found cause for losing manifold pressure

[johnny]

After replacing the turbocharger, the wastegate, the controller and the entire exhaust system we found the problem, which had nothing to do with any of the items we replaced. The first problem was my first mechanic did not have a clue. I ended up going to Million Air and after only two test flights they found that the problem was with the rear pressurization system dumping bleed air out at an extreme rate. They are now working on sealing up the leaks and we should be flying any day. We tested the plane with the rear pressurization system blocked off and the manifold pressure never dropped an inch. I will keep everyone informed about the final outcome later this week or next.

[Jerry De Santis]

Johnny, That does not mean they cured the problem. I'v done that to see if the turbo is putting out a good boost and eliminate the turbo as the problem. You should get full turbo pressure when the pressurivation is in dump mode. In my case, we found the exhaust tree feeding the turbo had large cracks.

Wow, I find it hard to believe that your cabin leaks are so big it effects the MP.

Anyone else out there have any thoughts on this?

Jerry

[SkyKing]

Johnny,

You said that Million $ Air discovered the rear pressurization system dumping bleed air... but from exactly where? I presume that both dump valves on the lower pedestal were pushed-in all the way when they discovered this.

Next time you have the rear cowl sides off check all the scat tube connections to be sure the band-clamps at the connection points are all snug and that you don't have any wear-rub holes in the tubing.

Gee, maybe your mechanic should be "charged back" for the turbo, wastegate and controller! I presume you had these overhauled? How much was the whole ticket?

SkyKing

[Jim Rainer]

I too don't think that will cure your problem. The turbos should make normal MP with the presurization system turned off just as in the un-pressurized turbo mobel BUT I have been wrong before and I'd be inetereted in knowing the outcome.

[WebMaster]

OH, which MillionAir??
I frequently use them at TEB and MDW, they are nice folks, and have done some minor maintenance for me (at TEB).

[kevin]

Re: Jim's comment about the pressurization being a problem

Maybe I am thinking about this wrong, but I think I disagree. Here's why:

A pressurization system is essentially a controlled exhaust leak. It takes a larger turbo to make enough MP to overcome this leak in the P models. The turbo is not the same one as on the non-P versions, it is physically larger.

If you make the exhaust leak too large, you will be unable to maintain MP at altitude. The balance is so delicate that above 16000', at 65% power, I used to have to use 2450 or 2500 RPM instead of 2400 to maintain MP in my '73 P. Also, an incorrectly adjusted wastegate (not 100% closed) will prevent maintaining MP at high altitude.

Many times I have spoken to owners about P337's that were not tight enough to allow operation at high altitude, even at 75% power.

There is a mistaken impression that if you pull the pressurization dump valves, you are just opening the cabin to the outside air, and folks who have done this notice that the engine still makes good MP. But when you put those valves in the dump position, they also *close* a valve that seals the cabin off from the exhaust system, thus plugging the exhaust leak entirely.

I think it is quite reasonable that an improperly sealed cabin, or inproperly operating pressurization valves, could make a P337 unable to maintain manifold pressure due to what would essentially be an uncontrolled, excessive exhaust leak.

Kevin

[johnny]

For all that are interested I will find out in the next day or so exactly what they did when they disconnected the pressurization. I will also let you know exactly what corrected the problem on the pressurization side. To answer a few questions from several individuals here we go:

1. The dump valves were all the way in
2. From what I understand they did not disconnect at the cabin but rather somewhere near the engine – I will find out for sure in the next day or so exactly what part they disconnected.
3. It was the Richmond, VA Million Air.
4. Yes I am considering charging the mechanic for the items replaced for no reason.
5. You really don’t want to know how much the whole thing cost
6. All I know for sure at this point is that with the rear pressurization system blocked off somewhere near the engine the MP held (31 in) regardless of RPM (2300 to 2550) up to 14,000 – we did not go any higher that day – before it lost MP at 6,000 – Max was 26in with WOT at 2300 RPM – 29 in max with WOT at 2500 RPM.

[Jerry De Santis]

On the top of the rear engine on the right side of the plane there is a alum cone shaped tube with a flange that connects to a hose that is attached to a bulkhead fitting entering the cabin. There are four small screws that hold the tube in place by the tube flange. What they do is remove the alum. tube and place a blank over the port that the tube was fed by. This elimates anything involving the cabin.

What I find interesting is Kevin's description above. He has a lot of knowledge on the Skymaster and what he said regarding opening the cabin dump cable(rod) which in turn dumps cabin pressure but also closes a valve preventing loss of MP. Yes, this is true--- so, did Millionaire do the check that way or did they first remove the cone tube and installed a blank.

BTW, that alum. cone type tube often gets cracks and leaks in them and indeed, that alone can be the problem. Had my repaired welded twice. Lot of vibration between engine and firewall. And yes, I don't want to know the cost! I am still sick over the cost of mine.

Thanks
Jerry:

[SkyKing]

Jerry,

I think what you're trying to describe is the sonic venturi, which actually serves as an air flow limiter, that comes off the engine's induction system - just before the pressure relief valve - and routes over to the air selector valve with two possible positions, (1) going to the heat exchanger if one desires cool air for the cabin; or, (2) going to the heater if one wants to fire-up the Janitrol. This venturi, which is a contoured, cone shaped aluminum tube, LIMITS the amount of pressure to the cabin. If that device wasn't there, the turbo system on the P's would be MORE than capable of OVERpressuring the vessel far beyond the design limits of 3.35 PSI at max differential.

As I recall, the sonic venturi device connects to the induction system via a short piece of rubber hose and a couple of band-clamps. IF there were a leak AHEAD of this device, then yes, you would definitely have a difficult time maintaining sufficient MP at altitude. But, if the leak was somewhere on the other end of the venturi, then it wouldn't matter, as it would be the same as pulling the dump valve and letting that 'limited' amount of pressurized air flow overboard.

With regard to Kevin's remarks, insofar as being able to maintain MP at critical altitudes, that has nothing to do with the cabin pressurization... they are as separate as apples and oranges and one of the reasons why I was having trouble with the purported diagnosis of Million $ Air, i.e., that they could seal off the cabin and wa-la, we now have sufficient MP from the turbo on the gauges at altitude. Something doesn't jive here, as like Kevin says, when you dump the cabin pressure with the dump valve, the already limited air pressure to the cabin coming from the induction system through the venturi is being dumped overboard... and at the same time there is a check valve in the plenum on the firewall side that seals shut... making it possible for the other engine's independent sonic flow limiter to sufficiently pressurize the cabin.

Either of the engines will sufficiently pressurize the cabin, that is, IF the engine is developing proper amounts of MP/RPM and you're not bootstrapping. UNLESS, of course, if the pressurization switch is in the OFF position... if it's OFF, electrical power is being applied to the safety/dump valve solenoid on the rear firewall and the valve is held in its OPEN position to prevent pressurization of the cabin. This is not the same thing as the dump valve controls on the lower pedestal.

If the turbo, wastegate, controller and pressure relief valve are all working like they're suppose to, then my bet is that there is either a leak in the upper deck reference lines -OR- a leak in the engine's exhaust system; or, as mentioned earlier, possibly a leak just ahead of the sonic venturi.

What Million $ Air should really do is perform the check as in the service manual to maximum critical altitude to help identify the problem.

SkyKing

[stackj]

Johnny,

You indicated Million Air in Richmond, VA. Are you base here? I am based at Chesterfield, 11 mi from RIC.

[johnny]

I hope to know by early week what was diconneted to make the system work properly. All I can say at this point is that Million Air Richmond is the only one who new and understood the system and what they were doing. They studied the problem made a few calls and bingo for the first time since I purchased the plane I was able to maintain MP. I will keep everyone informed as things progress.

[SkyKing]

Just wondering if you have a Service Manual for your Skymaster? Together with those sections relating to the engines and pressurization plus the "Airplane & Systems Descriptions" in Section 7 of the POH, you could probably do much of the diagnosis on our own... but education must come first. This is the primary reason why most mechanics are unable to deal with the problem... lack of education!

SkyKing

[hewilson]

I never fail to be both impressed and amazed at the cumulative knowledge of this site's members. This thread in particular reminds me of going on rounds at the university hospital when i was a student and intern. Everyone is well informed, has an opinion and is not afraid to express it. Kudos to all.

[GJ Humphrey]

I've just come through a very lengthy, very frustrating and very, very expensive trial in dealing with a rear engine manifold pressure problem on N3CU, a 73 T337G.

At altitudes above 10,000, I could not bring RPM below 2500 without the manifold pressure dropping suddenly by a couple of inches. At altitudes aove 10,000 the problem was even worse.

We went through all the usual tests looking for exhaust and/or induction leaks, blocked orifice, bad waste gate controller, sticking waste gate, bad turbo, bad Absolute Pressure Controller -- you name it, we did it. What an agony!

Finally, I think we found the problem. The rear fuel pump pressure was set too low. To increase (or decrease) pump output pressure is the simplest, fastest, cheapest operation you can think of. A slight twist of a setting screw.

The strange thing is, before the adjustment of the fuel pump I never had any problem in getting full take off power. I could even cause an overboost if I wasn't careful. Yet, using a pressure gauge attached to a T-fitting while the engine was at take-off power, the mechanic found the fuel pressure was below that specified in the service manual.

There is a procedure in the Aircraft Service Manual that details how to set the fuel pump pressure while the engine is runing at take-off power.

On the test flight to 10,000, RPM came back to 2300 with no problem. After the test flight, I flew N3CU home from Fort Pierce, FL to New Hampshire into fierce headwinds and climbed only to 11,000 on each leg. Again, the rear engine ran happily at 2350. The airplane went in for it's annual th next morning, so I haven't had the opportunity to take it to a higher altitude, but I think the problem is fixed.

Gordon Humphrey