Manifold pressure gauge

[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.