1997 Tbird Misfire - TCCoA Forums
 
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post #1 of 12 (permalink) Old 10-17-2012, 10:06 PM Thread Starter
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1997 Tbird Misfire

My buddy has a 1997 bird with a 3.8. He was getting a rough idle, so he swapped out the old plugs and wires. He's still getting a misfire on 1 & 5. Plugs got spark and compression in the cylinder. Thought it could be the coil pack but from what ive read, chances of it being a coil pack is slim to none; any idea as to what could cause the misfire? Could it be a ECM issue? Bad wire/plug? Any information to check for the issue would be greatly appreciated!

1996 Ford Thunderbird
170k miles 4.6V8 Auto
project started: 12/10 - project ended: never
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post #2 of 12 (permalink) Old 10-17-2012, 10:20 PM
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1&5? Firing order for a 1997 TBird 3.8 is 1-4-2-5-3-6 - and oh, look, 1/5 are on the same coil (it's distributorless). What you look for is what's in common, then work from there.

Yep, most likely the coil. Or the ECU, but swap the coil pack - it's quicker and easier. And more common.

One quick test is to disconnect 1 from the coil pack and see if 5 quits misfiring, then connect 1 and disconnect 5 and see if it quits then (since a bad wire and/or plug can load it down to where it won't fire the OTHER one ... ) But with new wires and plugs, that's doubtful.

Silly question - you DID use dielectric grease when you connected it all, right?

One more thing is to pull the plugs and see what they look like. There's plug reading charts all over the Internet.

Getting back to the coil pack - Try grabbing a coil pack from the wrecking yard, and see if it a) fixes it or b) at least moves to a DIFFERENT pair.

RwP
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post #3 of 12 (permalink) Old 10-17-2012, 11:17 PM
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What's wrong with the spark plug chart we have here? http://forums.tccoa.com/showthread.php?t=138950

Isn't this the second time this month I reminded you of that post, Ralph?

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post #4 of 12 (permalink) Old 10-17-2012, 11:37 PM
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Originally Posted by NetKeym View Post
What's wrong with the spark plug chart we have here? http://forums.tccoa.com/showthread.php?t=138950

Isn't this the second time this month I reminded you of that post, Ralph?
Possibly. I couldn't be arsed to check what the actual link is

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post #5 of 12 (permalink) Old 10-19-2012, 04:57 AM Thread Starter
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Quote:
Originally Posted by RalphP View Post
1&5? Firing order for a 1997 TBird 3.8 is 1-4-2-5-3-6 - and oh, look, 1/5 are on the same coil (it's distributorless). What you look for is what's in common, then work from there.

Yep, most likely the coil. Or the ECU, but swap the coil pack - it's quicker and easier. And more common.

One quick test is to disconnect 1 from the coil pack and see if 5 quits misfiring, then connect 1 and disconnect 5 and see if it quits then (since a bad wire and/or plug can load it down to where it won't fire the OTHER one ... ) But with new wires and plugs, that's doubtful.

Silly question - you DID use dielectric grease when you connected it all, right?

One more thing is to pull the plugs and see what they look like. There's plug reading charts all over the Internet.

Getting back to the coil pack - Try grabbing a coil pack from the wrecking yard, and see if it a) fixes it or b) at least moves to a DIFFERENT pair.

RwP
Yep dieelectric grease is on em, plugs are good, connections are tight, double checked wiring, order is proper. Gonna throw a different coil on her today and see if it fixes it. If so were good lol if not ill post back. Thank you guys!

1996 Ford Thunderbird
170k miles 4.6V8 Auto
project started: 12/10 - project ended: never
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post #6 of 12 (permalink) Old 10-20-2012, 04:47 PM Thread Starter
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Threw a new coil pack in her, and she runs like new! My buddy though a trans mount was going because it would shimmy cus of its running rough, and thats gone as well. So a few birds gone with one stone. Thanks guys! Problem solved

1996 Ford Thunderbird
170k miles 4.6V8 Auto
project started: 12/10 - project ended: never
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post #7 of 12 (permalink) Old 10-20-2012, 05:21 PM
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Originally Posted by RalphP View Post

One quick test is to disconnect 1 from the coil pack and see if 5 quits misfiring, then connect 1 and disconnect 5 and see if it quits then (since a bad wire and/or plug can load it down to where it won't fire the OTHER one ... ) But with new wires and plugs, that's doubtful.
On a Waste spark setup, if you disconnect either plug wire in the Paired cylinders, Neither plug will fire. It fires through both plugs in order to complete the secondary circuit - you would need to short one of the wires to ground to get spark on the other cylinder. Brand new plugs can be bad - loose ceramic / electrode, or a plug wire that is not fully seated will fire 3 sparks, one inside the boot .. this just makes the KV going up, burntime goes down which can lead to poor combustion on both cylinders but its not going to make one spark plug not work.

Sounds like the OP had an open or weak secondary coil in the pack.

Last edited by dDUBb; 10-20-2012 at 05:27 PM.
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post #8 of 12 (permalink) Old 10-20-2012, 06:45 PM
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Originally Posted by SCTbird1994 View Post
On a Waste spark setup, if you disconnect either plug wire in the Paired cylinders, Neither plug will fire. It fires through both plugs in order to complete the secondary circuit - you would need to short one of the wires to ground to get spark on the other cylinder. Brand new plugs can be bad - loose ceramic / electrode, or a plug wire that is not fully seated will fire 3 sparks, one inside the boot .. this just makes the KV going up, burntime goes down which can lead to poor combustion on both cylinders but its not going to make one spark plug not work.

Sounds like the OP had an open or weak secondary coil in the pack.
OK.

"It fires through both plugs in order to complete the secondary circuit"

I'm sorry, I can't see how.

The BLOCK is GROUND. ALL voltages reference to it.

So you're saying that magically the electrons flow THROUGH ground ?

Instead of flowing into/out of as separate events?

What gives them the opposite polarities is on which side of the PRIMARY windings they're on, isn't it? One has the spike positive, one negative.

But both reference to ground.

RwP
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post #9 of 12 (permalink) Old 10-20-2012, 08:40 PM
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OK.

"It fires through both plugs in order to complete the secondary circuit"

I'm sorry, I can't see how.

The BLOCK is GROUND. ALL voltages reference to it.

So you're saying that magically the electrons flow THROUGH ground ?

Instead of flowing into/out of as separate events?

What gives them the opposite polarities is on which side of the PRIMARY windings they're on, isn't it? One has the spike positive, one negative.

But both reference to ground.

RwP
The block would be ground if this were a Distributor Ignition or Coil on Plug ignition system.

The Waste spark uses 1 coil between two cylinders.

The Circuit is completed with the + on one end of the coil on the first ignition wire and the - on the second, you are correct in the aspect that one is a Positive and one is a Negative firing spark plug - the direction the electricity flows through the circuit. Obviously if the block were always ground they would BOTH be positive firing plugs.

The Primary winding in the coil induces the magnetic field in the secondary coil. But the Secondary coil - the one the Plug wires are attached to is a circuit on its own.

By disconnecting One plug wire and not grounding it to the block, you have an incomplete electrical circuit. When you ground it to the block, it will go through the block as if it were going through the first spark plug on the Positive firing plug and still go through the Second plug to go back to the negative side of the coil.

You are correct on the positive and negative spike - the Firing line. They both share the ground to complete the circuit Between the two plug - they both fire at the same time, the one on compression recieves a higher KV due to the additional resistance of compressed air, and the exhaust side has a lower KV - they switch KV depending on the engines stroke. But the Positive and Negative firing plugs depends on which side of the Coil they are on - they would be reversed if you were to switch the wires on the shared Coil in the pack and it would be exactly the same.

Last edited by dDUBb; 10-20-2012 at 08:47 PM.
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post #10 of 12 (permalink) Old 10-21-2012, 01:55 AM
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You can also understand that because this system works like this, technically, three cylinders fire the spark from the electrode to the ground strap of the plugs while the other three fire from ground strap to the electrode.

That makes total sense about the waste spark system and tracking down a misfire. The easiest way is to take a test light and probe each boot (creating a short which prevents that cylinder from firing).

Maybe a diagram makes it easier to see (pics are ALWAYS awesome):


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post #11 of 12 (permalink) Old 10-21-2012, 02:29 AM
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That makes total sense about the waste spark system and tracking down a misfire. The easiest way is to take a test light and probe each boot (creating a short which prevents that cylinder from firing).
Picture is worth a thousand words, thanks ..

Although a couple things I do not recommend:

Un-plugging plug wires and running engine can put excess fuel into the cylinder chamber creating potential for catalytic converter damage, cylinder washing and pre-ignition.

Piercing insulation on plug wires can create secondary leakage, the Arc could possibly travel out of the boot later and cause a misfire that wasnt there before. Its an easy test, much like spraying your engine with water in the dark to see if arcing is present at the wires. A spark plug tester is probably the most cost effective way to go about testing without having more expensive equipment.

Scope analysis is the best way although most people here dont own or have access to a graphing multimeter, KV module, digital oscilloscope, etc. But if you know the KV voltage is excessive, you have high resistance in the plug wires or the spark gap is too large, too much air / not enough fuel (lean mixture). Lower KV voltage readings inducate a short to ground, secondary leakage or a weak coil.

Most people just shotgun it and replace the plugs / wires since most of the time its not the coil itself.
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post #12 of 12 (permalink) Old 10-21-2012, 08:56 AM
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Originally Posted by SCTbird1994 View Post
Picture is worth a thousand words, thanks ..

Although a couple things I do not recommend:

Un-plugging plug wires and running engine can put excess fuel into the cylinder chamber creating potential for catalytic converter damage, cylinder washing and pre-ignition.

Piercing insulation on plug wires can create secondary leakage, the Arc could possibly travel out of the boot later and cause a misfire that wasnt there before. Its an easy test, much like spraying your engine with water in the dark to see if arcing is present at the wires. A spark plug tester is probably the most cost effective way to go about testing without having more expensive equipment.

Scope analysis is the best way although most people here dont own or have access to a graphing multimeter, KV module, digital oscilloscope, etc. But if you know the KV voltage is excessive, you have high resistance in the plug wires or the spark gap is too large, too much air / not enough fuel (lean mixture). Lower KV voltage readings inducate a short to ground, secondary leakage or a weak coil.

Most people just shotgun it and replace the plugs / wires since most of the time its not the coil itself.
I got that trick with the test light from my boss that owns the local shop - I always thought it a bad idea to poke a hole in the boot for the test, but I've see him do it hundreds of times and not cause an arc condition (like common sense would tell you would result). Plugs are relatively inexpensive to replace, wires not-so-much, but a good set should last a long time.

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