Erratic idle... SOLVED!!

[Guitarmaster]

 

I have been dealing with an erratic idle for a long time now. Nothing seems to help.

It would only do it when the engine was hot.

There are no intake leaks so I have really been stumped.

At Oshkosh this year, I visited the Lycoming tent. They had a seminar on sticky valves, but that particular one got cancelled or rather rescheduled, and the one I sat in on was about induction systems.  I wasn't going to stick around, but I thought well, this might be interesting. And it was!

One of the things he talked about, was people pulling their fuel Servo and send in for overhaul when it is unnecessary do to idling issues.  Not that it can't be the problem, but it rarely is.

He said to run a clear tube from the fuel Servo to the spider and watch for bubbles.  If there are bubbles, then you have a airframe-side fuel leak.

I was all set to do this when I got back from Oshkosh but then he said something else that caught my attention.  He said they had Mooney come into the shop with an erratic idle problem but only when it was hot. after 3 weeks of diagnosis, they finally found that the intake side of the fuel pump had an O-ring that had to run up on the threads and when it got hot, created a small leak.

I thought to myself, "I have a Mooney that has an erratic idle when hot.

Upon his advice, I pulled the connection to the inlet side of the fuel pump and replaced the O ring that had been smashed and had a ridden up on the threads.

With the new o-ring in place, no more erratic idle! I can't even explain the joy of fixing this! It has been a huge pain in my butt for a few years now.

if you're having a problem with an erratic idle, and you have no intake leaks, take a very close look at the airframe side of your fuel system. the chances are very good you have a leak there somewhere and it most likely will not show blue staining since it's on the vacuum side.

 

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Edited August 17, 2018 by Guitarmaster

[carusoam]

Nice find, Matt!

Thanks for sharing it.

Best regards,

-a-

[Hector]

Good tip and thanks for posting.


Sent from my iPad using Tapatalk

[cbarry]

Thanks for passing along your good news!  This one of the awesome things about aviation— there’s always something to learn!

[Marauder]

I have been dealing with an erratic idle for a long time now. Nothing seems to help.
It would only do it when the engine was hot.
There are no intake leaks so I have really been stumped.
At Oshkosh this year, I visited the Lycoming tent. They had a seminar on sticky valves, but that particular one got cancelled or rather rescheduled, and the one I sat in on was about induction systems.
I wasn't going to stick around, but I thought well, this might be interesting. And it was!
one of the things he talked about, was people pulling their fuel Servo and send in for overhaul when it is unnecessary do to idling issues.
He said to run a clear tube from the fuel Servo to the spider and watch for bubbles.
I was all set to do this when I got back from Oshkosh but then he said something else that caught my attention.
he said they had Mooney come into the shop with an erratic idle problem but only when it was hot. after 3 weeks of diagnosis, they finally found that the intake side of the fuel pump had an O-ring that had to run up on the threads and when it got hot, created a small leak.
I thought to myself, "I have a Mooney that has an erratic idle when hot.
Upon his advice, I pulled the connection to the inlet side of the fuel pump and replaced the O ring that had been smashed and had a ridden up on the threads.
With the new o-ring in place, no more erratic idle! I can't even explain the joy of fixing this! It has been a huge pain in my butt for a few years now.
if you're having a problem with an erratic idle, and you have no intake leaks, take a very close look at the airframe side of your fuel system. the chances are very good you have a leak there somewhere and it most likely will not show blue staining since it's on the vacuum side.
 
Sent from my XT1585 using Tapatalk
 
 

When you say erratic idle, what did it look or sound like?

Also, got pictures and part numbers? I have always had a bit of stumbling after a hot start. Always was told it was the fuel still boiling in the lines.


Sent from my iPhone using Tapatalk Pro

[Guitarmaster]

1 hour ago, Marauder said:

 

When you say erratic idle, what did it look or sound like?

Also, got pictures and part numbers? I have always had a bit of stumbling after a hot start. Always was told it was the fuel still boiling in the lines.


Sent from my iPhone using Tapatalk Pro

 

When the plane was hot (after a flight), it would idle kind of like a big-block Chevy with a performance cam installed.  In other words, it would run smooth for a bit, then quit for a split second then catch.  Rinse and repeat about every five to ten seconds or so. During this time, there would be an occasional back-fire through the exhaust as well. 

I had heard the same thing about boiling the fuel in the lines to the injectors and was ready to accept that as the reason, but I couldn't make that reconcile with what I was seeing.  The fact it was only happening when the engine was hot, made it that much more frustrating. 

After a hot-start, I DO believe that is the reason for the rough running, but it should smooth out very shortly after start.

Had I not attended the Lycoming induction seminar at KOSH, I would have NEVER figured this out!  There was no visual indication of a leak anywhere.  The premise of the seminar was people go right to the fuel servo as the cause of rough idle problems, but the servo is almost never the problem and many people end up overhauling a perfectly good servo.  Not that it can't be the problem, but it rarely is, according to the Lycoming rep.
Strange thing was, that was not the seminar I was supposed to attend; it was a mistake in the schedule printing.  Everything happens for a reason.

I don't have pictures, but it was an easy job.  Pull the inlet side fitting from the mechanical fuel pump and match the o-ring.  I am very blessed to have an engine shop on the field with good parts availability.  Before going to the trouble of doing this, I would run the clear tube from the servo to the spider and see if there are any bubbles.  I only went right to the fitting because of his story of a Mooney doing the exact same thing as mine.

 

Edited August 17, 2018 by Guitarmaster

[Guitarmaster]

2 hours ago, cbarry said:

Thanks for passing along your good news!  This one of the awesome things about aviation— there’s always something to learn!

You're welcome.  I have learned so much from the free flow of information on this site and other sources.  I always try and pass on anything I learn.  

I believe the information flow has lowered my cost of ownership considerably.  

[Marauder]

Thanks for passing along your good news!  This one of the awesome things about aviation— there’s always something to learn!

 It’s done just the opposite for me. I now have a LED landing light, LED nav lights, LED strobes, LED tail light with strobes, LED recognition light, CiES fuel senders, JPI 900, Senscor CO detector, CYA 100 AoA indicator, Jaeger Spatial Design Interior, all new RG-400 wiring, a new CI-196 antenna, new Klixon switch covers, countless RAM clamps, brackets and assorted hardware and I’m sure a bunch of stuff that is off of my radar.

 

These people are evil!

[Guest]

12 hours ago, Marauder said:

 

When you say erratic idle, what did it look or sound like?

Also, got pictures and part numbers? I have always had a bit of stumbling after a hot start. Always was told it was the fuel still boiling in the lines.


Sent from my iPhone using Tapatalk Pro

 

The “O” ring should be an MS29512-6, inlet and outlet fittings require them.  I use engine oil to lubricate them so they don’t tear when you torque the check nut.

Clarence

[fuellevel]

Muuhh. Ha ha ha.   Picture me setting down my morning coffee and wringing my hands in contemplation of my evilness.

OK -  evil test period over., now back to adding Cessna Twins to the AML 

[dmevans]

I am going through the same issues right now. Headed to the airport to replace o-rings. Lol