1 Cylinder misfire spark plug destroyed lpg

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00lewisd
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1 Cylinder misfire spark plug destroyed lpg

#1 Post by 00lewisd » Sat May 11, 2013 10:32 am

I'm new to this site but would really appreciate some help in troubleshooting my engine problem on my Toyota Altezza RS200 running lpg.

After driving the car on European motorways I noticed that one cylinder was not firing at all, because at idle, the car was vibrating quite badly. I removed all the spark plugs and noticed that one of them was destroyed (see pictures below). It looks like the plug had shot out of the engine which had destroyed the thread on the cylinder head and on the spark plug and cracked the coil.

I know that I can use a helicoil to repair the thread on the engine but I don’t want to do that until I can figure out why the plug would shoot out of the engine as I think there may be more problems to be found. I have heard that if the spark plug is not tightened to the correct torque this can cause the plug to shoot out, but I don’t think this is what has happened because of the amount of damage to the spark plug:
- Both the plug’s electrodes have broken off and fell into the engine
- There’s a build-up of something (maybe aluminium) around the plug’s electrodes and the thread
- And the whole plug looks like it has been sandblasted.

It looks like that cylinder had got so hot that it weakened some some of the aluminium cylinder head that supports the plug; causing the plug to shoot out, given high enough pressure.

So I have two theories of what may have caused this:
1- The engine was knocking excessively; causing abnormally high pressures and temperatures. In which case this could mean that the lpg ECU was not properly reacting to the readings from the knock sensor. Although this doesn't explain why it only affected one cylinder.
2- The LPG was burning too hot only in that cylinder. This could be caused by a very lean air to fuel ratio.

My 3s-ge engine is quite powerful for its size (2.0L, 4 cylinder, 220bhp, revs to 7800rpm). Would there be any obvious problems with running this sort of engine on lpg?



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00lewisd
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Re: 1 Cylinder misfire spark plug destroyed lpg

#2 Post by 00lewisd » Sat May 11, 2013 11:05 am

I have used a boroscope to see inside the broken cylinder. A link to the video is below. Skip to 2.31min to see most of the damage to the cylinder wall.

http://www.youtube.com/watch?v=yE3oejMPju0

volvobaggen
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Re: 1 Cylinder misfire spark plug destroyed lpg

#3 Post by volvobaggen » Sat May 11, 2013 11:13 am

I believe all the damage seen is the result of a loose spark plug.

In the end stages where the plug is barely staying in the engine, the soft aluminium threads in the head are destroyed(shavings) and its no wonder that the electrodes broke off either. The plug windings are made from steel with a chrome coating, much harder than the soft windings in the head. Also super hot exhaust gases from the cylinder would leak by and burn the soft aluminium threads at the same time mechanical damage occurs.

Why should any of the theories you present make a spark plug loosen? The knocking/shaking is the symptom of a spark plug on the way out, not the cause. Lpg is no different from petrol except from slightly higher combustion temperatures.
Last edited by volvobaggen on Sat May 11, 2013 11:29 am, edited 1 time in total.
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Gilbertd
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Re: 1 Cylinder misfire spark plug destroyed lpg

#4 Post by Gilbertd » Sat May 11, 2013 11:22 am

I'll agree with volvobaggen, that looks like it was caused by driving the car with the plug being loose and the damage to the plug is purely physical. The blackening on the metal of the plug is where the cylinder was still firing as it was part way out and is the exhaust gasses making there way out round it before it was blown out completely. The force of it blowing out is what has damaged the coil.

The LPG ECU will not react at all to the knock sensors, it simply piggy backs onto the petrol ECU which will react so running on LPG will be altered just the same way as on petrol.
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00lewisd
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Re: 1 Cylinder misfire spark plug destroyed lpg

#5 Post by 00lewisd » Sat May 11, 2013 11:32 am

Thanks for your replies.

This is disappointing, although I didn't use a torque wrench to install the plugs, I did follow the instructions that were given: "Turn the plug until you feel the thread bite, then do another quarter turn".

Can you explain why this would cause the electrodes to fall off?

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Re: 1 Cylinder misfire spark plug destroyed lpg

#6 Post by volvobaggen » Sat May 11, 2013 11:56 am

00leswind - Almost the same thing that you happened to you, happened to me after I installed plugs. I´m no mechanic, I have just read a lot of engine and car theory mainly off the internet, and with little experience this can be a bad combination.

There are many, many factors to consider installing spark plugs. Ideally, all wrenching on an engine or a car should be done with metal tempereatures at about 20 degrees Celcius to ensure proper torque values. Then there is the question of using antiseize on the windings, which is a gigantic topic on the Internet. Then there is the question of replacing the plug entirely if you check them due to allready deformed crusher rings etc etc.

Anyways, I did the same as you. Changed spark plugs, the old ones where hard to get out with carbon deposists on the windings but got them lose. Installed new plugs as the NGK instructions said, but without torque wrench and with a little engine oil on the threads.

I did it on two cars, my uncles Golf and my own Volvo. Some months later, my uncle reported shaking and vibration from the engine, and my uncles mechanic said that "one of the spark plugs almost looked like it danced" in the spark plug hole, and was almost free. The threads were luckily not destroyed, and the plugs were tightened much harder than I did. Now it works fine.

Checked my Volvo on a hot engine right after I switched it off(which is not correct), and two of the plugs were looser than the others. Had to almost tighten them 3/4 of a turn each.

So changing spark plugs is not as easy as most people say it is, atleast that is my experince.

I believe the electrodes broke off your spark plug when the plug were holding on with its last windings, in this situation the plug should be able to rotate and twist sideways giving rise to a lot of mechanical wear and damage. Also keep in mind that the side electrodes are made from Nickel; Another thing to add into the equation.
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Re: 1 Cylinder misfire spark plug destroyed lpg

#7 Post by volvobaggen » Sat May 11, 2013 12:12 pm

Here is what NGK writes about the subject of broken electrodes

http://www.ngk-sparkplugs.jp/english/te ... index.html
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00lewisd
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Re: 1 Cylinder misfire spark plug destroyed lpg

#8 Post by 00lewisd » Sat May 11, 2013 1:50 pm

Thanks for the link volvobaggen, however that doesn't explain why both electrodes fell off. You can see in the images below that the centre electrode is also missing. Blobs of metal have also been melted to the ground electrode; this is not shown in the image by NGK. NGK show a clean spark plug where the only damage is a snapped ground electrode.

If the spark plug vibrates until it is loose, the cylinder would loose compression. Why would this cause abnormal temperatures which would melt part of the cylinder head?


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Re: 1 Cylinder misfire spark plug destroyed lpg

#9 Post by volvobaggen » Sat May 11, 2013 2:13 pm

Good point. I have no experience with this issue so I am only speculating now.

What if the plug is loose and and there is expanding hot gases rushing out of the sylinder past the sparkplug when the fuel/air mixture detonates. This would theoretically mean that air should also be able to leak into the cylinder past the sparkplug at the intake stroke when the cylinder creates vacuum. This would lead to a lean mixture and higher temperatures which could weaken metal and is the only way I can explain the thermically eroded tip of the sparkplug. Considering the numerous revolutions an engine makes pr minute, this could probably also happen rapidly in a cylinder.

Also, with the temperatures rising in the combustion chamber, predetonation would also occur contributing to more damage.

Maybe someone else here has an alternate explanation, but maybe it could also be smart to ask engine gurus at other forums and show them the pictures.
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00lewisd
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Re: 1 Cylinder misfire spark plug destroyed lpg

#10 Post by 00lewisd » Sat May 11, 2013 2:20 pm

This is another forum I have posted on:

http://www.tezzaworld.com/viewtopic.php ... 56&start=0

Their opinion is that LPG runs hotter and has worse cooling properties than petrol, combine this with the 3s-ge engine which runs quite hot anyway...

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Re: 1 Cylinder misfire spark plug destroyed lpg

#11 Post by 00lewisd » Sat May 11, 2013 2:30 pm

By the way, watch my video of the inside of the cylinder, you can see it looks like the piston is pitted. I believe this suggested knocking.

http://www.youtube.com/watch?v=yE3oejMPju0


This link explains the problems that can be caused by knocking/detonation:

http://www.enginelogics.com/detonation.html

"Engine Detonation causes three types of failure:

1. Mechanical damage (broken ring lands, Bent connecting rods, rod bearing failure)

2. Abrasion (pitting of the piston crown)

3. Overheating (scuffed piston skirts due to excess heat input or high coolant temperatures)"

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Re: 1 Cylinder misfire spark plug destroyed lpg

#12 Post by volvobaggen » Sat May 11, 2013 2:41 pm

I don´t agree with the people in that thread.

LPG is higher octane than most fuels out there with a rating of 113 for propane, meaning that the fuel will withstand higher temperatures and compression better than normal petrol and racing fuels like ethanol. Yes, lpg burns hotter and dryer than petrol. Yes lpg will wear more on the valve material and can cause your exhaust valves to be destroyed. But it can not explain a spark plug flying out of the head in an otherwise healthy engine.

Look at this video http://www.youtube.com/watch?v=WuCUGcqO5SE where predetonation is explained in detail.
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Re: 1 Cylinder misfire spark plug destroyed lpg

#13 Post by Tubbs » Sat May 11, 2013 2:43 pm

Loose plug, end of. Coil pack murdered the plug with excessive sparking, modern ecu's would shut the cylinder down in this event, perhaps saving the cylinder. Lpg doesnt really knock ? So how do you think its to do with the LPG ?

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Re: 1 Cylinder misfire spark plug destroyed lpg

#14 Post by Gilbertd » Sat May 11, 2013 2:50 pm

Typical old wives tales and misinformation from those that think they know about LPG but then prove that they don't by posting BS. There's as much truth there as saying that since running on LPG your budgie has died and you haven't won the lottery, you can draw assumptions from anything but they aren't always right. Yes, LPG does burn hotter but the difference is 1350 degrees for petrol and 1370 degrees for LPG, a difference of 1.5%. LPG also has an octane rating of 113 RON so you can run more ignition advance and a higher compression ratio before you get pre-ignition and, if an engine is tuned to the max on LPG will actually produce more power than on petrol. The argument that the engine needs to warm up on petrol before switching over to gas has nothing to do with the fuel, it is purely because the mixture needs to be richer on petrol for cold running but it doesn't on LPG. As the LPG ECU piggy backs on the petrol ECU, it would run badly on LPG from cold due to the unnecessary cold running enrichment strategy that the petrol ECU will apply. On a stand alone LPG system, such as an older single point system, you can run on LPG from cold.

I suspect the bits of molten metal on the end of the spark plug are actually small fragments of the aluminium head that have been melted onto it by the combustion process. When the plug was loose but still firing, you have been getting flames coming out of the hole around the plug. That is what has blackened the plug threads and hex. The dent on the side of the plug shows that it has been hit by, or hit something as it came out. It may have been, as has been suggested in the other forum, by the piston although that is highly doubtful as that part of the plug wouldn't have been exposed, it would be within the threaded portion of the head. It's more likely that it was dented when it hit something else as it shot out of the head. It was, presumably, still attached to the coil/HT lead when you found it, what did it hit as it was fired from the head? I suspect if you look around the engine bay you'll probably find a matching mark on something else where it came out and hit it. Volvobaggen's theory that it was drawing air in around the loose plug so weakening the mixture and making that cylinder run hotter, at least while it was still running, makes a lot of sense. That could explain the pitting on the piston crown due to pre-ignition although it is far more like that the pitting was caused by the spark plug electrode bouncing around on top of it after it broke off due to the excessive vibration. At 6,000 rpm it will be firing 100 times a second and it wouldn't take many seconds to do the damage.

Make that +3 for loose plug, either not done up tight enough or maybe even overtightened causing the threads in the softer alloy head to be weakened. What amazes me is that you had to wait until you stopped to detect that it wasn't running right. Didn't you notice the 25% drop in power and bang as the plug came out?
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Re: 1 Cylinder misfire spark plug destroyed lpg

#15 Post by rossko » Sat May 11, 2013 3:44 pm

Another experience-based resource
http://www.ngksparkplugs.com/tech_suppo ... p?mode=nml
"An improperly installed (insufficient torque) spark plug can also result in pre-ignition due to inadequate heat transfer. "

You might be underestimating just how quickly a loose plug will heat up, just because it is loose - no thermal contact with head to cool it, and flame leaking right past it. Because it's become too hot, it will then cause pre-ignition and get even hotter to melting point. People are wondering why you didn't notice while driving - but I'll bet it was all over in minute at speed.

Take no notice of folk who think the spark plug ought to come unscrewed without any damage. Take no notice of folk who think LPG causes pinking (it's the exact opposite). As you'll already know, you don't have a melted piston or broken big end etc.

While you've got the boroscope, look into the other cylinders. If you wanted to blame LPG you should be able to see the same pitting, and the other plugs should look overheated too?

As you've worried yourself about LPG, it'll obviously be worth having your gas system checked over after you've repaired the plug damage for peace of mind.
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Re: 1 Cylinder misfire spark plug destroyed lpg

#16 Post by classicswede » Sat May 11, 2013 9:40 pm

What a load of shite on that other forum. Asking them for LPG engine advice would be like asking a tree surgeon to work on your teeth!

It is quite clear that the problem has been caused by an incorrectly fitted plug. The answers to why have already been given.

I think an engine rebuild might be on the cards but you could start with repairing the plug thread and doing a compression and leak down test on all the cylinders
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Re: 1 Cylinder misfire spark plug destroyed lpg

#17 Post by 00lewisd » Sun May 12, 2013 9:09 am

Yes that is going to be my next step. I will get a helicoil kit to repair the broken thread, and do a compression test.

I also remember hearing a thumping/banging noise that happened once every revolution. I think this is most likely to be caused by play/backlash in the big or little end bearings on the conrod in the broken cylinder. So I should be able to remove the sump to access the big ends (providing the engine passes compression test).

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Re: 1 Cylinder misfire spark plug destroyed lpg

#18 Post by Gilbertd » Sun May 12, 2013 10:39 am

More likely it was the combustion coming out around the plug coupled with the plug itself flapping about ripping the threads out. I doubt you'll find any bottom end problems. A loose plug will come out by itself and a dodgy big or little end wouldn't cause a plug to come out.
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Re: 1 Cylinder misfire spark plug destroyed lpg

#19 Post by 00lewisd » Sun May 12, 2013 10:52 am

At the time, I removed the spark plug and disconnected the fuel injector to the bad cylinder to try to prevent more damage, and I drove it home on three cylinders. In this setup I heard the banging noise, but it didn't happen straight away.

I thought the big ends would have been put under a lot of stress from the pre-ignition/knocking, and if part of the spark plug had got wedged against the cylinder wall, this could cause extra friction and put more load on the big ends.

Anyway, after I helicoil the thread, I can find out if the noise is still there.

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Re: 1 Cylinder misfire spark plug destroyed lpg

#20 Post by hardcorepit80 » Sun May 12, 2013 3:09 pm

What a load of shite on that other forum. Asking them for LPG engine advice would be like asking a tree surgeon to work on your teeth!
Amazing how people who know nothing about running on gas blame LPG for engine damage....
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