172 Running Rough at Idle

172 Running Rough at Idle

Today I was getting ready to take my 77 C-172 for a quick out and back. As I was taxiing I noticed that the airplane was starting to run rough. I did a Mag Check and everything checked fine. When I did my mixture check it shook pretty good before it dropped off. I taxied back to the hanger and pulled the plugs-no fowled plugs. Did a compression check it was in the mid 70's. So I ran it again. This time I noticed that at idle my #4 cylinder was cold. When I get it above 1200 RPM it comes to life and starts to read like the rest. When I go back to idle (or below 1200 RPM) it starts running rough again and the temp starts to drop. Any ideas? Someone told me that I have a valve sticking at lower RPM and to add some marvel mystery oil until it breaks free.  I know there are other threads out there about this.  But if you suggest it do you put it in the oil or gas?

Anybody else heard of this. Good Compression, Lifters look good (pulled the cap), plugs are good (even moved them to different cylinders to be sure), idle it sputters and pops and #4 is cold, above 1200 everything works as advertised.

TBIRD718
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Re: 172 Running Rough at Idle

TBIRD,
Have you checked the ignition lead to that cylinder. It is rare but I have seen instances where they were breaking down and wouldn't fire at low rpm's. This is probably not the problem but it sounds as if you have checked everything else.
Reece
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Re: 172 Running Rough at Idle

I did check the lead.  All of that is fine.  The IA's at the airport seem to think its a sticking exhaust valve.  When its running rough at low RPM I am getting a lot of blow by.

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Re: 172 Running Rough at Idle

It sounds as if your IA is on to the problem. Thanks for posting the follow-up. It helps everyone to have an answer in the event they have a similar problem. I am an A&P but only work on my own plane and I learn from this forum every day.
Thanks,
Reece
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Re: 172 Running Rough at Idle

Yeah I am a new A&P.  I worked fighters my whole life and this prop stuff drives me nuts HA!  Not to mention its my airplane.  I just wanted to see if anybody else had any ideas.  They all say the same thing, try the Marvel Mystery Oil and if that doesnt break it free in a couple of hours then we will have to tear into the cylinder.  I did a "g**gle" search and found that about 75% of the people out there agree with that.  Even though it isnt FAA approved.

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Re: 172 Running Rough at Idle

Your deluding yourself by using the MMO..If it is indeed a sticking valve that is causing the problem you are risking a bent push rod, a cracked push rod tube, loss of engine oil leading to crash and burn. A compression check sould verify that a valve is sticking.I would also check for induction leaks.

The valve can be removed and the guide very lightly reamed  without demounting the cylinder assenbly. I'm sure there are threads on this forum and others covering the problem.

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Re: 172 Running Rough at Idle

Hi All;

Just my two cents, but I would take a close look at the magneto and harness.  The units generate a spark as they spin, and don't work efficiently below about 500 RPM.  There is a chance that a worn or failing component in the magneto may be causing it not to generate a spark until the RPM is 1200 or better on that cylinder. 

Also, the 77 C-172 has the H2AD engine with the dual magneto.  There are some ADs on this unit that require regular inspections.  You might want to verify these ADs have been complied with.  Also, most A&Ps have a way to bench test a magneto for proper function.  If the AD for the impulse coupling inspection hasn't been complied with, the unit will need to be removed anyway.

Jon

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Re: 172 Running Rough at Idle

I agreee with jplaxton,  Check out the magnetos.  If it is a dual magneto, dont bother messing with it, send it out to a reputible repair shop.  The single mags are much easier to work on provided you have the proper overhaul manuals.  Good luck and let us know how you make out.

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Re: 172 Running Rough at Idle

Check out Lycoming S.B. 388 and S.B. 301 get the latest revision.I believe they are 388B ands 301C Have your local A and P with the proper 388 tools accomplish this for you. This will tell you the condition with cold engine of your valve guides and the valve wobble in the guides. The parallel valve cylinders are easier to accomplish this on than the angle valve versions.

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Re: 172 Running Rough at Idle

CESSNADON hit on the first thing that I would check, An Induction Leak... These usually only show up at idle or below 1k rpm. One other note, if the dual mag shows signs of burned points, change the condensors located in the harness cap as these are ALWAYS overlooked and usually are problematic.

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Re: 172 Running Rough at Idle

I have to disagree with some of the comments about mags.  Ignition problems generally manifest themselves at higher power/rpm,higher humidity and higher altitude. 

This is because higher power/rpm creates higher cylinder pressure which increases the resistance across the spark plug gap encouraging electrical current to flow to ground by other available means or not flow at all if the mag is not generating enough power. 

Other available means for the current to flow to ground include, bad plug wires, carbon tracking on the distributor block inside the mag or at the plug connectors.  These "leaks" will get worse at altitude because the thinner air provide less resistance.  Humidity and moisture will also make matters worse (any who ever owned and austin can attest to this). But generally the symtom will be engin runs fine at idle but misfires at higher power settings and/or high altitude.

Finally while it is true the output of the mag is proportional to its rpm meaning the faster it spins the more power it produces, the same mag produces power for each cylinder so if there was not enough power from the mag for one cylinder, there isn't for the others either.  This would end to create random misfires of all cylinders rather than just one. Also even the d2000 apart from the input drive, is a dual ignition system.  For the engine to run rough at idle both ignition systems must fail on the same cylinder.

Air leak is a possibility but the 172 induction system is not typically a high problem area.  Also remember that the induction system is interconnected between cylinders and the single carb, so a leak anywhere in the system will cause all cylinders to run lean both at idle and higher rpm notwithstanding frictional losses.

I suspect the stuck or burnt valve (or seat) is the culprit.

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Re: 172 Running Rough at Idle

Hello TBIRD;

Please be sure to post your findings once you have solved the problem.  This kind of information will be helpful to the rest of us in troubleshooting problems in the future.

Thanks

Jon

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Re: 172 Running Rough at Idle

Its funny to read posts that all of you have left.  This is the exact same arguements we had at the hanger trying to figure out this problem.  We finally made a command decision and despite the good compression check started prepping the cylinder for removal.  Just before we got started a friend walked up and said "I know that it sounds like a valve, but its a whole lot quicker to check that Mag first".  That actually made sense.  So we pulled the mag and sure enough we had 1 solid bad set of points and the other was on its way.  Just got her cranked up last night again and she is purring like a kitten. 

Thanks for all the help!

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