Fouled plugs

Fouled plugs

1999C172S (1350 tach time): Left mag drop of 300rpm on run-up. If I run the engine at 2100 for a minute that usually clears up the problem. On occassion when that hasn't worked I have shut the plane down and re-started and there was no abnormal drop in rpm when running up again. My mechanic says there is a small amount of oil fouling the plugs and I need to have the rings replaced. Are there any other possibilities to look at before authorizing this costly repair? Anyone else have a similar experience?

JOHN MASTERJOHN
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Re: Fouled plugs

If the problem is just oil fouling and not lead fouling (idle too rich) then I would get a second opinion.  What are the compression readings?  Are all of the cylinders showing signs of oil fouling, or just one?  The rings on the cylinders are not fixed...they rotate during engine operations.  It is possible for two rings to align on a given cylinder.  When this occurs, oil will leak by both rings and get into the top of the cylinder causing fouling. How often does this fouling occur?  If it is a constant problem, your mechanic may be correct.

Mike Flanagan
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Re: Fouled plugs

John,
Check to make sure that this AD has been complied with. And although it is not a recurring AD, I would recommend periodic compliance. If you need the complete text, I can post it.

2001-06-17

CESSNA AIRCRAFT COMPANY

Amendment: 39-12164

Docket No. 2001-CE-14-AD

(a) What airplanes are affected by this AD? This AD applies to Models 172R and 172S, all serial numbers, that are certificated in any category.

(b) Who must comply with this AD? Anyone who wishes to operate any of the above airplanes must comply with this AD.

(c) What problem does this AD address? The actions specified by this AD are intended to detect and correct an over-rich fuel mixture (improper fuel flow settings), which could result in rough engine operation or engine stoppage.This over-rich fuel mixture also contributes to the engine not restarting during flight when using published in-flight restart procedures.

Del

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Re: Fouled plugs

I'm going to be with the mechanic when he pulls the plugs and checks compressions tommorrow.

The problem started about2 weeks ago and has ben constant since then (7 flights).

Thanks for your feedback, I'll post twhat the plugs look like and the compressions when I get then on Thursday.

John

JOHN MASTERJOHN
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Re: Fouled plugs

Thanks for the AD info. Yes that ad WAS complied with.

JOHN MASTERJOHN
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Re: Fouled plugs

John
Before doing anything, I'd want an explanation as to what is causing the scenario
'On occassion when that hasn't worked I have shut the plane down and re-started and there was no abnormal drop in rpm when running up again'.
This does not sound like a fix to fouled plugs.
I owned a 172 for 8-9 years, and increasing rpm with slight leaning for about one minute was the only way I could clean fouled plugs at runnup.
(I'm not an A&P)
Steve

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Re: Fouled plugs

Thanks Steve,
You're right it doesn't make a whole lot of sense to me either, hence the post looking for feedback. I really want to see the plugs for myself.

As to shutting down and restarting: I flew to 3bo last Sunday and before shutting down I did a mag check and got the rough left mag. I tried to clear it as you explained and nothing happened. I shut down and was about to leave when my brother arrived and wanted to see the problem, after re-start, no drop in rpm. On Monday my wife was practicing landings and we decided to check the mags before taking off again again it was rough with a 300-400 drop in rpm and we could not clear. I decided I would fly the few miles back to our home airport. We shut down and switched seats. On re-start the mags checked with no rpm drop.

I can't see how shutting down would help, but this is what's happening.

John

JOHN MASTERJOHN
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Re: Fouled plugs

I do not suspect plug fouling.

I suspect a bad mag.

FWIW.

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Re: Fouled plugs

I have had these sypmoms twice now.
Once it was a mag, once it was lead.

Some suggestions -
1, It could actually be a faulty mag or cracked wire.
2. It could be a timing issue.
3. It could be lead fouling the plugs
4. It could be oil fouling the plugs.

My bet is on lead fouling.

Tony
C-GICE

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Re: Fouled plugs

My IA tells me that mags should be rebuilt about every 500 tach hours.  You have 1350 Hours.  Have the mags ever been rebuilt?

When I get fouling of plugs, I notice this because my rpm drop is almost nil.  I then run the engine up to about 2000 rpms and with a lean mixture to "clear" the fouling then I get the rpm drop expected.

my 2 cents
Barry

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Re: Fouled plugs

Well John, some of us have mag/plug problems, and some of us have spouses that won't fly with us.
Sounds like you have the best deal.
Sounds more electrical/mag than fouling.

Good luck/keep us posted on problem.
Steve
KSGR

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Re: Fouled plugs

Steve,

Right you are on both counts:

Here's the update:

1999 c-172S RPM drop on run-up left mag

Today the mechanic's assistant (Mechanic is on vacation) and I removed thelower spark plugs. There was minimal fouling on all four plugs. There seemed to be a slight trace of oil on one plug, should not be significant.

On run-up: 1800rpm right mag 75 drop left mag 75rpm drop, however you could hear and feel a slight change like it was about to drop, almost a hesitation.

1200rpm- left and right both had drop less than 50 rpm

at idle (600rpm) right mag no indicated drop, left mag imediate drop to engine out.

This was repeated several times over about an hour. The results were the same until the last run-up when both right and left checked perfectly at all rpms.

All wire were checked without any indication of a problem. He feels that it is definately not rings- thanks for your input that helped me persue this.

Waiting for the assitant to get in touch with the mechanic for a diagnosis.

Any ideas other than a bad mag?

Thanks,

John

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Re: Fouled plugs

John

Ask your mechanic if this problem could be related to the ignition switch????
If so, should be relatively inexpensive to check.

Not an A&P
Steve

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