Turbocharged Manifold Pressure
 

My rear engine manifold pressure has been a little flaky lately. The gauge will fluctuate between say 29 and 31 inches in high-altitude cruise. I thought it was a sticky indicator gauge. It wouldn't do it all the time, but I did notice it.

The second to last flight, the rear engine would never make the same MP as the front at the same settings. It was about 2" lower. Again, I chalked it up to the gauge.

The last flight, once I got to cruise altitude of about 16K, I noticed the rear MP was at about 25" when the front was at 30". Again, it would fluctuate between about 23"-25".

My normal cruise speed was knocked down by about 15 knots, and the rear engine ran much cooler than the front at the same fuel flow settings.

The rear engine makes normal MP at lower altitudes.

So my question is this: Where do we begin troubleshooting?

The problem could be, IMHO, one of the following. Feel free to add to the list or expand on the correct testing procedures:

1. Cylinder(s) with low compression ( I'll do a leak-down test this week)
2. Intake manifold leak
3. Bad controller
4. Exhaust leak (prior to the turbo)
5. Bad turbo

Anyone with any experience chasing down this type of an issue, please chime in!!!

I’m normally aspirated so little experience with 337 turbo. But have good deal experience with ground based turbo diesels.

Looks like good list.

Exhaust leaks are under appreciated and can be hard to find with surprising effect on turbo performance. The exhaust system lives in a tough environment so very much at risk. Exhaust leaking at the cylinder of course are their own doubly problematic issue. Also, if you have intercooler don’t forget to look at that. They can shake and crack with a resultant leak.

Air filters get overlooked because they look fine but at a micro level can get obstructed.

Frequently there’s a perfect storm of a little bit of everything: declining turbo, induction leaking, exhaust leaking, dirty filter. Don’t just jump in and sentence your turbo as guilty until you look at everything.

Of course carefully check for turbo shaft play and any rubbing on the housing or oil leaking out of the turbo into induction or exhaust. Any increased oil consumption?

Thanks for the reply.

I wouldn't have thought about the filter. I'll add that to the list.

The rear engine has oil leaks like many seem to. I think it's the pushrod tube seals that are leaking, but I'll look more closely around the turbo.

Experienced input is always appreciated!

I am having the same issue but on my front engine. I do know when I bought the plane 2 months ago, that one of the cylinders has a low compression (55psi). I havent been able to go down the list you posted...so that give me some additional areas to check for myself.

I'll post my findings here. To anyone reading, please do the same.

The plane does have a data-recording engine monitoring system. I may spend the $100 and send the files from the last few flights to Savvy Aviation for their experts to analyze.

Anybody here ever use their service before?

Savvy
 

Your lists looks appropriate. I would use Savvy based upon what I have seen and been told about the companies reputation. I personally have no first hand experience. Best to analyze before "attacking with tools"

The fellow with a 55 PSI cylinder. I wouldn't condemn that cylinder right away without doing a borescope. Followed up by lapping the valves if the borescope inspections leads you to believe a valve problem exists. Burnt valves also can be corrected without removing the cylinder in various cases. Again look at the information from EAA/Savvy on YouTube and go from there.

I'm a SAAVY Guy; Pro and Breakdown. I haven't had any problems, just receive the interval engine reports. I'm going to turn the maintenance over to them as soon as I get the aftermath of my latest annual over and that portion of the records digitized.

I also use Blackstone for the oil analysis.

Compression in Context
 

Compression in Context | Savvy Aviation Resourceshttps://resources.savvyaviation.com › articles_eaaPDF

TCM issued Service Bulletin M84-15 to provide official guidance to mechanics about performing compression tests on TCM engines.

Very informative way to analyze cylinder health. Eliminates a lot of old wifes tales. Regards, Joe

Trouble shooting
 

I hope it is not the turbocharger. My planes has been down 10 months waiting for one and I paid dearly for it. Good luck!

I would first look for any intake leaks from the compressor discharge through the whole system including the sonic nozzle and the hose going to the pre cooler.

Compression checks like you have done, 55 should still be ok if there is not significant burning of the exhaust valve or play in the guides.

Check the exhaust by pressurizing from the tailpipe with your shop vac then using soapy water to check for leaks, bubbles are ok, but blowing bubbles away not good. I have a new Acorn rear exhaust and had to eliminate the springs.

Unfortunately if those things are ok, time to dig further, looking at the waste gate and turbo. You can take the tailpipe off and check the play of the turbine shaft. If there is significant play up and down of the shaft that is not good. Cessna does not object to using a front turbo on the rear engine as up to S/N 98 they used the same one. Not approval from them, depends on how your IA looks at that. My opinion is that it is a minor alteration and no other approval needed, however, I am not your IA.

The wastegate when closed may have significant erosion on the butterfly valve causing blow by.

And then of course it could just be the controller, or the waste gate actuator may have picked up some trash from the oil, take the two hose fittings loose and blow it out both ways.

The maintenance manual has good troubleshooting on the system.

Dan
N67S
Finally got mine flying after 2.5 years, flew it about 12 hours the last few days.

Congratulations Dan on getting your plane up into the air! Now starts the fine tuning process. About the same time it took me to get mine going.
Your going to love that plane!