Engine Problem

[Mark942]

LOW STATIC RPM, Engine Backfiring

Background:

Mooney 1963, M20E, IO 360-A-1-A with RaJay Normalizing Turbo, 4747 Total Hours, 178 SMOH, plane airworthy but bought in October 2015 to restore better than new as retirement project. 62 hours flown in 24 months since purchase while lots of down time for restoration work such as tank strip and reseal, etc.

Chronology of engine problems:

Aircraft has performed pretty reliably for having a period of little use for several years. Engine oil analysis at Prebuy=all in normal range, and two subsequent oil analysis the same. First engine issue was a failing left Mag due to an oil seal leak allowing oil into Mag housing (164 SMOH). Repaired and all was well.

About 3 months ago, (174 SMOH) on a night takeoff, a glance at engine gauges on the start of the takeoff roll showed RPM 2300, MP 28-29”. Run-up had been normal. I exited onto taxi way, and performed another run-up which again was normal. Did a full power static run-up and only got 2300 RPM. Tried running at 2000 RPM and peak EGT for a minute in the hope that a plug was fouled. Brought RPM down to idle then slowly advanced to full throttle. RPM only 2300 RPM / MP=28. While pulling throttle back, this time the engine began to back fire a few times. Headed for hangar about a half mile away, and by the time I was half way there the engine was VIGOROUSLY backfiring at 1000 RPM. Tried to lean to reduce backfiring but had little effect. Please keep in mind, that until this moment, I had never had a problem with backfiring on this engine.  A week later A/P arrived and after a day of checking various things, he felt that the problem was in the fuel system and suspected the fuel injection servo. I removed it and he took it to a repair shop that specializes in overhauling fuel system components. Full overhaul on servo, and 3 weeks later reinstalled and plane ran perfectly.

 

Fast forward 3.7 hours (178 SMOH) spread over 3 flights. Again, a night flight. Everything normal including starting, taxi to runway, run-up Mag check, and Prop check. On take off roll 2300 RPM, 28-29” MP. This time as soon as I brought throttle back, the engine started backfiring. Tried to clear plugs by 2000 RPM leaning, but as soon as throttle reduced, backfiring begins and continues. Back to hangar, and now A/P has spent 4 days with me spending much more time than that in between his visits, doing the manual labor. All intake gaskets and seals replaced. Fuel sumps double checked for water or sediment, lines purged, screens removed and cleaned, lines removed from fuel divider and fuel pumped into measured container and timed. All 4 lines equal. Fuel pressure on electric pump alone about 10 PSI. Fuel pressure with engine pump at full throttle 25-30 range and doesn't change with or without electric pump on. Fuel Servo removed and taken for bench flow testing with no issues. Fuel divider removed, inspected by A/P, cleaned, and new diaphragm installed just because we had one. Turbo has not been engaged at all in the past 10 hours of flight. On the ground, we have moved the turbo control lever and verified that the exterior turbo lever arm is going from stop to stop.

 

Compression checks are always in the range of 73 to 78. We have checked compression several times. A/P has rotated prop by hand and felt for proper compression feel and sound. He has inspected the valves with a bore scope and sees nothing unusual. I have asked about the Prop governor and he says the blade angle looks proper with engine off, and the prop would not cause the backfiring. He has 40+ years of experience working for several shops including a period at Purdue University and has owned his own shop for 25+ years. He says this has him stumped. My A/P is reaching out to discuss with Lycoming Engines, and a couple other A/P shops.

 

The plane was scheduled to go into an avionics shop today for the beginning of several upgrades including an Avidyne IFD 550, but that has now been rescheduled for January at the earliest. Any thoughts on contacting SAVVY aircraft maintenance management. I know it is “after the fact” but has anyone had experience with them being able to pinpoint a problem where an experienced A/P is stumped?? BTW, I only have an original analog EGT on cylinder #3. No fancy engine monitor for help. All other crappy original Money analog engine gauges are in normal range.

 

Ref:

 

Major Overhaul in November 2001 at (4569 TTAF). Hours flown per year:

Year   Hrs  

2002 22 (22 SMOH) (4591 TT)

2003 29 (51 SMOH) (4620 TT)

2004 16 (67 SMOH) (4636 TT)

2005 17 (80 SMOH) (4649 TT)

2006 7 (87 SMOH) (4656 TT)

2007 5 (92 SMOH) (4661 TT)

2008 4 (96 SMOH) (4665 TT)

2009 10 (106 SMOH) (4675 TT)

2010 2 (108 SMOH) (4677 TT)

2011 1 (109 SMOH) (4678 TT)

2012 0 (109 SMOH) (4678 TT)

2013 0 (109 SMOH) (4678 TT)

2014 0 (109 SMOH) (4678 TT)

2015 2 (111 SMOH) (4680 TT) Purchased plane in late October, 2015

2016 43 (154 SMOH) (4723 TT)

2017 24 (178 SMOH) (4747 TT) Waiting for a solution to problem

Left Mag Repaired 4733

Fuel Injector Overhauled 4743

Second issue +3.7

Thanks,

-mark

 

 

[Guest]

Plugs, wires and mags?

Clarence

[Mark942]

Thanks Clarence,

I'm an electrical engineer, and so always think everything is electrical , having said that I do want to talk with my A/P about simply replacing the plugs and wires (cheap compared to the last few weeks) and maybe he could even find some "loaner" Mags ???  Thanks for your thoughts.

-mark

[Yetti]

Seems like you are over 500 hours on the right mag.  Bendix or slick mags?

[Browncbr1]

Did you send the left mag for repair by a specialist or did you get a Kelley “overhauled” exchange?    What was the engine retimed to when the mag was reinstalled?  

What kind of plugs and what is the resistance of each plug?

 

[carusoam]

I seem to be bothered by the rpm situation... full power  & 2300rpm?

While on the ground, the prop won't need much power to go full rpm, 2700...

I don't have any experience of full power and Lower rpm...

Most Mooneys gov will fail to fine pitch even if you run out of oil...  Missiles and Rockets will fail to the feathered position.

It might be possible all the back firing could be related to trying to run over square...

Good time to check the mag timing. They have a tendency to slip and give funky results...

Get a JPI immediately.  Learn to post the results.  About 50 of us read and comment on what they see not working right... an one of is really good at it... quite Savvy I might say...   

I think I would start with why the prop is stuck in a place it doesn't belong.  Could be the gov not Working, or the prop not working but something is funny there...

Retirement and flying go Nicely together... so I've been told...

Best regards,

-a-

[cbarry]

I would consider checking the air filter/intake for a restriction or blockage.  

[Yetti]

Not sure why, but I am feeling an overly rich situation.  Do the Servo leak down test.

[carusoam]

With such low hours over the years... cam performance is always a question...

If the cam has been rusted, it usually leaves plenty of metal bits in the oil for analysis....

But cams don't usually fix themselves and then go back to misbehaving...

Have you seen the cam or any of the lifters? Any metal bits in the oil?

Best regards,

-a-

[larryb]

Engine monitor data?

[gsxrpilot]

If you had on of those "fancy engine monitors" you'd have solved the issue by now.

I don't understand people who think "fancy engine monitors" are a extravagance. 

I had an issue today with my 252. My EDM900 diagnosed the issue immediately and accurately. I'll write it up and post pictures tomorrow.

[Captnmack]

Once I flew a friends 172 to a Funeral with friends. When we came back out to fly back home it was rough and backfiring, etc. We rented a car and drove back in the night. The day I drove back down with owner and when we pulled the cowling off we discovered that one of the Intake Tube couplings had backed off just enough to cause the VERY lean issue. Under different circumstances I might have found the problem myself...

[Guest]

Here you go.  Hold this picture of an Insight G2 near your engine and all will be resolved!

Clarence

[Bob_Belville]

It should not be a factor in the current puzzle but I wonder about the boost pump.

You reported: "Fuel pressure on electric pump alone about 10 PSI. Fuel pressure with engine pump at full throttle 25-30 range and doesn't change with or without electric pump on."

On my E the electric boost pump will hold FP near 25 psi. In cruise, turning on the electric pump will cause the FP to bump up by 1 or 2 psi and make the displayed FP rock solid.

You may has told us but when you're taxiing back and experiencing "backfire" at low power is the mixture full rich? You report you've tried to burn off fouled plugs by running lean at ~2000 rpm but do you lean as SOP on the ground?

[Htwjr]

I think I would start with the mags and plugs.  When I first got my C model I was running way to rich on the ground and It would sound like it was backfiring but actually the unburnt rich fuel mixture was exploding in the exhaust making a popping sound that I thought was backfiring at first.  Does it backfire on both mags individually?   Also you may want to check for any exhaust restrictions.  Just some thoughts.

[Mark942]

Thanks everyone for your thoughts.  I am at work right now and stealing a minute to look at replies.  I will respond to some of the questions tonight if at all possible.  Sure appreciate the inputs.  My last day at "work" is the 22nd and as luck has it, I have been able to rent a heated hanger with a cement floor and nice lighting as of January 1st.  Working in a junky 50 year old T-Hangar with a salamander  blasting Jet-A fumes makes this a lot more memorable, but looking forward to the new digs.

-mark

[Marauder]

Good luck on solving this. As others have mentioned, have them install an engine analyzer while it is in the shop. Based on the number of hours your mechanic could charge you, it probably will have paid for itself.

The three symptoms that bother me are the low RPM on takeoff roll, the backfiring and the low fuel pressure.

You may be dealing with the worst of airplane troubleshooting - the multi problem.

I suspect that a mag problem could result in low rpm and backfiring. But this doesn’t explain the low fuel pressure when just on the boost pump. Like Bob, I will see 27 PSI with the boost on by itself, then the engine fuel pump will be in the 25 PSI range in cruise but will jump up to 27 when I turned the boost on again.

Hope this can be sorted out. As for comment about Savvy, I signed up for SavvyAnalysis and have had Paul help me on a couple of issues. The first being a set of plugs that were intermittently not firing. Was not an easy problem to see without the data being available.

The second issue was a recent power anomaly that only lasted for a few seconds but was very noticeable in the cockpit. I have the JPI 900 set to 1 second collection intervals and it showed very clearly that I lost about 100° degrees of EGT temp on one cylinder for about 5 seconds. I was worried about a stuck valve but the entire flight as well as earlier ones show no evidence of the valve sticking. We are working on the premise that something came through that injector that disrupted the fuel flow for a few seconds. Analyzers really help look at these issues. Good luck.


Sent from my iPhone using Tapatalk Pro

[mike20papa]

The back firing is definitely a "over lean" symptom.  It appears that all of your engine fuel systems have been checked/rechecked.  What about air frame fuel lines and vents?  My 180 hp A model has fuel tank finger screens and the very fine screen contained in the boost pump.  

[Guest]

Barring the obvious things like something broken in the engine compartment, it’s a process of elimination.

General engine health, cylinder compression.

Spark plug condition, internal resistance.

Ignition wire high tension check.

Magneto health, failing coils etc. last engine overhaul may not have included magnetos. Magneto to engine timing.

Buy a shop vacuum and hook the blow side to the tail pipe with duct tape, turn it on with the throttle closed this will pressurize the engine, look for leaks in the exhaust system, intake system, and upper deck air system.

Clarence 

 

[Dream to fly]

@Mark942,  You seem to be covering all the bases.  I would definitely continue to look at plugs, wires and the ignition system, However as others have stated the fuel pressure is low and that would cause a lean condition.  That being said that could cause a top end to get over heated and cause a valve to stick.  A bore scope may not show signs of the beginnings of a valve sticking.  Have you done a hot compression check?  Also where are you taking the Fuel pressure test from?  Last year I was battling a wild misfire and fuel leak that turned out to be a fuel servo. 

[Mark942]

23 hours ago, Yetti said:

Seems like you are over 500 hours on the right mag.  Bendix or slick mags?

Bendix 200 series

-mark

[Mark942]

22 hours ago, Browncbr1 said:

Did you send the left mag for repair by a specialist or did you get a Kelley “overhauled” exchange?    What was the engine retimed to when the mag was reinstalled?  

What kind of plugs and what is the resistance of each plug?

 

My A/P repaired it himself at his shop and returned it, and reinstalled.  He also set the timing at time of reinstall.

[Mark942]

21 hours ago, carusoam said:

I seem to be bothered by the rpm situation... full power  & 2300rpm?

While on the ground, the prop won't need much power to go full rpm, 2700...

I don't have any experience of full power and Lower rpm...

Most Mooneys gov will fail to fine pitch even if you run out of oil...  Missiles and Rockets will fail to the feathered position.

It might be possible all the back firing could be related to trying to run over square...

Good time to check the mag timing. They have a tendency to slip and give funky results...

Get a JPI immediately.  Learn to post the results.  About 50 of us read and comment on what they see not working right... an one of is really good at it... quite Savvy I might say...   

I think I would start with why the prop is stuck in a place it doesn't belong.  Could be the gov not Working, or the prop not working but something is funny there...

Retirement and flying go Nicely together... so I've been told...

Best regards,

-a-

Engine backfires at even low RPMs, below 1000. Just seems to get more violent at higher RPMs.

[Mark942]

21 hours ago, cbarry said:

I would consider checking the air filter/intake for a restriction or blockage.  

Forgot to mention that I removed the inlet filter, and looked up inside with flashlight as best as I could. Ran engine on tarmac without filter installed just to confirm no difference.

[Mark942]

21 hours ago, Yetti said:

Not sure why, but I am feeling an overly rich situation.  Do the Servo leak down test.

Servo was removed and overhauled at a D&G in Niles, Michigan.  Second time, removed and took it up to them and they bench tested it and provided me with all the actual data at no charge.  Servo working in spec.