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[dbdb]

Apologies for prolonging the off topic rambling but
misfire detection can be sensed either by
1. using the knock detectors to sense a normal firing pulse (as opposed to the larger signal given by a pre-ignition or knock) (as already stated this is the method used by EOBD compliant Omegas)
2. by sensing the angular acceleration of the crank by use of the crank sensor. The ECU knows which cylinder it wanted to fire because it has access to the cam position sensor. I am pretty certain that VW have used this method of misfire detection but as already stated THIS IS NOT THE METHOD USED IN THE OMEGA.

Well that's amazingly clever of Vauxhall since knock sensors were only first proposed for misfire detection in academic papers in March 2002, the only two methods at that time being crank speed and ionisation current through the plug.

[dbug]

Just give up gracefully, you're now just being arumentative for arguments sake, and none of this is helping the OP.  He has a 98 2.5V6 and mifire detection is not available on his car.  This originally started because you gave the OP incorrect advice re his car

Your posts are becoming boring now

[Kevin Wood]

Yes, indeed. Let's get back to trying to help the OP.

A couple of more things occurred to me:

Coolant temperature sensor: If this has failed the ECU might think it's got a cold engine, hence it deteriorates as it warms up, runs rich, and may have been what destroyed the cats.

Lambda sensors: These start to control mixture as the engine comes up to temperature. Any leak in the exhaust upstream of them (hence my earlier point about the SAI pipes) or tired sensors, will have an impact here (although I'd expect a code to be  stored). As the car has no cats, there's no harm in disconnecting the lambda sensors, accepting that there'll be codes related to that, but in doing so, forcing the system into open loop. If it drives better, that's an indication that the exhaust / lambda sensors are the culprit.

If that doesn't help, reconnect and try it with the MAF sensor disconnected instead.

Might be worth trying it with the EGR valve blanked off with a sheet of metal too.

Plus check the basics - plugs, leads, DIS pack. Cut it immediately after a run and see if the plug condition varies between cylinders perhaps?

[dbug]

Checked back exhaust box clear and not blocked/partially blocked?  Some members have had this in the past, with significant power loss

[Webby the Bear]

All things that I listed to check... although I think it may have been overlooked following the topic getting heated 

[Kevin Wood]

Yes, wouldn't be surprised to find he's gone to find a more helpful forum now.

[Michael2.6]

The only person on this forum that can answer this post correctly and end it is TB

[TheBoy]

The only person on this forum that can answer this post correctly and end it is TB

There are many people on this site who are far cleverer than I will ever be. 2 of them have already posted on this thread.


Poor running, with no codes, and fuel trims and MAF reading in normal range, I'd always suggest checking HT in first instance.

Obviously not everyone can read live data on pre EOBD Omegas, so trying with MAF disconnected and then Lamdas disconnected (and MAF plugged back in) is a way of getting a feel for what might be happening within the ECU. Caution has to be paid to cat health normally when doing this though, esp playing with lamdas.

[Webby the Bear]

The only person on this forum that can answer this post correctly and end it is TB

There are many people on this site who are far cleverer than I will ever be. 2 of them have already posted on this thread.


Poor running, with no codes, and fuel trims and MAF reading in normal range, I'd always suggest checking HT in first instance.

Obviously not everyone can read live data on pre EOBD Omegas, so trying with MAF disconnected and then Lamdas disconnected (and MAF plugged back in) is a way of getting a feel for what might be happening within the ECU. Caution has to be paid to cat health normally when doing this though, esp playing with lamdas.

I never knew you thought of me so highly 

[dbug]

The only person on this forum that can answer this post correctly and end it is TB

There are many people on this site who are far cleverer than I will ever be. 2 of them have already posted on this thread.


Poor running, with no codes, and fuel trims and MAF reading in normal range, I'd always suggest checking HT in first instance.

Obviously not everyone can read live data on pre EOBD Omegas, so trying with MAF disconnected and then Lamdas disconnected (and MAF plugged back in) is a way of getting a feel for what might be happening within the ECU. Caution has to be paid to cat health normally when doing this though, esp playing with lamdas.

I never knew you thought of me so highly 

Sorry mate think he means dbdb   Don't be upset though Webs

[Webby the Bear]

The only person on this forum that can answer this post correctly and end it is TB

There are many people on this site who are far cleverer than I will ever be. 2 of them have already posted on this thread.


Poor running, with no codes, and fuel trims and MAF reading in normal range, I'd always suggest checking HT in first instance.

Obviously not everyone can read live data on pre EOBD Omegas, so trying with MAF disconnected and then Lamdas disconnected (and MAF plugged back in) is a way of getting a feel for what might be happening within the ECU. Caution has to be paid to cat health normally when doing this though, esp playing with lamdas.

I never knew you thought of me so highly 

Sorry mate think he means dbdb   Don't be upset though Webs

 

can I have this on one of OOFs welcome messages....

''Webby the Bear is officially more intelligent than The Fat Shadow'' 

[dbdb]

In amongst the personal attacks and frankly 'technical crap' there have been sound and helpful posts on this. I wouldn't normally post again and frankly won't answer the posts 'playing the man not the ball'  but there is one that only Kevin and Webby have mentioned (plus FS with his HT) and is relatively easy and cheap to check/fix.

Burnt out cats as someone else said are likely the result of unburnt fuel.  Unburnt fuel would be the result of the much discussed misfires.  So check all the spark plugs as a high priority.  It's a bit of a pain on the pre facelift v6 mind.

[minerwilly]

Right everybody, sorry I've been mysteriously absent, but I've been away on business.

I've managed to find the problem and luckily it's perfectly logical and inexpensive.

I ended up driving the thing to work a couple of times earlier in the week through absolute necessity, and although it was as rough as it was before, it got a little bit better the more I drove it. I've now been driving it all week and it's pretty much fine.

Through my astonishing powers of deduction, I'm pretty sure that when the cats collapsed they choked the cylinders, valves, and plugs with exhaust gases which condensed and covered them in carbon. I think that was the problem that caused the loss of power (which I guess would have translated to random misfiring), and simply giving the car a good run has burned off any deposits that were causing the problem in the first place.

Obviously there is possibly still the underlying problem that caused them to disintegrate in the first place, but that should be a fairly easy thing to track down, and whilst it will do nothing for the fuel economy, at least the car is drivable. I'm really glad that I seem to have got to the bottom of the problem, and even more happy that it was nothing expensive. This weekend will see a change of plugs and full oil change for the engine and the gearbox.

Thanks for all of the good advice and even the bickering, because I've learned a lot about the car through it all. Hopefully given a little time I'll be able to contribute too.

Cheers.

Marc