Glow Plug

I been flying for 10 years and still do not know much about glow plugs. How do you know when one is bad other than the engine starts running bad. The other day I had an old OS .40 running sickly, I changed glow plugs once and was still doing the same thing. I changed to a Tower plug and the engine started running like a Singer sewing machine. Makes no sense to me how these plugs work. I usually use the o.s. #8 90% of the time. The two plugs that I removed seem to glow brightly with power applied. The Tower plug did have a Idle bar however, I really do not know what the diff is, maybe one of you engine geeks can explain these plugs a bit. Thanks, Brad

Look carefully when applying heat and see if there is a 'dust' look to the element. If so, it has been damaged by stuff in the combustion chamber and that is the common way they appear to be good and give bad results. This only shows up OUTSIDE the engine.
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Amazingly, glow plugs can be bad right out of the package. Used to be that plugs were a platinum alloy so there was some platinum all the way through the wire. Nowdays it seems that some manufacturers are cutting costs by only plating the wire. If this plating is bad, the plug doesn't work as well. The fact that a plug glows with power applied really doesn't mean much for runing the engine.
If you look at the wire under some strong magnification, you will notice that a good running plug has a smooth surface. The bad ones usually have a rough surface or may appear to have some foreigh coatings attached.
-- Paul McIntosh http://www.rc-bearings.com
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On 11/22/2004 4:21 PM Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge:
One of the ways I tell if a given plug is good or not is to look at the element when power is applied to it. On all the bad plugs I have seen, the coil is not lit up on the first coil or coil and 1/2. With the power off I look to see if the element is shiny or dull. A dull appearing element is frequently also a sign of a marginal or bad plug. Neither of these is fool proof, however they do eliminate some of the guess work.
Other factors also come into play with this. Have you recently changed brands or nitro or oil content of the fuel ? Has the manufacturer changed anything ? This could mean you need to go to a hotter or colder plug. The symptoms are similar to a marginal or bad plug.
I have found that for 2 strokes an OS #8 will handle 90%+ of the engines. An OS F type plug is the best I have found (so far ) for 4 stroke engines.
I realize there are people who will swear by (or at ) a particular brand of plug. There may be (and probably are ) other brands as good as the OS plugs.
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I have "heard" that the Tower (6 for $14) plugs are identical to the OS #8 plug. I don't know it for a fact, but they sure look the same. Big $ savings if so.
John VB
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Thanks for all the information guys, I will try some of the things you have mentioned above. Can anyone verify if the OS and Tower plugs are the same. I really do not think so as my Tower plugs have a idle bar on them. Which brings up a new question, what does a idle bar do and how does it work. I assume that it helps with the idle. Thanks, Brad
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An idle bar, theoretically, prevents droplets of unburned fuel from striking the plug element. Unburned fuel on the plug is bad. This can drown the plug or even damage it.
That said, an idle bar is only needed by people who don't know how to tune their engines, IMO. <flamesuit on> If both mixture needles are adjusted correctly, the idle bar is not necessary.
An idle bar can even be detrimental in that it restricts fuel mixture access to the plug element. It also partially blocks the flame front from reaching the fuel mixture at the plug element. This leads to the necessity for a hotter-burning plug than "normal".
Dr.1 Driver "There's a Hun in the sun!"

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I was given an old Enya .19 that hadn't been used for around 15 years or more, it had only run a few times from new. It came with an idle bar glow plug, started and ran O.K. Glow plug died after a few outings and it was replaced with an OS-8. Ran as sweet as a nut all summer and autumn and was used to power a big thermal glider on windless days.
Come the first really cold spell, down to near freezing and it wouldn't start. No amount of setting up would do any good, it would give an occasional cough and that was it. I couldn't find a source of idle bar plugs (can find them now). Weather went a bit warmer... engine O.K again. Reverted back to cold and ... no engine !
It gave the distinct impression that the glow was being cooled... quenched ? by cold fuel hitting the glow element.
No amount of setting it up at home would cause it to start when the temperature went down. Spent that long outside fiddling one day that 'er indoors said "forget it and go and buy yourself another en..." I was out the door so fast I never did catch the last bit >:-)
Reg
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For an engine to run, it's got to vaporize its fuel. It sprays out the spraybar into flowing air, some of it evaporates, and that cools it down. Add to that the higher surface to volume ratio of smaller engines, and that makes the engine too cool to vaporize the fuel, even with the glow igniter attached. You might check Allen's RC Links and see if he has some cold weather starting tips.
Morris
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I think that's the best description I have seen so far. Puts it in a nutshell.
Reg

Then, why do nearly ALL 1/2A engines NOT have idle bars? Idle bars were useful when the standard fuel was 25% castor oil and the balance methanol. Modern fuels work very well in most engines and don't require idle bars.
-- Paul McIntosh http://www.rc-bearings.com
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I don't see what the discussion about why an engine is hard to start in cold weather has to do with an idle bar. But as for Cox engines and most 1/2 a's, you don't see any with loop induction requiring a baffle, so usually an idle bar is not required. The only engines I recall needing an idle bar was the loop induction and then only if you wanted a low idle. One exception was a highly timed SuperTigre .60 Bluehead with STS induction. The long duration and upward slanting ports caused them to spit fuel on the plug requiring an idle bar.
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I was replying to the assertion that smaller engines have different cooling requirements that necessitated idle bars.
BTW, if there were drops of fuel large enough to quench a glow plug, chances are the engine wouldn't run as those drops would contain more fuel than would be required to burn. Idle bars served two purposes, to keep the blobs of castor oil from quenching the heat and to retain more heat at low speeds when using low/no nitro fuels. Kind of like the extended shroud four stroke plugs now popular. Look at the Fox Miracle plug. The coil is deep inside the center rather than perched on the outside edge. The Glo-bee is an extreme example of having the element out in the breeze. They also don't survive any kind of abuse! ;^)
-- Paul McIntosh http://www.rc-bearings.com
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"Kind of like the extended shroud four stroke plugs now popular." I have not seen a four stroke plug with an extended shroud. They are longer with a longer coil and more exposure to the fuel. This has nothing to do with keeping fuel off of the element.
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They have more of the coil deeper inside the plug body to retain more heat between the less frequent firings of a four stroke.
-- Paul McIntosh http://www.rc-bearings.com
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From a book written by a well known English modeller... first printed 1978 and reprinted many times upto present date.
Basically he states... There are 2 main types of plug, the plain coil plug and the idle bar (or shielded) plug. The theory of using the latter plug is that at idling it protects the coil from impingement of fuel droplets and possibly assists in keeping the plug hot. Recommendation is... there are many types of plugs but start with the manufacturers recommended plug.
I have seen the same info. in other places as well as that book. The engine I was mentioning was over 15 years old and came with an idle bar plug as standard. The piston had a ridge formed on top of it and I guess that was to help in compressing the fuel charge and also possibly directing it around the cylinder head and not directly into the plug ??? The ridge was off-centre and on the inlet side of the piston.
Knowing what I do now I would have changed the plug for a much hotter version... however I only knew at that stage that the plug fitted had an idle bar and I was assured that they were very dificult to get now. I later found out that they are not difficult to get at all.
Fact was... it ran great in reasonably warm weather, as soon as the temp went down to around freezing it wouldn't start at all. I reckoned the fuel charge was cold enough, combined with the cold metal, to prevent the glow getting hot enough for ignition... effectively quenching the glow. Temperatute went up... engine back to normal. No amount of retuning would cause it to start in the cold. I spent 1 1/2 hours on it on one occasion.
My modern replacement engine has much better porting and an improved 'squish' area in the head.
Reg

That ridged piston is an old form of loop charging. It was an attempt to make the incoming charge take a longer path around the cylinder so that less of it would immediately exit the exhaust. Modern multi-port designs use something that was developed by Yamaha (IIRC) where the opposing transfer ports caused the incoming charges to collide, thus stopping their motion (relatively!) and the boost port attempted to flush the remaining burned fuel out. Many call this Schneurly (SP?) porting but the concept predates that by several years. I was working for a racing shop when the new Yamaha roadracing engines came out. It was a bizarre concept back then, but they won races!
The most likely reason that glow engines won't start at freezing temps is because the methanol just won't vaporize properly at those temps. Add to that the extra cooling of drawing it through a venturi and it may even freeze or ice up. Sure, then you get raw fuel drops on the glow plug but I think the battery would easily overcome that. No vapor, no fire.
-- Paul McIntosh http://www.rc-bearings.com
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I don't have any specifics, but one can probably do like the truckers and "winterize" the fuel with some percentage of pump gas?
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Never seen anything along those lines mentioned anywhere. A couple of ideas came up earlier in the thread.. a shot of WD 40 or a squirt of lighter fluid as a means to ease cold starting.
Reg
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I have known a prime of model diesel engine fuel to help on a very cold day with a recalcitrant glow engine.
I assume it's the volatility of the ether content.
On that basis one of the aerosol "cold start" products for automobile diesel engines, if they are still marketed, - I seem to remember these were ether with some form of propellant - a squirt into the venturi might do the job.
Malcolm