Bad running after new head gasket - cause?

[Laird Scooby]

The theory i was working on when i suggested that cleaning goo is that if the HG went, there is a fair chance it spewed a strange concoction of water, oil and unburnt fuel into the exhaust before it was fixed. As such it may well have clogged something on the turbo.

As for what Steve said, i totally agree with what he's saying but if the fault hasn't been fixed it's a bit daft trying to charge you!

I still think it's something silly that the original (HG repair) garage did (or didn't do) that's causing the problem, unless of course it was there before the HG went. In which case was it mis-diagnosed and the HG repaired when it didn't need doing but i guess we're getting into uncharted territory there!

Out of interest, does it run cleanly through the rev range if you accelerate gently, gradually increasing the pressure on the accelerator to just prevent it changing up (if auto) or to marginally increase your speed if maual?

[Steve940estate]

All anyone on here can do is guess based on their own experiences I think the same can be said for a lot of garage mechanics.
I wonder what a senior Volvo technician would suggest and by that I mean one thats actually worked on 940's a lot, if there are any left that is.

After reading the last post I started thinking Lambda sensor but I'm sure they only affect the idle. It doesn't do them a lot of good getting covered in coolant if thats what happened.

You could just bypass the Turbo+ by just connecting the turbo pressure pipe directly to the wastegate acctuator.

I will have a look at my otp discs to see if there are any troubleshooting sections trouble is will most likely say to connect the car to a computer as step 1.

[Laird Scooby]

That's very true Steve about the Lambda sensor and coolant (or any other) contamination. As far as i know, the Lambda sensor is only "switched out" during warm up and wide open throttle (WOT) operation - the rest of the time it controls the mixture. Certainly how it works on most other cars i've come across anyway.

The longer this saga goes on, the more i think it's something ridiculously simple that has been overlooked and there will be a very red-faced mechanic when the problem is eventually found!

[Forrest]

Going back to about the middle of August did you ever confirm all the crankcase breather tubing was sound? You mentioned that the double-layer tube over the top of the engine was a bit iffy. Which layer were you referring to? I have made the mistake in the past of assuming the outer layer, supplemented with friction tape, will hold up when the plastic inner part was snapped following major engine surgery. This is not the case. The outer layer is a porous and just there for insulation/protection.

I would definitely recommend thoroughly checking the integrity of this tubing and cleaning out the little white y-piece with carb cleaner if you haven't done so recently.

[rogerthechorister]

She's had a lambda sensor.

IIRC the crankcase breather (whole system) has been checked.

She's had 3 meccos - lad at local garage, his boss, and current expert.

The bad running first appeared after I left her ticking over to charge a caravan battery, in a field at a folk festival - which I think was when I blew the H/G. According to first garage when I took her there on return from festival she was boiling up like a kettle - and the H/G was a right mess when removed.

My first grumble to them was that after they did H/G and clutch (nearly a grand) she was still running exactly as badly as during the trip back from the festival.

So it's probably not something the first garage did wrong, rather something caused at the folk festival. The other thing I did there was nearly get stuck in the mud and have a rather violent tow (much crashing and banging from underneath) across a muddy and bumpy campsite - eventually having to borrow a friend's 4*4 to finish positioning my caravan. So the original fault was either something from cooking her, or banging her about. No it can't be cat or downpipe getting obstructed, she's had both. Rest of exhaust is OK to look at and not apparently obstructed.

That does make it at least plausible that the problem is goo in the turbo. I will mention that to current expert in the morning.

[Laird Scooby]

At the risk of asking a dumb, fool question, when were the downpipe and cat renewed - before or after the festival and/or head gasket? If it was before, there is a chance, albeit small that the cat is blocked hence my earlier question about will it pull cleanly through the rev range if you're gentle on the loud pedal.

Also, given that extra knowledge about the circumstances etc of when the H/G went it is entirely possible the turbo is gunked up with some kind of goo - i know it's not the cheapest fix but that Revive turbo cleaner could well be worth a shot, preferably before changing the expensive stuff like ECUs etc. If nothing else it should eliminate the turbo as being at fault, shortening what seems to be a growing list of possible culprits.

[rogerthechorister]

Downpipe lambda sensor and cat - after the H/G.

No, flat spot is there even if I'm gentle (to the best of my recollection, I'm hardly ever gentle). Certainly on the way back from the festival with the caravan on the back I really had to wring her neck to overtake on the motorway.

[Forrest]

I am not that familiar with engine terminology, so thought I had better check what you actually mean by "flat spot" and found this article which I'm sharing for two reasons.

http://www.privatefleet.com.au/glossary/a-flat-spot/

Firstly, it lists a lot of possible causes some of which I don't think have been mentioned.

Secondly, I'm still chuckling at the advice "perhaps the best remedy for that irritating flat spot might be for the car to be taken in to a mechanic".

[rogerthechorister]

Hilarious!

[Laird Scooby]

This bit made me laugh :

" or the ECU random access memory being faulty are some of the possible causes for flat spots in a car’s acceleration. Hey, there are many other reasons as well, and perhaps the best remedy for that irritating flat spot might be for the car to be taken in to a mechanic."

It's hard enough for electronics engineers to diagnose faulty RAM chips at times - how on earth is a mechanic going to be able to diagnose faulty RAM chips???

Granted they may well be able to diagnose a faulty ECU but then it goes off to one of these firms that allegedly specialise in ECU repair (some are good, some aren't so good but i don't know which would be which) in the hope that after spending the equivalent to several tanks-worth of fuel they will be blessed with a born-again ECU and the faults will miraculously disappear.

There is a solution i have just thought of and that would be to get one of the aftermarket engine management systems that are available and install that - it can be interrogated and re-programmed with a laptop. Bit of a drastic solution perhaps but it should sort the fault. Failing that, do away with all the electronics, slap on a brace of Strombergs and a "normal" distributor and hope for the best!

Slightly tongue in cheek suggestions there but has there been any positive movement towards diagnosing what the actual fault is?