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Weird engine/glow plug trouble

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Old 11-17-2012, 09:42 PM
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Mr-Punk
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Default Weird engine/glow plug trouble

So I took the old Kadet Senior out today to get some relaxed flying. After 1 flight with the old Magnum GP40 running fine, I fueled it back up and tried to start it. It started and quit when I tried to run it up. I went to start it again and all it would do is sputter a little with the electric starter turning it over. I went to pull it through and noticed it had about zero compression. I tightened the glow plug and still nothing. Thinking this over 10 year old engine just bit the dust I put it away. Well I took it apart to see if the piston or liner was scored or damaged and nothing and still had the pinch in the top. Put it back together and turned it this time noticing a hiss coming from the head. Upon examination and replacing the glow plug compression is back. Seems the glow plug's little stud broke it's seal. The glow plug still lights but the little stud part is loose. I've never seen that happen. Has anyone else ever had a glow plug fail mechanically? and still light?

Peace,
J
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Old 11-17-2012, 10:03 PM
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Default RE: Weird engine/glow plug trouble

It isnt a common problem but it has happened to me, usually on Cox glow heads . Ihave seen a few Fox plugs do it. Never had a Merlin break its seal.
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Old 11-17-2012, 10:24 PM
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Default RE: Weird engine/glow plug trouble

Yeah I've never ran across that even with cox heads. It just so happened to be a Fox plug I think.

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J
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Old 11-18-2012, 12:33 AM
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Default RE: Weird engine/glow plug trouble

Some plugs are more prone to this than others. I won't name names.

Rough handling with ignitors can crack the seal and cause such a leak.
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Old 11-18-2012, 05:04 AM
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Default RE: Weird engine/glow plug trouble

I remember that happening a couple of times when I used to do some pylon racing. It happened with some expensive high end glow plugs too. I have seen glow plugs do weird things over the years. Once when I was really big into pattern flying, I bought a couple of cards of glow plugs made by a major brand engine company, and every glow plug lasted just one flight. They denied there was a problem, but a few years later I found out that they had quietly changed the design and fixed the problem. But I never have bought any of their glow plugs since.  I have had a few plugs go bad, in that they still have continuity on the glow ignitor meter, but the engine won't run. I pull the plug and look at it and there isn't anything obvious that is wrong, it lights up good, it just won't fire up the engine. This tends to happen rarely, so that you forget about it for a while trying to figure out what the problem it. When in doubt I put in a new glow plug. I keep the old ones, almost news ones, etc. in a small box to use for test running engines. Usually when I test run engines, the running in process is hard on glow plugs and they tend to go bad fast like that.  I normally just buy Fox glow plugs and except for pylon racing, where I used some Rossi plugs, the Fox plugs have given me good service.
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Old 11-18-2012, 06:40 AM
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Default RE: Weird engine/glow plug trouble

Plug posts do come loose occasionally and any time an engine suddenly loses compression it's the first thing to check. Just dribble a little fuel around the post seal and turn the engine over slowly. If it's leaking you'll see bubbles.
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Old 11-19-2012, 11:24 AM
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Default RE: Weird engine/glow plug trouble

I had the same thing happen with a os 8 plug on a os 55. It would start and run, but I noticed it had a power loss. After landing and getting on the stand I hand spun the prop and compression was almost gone. I thought it was dead, got it home messed with it and could hear the air coming out the top of the head and found it was coming through the glow plug. New glow plug and it runs great
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Old 11-19-2012, 11:57 AM
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Default RE: Weird engine/glow plug trouble

One Sunday, many moons ago out at the field, a friend was running-up his engine...getting ready for a flight...when POP...it sounded like a muted firecracker or something...
As the engine windmilled to a stop, we walked toward it, wondering what happened...
The glow plug core / center stem had popped out and flew off somewhere!
It was an upright engine, and by the sound it made, it musta went 20 or 30 feet in the air...
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Old 11-19-2012, 01:17 PM
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Default RE: Weird engine/glow plug trouble

Fireballs do it all the time.  I epoxied a couple of cards of them, and if the motors run hot or have high compression they will go anyway.  I might try swedging them .  I want to use them up, they are great otherwise.
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