so today i was going to work car drove fine then it sat for about 5 hours. the i go to leave and its miss bad so i pulled the plug out cyd4 and it was driping wet with gas. so i put a differnt set a of 750S in the car and the same thing happend. now could it be a ecu? there's spark on all 4 cylinder /ubbthreads/images/graemlins/banghead.gif

How do you know there's spark on all cylinders if cylinder 4 is wet with gas? Sounds like a coil pack or related ignition part to me.

How did you check for spark?

I've been fooled a couple of times by just laying the plugs on the valve cover and watching for spark as the motor is cranked over.

That test verifies there is "some" spark, but it still may not be enough to lite things up when the plug is in the chamber and under pressure.

I think Jeff's on the right track with following up on all the things that are unique to tossing spark at the affected cylinder.

The ecu fires #1 and #4 together, so if one plug of the pair is firing well, and the other isn;t, the ecu is probably okay, and taking a long hard look at the coil terminal/spark plug wire/spark plug is in order.

If you have a spare plug wire, toss it on and see if that clears things up.

One place to check for a single cylinder mis-fire is the coil terminal where the plug wire enters the coil. Those can corrode and introduce a whole sh*t ton of reistance into the circuit, leading to a weaker spark on that hole.

I've also had plugs give some weird problems after they get fuel soaked. The fuel can loosen carbon, and then it bridges the gap, leading to a no spark and a dead hole.

I've also had fuel soaked plugs glaze over when they got hot and all the softened deposits remelted on the ceramic. This can give an easier path to ground than jumping the plug gap under the higher cylinder pressures seen under boost.

When in doubt, change them out. /ubbthreads/images/graemlins/wink.gif

thanks guys i'll have to go and try suggestions. sorry about the spelling errors i was in a hurry this morning so i wouldn't be late for work.

Quoting toybreaker:

The ecu fires #1 and #4 together, so if one plug of the pair is firing well, and the other isn;t, the ecu is probably okay, and taking a long hard look at the coil terminal/spark plug wire/spark plug is in order.



The cylinder pairs actually fire in series with both plugs forming a complete path. It is impossible for one of the pair to fire and not the other. That is, if 4 is having a spark issue then so to is one. The only exception is if one of the plugs is so badly melted that it forms a short and carries current without an open spark. So, if 4 is missing, but not 1, then it is almost certainly NOT an ignition issue. It could be overfueling or maybe a compression issue. Also check your cables. If the cable to the bad cylinder had a short and were grounding straight into the head it would bypass the plug. That would be effectively the same as a melted plug since it would carry current without an open spark.

Quoting belieze1334:
It is impossible for one of the pair to fire and not the other. That is, if 4 is having a spark issue then so to is one. The only exception is if one of the plugs is so badly melted that it forms a short and carries current without an open spark. So, if 4 is missing, but not 1, then it is almost certainly NOT an ignition issue



... you should get out to the shop more often ...

As a tech, I can tell you we see it all the time.

Mostly, it's glazed plugs, but it can also occur from burned coil terminals.

It'll throw spark sitting on the valve cover, but put the plug back into the hole and put some compression pressure/increased density into the mix, and all bets are off.

A plug will fire at less that a couple of kv in free air, but it takes many thousands of k.v. to toss a spark under pressure.

There can be many paths that have less resistance than that voltage level.

The spark energy can trickle down the side of the ceramic, due to the coating of carbon that's left on a "fuel fouled plug" after the fuel is burned off and a layer of carbon glaze is formed on the ceramic.

And, my personal favorite is when the coil tower/plug wire corrodes over time, and eventually the resistance gets high enough the spark punches out the side of the terminal. (I've got a pic of that failure on my old laptop, and I've seen it several times on these cars.

Both of those scenerio's present a single cylinder misfire in service, and yet will often pass the ole spark plug on the valve cover test.