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[tunnie]

As discussed, I'm home soon so will post coil packs, it's something else that you can rule that out.

Interesting misfire on pot 4, had you not already put new plugs in?

[jonny2112]

Thanks mate 
When I first went to get the plugs, dealer only had two  . I fitted these to one and three.
Got the others last week but didn't fit them 
There was a misfire previously on four, and on one, which has now disappeared. Also both knock sensors previously showing, with only one now.
Strange though that the misfire and knock sensor currently indicated are on opposite banks 

[Andy H]

also to work out roughly what psi youre looking for on each pot....

atmospheric pressure (14.7) X compression ratio (which i believe is 10.8:1) = 173psi

someone may need to double check that though as i have been drinking 

It is a bit more complicated than that.

When the air (& fuel) is drawn into the cylinder the restriction of the inlet passages means that the pressure in the cylinder when the piston is at the bottom of the stroke is a bit less than atmospheric which therefore reduces the maximum pressure at the top of the stroke.

On the other hand.......  as the air fuel mixture is compressed it gets hot which in turn raises the maximum pressure. The heat generated quickly transfers to the cold cylinder walls so the effect is greatest with a good battery and high cranking speed.

When I have carried out a compression test I have measured a higher pressure than the simple maths (atmospheric pressure X compression ratio) would suggest. The key is that all cylinders should give a reading within about 10% of the others.

[Kevin Wood]

Other problem with calculating compression pressure is that there will always be an intake valve open until a fair way up the compression stroke so, depending on the cam timing, this will reduce the pressure.

[05omegav6]

What you will also find is that the knock sensors are very sensitive. They pick up the misfire, send a signal to the ECU which then tries to control the knock by adjusting the fueling. This only makes the engine misfire more.

Change the plugs, disconnect both knock sensors, reset the light and take it for a spin. The knock sensors will put the light on, but you can ignore the codes as you know they are disconnected, any other codes should point towards the problem.

If the only codes you get back are 0300 and 0304 the that only leaves the coil pack and the wiring

[jonny2112]

Thanks al. I'd hoped to get this done this evening but it didn't happen.
Tomorrow morning 

[jonny2112]

Well changed the plugs and had a quick spin with the knock sensors disconnected and them connected. No change 

Codes -
0300
0304
0325
0340 (I disconnected wrong sensor for test so can disregard  )
0325 again!?
0330

Nothing new or unexpected, except 0325 showing twice.  I'll need to nip it down to the garage to reset the codes tomorrow to see if the new plugs cured the misfire though not convinced.
Idles as normal at around 500 rpm, but a little lumpy. Seems a bit reluctant to change gear, revving to about 2500-3000 from 1st before changing. Down on power too obviously.
Wasn't able to get the compression tester for today, which was a mistake on my part as it could have been tested while changing the plugs (especially no 6 !!).
For the compression test I guess the top end needs to be complete except for the plugs?

[05omegav6]

Either coil pack or wiring then.

Try a pair of known good coil packs, ( I know only pot 4 playing up, but if one is on its way out, tother side won't be far behind ).

If that doesn't work, then a new loom is required.

Others can recommend coil pack providers, new about £60-80 each, loom is GM only and £120 ish from VX.

A second hand loom in mine is better than the old one but still not perfect. Only fitted because 3 sets of coil packs, including new all gave the same symptoms.

[jonny2112]

Cheers al. You seem confident that its electrical, and I'm making arrangements with tunnie to try some coil packs, so fingers crossed 

[05omegav6]

Pretty much everything else has been ruled out, so hopefully nearly there

Suspect my problem stemmed from the plugs being loose, drawing a bit more current through the coil looms, not helped by one of the plugs breaking up. Coil packs were ruled out as well, leaving the loom, even tried a different ecu to rule that out.

[jonny2112]

And was it sudden? Or did you have symptoms beforehand?

[05omegav6]

First sign of a problem was the core electrode (and half of its insulation) of spark plug number 6 being swallowed by that cylinder All the other plugs were loose, but that one failed first.

Had a persistent, but variable misfire from then on. Was fine from start if you drove off straight away, but as soon as the revs dropped below about 2k, it would start missing. Then, if it was left to idle it used to shut down at least 2 cylinders, which confused the knock sensors causing the ECU to try to adjust the fuelling to compensate for ignition knock that didn't exist. Disconnecting them helped, but obviously put the spanner light on.

The only two measurable things that stood out were: 1. Fuel trims were all over the place, and 2. The spark at cylinder 6 was only 40kv, rather than 60kv that the other 5 were managing.

Having ruled out most everything else, I tried changing a section of the coil pack looms to see if that helped as both plugs were corroded, but no real change. Hence ending up with a loom change. Not perfect now but notably better that it was .

Not sure why, but this car has never idled particularly smoothly, but it least it does now idle

Was a mare of a process tbh, changed the plugs, both coil packs-twice, inlet manifold+injectors, throttle body, MAF sensor, ECU, radio suppressor and eventually the loom. The only consistent codes were 0300, 0306 and, if it got really arsy, 0304 and 0305

[jonny2112]

Sounds like a nightmare 
I had no symptoms or problems at all. If it hadn't been for the leaking gaskets, oh and the 'dodgy'? stat, I wouldn't be in this mess 
Getting my hopes up again that the coil pack will make a difference 

[05omegav6]

Hope so

The weather's turning a bit too Autumnal to be messing around with stone cold engines

[jonny2112]

Indeed it is! And I still never got round to fixing the drivers heat pad