Explain the physics behind an intermittent misfire.

[Yota Up]

When it comes to regular misfire, it makes perfect sense that a part isn't working anymore.
You hit plugs, wires, coils, injectors, etc. You get a code for where it's at and tackle it.

But if you've got a situation where you fire it up and after it warms up, all of a sudden one of these things decides to work only 95-99% of the time?
I'd like to understand the concept of how this happens. Most things either work or they don't. Yet here you've got an engine running that skips a beat every 10 seconds or 20 seconds. A cylinder is firing just fine, over and over, and every now and then it decides to not fire.

Explain the concept of it. Explain the physics behind this.

If I was responding to this thread with my best guess the closest I can compare and guess at this concept is a light bulb that works most of the time because the wires are loose but still touching. So every now and then if you hit the switch the wires were moved a tiny bit by you hitting the switch that made them separate so they no longer have connection and it doesn't work on that try. You hit the switch again, and by that touch the force applied moved the wires a little bit to the point where they touch again so now the light turns on. <- that's my best attempt at explaining this concept and if something like this could apply to an engine misfire.

3rd gen 4runner, 3.4 engine. I'm good on fixing it so that's not the point here. The point is to understand how something like this can work and function but fail only a fraction of the time for what reason. Is it a wire that's barely touching? Is it a computer thing that decides to shit every now and then for no reason that only a computer engineer would understand? Is it a buildup of stuff that reaches max point to vent and then resets until the next time the shitter is full?

 

[Roc Doc]

Vomit

 

[Yota Up]

For most of us, it's likely that the very first time we were introduced to this concept was when we got a warped wheel on our bicycle. Every now and then, the brakes would get pinched. It was based on the rotation of that part of the wheel making a full circle. So the tire worked just fine most of the time, but when it hit that damaged part it would pinch.

 

[Roc Doc]

More vomit

 

[Roc Doc]

 

[Ted_Kaczynski]

It's like your posting lately. You run along fine then misfire. Search within for your answers Daniel-son.

 

[Gbkeith]

Well, I never could figure what the sky was thinking, but the soil, she don’t keep too many secrets.

 

[Ranchtruck]

When it comes to regular misfire, it makes perfect sense that a part isn't working anymore.
You hit plugs, wires, coils, injectors, etc. You get a code for where it's at and tackle it.

But if you've got a situation where you fire it up and after it warms up, all of a sudden one of these things decides to work only 95-99% of the time?
I'd like to understand the concept of how this happens. Most things either work or they don't. Yet here you've got an engine running that skips a beat every 10 seconds or 20 seconds. A cylinder is firing just fine, over and over, and every now and then it decides to not fire.

Explain the concept of it. Explain the physics behind this.

If I was responding to this thread with my best guess the closest I can compare and guess at this concept is a light bulb that works most of the time because the wires are loose but still touching. So every now and then if you hit the switch the wires were moved a tiny bit by you hitting the switch that made them separate so they no longer have connection and it doesn't work on that try. You hit the switch again, and by that touch the force applied moved the wires a little bit to the point where they touch again so now the light turns on. <- that's my best attempt at explaining this concept and if something like this could apply to an engine misfire.

3rd gen 4runner, 3.4 engine. I'm good on fixing it so that's not the point here. The point is to understand how something like this can work and function but fail only a fraction of the time for what reason. Is it a wire that's barely touching? Is it a computer thing that decides to shit every now and then for no reason that only a computer engineer would understand? Is it a buildup of stuff that reaches max point to vent and then resets until the next time the shitter is full?

Exhaust valves have worn into the seats making the lash clearance too tight I bet. Valve can't make a good seal when it's warm because it grows as it warms up (which is the point of having lash clearance) and the compression leaks out the exhaust.

What sucks on those is the lash is done with shims so it's a bit of work & math to get the right size in the right spots.

 

[alk1174]

There is a timmy the tool man video on YouTube that runs through the above scenario in pretty good detail .

 

[lagunaMS]

There is a timmy the tool man video on YouTube that runs through the above scenario in pretty good detail .

He doesn’t want that he wants people to interact with him

 

[Yota Up]

Exhaust valves have worn into the seats making the lash clearance too tight I bet. Valve can't make a good seal when it's warm because it grows as it warms up (which is the point of having lash clearance) and the compression leaks out the exhaust.

What sucks on those is the lash is done with shims so it's a bit of work & math to get the right size in the right spots.

If compression leaks then why would it fire perfectly for 20-30 seconds at a time before misfiring a single time, only to go back to firing fine again for a while?

alk, got a link to the timmy tool man video that talks about this?


Only 2 responses that even tried to offer a guess or talk about it. Everyone else just spews brainless bullshit as always. It's ok if it's out of your league, just keep your mouth shut and nobody will know you're too dumb to even offer a guess at it.

 

[Yota Up]

This thread has nothing to do with 4x4, by the way. It applies to 2wd vehicles just as much.
Who are the mods anyway?

 

[BDK]

Ummmm, maybe there is a part that is starting to fail but hasn't completely failed yet? Kind of like the OP's brain.

Had a GM 3.6 with a intermittent misfire in one cylinder. Turns out one of the new spark plugs had a hairline crack in the insulator.

 

[YotaAtieToo]

My samurai had a bad coil that would work fine, then stop when warmed up. My guess was a broken connection that could jump the gap when cold, but the gap would grow just enough to make it not work. No idea if that true, but makes sense to me.

 

[PAE]

Dont we all know to always to state year, make, model.

Codes ?

 

[Yota Up]

Ummmm, maybe there is a part that is starting to fail but hasn't completely failed yet? Kind of like the OP's brain.

Had a GM 3.6 with a intermittent misfire in one cylinder. Turns out one of the new spark plugs had a hairline crack in the insulator.

Something has to be changing with that crack to allow it to work 50-100 times in a row before failing once, and going back to function again.
Maybe the heat expands the crack enough to cause a misfire, and then it tightens again to resume working until the heat builds up again?
This is the type of stuff I'm trying to wrap my head around, to understand exactly what is going to produce these results.

My samurai had a bad coil that would work fine, then stop when warmed up. My guess was a broken connection that could jump the gap when cold, but the gap would grow just enough to make it not work. No idea if that true, but makes sense to me.

Don't those have a little microchip? If so, all bets are off to understanding that. Seems like any type of ghost can allow a processor to work work work and randomly decide to not work for whatever reason.

Dont we all know to always to state year, make, model.

Codes ?

Original post, see above.
I said normally you get a code, but not in this case.
Also stated it's a 3rd gen 4runner with 3.4.
I don't need help fixing the truck.
I just wanted to talk about the concept of this stuff.

 

[BDK]

Something has to be changing with that crack to allow it to work 50-100 times in a row before failing once, and going back to function again.
Maybe the heat expands the crack enough to cause a misfire, and then it tightens again to resume working until the heat builds up again?
This is the type of stuff I'm trying to wrap my head around, to understand exactly what is going to produce these results.

It was mostly under hard acceleration so I imagine the rapid firing caused electrial charge to find a route out the crack instead of to the ground, which caused a intermittent misfire.

Or the crack in the insulator caused the magic smoke to leak out of the side...

 

[Will Scarlet]

 

[Gdogg]

You need to understand what fuel trim is and how it works, then open loop and closed loop, there's way to much to explain in a forum go to youtube and watch some videos. I use a scanner daily and do a fair amount of driveability.

 

[Fishnbeer]

I dont have any saved scope patterns on my phone, have some stored on the laptop at work... But I can try explaining secondary ignition.

If you have used a voltmeter to check resistance on something before, try to think of this being very similar - think of the compressed charge or air and fuel in the cylinder being a conductor or the 'load' in a circuit... think of checking resistance of the spark plug gap in a cylinder with a compressed air/fuel charge.

More fuel/rich mixture = more conductive (fuel atoms are closer together... a better electrical connection). A lean charge is the opposite, harder for electricity to go through - harder to jump the spark plug gap (high resistance). Compression factors into this too, squeezing the atomized fuel gets the molecules together - a better electrical connection, easier for the spark to jump the gap - low resistance.

Now when the engine is running lets just say for now the fuel/air mix is a fixed ratio so we can focus on compression. When is compression at its highest? Engine cranking, when engine speed is slowest. When is compression lowest? High rpm. So high rpm is low compression = harder to jump the gap.

Soooooo any time it is hard to jump the spark plug gap is when you are likely to see a misfire. If your ignition is aged, old spark plug wires, old ignition coil boot, etc... there are going to be itty bitty holes in the rubber. Those itty bitty holes are the weak link. When it is hard to jump the spark plug gap, the spark will find a new path through the weak link.

 

[Skinny]

Random mis can be anything. Chaffed wire causing an erroneous signal, bad MAF, air filter fuzz on the MAF hotwire, filthy injectors, coil starting to fail, pickup sensor going out.

A component failing doesn't always fail 100% and just die. They can certainly get finicky before going out completely. This is where a scan tool comes in and knowing how to read the data, figuring out what cylinder it is, swapping parts from one to another, etc.

 

[Fishnbeer]

Here is an image I pulled from the web. Ignition patterns are all very similar but this appears to not be points type ignition and a modern transistorized ignition. Either system will have this same basic pattern.

At the left is the "spark is established" line. Then the flat line is the "spark line". This is a pattern of a good running engine.

If you had a car with old spark plug wires, and had a misfire, what do you think would happen to that 'spark line'?

 

[Fishnbeer]

Here is something neat I put together years ago. This is what it looks like when you thread an adapter into the spark plug hole, mount a pressure transducer and connect it to a scope.

 

[Yota Up]

A lot of these things can't even be seen by the eye. Funny you mentioned a tiny hole in the insulation of a plug wire. I had a coded misfire long ago and when I checked the wires I could actually see the hole. But that was a consistent failure. With something like this I'm sure it won't even be visible. So scan tool/scope or wild guesses is where it ultimately settles then. Thank you.

 

[Fishnbeer]

well you could also just swap parts. Swap coil #1 with #3 and see if misfire follows. Swap spark plugs, see if misfire follows. An ignition coil or plug wire may have 'arc marks' where spark is going through insulation. Looks like a white/grey spot that would be close to something metal

 

[Yota Up]

plug wire may have 'arc marks' where spark is going through insulation. Looks like a white/grey spot that would be close to something metal

 

[Fishnbeer]

Yeah, like that

 

[Winchested]

Something has to be changing with that crack to allow it to work 50-100 times in a row before failing once, and going back to function again.
Maybe the heat expands the crack enough to cause a misfire, and then it tightens again to resume working until the heat builds up again?
This is the type of stuff I'm trying to wrap my head around, to understand exactly what is going to produce these results.


Don't those have a little microchip? If so, all bets are off to understanding that. Seems like any type of ghost can allow a processor to work work work and randomly decide to not work for whatever reason.


Original post, see above.
I said normally you get a code, but not in this case.
Also stated it's a 3rd gen 4runner with 3.4.
I don't need help fixing the truck.
I just wanted to talk about the concept of this stuff.

Ya shit gets hot then doesn't work. It's not rocket science. Coils heat up they fail when hot. Plugs get hot, they fail when hot. Valves get hot spots, it's like dieseling with carbs that continues to vacuum suck fuel out of the carb.

Heat kills there's the physics. Problem solved. All electrical/ mech things fail.

Life sucks.

 

[Ranchtruck]

If compression leaks then why would it fire perfectly for 20-30 seconds at a time before misfiring a single time, only to go back to firing fine again for a while?

Because the valves rotate on the seat. So if you have a tight valve that's barely seating it's going to rotate and line up on some of the worse damaged areas intermittently. That will give an occasional miss until you put the engine under enough continuous load that it burns the valve because it can't dump the heat to the seat. Then you get a constant miss because all the compression gets past the valve.

I read your initial post as you had thrown plugs, wires, coils & injectors at it and it was still missing. So seeing that you had an arced out plug boot means that you were just looking for a mental masturbation session.

You should probably still check the exhaust valve clearance, high mile 5vz's are known for tightening up and replacing shims is cheaper than a head job for a burnt valve.

 

[Yota Up]

I read your initial post as you had thrown plugs, wires, coils & injectors at it and it was still missing. So seeing that you had an arced out plug boot means that you were just looking for a mental masturbation session.

Never threw plugs and all that at it. Why would you think that when the first thing you see is a bad wire?
The plug boot was from way before, and it was a constant misfire throwing CEL, not an intermittent like we're talking about here.