Rpm Issues

[druidjaidan]

Hi folks we got AOG with a RPM issue that's proving hard to work on.  We were on our way back from Oshkosh Saturday and decided to make a precautionary landing in Minnesota due to what we thought was a governor issue. We were cruising along at 8500, 2500rpm WOT.  We suddenly had a small loss of rpm like 25-50.  Along with it we had a very minor vibration, more like a deep hum not really like a lost cylinder. That lasted like 5-10 seconds and then resolved on it's own. About a minute later it happened again.  No apparent changes in cht, egt, oil temp, oil pressure etc (however we have one of the shitty JPI classic scanners so it's hard to be sure with a transient issue). Additionally, a few months ago we noticed we were only getting 2650 or so on the takeoff roll and we reported at our last annual that our prop wouldn't hold 2500 rpm and the control would rotate up to 2580ish. We're at an airport with basically no services, but we were able to get a mobile mechanic to come out. The first said fly it home, the risk is minimal. We got delayed by weather and another machanic called us back before we left and said basically you don't really want to risk that failure over the Rockies we have a prop/governor shop 50 minutes away, and they can turn it around in a couple days. 

So we had him pull it.  Drive it to Maxwell Aviation services and they bench test it. They say it's fine, nothing wrong with it.

The mobile mechanic that pulled it is unavailable until Thursday. So now I'm here just going fuck. If I fly commercial back I suspect nothing will really get done, if we put it back together and don't find anything else wrong I have to cross the Rockies and Cascades back into Seattle with a machine I don't really trust.  

We've called a bunch of local shops and nothing great. Maybe an intermittent stuck valve (the loss of rpm is too small for that, the vibration too faint and we have no other signs of that), an intermittent plugged injector (once again I dont buy we were losing a whole cylinder). Really grasping at straws.  Current theory is the loss of takeoff rpm might be the prop control, it might have slipped we didn't check. The vibration and loss of 50 rpm in flight is very fuzzy what could cause that. Basically the concensus is it might he a fuel issue, it might be a spark plug, it might be the mags, it might be a sticky valve, it might be related to a intake o ring we lost a chunk of a few months back (valve or cylinder damage), so on and on. 

Edited July 30, 2019 by druidjaidan

[Prior owner]

Might be time to flush the prop and wipe out the front of the crank- When was the last time your prop was removed?

Guessing here, but any engine issues (intermittent or not) should not cause a a sustained rpm loss- the governor would adjust the pitch accordingly and bring the prop speed right back up.

What was the MP doing when this happened, and generally how are your MP indications?

I'm no authority, just thinking out loud while we wait for the more seasoned guys to chime in...

D

 

Edited July 30, 2019 by PilotCoyote

[Yetti]

Sounds ignition.   Since you are bored, pull the plugs and see what they look like.

tell us more about the "intake o ring"

Bad injector.

Do a baby jar test to eliminate the flow divider.

Sumped the tanks really good lately?

 

 

[Ragsf15e]

I agree that the governor should cover some engine issues by keeping rpm constant, but 50 is a tiny change to think you noticed.  I have a JPI930 and I’d be hesitant to say I notice a 50 change without looking back at the data.

So, I had 2 cylinders with exhaust valves that were just starting to stick/wobble.  I’m not saying that’s what you had, but it gave me a faint vibration, I thought I should get my prop balanced.  Looking at the data afterward, it was more clear as the egt went up on both (above “peak”) a couple minutes after I leaned as the valve stuck open a little.  

The real indicator of power issues in cruise is airspeed.  Did it slow down when you noticed the perceived problem?  With the above valve issue, I immediately noticed about 5-7 knots loss and had to retrim for my autopilot.  If you maintain speed, power is still good, probably not something real significant... that being said, I’m not the one who has to fly it over the Rockies.

Where in Minnesota... there’s an MSC up there if you’re more confident flying it there?

Edited July 30, 2019 by Ragsf15e

[Yetti]

And if you have the throttle quadrant do you know about the friction knob.   How do you know the Prop control did not slip?   How about the other end of the cable slipping?  

[druidjaidan]

13 hours ago, Ragsf15e said:

I agree that the governor should cover some engine issues by keeping rpm constant, but 50 is a tiny change to think you noticed.  I have a JPI930 and I’d be hesitant to say I notice a 50 change without looking back at the data.

So, I had 2 cylinders with exhaust valves that were just starting to stick/wobble.  I’m not saying that’s what you had, but it gave me a faint vibration, I thought I should get my prop balanced.  Looking at the data afterward, it was more clear as the egt went up on both (above “peak”) a couple minutes after I leaned as the valve stuck open a little.  

The real indicator of power issues in cruise is airspeed.  Did it slow down when you noticed the perceived problem?  With the above valve issue, I immediately noticed about 5-7 knots loss and had to retrim for my autopilot.  If you maintain speed, power is still good, probably not something real significant... that being said, I’m not the one who has to fly it over the Rockies.

Where in Minnesota... there’s an MSC up there if you’re more confident flying it there?

The 50 was only noticed because the rpm was set at 2500, then the change in noise/slight vibration drew my attention and when I looked at the rpm it was below 2500 line a smidge.once it resolved it was back on 2500.  Similar, but different numbers on the later issues in descent.  No real opportunity to notice an airspeed change, and definetly nothing severe enough to feel a deceleration. 

13 hours ago, PilotCoyote said:

Might be time to flush the prop and wipe out the front of the crank- When was the last time your prop was removed?

Guessing here, but any engine issues (intermittent or not) should not cause a a sustained rpm loss- the governor would adjust the pitch accordingly and bring the prop speed right back up.

What was the MP doing when this happened, and generally how are your MP indications?

I'm no authority, just thinking out loud while we wait for the more seasoned guys to chime in...

D

 

AFAIK it hasn't been removed in the 3+ years we've owned it.  I'd have to have a good look through the logs to see the last time. 

13 hours ago, Yetti said:

Sounds ignition.   Since you are bored, pull the plugs and see what they look like.

tell us more about the "intake o ring"

Bad injector.

Do a baby jar test to eliminate the flow divider.

Sumped the tanks really good lately?

 

 

Ignition and fuel supply issues are high on the list of things to look at. 

O ring pictures attached.  That was fixed mid June 

[Yetti]

What caused the one O ring to go south.   Did you replace all of them or just that one?

Not that an F is prone to Ice up but could it have been moisture?   Were you flying with RAM air open?   You are supposed to shut it in icing conditions.   Check intake path for obstructions.  

[jetdriven]

Shoving the tube too far in, the o-ring expands and then when you pull it back into the proper position, the sharp edge inside cuts the o-ring. If you ever do this, remove the pipe and inspect the o-ring. 

Edited July 30, 2019 by jetdriven

[druidjaidan]

6 hours ago, Yetti said:

What caused the one O ring to go south.   Did you replace all of them or just that one?

Not that an F is prone to Ice up but could it have been moisture?   Were you flying with RAM air open?   You are supposed to shut it in icing conditions.   Check intake path for obstructions.  

Suspicion is that it was caused during installation. It was soft and pliable and didn't seem likely to be aged out. The rest have been looked at and don't show any signs (not removed however). Also this was at WOT for the first incidents, so limited suction. 

Icing was a thought, but we were in VMC above the clouds.  I did check OAT because we were considering if we would need to pick up a clearence to get back down (we didn't). Ram air was open for the cruise incidents and closed for the descent ones.

One thought I had was maybe we had some water in the tank that didn't shake out during the preflight sump.  It's possible I guess, but seems unlikely. We sumped in La Crosse, did not fuel at Oshkosh, had only a miniscule amount of rain (and our fuel cap o rings have been solid and were replaced in Feb at the annual) and we sumped at Oshkosh before departing. 

[Prior owner]

Was it just takeoff rpm that suffered, or were you not able to attain max rpm in flight? 

[Yetti]

14 minutes ago, PilotCoyote said:

Was it just takeoff rpm that suffered, or were you not able to attain max rpm in flight? 

I think we are talking cruise and descent RPM Burble.

[Yetti]

Do a leak down test on the mechanical pump.

has the fuel system been touched recently.    Could be sucking air on a fuel line.   It is bad form to retourge fuel oil line fittings.   But you can put a wrench on all line fittings to check tightness.

[druidjaidan]

Well we found something.  All the plugs were normal or a touch lean looking...except the #4 those were black and oily. So the suspicion is that cylinder wasn't firing all the time for some reason. Suspicion is the drop in rpm and slight vibration was the bottom plug not firing. 

 

 

Edit: diagnoses is a cracked or stuck ring. Pulling the cylinder =(

Edited August 1, 2019 by druidjaidan

[Skates97]

1 hour ago, druidjaidan said:

Well we found something.  All the plugs were normal or a touch lean looking...except the #4 those were black and oily. So the suspicion is that cylinder wasn't firing all the time for some reason. Suspicion is the drop in rpm and slight vibration was the bottom plug not firing. 

 

Hard to tell from the picture, but in the plug on the bottom in the picture is it grounded to the center electrode?

On a recent flight I got some slight vibration, no RPM loss, no change in airspeed, but I have an engine monitor and saw the EGT go up and CHT go down on #4. Mag check showed the bottom plug on #4 not firing. After landing it was a quick 10 minute job to pull the plug, which was grounded, and swap it out with a spare I carry. 

 

[DualRatedFlyer]

Not sure what causes this spark plug bridging, but it cost the company I used to work for about 3 millions in UAV losses overseas. The issue popped up out of no where and disappeared just as quick. We tried everything under the sun to figure out what was causing it to no avail. 

Edited August 1, 2019 by DualRatedFlyer

[ArtVandelay]

Can’t bridging be fixed by just ground running it while severely leaning it?


Tom

[Yetti]

2 hours ago, ArtVandelay said:

Can’t bridging be fixed by just ground running it while severely leaning it?


Tom

Probably not.   I don't think it gets hot enough to melt the lead..    I have been wrong before.

[carusoam]

+1 on Richard’s observation... check to see if something is bridging the plug’s gap.

The typical mag check, or inflight mag check should be able to pick that out with an engine monitor...

a bridged plug will not be able to spark, so... the EGT will be hotter than normal as more fuel will be escaping the cylinder unburned...

And on a single mag, the cylinder will go cold when the good plug is turned off...

PP thoughts only...

Best regards,

-a-

 

[Ragsf15e]

8 hours ago, druidjaidan said:

Well we found something.  All the plugs were normal or a touch lean looking...except the #4 those were black and oily. So the suspicion is that cylinder wasn't firing all the time for some reason. Suspicion is the drop in rpm and slight vibration was the bottom plug not firing. 

 

 

Edit: diagnoses is a cracked or stuck ring. Pulling the cylinder =(

Did you notice any changes in oil consumption or oil analysis?

[ArtVandelay]

5 hours ago, Yetti said:

Probably not.   I don't think it gets hot enough to melt the lead..    I have been wrong before.

Melting point of lead is 327.5°C

Combustion temperatures (not EGTs which is has cooled down) run around 2500°C

 

[N201MKTurbo]

I have had that intermittent vibration thing before. It has always been caused by a set of points that have worn to the point that they are barely opening. They will intermittently stop opening causing that mag to stop firing. Resetting the point gap fixes the problem.

[Yetti]

3 hours ago, ArtVandelay said:

Melting point of lead is 327.5°C

Combustion temperatures (not EGTs which is has cooled down) run around 2500°C

 

True but since there no sparky in the area and the combustion is a flash.....    I can see when the sparky is happening and the tip of the plug is hot (like a soldering iron)  but with it shorted it be cool. 

For the purpose of this thread.   I say that is oil bridging the gap.   So hopefully a new set of rings, lap the valves since it is off and should be good to go.    

 

  Note my very first suggestion was to pull the plugs.   They usually tell the story.

[N201MKTurbo]

I have examined those deposits under a microscope and they are not pure lead. They appear to be lead oxide which has a very high melting point. You will often see that they have a metallic appearance, but you will find it is a thin layer of metal over the oxide. I believe that the metal comes from a reduction reaction when running rich. The oxide is formed while running lean. The oxide is probably a very poor electrical conductor while the metallic film is a good conductor. Running lean to clear a fouled plug most likely is just oxidizing the thin metal layer.

[jetdriven]

The Lycoming O-235  runs a really cool combustion chamber, so much so, that the bottom plugs fill up with lead and eventually short out the plug. They developed the BY plug for those. 
how much oil is it using? You may be able to hydro lock the cylinder with Berrymans B12 chem tool and free the rings. 

[MB65E]

What is the condition of the mags?

-Matt