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How do it know?

Fordum

Fordum

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Thats the punchline of an old joke about Thermos bottles.

What I am trying to figure out is different.
Got a friend with a Lincoln Town Car. He has been getting an intermittent skip, so he borrowed my code reader.
It reported an intermittent misfire on No. 5 Cylinder.

Now there is a test procedure for some Fords, that will find a dead cylinder. You put the computer into test mode, and it sets a constant engine speed, then turns off the fuel injectors one at a time and looks for an RPM drop.

If it does not see a drop, that means that its a dead hole.
If it has individual coils, it turns them off on at a time.

Fine, I get that. I have run that test, and its obvious that its running the test.

What I want to know, is how it detects a misfire on a cylinder while driving down the road?
It doesn't have an oxygen sensor in each exhaust port, so it cannot see a rich or lean condition.

If it uses sequential injection, and/or individual ignition coils, it knows when each cylinder fires, but is it quick enough to detect a momentary loss of power at speed?

Or does it only check when doing a hard acceleration? Driving down the road at cruise, its hard for me to feel a skip, but when I am pulling away from a light or passing a car, it obvious.
 
That one is so old its got moss on it.
I thought everybody had heard it.
I'm sure it was originally an.....ethnic....joke. I'll modify it.

Old backwoods logger was given a Thermos bottom for his birthday.
After he had had it for a few months, a friend saw it one day and asked about it.

He told him that it was an absolute miracle.
Said when he first got it, his wife would put hot stew in it for his lunch during the cold days.
Then, as the weather warmed up, she started putting ice tea for a cold drink when he was overheated.

His friend said that that was nice of his wife, but what was the miracle?

The old man started getting agitated, waving his arms around.
Said you don't understand.
She puts in hot stuff, and it keeps it hot. She puts in cold stuff, and it keeps it cold for me.
Friend says "So?"

Old man says you still don't understand, it keeps hot stuff hot, cold stuff cold............... changes depending on what you put in it........HOW DO IT KNOW?!!
 
We get misfire codes all the time on cars with dead coils and such. This on a car with, for example, 5 cylinders exhaling through a manifold, then a turbo before it gets to the first of (at least) 2 oxygen sensors. They just know. It's like majic.
 
Fordum said:
Thats the punchline of an old joke about Thermos bottles.

What I am trying to figure out is different.
Got a friend with a Lincoln Town Car. He has been getting an intermittent skip, so he borrowed my code reader.
It reported an intermittent misfire on No. 5 Cylinder.

Now there is a test procedure for some Fords, that will find a dead cylinder. You put the computer into test mode, and it sets a constant engine speed, then turns off the fuel injectors one at a time and looks for an RPM drop.

If it does not see a drop, that means that its a dead hole.
If it has individual coils, it turns them off on at a time.

Fine, I get that. I have run that test, and its obvious that its running the test.

What I want to know, is how it detects a misfire on a cylinder while driving down the road?
It doesn't have an oxygen sensor in each exhaust port, so it cannot see a rich or lean condition.

If it uses sequential injection, and/or individual ignition coils, it knows when each cylinder fires, but is it quick enough to detect a momentary loss of power at speed?

Or does it only check when doing a hard acceleration? Driving down the road at cruise, its hard for me to feel a skip, but when I am pulling away from a light or passing a car, it obvious.
Click to expand...

It was explained to me like this many, many moons ago. The "dwell" time in the ignition coil will change on a cylinder that is not firing. Supposedly, it take a certain amount of Dwell in the ignition coil during the normal combustion process, and that changes on a non-firing cylinder.

Now, whether that is correct or not, I cannot say, but, it sure would go along way to explaining a few things.:dunno:
 
Boondock is right, it has to do with the time and current that is drawn from the coil. With the correct amount of fuel, air and compression, there is a pretty normal dwell and current draw curve every spark, but when something isn't right, such as the amount of fuel, or the pressure, or the plug/coil itself it changes the amount of electrical energy required and the time it takes to build up and release. The computer knows based on monitoring the coils and/or coil drivers. I could find out more detail, I have some friends that I used to work with in the ignition department at Chrysler and GM. Might be a few weeks before I see them again though.
 
i'd wager if he pulled that coil off, he would find the boot burned through. put a new spring and boot on it and have a nice day.
 
Confedneck said:
i'd wager if he pulled that coil off, he would find the boot burned through. put a new spring and boot on it and have a nice day.
Click to expand...
I figure the same thing, and the boot and spring are cheap.
But, unlike some cars, this one is a real pain to get to the coils. Covers must be removed, and maybe a line must be disconnected.
He figured he would just go ahead an order a coil with the boot so he would not go to all that trouble twice.

Its just nice that the computer told him which one it is, if its right......
 
a town car? it's one cover over the engine, and you use a 7mm 1/4" drive swivel socket on a 6" extension with a 1/4" ratchet and you can get the coils out without any other work.
 
I'll tell him. I haven't looked at the car for that problem. He just mentioned that it looked a lot harder than his old town car.

I guess I'm going to have to get more involved instead of just listening to what he says.......


Thanks
 
I know better than to post something here without all the facts at hand........
Just a guess, 4.6L.

2002 town car is all I know for sure.
 
Going fishing tomorrow, I'll tell him when I get back. I think he has the coil on order from Rockauto with boot.

I'll probably wind up doing it for him.......
 
all you need is a 1/4" ratchet to take the top cover off, as it's held on with a large headed bolt with a 1/4" square drive, and the coil is a 7mm, best is a 1/4" flex socket on a short extension. if you want, either 8 or 10mm to loosen the fuel rail
 
Wanted to update this.
The part came in today, and my friend called to ask which was No.5 cylinder. He thought he knew, and so did I, but I went online to be sure.

He called back, and said to tell you that he did not beat your time of 4 minutes, but he was under 6, and to be sure to thank you.
He said the boot and spring looked OK, but he replaced the coil and boot.
He used my scan tool and cleared the code, and said no more skip or codes.

I told him I would pass along his thanks.

Thanks from me too..
 
easy peezy. tell him he might as well buy the other plugs and 7 more boots and springs. get it done up nice. a little dielectric grease and he should have no worries.
 
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