misfire diagnosis w/ vacuum gauge - Page 2 - Off-Road Forums & Discussion Groups
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post #11 of (permalink) Old 07-11-2009, 08:07 AM
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Anytime you pull a plug wire off, you run the risk of getting ground fires inside the distributor,
Or worse yet, inside the ignition coil.

A ground fire inside the ignition coil leaves permanent damage since a ground fire inside a coil leaves burned insulation on the windings.

And now that we are dealing with much larger energy discharges than ever before in ignitions, risk of spark energy discharging in the wrong place increases...

An HEI module will make for larger gap jumps because of the way the module 'Opens' and 'Closes' the coil circuit,
CDI modules will easy make for 3" or 4" discharges with no problems,
And all that energy can cause problems if you let it get away and do what it does naturally...

Anyway, enough with common electrical basics,
Let's get specific...

One thing you need to realize is,
Not all ignition modules are equal...
Cheap modules won't always fire every time they are 'Supposed' to...
So you will get occasional misfires from cheap ignition modules.
(...you get what you pay for...)

$20 'Import' modules like to miss firing cylinders REGULARLY, but randomly.

The same is true with $15 trigger stators in the distributor...
(... Again, you get what you pay for, 'Import' = Low quality.)


Poor grounding of the engine head will cause random misfires.

Not using a metal bearing 'Never-Seize' on the plug threads can cause rust on the head threads,
And rust will keep plugs from grounding properly.

Rust and corrosion also keep head bolts, grounding lugs and brackets from providing an adequate electrical 'Ground' to the engine head...

And then run over to the ignition module is the ONLY way to ensure your primary and secondary ignition system has adequate electrical 'Grounds' to operate properly at all times.

One way to minimize the possibility of ground fires inside the cap and coil is to provide a tempting electrical ground OUTSIDE the distributor cap.

Make a loop of bare (Stripped) wire that will fit around the center distributor cap (Coil wire) terminal.
Make it large enough that it will touch the plug terminals on the cap.
Ground the other end to a SOLID, known good electrical 'Ground'.

Pull the coil wire, place your 'Ground' on the distributor cap, and put the coil wire back on...

Now when you pull the plug wires, the spark energy can discharge to the 'Ground' you provided just an inch or so away,

That will keep the spark energy from blasting away at the coil, rotor, distributor cap, wrong spark plug terminals, ignition trigger or in the case of HEI ignitions, the module it's self.

Also helps keep you from getting 'Zapped' when you pull the plug wires and put them back on with the engine running!
That alone should be worth a 3' piece of insulated, grounded wire!

One way to find a bad plug wire/plug is to get another 'Test' plug and plug wire, and move it from cylinder to cylinder to see if your 'Miss' goes away...

Just remember to get a plug wire long enough to reach all cylinders.
It's cheap, effective, but somewhat time consuming.

You also need to keep in mind that this could be a number of other things than vacuum leak, bad plug wire or plug...

Bundling your plug wires together will cause INDUCTION FIRE problems...
Trying to fire more than one cylinder at a time.
You need AT LEAST 3/4" between plug wires, and an inch would be better...

Plug wires CROSSING each other (a "X" crossing is OK), but running them together is suicide for your ignition system...
All the hard work you put into it is wasted since you are going to get crossfires all over the place!

The farther they run together, the worse the induction fire will be...

Another problem is running coil, coil wire, or plug wires too close to the distributor trigger wires.
It's called 'False Trigger' or 'Induction Trigger'...
And it's the same principal (electrical induction) as getting multiple fires from running the spark plug wires together.

The trigger runs on MILLIVOLTS, and can fire the ignition with less than 0.2 volts on some modules.

Induction trigger from the wiring being too close to plug wires, coil wire or ignition coil/feed wires can cause REAL false trigger problems!

With the trigger signal voltages being so low,
It doesn't take spark plug wire voltages to cause induction 'False' triggers in the 'Stator' pair from the distributor trigger...
Even the 12 volts (or less) switching on and off to the coil primary can cause REAL PROBLEMS if those wires are bundled with the distributor stator wires...

From the factory, the (stator) trigger pair were twisted together, wound around each other.
This minimizes the induction differential created by running them in the same harness as one of the coil wires.

(Only running ONE of the coil primary wires in with the trigger pair cuts the current differential down by more than 50%)

Then the trigger pair were shielded by using a foil lined tape on the trigger pair before they were inserted into the harness.
This further shielded them from false triggers.

If you have re-routed or re-wired your ignition, you need to keep in mind that the trigger pair need to be kept next to metal...
They need to be twisted around each other,
And they need to be shielded.

Running a grounded wire in with them also helps to shield them, or grounding the shield helps...
IE: I often run the trigger signal pair in a piece of metal fuel line as a shield when I'm using really high powered ignition systems,
OR when I'm installing an ignition in something with very large radio transmitting potential...

A word about plug wires here....

NOT all plug wires are created equal.
There is a reason I recommend MSD plug wires above all else, and shielding from emitting EM and RF noise is one of the reasons!
(EM= Electro-Magnetic, RF= Radio Frequency)

Cheap plug wires Transmit in all kinds of EM and RF frequencies, and they case all kinds of induction problems.

MSD wires and specifically designed to cut back on much of the problems the $20 and $40 'Import' plug wires crank out.

EM & RF 'Noise' can cause random misfires and false fires at any time on any cylinder, and are usually the hardest problems to find with someone has 'Random Misfire' issues...

One way to find a bad plug wire, or RF/EM problems is to take one of the small, cheap (no external antenna) and tune it to an AM frequency that IS NOT broadcasting (Dead Frequency)
Wrap that little AM bugger in layers of tin foil,
And then pass it around your plugs/plug wires.

If you hear a steady 'POP' in the radio when it's around one certain plug/wire then you have found a bad plug wire/plug.

If you get a RANDOM Popping, with no speicific location, but strongest around the distributor (Farther away from the plugs as possible) then you have found cross fires/bad rotor or cap,
Or spark energy jumping to ground inside the distributor cap.

If you still don't hear any 'Pop' that doesn't sound like the rhythmic firing of the engine, but you are still getting 'Missing',
Then you have an induction misfire problem somewhere...

Either the coil primary, coil secondary (coil wire) or spark plug wires are causing an induction misfire/false trigger...

So Many Cats, So Few Recipes...
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