Xantia V6 broken exhaust (update: and rough low rpm running)

[myglaren]

Pleased to hear it is sorted at long last Simon.
Haven't a clue as to what may have been at fault. The widget that controls the doodaah probably.

[Xantidote]

FIXED??? That's great news

And now, after 27 pages of suggestions, thoughts and posts, you're teasing us to guess the final problem - I'll watch this space

[mooseshaver]

Flux Capacitor?

[Mandrake]

I'll not give the game away or say anything to hint but I will say that it is certainly driving like a fixed car!

I took Simon for a run in mine and then let him have a drive of it. I then took his for a drive and the power/performance of both cars was identical. The gear changes on Simon's were smooth as silk apart from when it had a hiccup going from 2nd down to 1st. It rode beautifully as well.

Thanks David, I enjoyed the trip there and back immensely too because it was running so well for the first time in many months. The V6 is a beautifully flexible engine with a wide power band (when running properly!) and although the standing start performance is very good its the foot down 50-80 (or more ) acceleration that really puts a smile on your face. It just goes and goes and hills don't slow it down...

That lurch from 2nd to 1st its done a couple of times recently once quite violently, its always just as you slow down to nearly stop then apply sudden heavy throttle it feels like its in neutral and then suddenly engaging 1st gear with a jolt. I suspect that I'm catching it with the throttle exactly as it is shifting between 2nd and 1st when neither gear is fully engaged.

I did an auto-adaptive reset before I left and I could tell it spent the first half of the journey home learning and adapting with slightly rough changes, the gear changes were smooth again by the time I got half way home. Going between 3rd and 4th at 60mph you could barely feel the gear change, you'd just see the revs change, which I assume is how it should be.

The exhaust is definitely a bit boomy at low rpm open throttle compared to yours, I could really notice it when I got to the 30mph roads at the far end of the journey whereas there was no boom from yours at all. Looks like it could do with a new back box and centre silencer next summer.

The only niggle with the engine now (apart from the coolant expansion tank needing replacing) is the tendency for the idle to stagger sometimes when first starting - you saw it doing that when you started it to take it for a drive, it only does it when its hot but has sat for a little while, the revs drop and stagger, recover, drop and stagger a few times then settle. I see other people have come across this before too, I wonder if its a sticky or faulty idle control valve ?

Interesting to note that the steering on my V6 was heavier then it was on Simon's V6. I've never thought mine was heavy but it was fairly noticeable that Simon's was a little lighter.

Yes there was quite a difference, as I said in another thread it has lighter steering than all the other Xantia's I've driven. I wonder if yours has had a split boot on the steering rack and let some dirt and muck into the rack teeth ? It felt a bit like that.

Looking forward to meeting again in the summer on a good day so I can bring the XM out of the garage without it getting wet

Yes definitely, will catch up again in the good weather, it would be nice to see the XM going and also have a bit more of a look at the Activa, which is another Xantia I've never seen in person before. (There were only 10 in NZ!)

[RichardW]

Good work Simon!

Fuel side? You've been over almost everything else! Something stupid like blocked fuel filter, or the vac pipe dropped off the fuel pressure reg probably....

[DHallworth]

Yes definitely, will catch up again in the good weather, it would be nice to see the XM going and also have a bit more of a look at the Activa, which is another Xantia I've never seen in person before. (There were only 10 in NZ!)

If time permits by the summer the Activa might be wearing some of the V6 clothing...

David.

[lexi]

Well if the problem has ran for 27pages we are hardly going to guess what it is now if we couldn't' tell before

My guess ? something you overlooked

[Mandrake]

FIXED??? That's great news

And now, after 27 pages of suggestions, thoughts and posts, you're teasing us to guess the final problem - I'll watch this space

Ok ok I better put you all out of your misery. Nobody would have guessed it anyway as it was something I thought I'd already eliminated.

Yesterday my fuel pressure gauge arrived so of course I was eager to test the fuel pressure, trying to get to the valve was interesting, I took the top plastic cover off the engine to try to get access to the schrader valve, no dice, removed the air filter box, now I can see it but the adaptor can't screw in because there isn't enough clearance from the bracket that holds the cruise control diaphragm, so I had to loosen that and push that out of the way too.

Connected the valve and switched the key on and the pressure only went up to 1 bar. Bingo! Or so I thought... After I started the engine it went up to about 2.2 bars which fell to 2 bars at about 3000rpm which seemed a bit on the low side. (quoted is 2.5 bars at idle, 3 bars at wide open throttle) Disconnected the vacuum line and its putting out a solid 2.8 bars with absolutely no fluctuation, I could fully blip the throttle with no drop in pressure at all.

The pressure with the vacuum hose off should be 3 bars but I think 2.8 is within tolerance, and the fact that it held up steady under full throttle and also held its pressure for several minutes with the engine turned off with no visible drop suggests that the fuel pressure is absolutely fine. The vacuum hose worked fine too, dropping the fuel pressure by an amount equal to the manifold vacuum. So as far as I could see the pump, filter and pressure regulator are all ok, at least when I tested them.

I didn't have the Lexia down at the car to perform an injector pressure drop test and it was getting a bit dark and cold so I thought I'd leave that to try at David's the following day, although my hunch was the injectors were ok.

As I was packing up I glanced at the exposed ignition wiring and a nagging thought came back to me. I remembered that when I fitted the new ignition leads (back on November 17th according to this thread) I'd had one hell of a time getting the wires into the guides, especially the ones near the coil pack - the grooves have little sharp "teeth" for lack of a better word to hold the wires in place and it was very difficult to get them in no matter how much I rolled the wire around or squeezed and prodded it, the teeth kept digging into the wire, and I remembered that I'd nicked one slightly and had been meaning to try taping it up but never got around to it.

It seemed to be back to symptoms of misfiring in the last few weeks, (which I had thought was fuelling) surely it couldn't be that simple ? So I lifted the wires out and inspected them, one was damaged, it wasn't just a nick it was a tear, like a loose flap of skin and when I pulled it back slightly I could clearly see the wire inside.

So I covered a 2" length around the damage carefully with some 3M self amalgamating rubber tape and put it back in. As soon as I started the engine I noticed a difference in the beat of the engine, and also that there was no vibration when revving it up to 2000-3000rpm. Revving the engine was a lot more responsive that previously with no "bogging" when blipping the throttle so I thought it was time for a test drive.

Before I had even got onto the main road I had my answer - trying to turn out of the car park onto the road I put my foot down my normal amount to get across in front of an approaching car and it spun the front wheels. It hasn't done that in a long time... needless to say I went for a longish test drive and the difference in performance is simply phenomenal, all thanks to a little piece of insulation. I did maybe 20 miles yesterday and a further 60 miles to David's and back today, all flawless with no regression back to its old ways, whats more the weather has been very wet and damp (which previously made the performance even worse than dry weather) with no loss in performance.

So it looks like the spark plug leads were the major problem all along. Now, the original leads were definitely faulty, it was misfiring badly before I replaced them - two of them were open circuit with breaks near the spark plug end, so I was absolutely right to replace them, however in the process I damaged the new ones introducing a new fault. Because I'd just replaced them I kept ruling them out in my mind and looking for other problems.

At idle the required spark voltage would have been less than the voltage to jump the gap from the hole in the damaged lead to the chassis, so the cylinder would fire. As you applied more throttle and the combustion pressures in the cylinder increased it would require more voltage to fire the plug, a point would be reached where it was easier for the spark to jump from the hole in the lead to the chassis, thus limiting the maximum available voltage and causing the plug to no longer fire, thus under load one cylinder was not firing at all.

As soon as that cylinder stopped firing not only did you loose 1/6th of the power, the unburnt oxygen would send the ECU into a tizzy because the oxygen sensor was reporting a very lean mixture, which would then cause the ECU to retard the timing by 20-30 degrees to protect the engine, whilst simultaneously trying to richen the other 5 cylinders...no wonder there was no power and terrible fuel economy.

As for a long term fix, although the 3m tape is rated to 69,000 volts breakdown and forms an air tight seal, I'm not sure that it can stand the temperature on top of the engine indefinitely, so when warm weather returns in a few months I may need to simply replace the spark plug leads again and be much more careful fitting them this time. I think I'll file the sharp edges off the little teeth in the guides and maybe put a bit of vaseline (?) on the wires as I fit them in place so they slip in a bit better without damage.

So it looks like on the engine the only major problems were faulty spark plug leads and missing intake manifold gasket, possibly the plugs weren't as good as they could have been either. Things that weren't faulty - MAP sensor, coolant temperature sensor, coil pack, no oil on the plugs (although a couple of the wells were building up oil) and nothing wrong with the exhaust, at least nothing performance affecting.

One question remains as to whether the cat has been permanently damaged by a few months of running with a misfire, it will have been running hot thats for sure. The next MOT is August but I think its worth me getting an emissions test done a few months before then to see whether the cat is still working so I have time to plan for a replacement if it is damaged. Just because its not blocking the flow or affecting performance doesn't mean its reducing emissions. My gut feeling is its probably ok because I haven't done a high mileage since November, maybe 500 miles or so, and haven't pushed it hard because it hasn't been running well.

As for the gearbox, 4 oil changes and it seems to be working pretty well now, with no groaning noises under load even with the engines new found power. Changes are usually very smooth, it does occasionally snatch down into 1st, and I suspect the lockup clutch linings are getting down to the minimum (it sometimes makes a little bit of noise at low speeds just as it locks up, and is a bit slower adjusting to sudden torque increases than I think it should be) but I think its got a good many thousand miles left yet thanks to the fresh oil and probably isn't in imminent risk of failure as I originally thought. I'll do another oil change on it in a few months to see what the oil is like then.

The end.

[KennyW]

Well done Simon

It reminds me what I say to myself when fault finding, it is usually the most simple thing.
My example was the problems I had trying to start my Old TD after changing the ignition barrel to find the original problem was a wire had been cut by someone trying to steal the car whilst parked at a garage for re coding the new ignition.

I think it is lesson everyone can learn.
Kenny

[Mandrake]

Well, a small addendum to add to the "conclusion" in this thread, it looks like there is still another intermittent problem with the engine...now that I'm not driving to work any more I don't get to drive the car very often so it's taken a while to notice, but the last couple of times I've driven it all is not well in the performance department.

As far as I can tell its not misfiring like it was before, the engine is still very smooth (it was noticeably rough when it was misfiring, enough that the engine was vibrating at 2000rpm etc) however intermittently the power is down a little bit (nothing like before though) and there is quite a big throttle lag.

For example I could be in sport mode in 2nd gear at 3000rpm and put my foot down suddenly and instead of the expected instant torque that pushes you back hard in the seat I get a good half a second delay where the power gradually ramps up, which I can tell both from the delay in acceleration and the gradual change in the engine power note. The throttle response feels "soggy" for lack of a better word.

This intermittent throttle lag is a problem that has come and gone many times in the past as well, now that the misfire is fixed it looks like it has a separate cause. There was absolutely no sign of this throttle lag when I drove to David's and back, but its there again now. In a half hour journey the problem might be intermittently there for half the journey - sometimes the throttle response is instant, sometimes the lag is there...it can come and go in a period of a few minutes.

I have a new fuel filter that I haven't fitted yet so I'm inclined to hurry up and fit that just in case it is a bit clogged up. I measured the fuel rail pressure the day before I drove to David's and it was fine, however that is also the same day that I fixed the ignition wire and the car was performing brilliantly that day. Just because it measured ok then doesn't mean the fuel pressure is still ok at the times when the throttle laggy, it hasn't been measured yet under conditions where the lag is there. (A bit awkward really as you have to take out the air filter box and move the cruise control diaphragm to get access to it, and you can't judge the performance of the car with the air box off)

Anyone have any ideas what might cause an intermittently laggy/soggy throttle ?

[Mandrake]

Needless to say I've missed a lot of the not so important bits but fully intend to sit down and read the full thread again so that it actually goes in this time, mainly because I have a v6 and any info is good info in my book. It'll take some reading though, its a mighty volume, maybe tonite .

Well you have your work cut out for you considering all the crazy theories and blind alleys that were travelled.

If I had one piece of advise though it would be don't leave your gearbox oil to chance, just because there are no symptoms with the gearbox doesn't mean the oil isn't filthy and doing damage to the box... I see yours has done 95k, it's getting to "that age". I would definitely be doing some oil changes and checking the state of the oil that comes out. If you're lucky and yours has the dipstick fitted its not a difficult job.

My other piece of advice would probably be if you have any signs of misfiring etc, check the spark plug leads before the coil pack.

[Xantidote]

considering all the crazy theories and blind alleys that were travelled

We were doing our best, honest

[Mandrake]

considering all the crazy theories and blind alleys that were travelled

We were doing our best, honest

I'm talking about my crazy theories and blind alleys.

[CitroJim]

considering all the crazy theories and blind alleys that were travelled

We were doing our best, honest

I'm talking about my crazy theories and blind alleys.

No, not at all. I'd describe it as thinking aloud... Nothing wrong with that

[Mandrake]

No, not at all. I'd describe it as thinking aloud... Nothing wrong with that

Yes, that's it, thinking aloud, thanks Jim.

Well I just braved the cold to fit the new fuel filter - I've only been for a short trip and I can't say I've noticed any real difference. The old one looked in pretty bad shape, it was very old based on the corrosion and had a strange wax like substance pooling at the bottom of it...it needed to be replaced so I'm glad its done even if it didn't affect the problem at hand.

There is still intermittent throttle lag, although power seems to be good especially at higher revs, and there is no sign of misfiring - the engine is perfectly steady as you raise the revs up through 2000 rpm or more without any of the tell tale vibration from a misfire that was there before.

My current "thinking aloud" is it may be related to my sluggish oxygen sensor - it only manages about one to two transitions per second at idle which is apparently far slower than it should be, and it lingers a long time at each end of the range. (If you were to graph it it would look more like a square wave than a sine wave)

I now understand from the false lean reading I had during the misfire that when you accelerate heavily the ECU goes into open loop mode and deliberately errs on the side of fuelling rich, and expects the oxygen sensor reading to very quickly read rich and stay there. If the reading goes lean or lingers on lean for any length of time it backs off the timing to prevent detonation.

My thought is if the oxygen sensor is taking a good half second to a second to swing to a rich reading when flooring the accelerator the ECU is playing it safe and backing off the timing until it gets a steady rich reading from the oxygen sensor at which point it advances the timing again - thus the half second or so power delay with sudden throttle opening.

One way to test it would be to temporarily disconnect the oxygen sensor again (I haven't tried it since the misfire was fixed last week) which removes the ECU's ability to get feedback on whether the mixture is rich or lean...thus in theory it shouldn't back off the timing and shouldn't cause throttle lag.

If the throttle lag is gone with the oxygen sensor disconnected it could be time to change the oxygen sensor...I've known for a while now that it is sluggish and also suspected it is reading slightly false rich (causing the ECU to lean the mixture) as my MOT lambda reading was 1.02.