Fluctuating MP on rear engine

Ken MacLean · #1 03-23-04, 07:07 AM

Yesterday I was flying my T337C and began to experience fluctuating MP (22"-26")on the rear engine. I had been flying at FL220 and was doing descending holds to 12,000 when the problem appeared. OAT was -35 Fahrenheit, and no problem with the front.

I tried adjusting mixture (no effect) and throttle position (raised upper and lower limits of fluctuation range ). In descent to 8,000 the needle steadied, then below 7,000 fluctuation reappeared with fuel flow reading 12 GPM (front engine was 8.5). I went down to 4,000 and was able to continue to an uneventful landing with no further problems, except that the fuel flow remained stuck on 12 GPM (rear engine) even after shutdown.

All other indications were normal - EGT, oil pressure and temp and cylinder head temp.

During the worst of the fluctuations the engine seemed to be surging.

I remember reading a thread about folks with a similar problem that ended up being rather simple - some screw or something in the fuel control unit. Do I remember correctly? Any other ideas?

Thanks.

skymaster · #2 03-23-04, 08:44 AM

i sprayed some mouse milk on the turbo ? what was your rpm? at higher alts mine needs high rpm. or it will bootstrap/surge. J 73p

kevin · #3 03-23-04, 09:59 AM

Ken,

I have had the surging you are talking about in my '73 P, but 4" seems a bit excessive. 2" surges have turned out to be the wastegate, as the other gentlemen suggested. Just lubricate it.

But I have also had problems like this (once) indicate a failure of the turbo. So I would have your mechanic check that as well, make sure the blades are still intact, will spin freely, etc.

Since you have a turbo, not a P, RPM is not likely to be a factory, your turbocharger is not working nearly as hard as the P model's do.

I am sure mechanics on the board can suggest other possibilities, I am just reporting to things that have happened to me as an owner.

Kevin

Ken MacLean · #4 03-23-04, 10:30 AM

MP 26", RPM 2400

Richard · #5 03-23-04, 12:04 PM

I had the same problem with my rear engine. Turned out to be the turbo seperating from the center section and needed to be replaced. I agree that it could be the wastegate, but the range 4" = 2 psi would suggest the turbo. If the turbo is fine (end play, etc) then I'd look at the controller on the top of the engine. Those old turbos are also suspect to coking on the bearings. The turbo should spin freely and without effort. If it seems difficult to spin or a rough feel, then you need to overhaul the turbo.

Paul Sharp · #6 03-23-04, 05:16 PM

The controller can also cause those kinds of problems.

As for the high altitude surging, at 12K I don't think that is high enough to cause it anyway; plus you said you had it set at 2400 so that should keep it with enough drive to take you up to your ceiling limits.

Ken MacLean · #7 03-25-04, 08:48 PM

Here's what they found, although I can't confirm it has fixed the problem. Apparently, just ahead of the turbo is a clamp holding a boot in place and although the clamp was tight, the boot was not attached at the rear which the mechanic detected when inspecting with a mirror. We believe this was allowing the introduction of air to the system and causing the fluctuation. We'll see when I fly it next. Could the -35 F temp have caused this? This was my first venture into the flight levels.

The fuel flow gauge turned out to have a failed spring in the unit so the reading just settled at 12 GPM. Seems odd that the two problems would appear simultaneously, but as I have read in other posts working the bugs out of a 337 can be time consuming as the bugs just keeps appearing.

My faithful Skylane took me back yesterday to finish what I had started with the 337. But then again, it took awhile to get the bugs out of the Skylane, too. Now I can't bear to part with it.