropping IAT by 50-75 degrees. So you still have good margins against detonation.  

That said, the CDT’s you are seeing don’t seem normal to me and are a symptom of something, the question is what.  I occasionally see redline CDT’s during a climb into the flight levels during the summer, but it is not normally an issue until the aircraft is above 18k MSL, not 8k.  

The thought I have is to have your Merlyn checked.  I suppose it is possible that it is not opening (the wastegate is not opening sufficiently), which would make your turbo work harder than normal, which would heat the turbo up, which would heat the discharge air.  I suppose it could also be a significant induction leak, which would allow induction air to be blown overboard, and the turbo works harder to make up for it.  Those are my thoughts, since the turbo and probe are normal. Unlikely it would be hotter than normal exhaust gases, but possible.  Do you have an engine monitor, if so, you would see that on the monitor, and in any event it would take some very hot TIT’s to warm the discharge air that much.  I routinely operate at around 1600 and don’t see redline TIT’s except towards the end of a high climb in the summer, as I said. 

[carusoam]

What did the first shop see that Main Turbo didn’t?

Got Somebody that can review maintenance on the Merlyn?

Are you familiar with restarts after a turbo failure at altitude?

Pp thoughts and questions that come to mind... not a mechanic.

Best regards,

-a-

[Mikey]

I am going to have the wastegate checked. That seems like the next logical step. I’ll update when I know more.

[Austintatious]

any update?

 

[kortopates]

any update?
 

I don’t recall, but he flies a Lancair IV these days.


Sent from my iPhone using Tapatalk