Bad running after new head gasket - cause?

[rogerthechorister]

Does '96 have a coil as well as an ignition amp on the inner front wing? If so I'm pretty sure it is one of the many things that have been changed. Would it not show up with a fault code on the pulse reader or indeed the expensive Volvo diagnostic thingy?

No, the variable flat spot and holding back is still there.

[Steve940estate]

My car has a coil and it has the engine out of a 95 car so I would have thought yours is the same.

It was the loosing spark comment that made me wonder.

I'm not sure exactly what the unit on the inner wing is and what it does other that it's a part of the ignition system. When my car wouldn't start I swapped various components over until I found the problem, this was one of them but I eventually found it was a faulty injector relay.

The engine management ECUs should all be found in the drivers footwell area. The fuel injection one should be on the a panel by your right foot and the ignition ECU or EZK as it's also known up above it. I mistook the ABS unit for the ignition one and ended up driving my car round with the non turbo ignition ECU in place. This resulted in far too much ignition advance so it went like stink but couldn't cope unless I ran it on super unleaded.

Going back to your problem a weak spark would affect performance by creating a flat spot and holding back. Too much pressure in the cylinder effectively blows the spark out.
A few years ago now I had a Fiat Panda. It gradually died one day on a dual carrageway and the cause of that was the coil overheating. As soon as it cooled it was fine again. It felt like I had pulled the choke out. I have always suspected coils as being faulty since.

[Laird Scooby]

Can't answer your question on whether 96 models have a coil etc but chances are a faulty coil won't show up on the OBD system for the simple reason coils very often have other problems that mean their output is reduced but without exhibiting a fault that could be measured.
I had a similar fault on a n/asp Cavalier Mk1 many moons ago (25 years ish) where all of a sudden, it would give a monumental flat-spot then pick up again and be fine for a while or the flat-spot would last from about 1500 to about 4000 rpm then at just over 4000rpm it was like a turbo and nitrous kicking in all at once. The engine had a high lift cam amongst other go-faster mods so when it came on the cam it went and it came on the cam about 4000rpm, just the end of the flat-spot.
Took me ages to realise it was the coil - i replaced the standard AC Delco coil with a Lucas Sport Gold High Output sports coil (12V, no ballast resistor) and once more it drove like i wanted it to.

[rogerthechorister]

Coil has been changed. Knock sensor has been changed. ECU now away for checking.

No other brains near ECU nor above driver's left foot - only the thingy on the left hand inner wing panel. Which has been changed.

Flat spot still present. Expert seems now to be convinced it is fuelling not weakening spark.

[Laird Scooby]

At least now you seem to be eliminating what it's not, hopefully soon all you'll have left is what it is!

Positive steps at last! Very pleased for you!

[rockheadrumble]

As i mentioned a while ago your car has the same symptoms mine had when it was running lean. power would drop off mid rev range when flooring it.

I had a adj fuel pressure reg and that was the problem, it was nakered and even at 8bar it was still lean!

If youve not done so id try a new fuel pressure reg and different injectors, they may be partially blocked. Im sure you will off checked this and iv not read every page of the thread but have you checked the vacuum tube from the fuel pressure reg to the inlet?

Another thought is that the injectors may not be fully home in the head, it is possible to fit the fuel rail with them not seated correctly. put some grease around the O ring and push them in.

Joe

[Laird Scooby]

I just bought a few sundry items in my local auto-factors and unpacking the bag when i got home, i found they had put a leaflet on Revive Turbo Cleaner.
Aimed primarily at the variable-vane geometry turbo market, at first glance it might seem as if it won't help much on yours.
It has just occurred to me that perhaps your wastegate is sticking internally, causing either under or over boost pressure. Given the engine speed, throttle position etc, this may well trigger the knock sensor on yours resulting in a retarded, weaker spark, especially if yours has the turbo over-limit sensor (that might be diesels only, not sure).
If that goo does what it's supposed to do and the fault is something internal in your turbo, there's a good chance it might just sort it. I appreciate it's a long shot but it seems to me everything else so far hasn't worked so maybe a long shot is the answer?

Have a read of the blurb on it via the link, you may decide it's not worth trying but given the fact you seem to have tried everything else it might be.

[rogerthechorister]

Good idea but she had a recon turbo and a 90+ manifold as good as I could get about a year ago. Anyway, this is what the latest expert has to say so far: -

"2 ECU (that's fuel and ignition) £ 132 + postage £16
New crank shaft sensor - £42
Ignition amplifier - £96
NGK platinum spark plugs £ 8.56 * 4
New coil lead as corroded - £5.40
Knock sensor - £32
New earth strap - £7.20


Total : £ 364.84

Now we have to move to fuel pressure check , she run better but not perfect, still waiting for the diagnostic cards to be returned to me."

That's on top of the head gasket and clutch at the previous garage and other stuff listed above. Getting a bit fed up to be honest.

[Steve940estate]

I realise it's how it works but why are you expected to pay out that much money when they still haven't fixed it. You could have just swapped all those parts yourself, you don't need a so called expert to do that.

I'm not surprised you are getting fed up with it now. What exactly did the garage that fixed the head gasket say when you told them something was wrong ?

I used to work in a garage so I have witnessed both sides of the counter but to be honest most are either pretty f#####g useless or just don't give a t##s.

Sorry for the outburst but have spent a few days trying to find someone a decent used car only to have been messed about by local traders selling rubbish.

[rogerthechorister]

Well, so far nobody has (with the benefit of hindsight) a foxing clue what it is - only theories - and the only way to test the theories is to change the bits. I used to be a keen amateur mecco and the theories all make sense (I was convinced it was the TPS or the EZK - but no) and they have always discussed it with me and I have always agreed to go for the new bits, and they can't send them back once they've fitted them. At least I'll have a stock of probably good spares once (if ever) the job is finished. That's why I am getting glum - we seem to be getting to the point when there will be nothing left to change, and still a large flat spot.

What does anyone think to the possibility it might be the turboplus relay? I don't think it can be as mostly the flat spot is below where the turboplus arrives, and also the usual fault on them is that they full stop do not work rather than work but a bit feebly some of the time. I suppose I could definitely rule the turboplus system out by removing the relay and popping an MBC in.

The other oddity is that the flat spot is not consistently in the same place in the rev range.