Low RPM

[Skylane81E]

TSIO360 down 400 RPM from max, propeller govenor not holding it back.

Fuel pressures just a few PSI off of the 2500RPM values despite only turning 2100 so I don't suspect a fueling issue. MAP is good at 40". I did correct a couple of sticky valves but the low RPM remains. Two cylinder have low compression, one 26PSI and the other 48. Would these values really bring the power down that much or is there a bigger issue here?

PS engine runs smoothly at all RPMs

 

[dell30rb]

26 PSI and 48? I would think that you're not getting too much power out of those cylinders.. probably why you cant turn 2500 rpm

Correct me if I'm wrong but compression should be around 80 psi.. if you have 48 that's a reduced power cylinder and the cylinder with 26psi is probably not doing much of anything for you. 26 is real low and you probably have a serious problem there.. badly scored cylinder, melted piston or a broken ring...

Not an aircraft mechanic but If I had low compression in two cylinders and the motor was not turning out good power....

 

[Ted]

Correct me if I'm wrong but compression should be around 80 psi.. if you have 48 that's a reduced power cylinder and the cylinder with 26psi is probably not doing much of anything for you. 26 is real low and you probably have a serious problem there.. badly scored cylinder, melted piston or a broken ring...

Not sure what your background is with aircraft engines, but what the aviation world calls "compression" is really more of a leakdown. 80 psi is put in through a calibrated orifice, and the question is how much of the air stays in the cylinder. Standard convention says that 60 psi is the min limit, but for Continentals I forget what they say is allowable.

The two cylinders with low compression might impact power, but I would not expect it to be to the point of being 400 RPM low. The difference is usually not very noticeable. I've flown airplanes that were later found to have low compression and a top overhaul was done that brought them right up to high 70s. No noticeable difference in takeoff, climb, or cruise performance.

Makes me wonder if there isn't an ignition problem if the fuel flow is really sufficient and the engine is running smoothly.

Also, what plane is it on? Are you sure the prop isn't stuck a bit more coarse than it's supposed to be?

 

[Dan Thomas]

Ignition timing. Late timing costs power. Spark plugs (test them under pressure). Weak plugs might fire on one mag but can fail when both mags are running if one is firing just a little ahead of the other and raising cylinder pressures before the second mag fires.

Can you reach max RPM once in flight? If not, the governor is probably out to lunch. Broken speeder spring, maybe.

Dan

 

[dell30rb]

Not sure what your background is with aircraft engines, but what the aviation world calls "compression" is really more of a leakdown.

Not much of a background with aircraft engines, although I have lots of time working on automotive and motorcycles.

That's news to me, when I hear compression check I think of the compression test done on a car or a motorcycle - hook a tester into the spark plug hole and crank it over a few times to see how much pressure is produced. 26psi would basically mean something catastrophic

 

[Bob Noel]

Caveat: I'm not a mechanic.

Another thought: something broken/blocking the muffler.

That said, being a cowardly pilot, I wouldn't fly an aircraft with a compression of 48 and 26 in two of the four cylinders

 

[Capt. Geoffrey Thorpe]

Not much of a background with aircraft engines, although I have lots of time working on automotive and motorcycles.

That's news to me, when I hear compression check I think of the compression test done on a car or a motorcycle - hook a tester into the spark plug hole and crank it over a few times to see how much pressure is produced. 26psi would basically mean something catastrophic

http://www.aircraft-tool.com/shop/detail.aspx?PRODUCT_ID=2E-14

26 is not something I would be happy coming out of a leakdown test.

Lets see, low compression, "I did correct a couple of sticky valves", low power? And you are still flying it?

 

[Skylane81E]

http://www.aircraft-tool.com/shop/detail.aspx?PRODUCT_ID=2E-14

26 is not something I would be happy coming out of a leakdown test.

Lets see, low compression, "I did correct a couple of sticky valves", low power? And you are still flying it?

Wasn't, we grounded it at the first hickup, but we will be now

Retest put all in the 50s or higher (good for a TCM today) and after a chat with TCM I found I wasn't reading meetered fuel pressure correctly. (Overhaul manual not as clear as SID97-3, so allways use the SID!!)
Got that fixed, found fuel pressure still to low, bumped it up to spec and viola!

Kinda feel like an idiot but the TCM guy made it seem like a common goof.

 

[Skylane81E]

Ignition timing. Late timing costs power. Spark plugs (test them under pressure). Weak plugs might fire on one mag but can fail when both mags are running if one is firing just a little ahead of the other and raising cylinder pressures before the second mag fires.

Can you reach max RPM once in flight? If not, the governor is probably out to lunch. Broken speeder spring, maybe.

Dan

Did not know that, will file that away for future refference

 

[Skylane81E]

Caveat: I'm not a mechanic.

Another thought: something broken/blocking the muffler.

That said, being a cowardly pilot, I wouldn't fly an aircraft with a compression of 48 and 26 in two of the four cylinders

Turbocharged engine so no muffler, and she's a 6 cylinder. The 48 is "ok" though not what I'd like but after a good looooong run up both were higher.

 

[Skylane81E]

The two cylinders with low compression might impact power, but I would not expect it to be to the point of being 400 RPM low. The difference is usually not very noticeable. I've flown airplanes that were later found to have low compression and a top overhaul was done that brought them right up to high 70s. No noticeable difference in takeoff, climb, or cruise performance.

That's why I posted, just didn't seem right to me.

Makes me wonder if there isn't an ignition problem if the fuel flow is really sufficient and the engine is running smoothly.

it wasn't

Also, what plane is it on? Are you sure the prop isn't stuck a bit more coarse than it's supposed to be?

Thought that too, it's a Seneca so I compaired it to the right engine, besides being backwards they had the same pitch.

 

[Tom-D]

TSIO360 down 400 RPM from max, propeller govenor not holding it back.

Fuel pressures just a few PSI off of the 2500RPM values despite only turning 2100 so I don't suspect a fueling issue. MAP is good at 40". I did correct a couple of sticky valves but the low RPM remains. Two cylinder have low compression, one 26PSI and the other 48. Would these values really bring the power down that much or is there a bigger issue here?

PS engine runs smoothly at all RPMs

Has any one worked on the aircraft lately?

have you checked the tach ?

 

[Tom-D]

Poor compression will not cause a low RPM, Dynamic compression will carry the cylinder.

Any Ignition problems will show high mag drops.

Low RPM with good performance is usually a tach.

twins will yaw when they really have a problem with making power.

singles will climb poorly when not making power.

Throttle rigging and governor adjustments will show low RPM when they are played with.

Dynamic compression... think how long does it take to travel 180 degrees on compression stroke at 2400RPM? How much loss of air/fuel mix can escape from the cylinder in that amount of time?


answer ? not enough to matter.

 

[Ted]

Not much of a background with aircraft engines, although I have lots of time working on automotive and motorcycles.

And most of it does carry over, but not all. But there are a lot of things that either have different conventions in the aviation world or different nomenclature.

That's news to me, when I hear compression check I think of the compression test done on a car or a motorcycle - hook a tester into the spark plug hole and crank it over a few times to see how much pressure is produced. 26psi would basically mean something catastrophic

Correct. In a car or a motorcycle, that would mean a catastrophic failure. In an aircraft engine (especially a TCM engine), 26/80 does frequently mean the cylinder needs to be serviced, but usually doesn't mean a catastrophic failure. A lot of times just running the engine a little bit will get the compressions back to where they should be. TCM engines especially have low compression checks. I remember hearing that TCM actually did a dyno test where they wore out the engine to get it to the point where compressions were as low as about 10/80 (they did it in increments of 10 from 80 on down) and showed that about 20 was where the power finally started dropping off.

 

[Dan Thomas]

Retest put all in the 50s or higher (good for a TCM today) and after a chat with TCM I found I wasn't reading meetered fuel pressure correctly. (Overhaul manual not as clear as SID97-3, so allways use the SID!!)
Got that fixed, found fuel pressure still to low, bumped it up to spec and viola!

I know what you did. You weren't using a differential pressure gauge to get the fuel pressure reading above upper deck pressure. You were reading fuel pressure above ambient pressure. Makes a big difference.

Dan

 

[Skylane81E]

I know what you did. You weren't using a differential pressure gauge to get the fuel pressure reading above upper deck pressure. You were reading fuel pressure above ambient pressure. Makes a big difference.

Dan

Yep, surely does

 

[Skylane81E]

So the plot thickens, ran wonderfully in several ground runs so we cowled it up and went for a test hop.

Take off was great but shortly after the engine started acting lean again. As soon as it's cooled enough to put the guages on again I'm going to recheck the fuel pressures but if they've changed on their own I'd assume we're looking at a bad fuel pump.

 

[Skylane81E]

Guages hooked up and she won't even idle well it's so lean. checking fuel supply next.

 

[Skylane81E]

Didn't measure it but my "calibrated" eye tells me that there is plenty of fuel to the engine driven pump. I'm really leaning towards a problem there but a call will be placed to TCM in the AM.

 

[Dan Thomas]

So the plot thickens, ran wonderfully in several ground runs so we cowled it up and went for a test hop.

Take off was great but shortly after the engine started acting lean again. As soon as it's cooled enough to put the guages on again I'm going to recheck the fuel pressures but if they've changed on their own I'd assume we're looking at a bad fuel pump.

Check the upper pressure reference line to the fuel pump. This line feeds turbo pressure to the pump's regulator to increase fuel flow as MP comes up. A leaking line will fool the pump into reducing flow.

Dan

 

[Skylane81E]

Check the upper pressure reference line to the fuel pump. This line feeds turbo pressure to the pump's regulator to increase fuel flow as MP comes up. A leaking line will fool the pump into reducing flow.

Dan

Wilco, would that cause too little flow to idle however?

 

[Dan Thomas]

Wilco, would that cause too little flow to idle however?

Idle is a separate setting on the pump, and at idle the upper deck pressure isn't a factor.

Check the SID real good for the settings.

Dan

 

[Skylane81E]

I did and had it running sweet, then one short hop and it's almost dead

 

[Skylane81E]

So the TCM guys are thinking that it's the throttle body fuel control, metered pressure is about 3PSI and unmetered has gone UP to 18 at 700 RPM.

 

[Dan Thomas]

So the TCM guys are thinking that it's the throttle body fuel control, metered pressure is about 3PSI and unmetered has gone UP to 18 at 700 RPM.

Probably some crud in there. It will need overhauling.

Dan

 

[Skylane81E]

yep, and a L/R swap makes it run well.

guess that's it.

 

[Skylane81E]

Ok, new (rebuilt) throttle body installed, retuned and A-Okay