Manifold pressure gauge

[Mason]

Aircraft C401 TSIO-520

At idle, one engine indicates about 15in MP (normal) and the other indicates around 20in MP with RPM equal on both engines. At t/o and cruise, all indications are normal. I'm thinking that it is some sort of leak in the MP gauge or line. Anyone seen this before? What could be some other causes.

 

[Torqued]

There's a couple of things that could affect your idle MP. One is the idle mixture setting. A too rich mixture can give a high MP. Try leaning the high MP engine and see if that reduces the MP.
Next, it could be the prop low pitch setting. During runup, does the split in MP continue as you increase RPM until the prop governors start to limit RPM? If so, the side with the higher MP may have its low pitch set a little coarser than the other. I'm not familiar with the C401 so I can't say whether it could be adjusted on the aircraft or not.

 

[erj-145mech]

I wouldn't spend a lot of time or effort to marry up the needles at idle, since you don't have any specs at that power setting. But it does sound like an idle mixture issue.

Does the engine sounds like its "loping"?

 

[bocefus]

Never saw a TCM fuel injected engine that didn't "lope" at idle

 

[RedGuy]

I fly 310's and 402's both with IO/TSIO 520's and I can tell you that the MP very rarely matches up at idle. It's not uncomon to see a 5 in split at idle, but usually it's more like 2-3 inches. I suspect it's a mixture issue since once you start bringing the power up the needles keep getting closer and closer as the power is increased.

 

[Bus Driver]

Having absolutely no experience with the TSIO- 520, I am curious as to why you gentlemen are leaning toward an idle mixture problem vs. an induction leak or possibly a wastegate problem.

 

[avbug]

There's a couple of things that could affect your idle MP. One is the idle mixture setting. A too rich mixture can give a high MP. Try leaning the high MP engine and see if that reduces the MP.

What??? How do you suppose mixture has any effect on manifold pressure? It doesn't. Mixture has an effect on power, and temperature, but not on manifold pressure, unless it changes RPM. For a given RPM, mixture has no effect on manifold pressure.

Manifold pressure is a function of throttle position and engine RPM, period. For a given engine RPM, increasing throttle will increase manifold pressure by reducing the obstruction to engine suction through the induction. For a given throttle plate position setting, increasing RPM decrease manifold pressure.

In normal operation, increasing throttle plate position increases manifold pressure in much the same way that removing an obstruction from a vacum cleaner hose increases the pressure in the hose...by removing the obstrution to the hose. Your engine is an air pump, and consequently a suction machine. The induction represents the vacum cleaner hose, or suction end, and the exhaust represents, well...the exhaust. Put your hand over a vacum cleaner hose, pressure drops in the hose, just lke closing the throttle on an engine.

For two otherwise identical engines to have differing manifold pressure for a given throttle setting with identical RPM, you either have an induction leak or an indication error. This assumes the same engine RPM.

Take the theoretical shopvac, turn it on, put your hand over the hose. Measure the pressure in the hose. There's your manifold pressure; the hose is the induction manifold. Cut a hole in the hose. Hose pressure will rise, and there's your induction leak.

If it's an indication problem, you may see a higher manifold pressure, but you're also going to see temperature differences if you have a means of monitoring them. The same is true of an induction leak.

How matched are the engines at idle?

 

[Mason]

What??? How do you suppose mixture has any effect on manifold pressure? It doesn't. Mixture has an effect on power, and temperature, but not on manifold pressure, unless it changes RPM. For a given RPM, mixture has no effect on manifold pressure.

Manifold pressure is a function of throttle position and engine RPM, period. For a given engine RPM, increasing throttle will increase manifold pressure by reducing the obstruction to engine suction through the induction. For a given throttle plate position setting, increasing RPM decrease manifold pressure.

In normal operation, increasing throttle plate position increases manifold pressure in much the same way that removing an obstruction from a vacum cleaner hose increases the pressure in the hose...by removing the obstrution to the hose. Your engine is an air pump, and consequently a suction machine. The induction represents the vacum cleaner hose, or suction end, and the exhaust represents, well...the exhaust. Put your hand over a vacum cleaner hose, pressure drops in the hose, just lke closing the throttle on an engine.

For two otherwise identical engines to have differing manifold pressure for a given throttle setting with identical RPM, you either have an induction leak or an indication error. This assumes the same engine RPM.

Take the theoretical shopvac, turn it on, put your hand over the hose. Measure the pressure in the hose. There's your manifold pressure; the hose is the induction manifold. Cut a hole in the hose. Hose pressure will rise, and there's your induction leak.

If it's an indication problem, you may see a higher manifold pressure, but you're also going to see temperature differences if you have a means of monitoring them. The same is true of an induction leak.

How matched are the engines at idle?

This issue occurs at idle RPM (1000), approx 5+ inch MP split. As the throttles are advanced the split decreases until the turbos really start to come online around 29-30in MP then there is no split up to full power.

 

[Torqued]

What??? How do you suppose mixture has any effect on manifold pressure? It doesn't. Mixture has an effect on power, and temperature, but not on manifold pressure, unless it changes RPM. For a given RPM, mixture has no effect on manifold pressure.

I agree with the theory of what you said but I have seen this many, many, many times. I can only surmise that the rich mixture must have something to do with the combustion efficiency and that affects the airflow through the engine. I must repeat also that I'm not familiar with his particular aircraft so other things may apply.
I'm not posting here to start an argument. I only post to share some experience and maybe help someone. If the shoe fits...

 

[avbug]

If you see a change in manifold pessure when adjusting the mixture, you've chaned the engine RPM to do so. If the RPM stays constant the the manifold pressure stays constant.

What type of temperature monitoring do you have onthe engine and what temperatures are you seeing?

At 1000 RPM, you are seeing a 5" split in manifold pressures? Which way, and what values? The correct value should be approximately 12" of manifold pressure. What is being displayed?

 

[Torqued]

If you see a change in manifold pessure when adjusting the mixture, you've chaned the engine RPM to do so. If the RPM stays constant the the manifold pressure stays constant.

When the mixture is enriched the RPM drops. The idle speed is then adjusted up by opening the throttle plate a little. So you end up with both engines at the same RPM but at different manifold pressures. Works every time. The split in MP will decrease as the throttles are opened further and the main metering system comes into play and the idle system becomes irrelevant. Can't say for sure at what RPM this will happen.
Since it's cheap and easy and a likely cause, I'd check the idle mixture first.
As for the prop low pitch stop, it will usually give a steady split in MP until the governors move the blades off the stop to a coarser pitch to control RPM. So you should see the MP split go away at TO RPM if that's the problem.

 

[Torqued]

Having absolutely no experience with the TSIO- 520, I am curious as to why you gentlemen are leaning toward an idle mixture problem vs. an induction leak or possibly a wastegate problem.

It could be any of a number of problems but it's always best to start with the simple and easy first and get them off the list. Idle mixture is easy to check and causes this type of issue (on some engines) regularly.

 

[avbug]

Idle mixture is checked with the throttle plate closed. Clear the engine at runup RPM first, leaning it thoroughly. Then enrichen the mixture and retard the throttle to idle. Note your manifold pressure. It should be approximately 12" at an engine RPM of 800. (A thousand is normally a little high). With the throttle closed and mixture in the full rich position, begin to retard the mixture lever toward cutoff. While doing so, watch the RPM for an increase. You should see an increase of approximately 25 rpm before the engine dies.

More than this indicates too rich an idle mixture. Less RPM indicates too lean an idle mixture. This check should be performed by the pilot at every engine shutdown along with a preshutdown runup and mag check.

The fact that this is a turbo'd engine adds some additional complication.

A 5" manifold pressure split with closed throttles is not a mixture adjustment, though the higher idle RPM might indicate that it's a contributing problem.

The indication of a significantly different manifold pressure suggests an indication error, though not necessarily so.

So again...what are the indications being seen?

 

[avbug]

,,,,,

 

[erj-145mech]

Its in the first post, 15" and 20" MAP.

 

[avbug]

Yep...just noticed that, thanks.

With the throttle closed at idle, one instrument indicating fifteen inches and the other indicating higher, suspect the higher one, especially when they're both showing the same RPM.

Remember that gauges seldom indiate correctly. A check of the manifold pressure gauges for proper calibration, for example, will generally show them off by a given value on almost any aircraft. Likewise, the RPM is often not what's being read, and the error changes throughout the indicating range. Checkin that calibration is also wise, but seldom done. Several things spring to mind here. One is a leaking alternate air induction door. You should be seeing a very lean idle mixture of that's the case, whereas fuel increase isn't occuring due to throttle positioning. Perhaps it's just a weak door that's being overcome by turbo induction; in automatic doors this could be simply a weak or missing magnet. It may also be the case of an open overboost valve, or a sticky one, though this normally means a mismatch in the boosted range (eg, above barometric pressure, such as above 29"-30" at sea level).

The engine induction on the 520 involves rubber couplings that are hoseclamped, and leakage isn't uncommon. Leakage symptoms vary depending on where in the system they occur. The rubber coupling hoses tend to take a set, and generally must be replaced if they leak. The common fix is to tighten the clamps, though this seldom works for long, if at all. The leakage checks are done using a vacum cleaner and soapy water, or in some cases, talc. Other problems may also exist.

In any event, until the system is inspected, and the cause determined and fixed, the engine shouldn't be operated in flight.

 

[cl_robster2481]

I work on a fleet of 402 Cs witch have TSIO 520 and have seen this often. I would suspect an iduction leak if you have higher MP at idle and lower MP at high power. Adjusting the idle mixture so that you get the proper mixture rise should take care of this and bring your neddles closer together.

The MM tells you to adjust this to get a rise of 25 RPM at idle. It is a little easier to see if you set it at 1300 RPM and slowly lean till you get a rise. You should have a mixture rise of about 75 to 100 RPM at this setting.

you may have to tweek on the engine a little to get the RPMs to match up

 

[cl_robster2481]

Avbug has a good point about the alt air door. Definatly worth a look.