Frosted Plugs

[JRitt]

My brother in-law was flying my plane (C172N) and stopped for 1.5hrs (outside temp was ~20deg F dew point ~0deg F). When he did the runup to leave the left mag failed (ran rough and lost ~400rpm). There was a mechanic there (he is lucky like that) and he pulled and cleaned the plugs on the left mag and said they did not look bad and they were probably frosted. The runup was fine after the reinstall and he flew home with a 50kt tail wind.
Can someone explain the cause of frosted plugs?

 

[Michael]

I have never heard of this, but couldn't you just run it lean and treat it like fouled plugs to burn off whatever...?

 

[SkyHog]

Frosted plugs == carboned over plugs.

Look in the back of any haynes automobile manual, and it explains what frosted means. It doesn't mean frozen frost, it means white carbon build up.

edit: here's a pic:

 

[JRitt]

When he called I had him run it lean for 45sec @1700rpm but it did no good. Why do they use a term like "frosted plug" when its just a carbonized plug. Well they are clean now and the mechenic did not want to charge for it (will wonders never cease) but I insisted that he get paid (thats his job and I'm sure he has mouths to feed even if it is just his own). I was thinking there was ice across the electrode to ground and could not figure out how this could happen in a running engine.

 

[skyflyer8]

Frosted plugs == carboned over plugs.

Actually, no, it is frost, as in ice. See this FAA web site for details.

Another cold start problem that plagues an unpreheated engine is icing over the spark plug electrodes. This happens when an engine only fires a few revolutions and then quits. There has been sufficient combustion to cause some water in the cylinders but insufficient combustion to heat them up. This little bit of water condenses on the spark plug electrodes, freezes to ice, and shorts them out. The only remedy is heat. When no large heat source is available, the plugs are removed from the engine and heated to the point where no more moisture is present.

Brrrrrr.

 

[JRitt]

But it couldn't have been real frost because the engine was running and the right mag checked ok (unless H20 can be in ice form at a few thousand degrees). But I can see ice formation under the conditions the FAA spelled out

 

[Tim]

sounds like the mag may be getting weak

 

[apg86]

He did not lean the mixture when taxing, was some lead build-up on the spark plugs, to fix the problem next time just go full power and lean to peak and wait for a full 1 minute, and re-do the mag check. if the problem persist check with a A&P

Andrew

 

[gismo]

He did not lean the mixture when taxing, was some lead build-up on the spark plugs, to fix the problem next time just go full power and lean to peak and wait for a full 1 minute, and re-do the mag check. if the problem persist check with a A&P

Andrew

I wouldn't lean to peak EGT on a full power ground run, the CHTs will go through the roof.

That said, I've had more luck clearing carbon fouled plugs by running at full power for about 20 seconds (with no leaning at low DA) than by running lean at 1700 RPM. Typically that (full power) is about 1300 F on the EGT.