losing manifold pressure at altitude

[johnny]

I have a 1976 Riley P337 that I keep losing manifold pressure on the rear engine as I climb. we have swaped controllers, installed a new wastegate and the turbo seems to be within tolerance. I can only get 28" at 8500 feet while the front engine will go to red line. We checked as much as possible for any inductions air leaks. Does anybody have any suggestion - does anyone know anyone that can check the turbo before I purchase a new one?

[kevin]

If you are running climb RPM, and running out of boost at 8500, I don't have any suggestions other than what you have already listed. To get cruise power above 16,000' or so, you generally need to be running 2500 RPM (for 65% power) or so, and you must make sure the wastegate is adjusted to *completely* close on the rear engine. It does not come this way when you buy it, and your mechanic might not be changing it because very few airplanes other than the P337 require the wastegate to completely close at high altitude.

But it sounds like you have a more severe problem. If you have no leaks, then perhaps you should send the turbo for overhaul to someplace like Main Turbo, and see if they can see a problem with it.

I am not an A&P, just a former owner who has experienced this problem, as have many at one time or another. Perhaps others on the site will have a better suggestion.

Kevin

[johnny]

Kevin,

Thanks for the info - I think you are right - My AI is removing the turbo as I am writing this and is going to send it off to be checked and overhauled if needed - I will let you know what the problem was - Thanks - Johnny

[kevin]

Glad you found the information helpful.

Don't forget to make sure your AI sets up that wastegate correctly... Mine couldn't believe it was supposed to be that way until he reread the book.

Blue skies,

Kevin

[Pat Schmitz]

Johnny -

I had the same problem w/ My 337 when I did the pre-purchase inspection...

When we did some hight altitude flying, we discovered (barely) that the induction tube on the back was just slightly loose, and it must have been leaking just enough to keep the engine from developing full boost above 11,000'.... When we would get to that altitude, the rear engine would suddenly get quiet...almost as if it had quit... It was still running, just not at the same power as the front...

We descended, and engine would develop full boost again..

It turned out to be a loose fitting... Since you have already sent your turbo out... its probably moot, but it sounded like you have already taken it apart/reassembled... I would assume you would have found any obvious issues/cracks, etc. etc..

Thankfully, mine was an easy fix...

[GJ Humphrey]

I'm having the same problem right now with my 73 T337G. This is a problem that has grown worse lately.

At altitudes above 10,000 feet, when I start coming back to cruise power, the rear engine will not hold manifold pressure at less than 2500 RPM.

My mechanic tested for leaks in the induction system by pressurizing with a vacuum cleaner exhaust, and he replaced one of the rubber hoses that connect that sections of metal ducting. He cleaned out the waste-gate actuator, from which he removed a rather large particle of carbon. Yet, on the test flight today at 17,000, the problem remains.

In his reply below, Kevin says that one has to use 2500 RPM if one wants to hold manifold pressure at the higher altitudes. Well, yes, 2500 works. But, surely, it's not supposed to be this way. The front engine can be brought back to 2350 without losing manifold pressure.

Any advice?

Gordon

[SkyKing]

Just curious if those of you who are having turbo problems have reached into the compresser side and felt for axial play in the shaft and also inspected the impeller for nicks, etc? For the rear turbo this can be accomplished by loosening the clamp that holds the rubber bellowed boot onto the compressor side, i.e., the "AIR" side. for the front turbo, you need to remove the air cleaner and the inlet scoop from the front cowl to gain access.

Let us know what you find.

SkyKing

[GJ Humphrey]

Thanks, Sky King. I will check the issue of axial play. I've seen that problem described in some of the archives.

[bawb]

Don't forget to check the upper deck reference lines for leaks while doing the shop vac test. It only takes a small leak to give bad information to the controller.

[GJ Humphrey]

Thanks for the suggestion to check the reference lines. We're going to go over the entire system again next week. There must be a leak or leaks, maybe smallish individually but adding up to enough to cause the problem.

[KyleTownsend]

We are currently experiencing this problem on the front engine. Did you ever find a cure?

[GJ Humphrey]

What would you do next?

Since purchase of this 73 T337G (P-model) three years ago, the rear engine has underperformed the front engine at altitudes above 7000 feet unless RPM is 2500 or higher.

At take-off both engines produce full power. Even a recent take-off from Gunnison, CO, field elevation nearly 8,000 feet, produced full takeoff RPM and MAP. But at climb or cruise power, the rear engine just doesn't have the power of the front engine.

In climb, in order to maintain something like equal engine output, it's necessary to advance the rear throttle farther and farther ahead of the front. In cruise at higher attitudes, the rear engine settings won’t stay put. Bootstrapping all over the place. I have to constantly adjust RPM and MAP.

Leaks? The exhaust and induction systems have been tested multiple times for leaks. Minor leaks discovered and eliminated.

The turbo has been replaced. Ditto the wastegate, wastegate actuator, and variable absolute pressure controller. No change in the symptoms.

Recently, with the wastegate wired closed by an A&P the engine exhibited no improvement on a test flight.

Believe me, there are no leaks left in the exhaust system or induction system. The most recent leak test was accomplished using a shop air compressor (not a vacuum cleaner) attached to a special metal adaptor made for checking turbo systems (the adaptor attaches to the intake side of the compressor). The one uncertainty remaining is the integrity of the intercooler. I have no reason to suppose a leak or restricted internal airflow, but can’t rule out either since the unit is difficult to check without removal from the airplane. As for the pressurization system, we capped that off upstream of the flow limiter, with no improvement noted on a test flight.

Have we missed any remedial action?

Is it possible the problem doesn't lie on the "air side" at all, but on the "fuel side?"

Has anyone solved a problem like that described above by addressing the fuel side of the equation? The fuel pressure settings were recently checked and readjusted at idle and at full throttle. Still the underperformance problem remains.

Several really good mechanics so far have not been able to find the problem.

Does anyone have experience with this set of symptoms? All advice welcome.

GJ Humphrey
N3CU, SN P3370063
Concord, NH

P.S. Disregard my posting of two years ago (below). Adjusting the fuel pressure did not cure the problem at altitude.




.

[Roger]

First off, I can't speak about the boost in my 337, as I own a normally aspirated one for specifically the reasons some of you are talking about. I just didn't want the hassle anymore. But I have experience, as my previous Navajo Panter was blown, as is my boat, so I know the drills.

1st, never give up on checking the exhaust side. You may have some obstruction or blowby reducing the exhaust velocity and subsequent rpms of the turbo.

2nd, use the one cost savings measure that twins actually have over our single engine wannabees, ie.. swap parts. If you think it's the intercooler, don't buy a new one, swap it out with the front. Same with the wastegate, etc. It the problem changes with the swap, you've identified the part, if not, leave it alone.

3rd. Dont change anything untill you've swapped out your guages. My MAP and RPM's are all over the place at times as well (typically on my front engine, but it isn't a blower doing it, it's the guages.

Just some ideas/R

[Jerry De Santis]

Good suggestions Roger.

Jerry
N34EC

[GJ Humphrey]

Thanks, Roger and Jerry,

I'll have the tachs checked with one of those devices that counts the blades of the prop. Certainly worth checking.

Honestly, I don't think it's the gauges. I synch the props by ear, and when they're in synch the RPMs read the same, front and rear. If the gauges are the problem, both gauges must be wrong by the same amount, and that seems improbable.

And the problem is not just underboost of the rear engine, it's also frequent bootstrapping. The rear engine power won't stay set for more than a few minutes before, totally on its own, the RPM will either rise or fall and the MAP will follow suit.

Swapping parts is a great idea, but on our weird bird the front and rear turbo plumbing is different. I've just replaced the rear turbo, so there's no point in swapping turbos. I'll look into the practicality of swapping the intercoolers.

Elsewhere on other threads, I've seen reference to leaking fuel strainer gaskets introducing air into the fuel and causing engine surging and/or underperformance. Ditto, reference to malfunctioning fuel tank vents and/or filler cap vents. Could one of those cause the underperformance and bootstrapping of the the rear engine?

And I continue to wonder about possbile leaks in the air reference lines that, as I understand matters, tell the fuel pump how much output is needed.

There's something wrong somewhere, and it has eluded the best mechanics I can find. I just know it's some darned thing we're overlooking. Wish I were an A&P and AI, maybe I could solve the problem on my own!

Thanks for your concern.

GJ Humphrey
N3CU
Concord, NH