C172N Running Rough

C172N Running Rough

Hi -

I am having a 172N (0-320-H2AD) delivered but it has been delayed due to its engine running rough. The problem has been described as 'The roughness consisted of erratic rpm changes between 50 and 200 rpm in short bursts.'. All the usual inflight checks were carried out, carb heat, fuel, magnetos, etc.

The aircraft has just had its annual (20hrs flying time ago) where its oil analysis came back clear. It has 1600hrs since factory T-mod engine and all its ADs are up to date. It is currently in Canada so maybe the cold, icey conditions could be a contributary factor?

Although it will be looked at shortly I wondered if anyone out there has experienced a similar problem or if anyone would have an idea as to its cause.

Thanks very much.
Regards.
Chris.

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Re: C172N Running Rough

I have a '77 N  model which had the same problem.  When it was flown to NY from Texas at purchase, the Pilots stopped at an FBO along the way to have an intermittent miss checked.  The FBO charged for a new plug wire....

,....the problem was one of the lower plugs would short every now and then and foul.  The plane has been flown for about a year before my A & P finally found the bad plug.  Replaced and all is well!

I don't know if this is your problem but it's worth checking.

Gary

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Re: C172N Running Rough

Thanks for that Gary.

I passsed this idea onto the ferry pilot. He said he had the same problem on boths magnetos so had discounted that possibility. Since then he has told me that an engineer had said that it COULD be a sticking inlet or exhaust valve. Does anyone know whether this sounds feasible and if so the ballpark cost of fixing it?

Chris.

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Re: C172N Running Rough

I had this happen in my Cessna 175.  Running smooth one minute and rough the next.  After changing all spark plugs and having the mags checked I decided to troubleshoot the problem myself.  I started removing plug wires trying to figure out which cylinder was causing the problem.  I narrowed it down to my #4 cylinder, I took the valve cover off and found I had an exhaust valve stuck open.  I was real close to my oil change time so my mechanic unstuck and lubricated the valve stem, checked the valve spring tension and valve guide.  I changed the oil and it's been working great since.

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Re: C172N Running Rough

Chris, Very feasible that it could be a sticking vavle, most likely the exhaust valve.  A compression check will identify the faulty cylinder.  Worst case would be removing cylinder for repairs. Cheers !

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Re: C172N Running Rough

Thanks for the responses.

I am getting a compression test done on the engine to see if that provides any clues. I'll post a message when I hear the results.

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Re: C172N Running Rough

**,....the problem was one of the lower plugs would short every now and then and foul. The plane has been flown for about a year before my A & P finally found the bad plug. **


If you had had an engine monitor, you would have found the problem in 30 seconds, rather than a year.  And, the engine monitor would have cost less than what you spent trying to chase down the problem.

Something to ponder.

Walter Atkinson

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Re: C172N Running Rough

Just had the compression test done. Results were not too bad at ...

NO. 2 = 70/80
NO. 4 = 76/80
N0. 1 = 70/80
NO. 3 = 72/80

When tested at 1700rpm and 2300rpm the engine ran smooth with no mag. drop. Seemed to be ok.

I'm still considering having 2 cylinders out, maybe the lower compression ones to check the cam and valves.

Chris.

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Re: C172N Running Rough

i am having the rough engine problem with a 2001 172s with 500 tt  this engine will run rough during the first power reduction .  i always reduce power very slowly approx. 20 seconds from full power to cruise.

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Re: C172N Running Rough

Chris , Those compression readings are not so bad.  So the ferry flight was uneventful except for the one incident?  I would be hesitant to go pulling cylinders if engine is running good with good compression readings.  Maybe there was a slight amount of water in fuel that caused a momentary hick up.

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Re: C172N Running Rough

I wouldn't make too much out of the difference between 70 and 76 in the compression check and call one cylinder worse than the other.  If you review your old compression checks you may find that the high cylinder was lower previously.  The bad spot on the contact surface may not be where the piston is positioned at the time of the pressure check.  Or the tension of the valves may not be the same.  My sense is that this check is looking for gross differences.  comments?

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Re: C172N Running Rough

I know, Walter, I know!  smile  A moniter is on my "to-do" list.  I put the plane on leaseback, though, and am hesitant to put too many electronics in it as it's getting beaten up by renters.

The engine runs fine otherwise, though, so all is well now.

Gary

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Re: C172N Running Rough

John Lynn,
There is an AD on your injected lycoming. 2001-06-17 corrects an over rich mixture condition that can cause engine roughness, engine stoppage, and the inability to restart in flight when using published procedures in the AFM.

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Re: C172N Running Rough

Chris,
I would check your P leads to make sure their not chaffed thru somewhere and shorting out. If you dont find the problem, and your sure your plugs and harness are OK, then remove that dual mag and have it bench checked.

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Re: C172N Running Rough

Frank,

We have discounted water ast the cause, as the roughness started 30 minutes into the flight on the same tank and it lasted for a good hour or 2 until the pilot could get back to the point of departure. Unfortunately this happened while he was just clearing a 60 mile wide stretch of water and decided it was sensible to follow the coastline back just in case!

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Re: C172N Running Rough

This is a very interesting web link.

http://www.sacskyranch.com/eng37.htm

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Re: C172N Running Rough

<http://www.sacskyranch.com/eng37.htm>

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Re: C172N Running Rough

Del,

The engineers are now checking the P leads (which they describe as looking the worst for wear!) and the mags. I read the SACSkyranch site and they describe mag coil problems very much like the symptoms experienced. However, the pilot had the same roughness on both mags, could they both have problems simultaneously?

Chris.

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Re: C172N Running Rough

It is very pobbible you problem is a sticking valve. I would recomend that you have your mechanic perform Lycoming mandatory service bulletin SB388C. The bulletin can be downloaded free from Lycoming at the following link.

http://www.lycoming.textron.com/support … SB388C.pdf

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Re: C172N Running Rough

Please excuse my typing errors on the above post. wink It seems it is always best to proof read before posting.

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