Low RPM Misfire M20F

[Tony Starke]

14 minutes ago, M20Doc said:

If you’re looking for induction or exhaust leaks, why not just connect the outlet side of a clean vacuum cleaner to the tail pipe, close the throttle and turn on the vacuum cleaner.  Now spray soapy water on all of the flange gaskets and intake pipe O rings or couplings.

Clarence

I've got to tell you Doc, that method really sucks...ah actually it blows. Works great though. I was able to locate an exhaust leak using this trick.

[N201MKTurbo]

9 hours ago, jetdriven said:

Well, explain the mechanism a vacuum leak makes it run richer?  Beucause I’m a mechanic and i’m not following you here. 

I can see how, on a turbo engine, a boost leak makes the whole engine run richer. But a NA engine has intake vacuum. 

Here’s some more info.  From: https://www.savvyanalysis.com/articles/in-flight-diagnostics

 

 the principle behind this test: During the high-MP test, the induction manifold pressure is very close to outside ambient pressure, so any induction leak will have little or no effect on engine operation. During the low-MP test, the manifold pressure is significantly lower than outside ambient (by about 10 inches), so any induction leak will cause the affected cylinder (or cylinders) to run substantially leaner than the others, resulting in a smaller drop in EGT than the others.

On a carbureted engine what you say is true. The stuff flowing through the intake risers is a precise mixture of fuel and air. If you introduce additional air to the flow to one cylinder it will significantly affect the fuel/air ratio going to that cylinder, making it leaner.

In a fuel injected engine the stuff flowing in the intake risers is just air. The air flows through the servo where the airflow is measured and the servo produces an outlet fuel pressure proportional to the airflow. This fuel is applied equally to the injectors. The injectors convert this fuel pressure to a fuel flow through their orifices. This fuel is mixed with the air just before the intake valve.

the amount of air drawn into the cylinder is controlled by pressure differential between the descending piston and ambient outside pressure. This flow is controlled by the restriction caused by the throttle plate which is down stream from the flow measuring parts of the servo.

Any leak between the throttle and the cylinders will increase the absolute pressure in the whole manifold. The only pressure differential between the individual cylinders and the leak will be because of the restriction differences of the length of pipe between the leak and the individual cylinders. Compared to the restriction caused by the throttle, which is huge at idle, the flow differences are neglable.

So even with a substantial leak the airflow to the individual cylinders is still essentially equal. With the fuel being introduced at the intake valve the fuel air ratios will not be different between the cylinders. The fuel air ratio will not be what is normally expected because air is being introduced downstream of the servo, but it will be equal between cylinders.

So the leak won't make one cylinder either richer or leaner than the others.

[Hank]

2 hours ago, M20Doc said:

If you’re looking for induction or exhaust leaks, why not just connect the outlet side of a clean vacuum cleaner to the tail pipe, close the throttle and turn on the vacuum cleaner.  Now spray soapy water on all of the flange gaskets and intake pipe O rings or couplings.

Clarence

Wonderful voice of experience! KISS, too.

[Mooneymite]

2 hours ago, Hank said:

Wonderful voice of experience! KISS, too.

Yeah....  I discovered a huge hole in my vacuum cleaner hose!  

[tylerkocon]

22 hours ago, bradp said:

$15 endoscope / borescope / dental camera can tell you a lot about the condition of your exhaust valves prior to any potential need for work.  

Usually valve related failure modes while at idle are more commonly associated with morning sickness.  There’s a saavy algorithm called FEVA that @kortopates could probably elaborate on for prediction of exhaust valve failure.  

What exactly do you mean by morning sickness? 

 

After more investigating my mechanic could not find anything unusual. All the spark plugs are in good condition and compressions were 79, 78, 79,76.

 

He shined a light into the cylinders and said he couldn't see anything unusual. We checked for fuel leaks and there was no blue staining anywhere. 

 

Before I got the plane, the previous owner only put 80hrs on it over about 6 years. He tried to fly once a month. Now that I am flying a lot more is this common? My mechanic said it may fix itself after being ran some more. 

 

I did not have a chance to fly last night and get a video but I should be able to in the next couple days. I'm sure it isn't anything major I just get very particular when it comes to the engine and figured I would pick your guy's brains. 

 

I appreciate all the help!

[Stephen]

23 hours ago, tylerkocon said:

He flew the plane with me last week and was thinking an exhaust vulvae may need to be bored, but he wanted to look at a few other things first. 

 

http://www.dictionary.com/browse/vulvae

I believe this was the origin of the "morning sickness" offering...

[Marauder]

What exactly do you mean by morning sickness? 
 
After more investigating my mechanic could not find anything unusual. All the spark plugs are in good condition and compressions were 79, 78, 79,76.
 
He shined a light into the cylinders and said he couldn't see anything unusual. We checked for fuel leaks and there was no blue staining anywhere. 
 
Before I got the plane, the previous owner only put 80hrs on it over about 6 years. He tried to fly once a month. Now that I am flying a lot more is this common? My mechanic said it may fix itself after being ran some more. 
 
I did not have a chance to fly last night and get a video but I should be able to in the next couple days. I'm sure it isn't anything major I just get very particular when it comes to the engine and figured I would pick your guy's brains. 
 
I appreciate all the help!

Shining the light into our Lycoming cylinders isn’t going to do much. The intake and exhaust valves are on the top of the cylinder head and you need an articulating scope to see them.

Morning sickness is referring to a stuck exhaust valve that you would typically see on the first startup of the day (hence “morning sickness”). The plane will run rough but often will smoothen out.

A scope is needed to see issues like this one on Dev’s airplane:
Where there is evidence of exhaust leakage around the edge of the valve.



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[Guitarmaster]

@tylerkocon, if it makes you feel better, mine will do the same thing.  Low rpm usually ends up with a lope.  I have no induction leaks using a similar test to Clarence's.  There are no crankcase leaks.  No fuel leaks.  Both in-flight and ground mag checks are purrfect.  LOP mag stress test is perfect.  My valves look good with no leaks per the borescope.

I recently had a problem with a slightly elevated #3 EGT (love my EDM-900) and I had a hard time running LOP.  In addition, I was having some lean after-fire activity.  A good ultrasonic injector bath took care of that problem.  

When I changed the intake on the airplane, the idle smoothed out tremendously.  Check your intake boot for rips.

Here's my theory... 1.) Rips in the intake boot cause turbulence that disrupts the airflow at idle.  2.) The fuel injector lines are on top of the engine and heat may cause bubbling in the lines.  I came up with this since my plane only does it when its hot.  3.)  Dirty fuel injectors.  That being said, it's easy enough to diagnose this with an engine monitor.

Overall, I have talked to many A1A owners that seem to have the same symptoms.  

[Sean S]

In Looking at the Lycoming trouble shooting guide, the first place it directs you to is checking the idle mixture. As mentioned above, if you don;'t get the RPM rise on shut down at idle your idle mixture is likely too rich. If you decide to lean the idle mixture, you will then need to verify your idle RPM and adjust it to the spec which I believe on the IO-360-A1A is 600-650 RPM. Check your manual to verify..

[jetdriven]

16 minutes ago, Bayern Speed said:

In Looking at the Lycoming trouble shooting guide, the first place it directs you to is checking the idle mixture. As mentioned above, if you don;'t get the RPM rise on shut down at idle your idle mixture is likely too rich. If you decide to lean the idle mixture, you will then need to verify your idle RPM and adjust it to the spec which I believe on the IO-360-A1A is 600-650 RPM. Check your manual to verify..

Precision has service letter SIL RS-67 which tells you how to set the idle mixture. Even a zero RPM rise is allowable

[Sean S]

37 minutes ago, jetdriven said:

Precision has service letter SIL RS-67 which tells you how to set the idle mixture. Even a zero RPM rise is allowable

That is a very detailed SIL! It does recommend a 10-40 RPM rise. 

[Guest]

5 hours ago, Bayern Speed said:

In Looking at the Lycoming trouble shooting guide, the first place it directs you to is checking the idle mixture. As mentioned above, if you don;'t get the RPM rise on shut down at idle your idle mixture is likely too rich. If you decide to lean the idle mixture, you will then need to verify your idle RPM and adjust it to the spec which I believe on the IO-360-A1A is 600-650 RPM. Check your manual to verify..

If you don’t get a rise in RPM at shut down, wouldn’t that indicate a lean mixture?  A large rise would suggest too rich.

http://www.precisionairmotive.com/Publications/SIL RS-67.pdf

Clarence

Edited May 26, 2018 by M20Doc

[N201MKTurbo]

If it is too rich, you can lean it out with the knob. If it is too lean there is nothing you can do.

[Sean S]

If you don’t get a rise in RPM at shut down, wouldn’t that indicate a lean mixture?  A large rise would suggest too rich.
http://www.precisionairmotive.com/Publications/SIL RS-67.pdf
Clarence

Maybe I didn’t articulate my thought very well. I am in agreement with you Clarence. One should expect a small rpm rise on shut down indicating the tendency toward lean. No rise, too lean, big rise too rich.
Sean


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