Simon's new Xantia V6 and Leaf blog

[lexi]

Have you moved yet with a place to work on car? I am rather behind things with a travelling and fishing season just finished.

[Mandrake]

Nope, the house move has been delayed a few weeks while the sellers get an engineering report done on a loft conversion whose documentation has gone missing

All going well we move mid/late November, and yes there is a reasonable size garage that should be big enough to do car work in as well as a driveway/back yard.

[lexi]

Brilliant. Wish you and Lady atb with moving in.

[bxzx16v]

Good luck with the move Simon.

Mark

[Mandrake]

Thanks guys.

In other news the Xantia is still going like a champ since the last ignition tinkering on Sunday - today has been wet and miserable, (bad conditions for secondary ignition problems) the car has had two short trips and two longer trips, the engine performed flawlessly for all four trips - extremely eager and responsive to the throttle, tons of power right across the rev range. Can't fault it at all. Earlier in the week it has been consistently good too.

It helps reassure me that the problem all along has indeed been a secondary ignition problem. I'm not 100% sure exactly where the problem is in the ignition system, because a number of things were done when I swapped the coil back over on Sunday:

1) Two rather wide cracks on the top of the coil pack were epoxied.
2) One slightly swollen spark plug boot (a loose fit on the spark plug insulator) on the bottom of the coil pack on the oil filler side (cylinder 1) was swapped with a better condition spare.
3) When I had the spark plug boots off the bottom of the coil pack I also put a small wipe of silicone grease around the outside of the nub that each boot presses onto to help seal the junction between the two.
4) When I refitted the spark plug leads to the top of the coil pack I gave the outside of the posts a wipe of silicone grease to help the rubber boots seal on the posts. (They're a loose sloppy fit)
5) I straightened out the kink in the right hand spark plug lead that passes by the earth terminal and re-oriented the protective trunking, and also made sure to push the wire aside when refitting the top cover so it doesn't get crushed and stretched again.

So one or more of the above seems to have fixed it although I still think the spark plug leads need replacing, especially at 17 years old and probably being factory originals. There is a good chance that the wire is broken where it was crushed under the top cover and that straightening the wire and re-positioning it has helped it to make contact again but that may not last. Also I suspect a good part of the problems could be the very poor fit of the plug lead boots onto the coil pack posts.

These old leads just seem worn with the rubbers stretched and saggy looking causing quite a gap between the boot rubber and the post in places. That joint really needs to be snug and air tight to ensure there isn't any high voltage leakage. A wipe of silicone grease will help bridge the gap and provides an air tight insulating seal but it can only do so much if the gap is too large.

The funny thing is the replacement leads that I bought for the other car also aren't a great fit on the posts - and lack the white spacers the originals seem to have which allows the joint to wobble around a bit unsatisfactorily. The routing of the cable where it easily gets crushed under the top cover and the poor fit of the rubber boots seem to be design weaknesses IMHO. Both cars have suffered from both of these problems.

Fingers crossed that my temporary repair lasts for a while as the car is an absolute joy to drive when the engine is so eager, however I suspect I won't get permanent relief until the leads are replaced. It will be one of the first jobs I do when I start using my new garage!

[CitroJim]

Simon, excellent news

[Mandrake]

Yes, I'm well chuffed.

Again this morning on the way to the train station it was absolutely perfect right from a cold start even though conditions outside were wet and damp. You can tell the engine is happy when you're pulling away and changing up from 1st to 2nd on a light throttle and it surges forward eagerly as 2nd engages... in fact in general, pickup from slow speeds in 2nd is a good indicator of how well the engine is performing.

Both V6's of mine have had problems where the performance is intermittently bad which seems to be caused in large part by the timing becoming very retarded in response to knock sensor activity - anyone who followed my old blog will know I battled that problem relentlessly on the old car and while I improved matters somewhat I never really solved it - all I knew for sure was that the timing was getting retarded and that resetting the ECU temporarily restored most of the lost performance because it restored the timing to normality for a while, but eventually it would get retarded again and tend to stay that way without intervention.

Anyone reading the beginning of this blog will know that I was a bit gutted when I first drove this car back home because the performance on that first trip was really flat and had that same retarded feel to it - low mileage and otherwise general excellent condition notwithstanding I was actually worried that I might have bought a lemon! It too had a sudden major increase in performance the first time I did a battery off reset which lasted a number of weeks but then it deteriorated again. Wiggling the spark plug leads around seemed to help for a few weeks then it went flat again until I fiddled with it, etc... Never as severe as the first car but the symptoms were eerily familiar...

I've been giving a lot of thought to this in the last few days and I think I've finally cracked the cause of the timing getting so badly retarded on both cars. "False knock" is a well known and problematic phenomenon on some cars, and is anything picked up by the knock sensor that the ECU interprets as knocking which is NOT genuine pinking/detonation/pre-ignition. Both cars have nearly always been running on 99 octane petrol so they shouldn't be knocking, period.

So what is the source of the "false knock" then ? I believe it's caused by a misfire! Not a dead misfire where a cylinder just isn't firing at all for example no spark or injection at all on a cylinder. I mean a partial misfire where you get some combustion but it is incomplete and chaotic. For example due to a lean mixture, weak spark, or poor compression due to a leaking valve. I believe this generates a vibration in the cylinder that is similar enough to actual knock that the ECU is fooled. The normal response to detecting knock is to retard the timing by 2 degrees per (4 stroke) cycle each time knock is detected up to a maximum of 12 degrees or until knock is no longer detected. In the absence of knock the ECU will slowly advance the timing by 0.2 degrees per cycle until it detects knock again or until the timing is fully advanced again. Thus it is quick to retard to immediately halt any detonation/pre-ignition but quite slow and cautious to advance again. These values are memorised separately for each load/rpm cell in the knock retard table.

However unlike real knocking "false knock" doesn't go away when the timing is progressively retarded, so it just keeps retarding it more and more every engine cycle until it hits the full 12 degrees allowed, and probably does it in the space of only a second or so. Provided that an intermittent misfire occurs at least once a second or so it remains pegged at 12 degrees of retard. On the old car I measured a constant retard of about 12 degrees below 2500 rpm when it was symptomatic. So a relatively minor misfire on one cylinder that would only cause a modest loss of power without a knock sensor ends up causing a massive loss of power because all 6 cylinders are now running retarded.

But wait - it gets even better! Another revelation I've just had over the last few days of thinking is that in the case of an ignition misfire where the cause of the problem is a weak spark, (low spark voltage essentially) retarding the timing actually makes it misfire even worse! Why ? Because the cylinder pressure is higher the closer you come to TDC on the compression stroke. So if you fire the spark with a normal advance of say 20 degrees then the spark is trying to fire against a certain cylinder pressure. However if you fire the spark later, say only 8 degrees before TDC the pressure will be that much higher, the higher pressure requires an even higher spark voltage which may not be available so the result is a misfire. So as it detects false knock caused by an intermittent misfire it starts to retard the timing but the spark has an even harder time firing with retarded timing, misfires even more often and you end up in a runaway situation where you're pegged at the maximum amount of retard available with a pretty nasty misfire on the guilty cylinder.

What led to me thinking about the spark being harder to fire with retarded timing was that I had noticed this car had an obvious intermittent misfire at idle in neutral with no load - about once a second it would give an audible and visible kick from a misfire. And yet put it into gear with the load of the torque converter while stationary and it would idle perfectly without misfiring! Put it back into neutral and it would start misfiring again...Why ? Because at idle in Neutral the ECU retards the timing on purpose! It uses ignition timing for fine idle speed control - varying the timing retard from about 0 to -25 degrees (relative to normal advance) with an average of about -15, to make small but very rapid corrections to stabilise the idle speed. You can clearly see this happening on a Lexia if you watch the "torque reduction" timing figure while idling in neutral then put it into gear. I suddenly realised that the (perfectly normal) retarded timing at idle was promoting a misfire because the spark is harder to fire the closer you get to TDC, and at idle in neutral it's sometimes firing the spark very close to TDC.

So there you have it. In the case of this car the misfire appears to be entirely ignition related - solve the misfire, the "false knock" goes away, and the timing eventually goes back to normal. It's taken a couple of days of driving after swapping the coil for it to go from good to excellent and it seems to be staying there. On the old car it did have ignition problems at some points in time but when I eventually solved the ignition problems it was still misfiring intermittently - my suspicion is a valve gear related problem on that car was causing an intermittent drop in compression of one cylinder, so the cause of the misfire was different but the end result of the timing getting crazily retarded is the same.

[RichardW]

You have too much time to think - you need a house and / or a family to absorb some spare time

Interesting all the same. Pity we never got a chance to strip the engine on the problem child to see if there was a broken spring or something.

[CitroJim]

Simon, an excellent dissertation indeed and it makes perfect sense. A brilliant piece of detective work there...

I'm amazed they use timing retard for fine idle control. That realy is a new one on me. How on earth did you find all this out? That and the timing retard method in case of knock?

Is that unique to the MP7 or applicable to all engines with knock sensors?

So what do you reckon was the root cause of your misfire then?

[Mandrake]

You have too much time to think - you need a house and / or a family to absorb some spare time

We'll see! A lot of the thinking about things like this occur on the commute to work and back or in the shower...

Interesting all the same. Pity we never got a chance to strip the engine on the problem child to see if there was a broken spring or something.

I know, I would have liked to have found the exact cause.

I'm amazed they use timing retard for fine idle control. That realy is a new one on me. How on earth did you find all this out?

I've had to piece it together from various sources - there's precious little information about the nitty gritty details of the ES9J4 / Bosch MP7.0 engine management design and ECU... but a lot of the principles are also quite generalised and applicable to most similar engines from the same era once you understand the principles...

The fine idle control information came from one of the principles of operation documents in Docbackup - there's about 10 documents for the ES9J4 in docbackup with little nuggets of information that I've exported as PDF's, this is the one that mentions the fine idle control in section 2:

https://dl.dropboxusercontent.com/u/782 ... %207.0.pdf" onclick="window.open(this.href);return false;

"2. Calculation of the ignition advance. A dynamic correction of the injection advanced is applied at idle.
This correction stabilizes the engine through advance variations from one TDC to the other, in positive or negative, in relation to the
cartographic value. Corrections of the injection advanced are equally applied during transitory phases."

I'm not quite sure what the pigeon French in the last sentence means but the rest speaks for itself. You can see the timing value jump all around on the Lexia when idling in neutral too, but as soon as you press the throttle slightly or put it into drive it reverts to normal fully advanced timing.

That and the timing retard method in case of knock?

The 12 degree maximum knock retard came from this document:

https://dl.dropboxusercontent.com/u/782 ... or%203.png" onclick="window.open(this.href);return false;

A rough google translation of the interesting bit is:

"These sensors can detect knocking, vibration phenomenon due to an explosive ignition of the mixture in the combustion chamber.

This repeated phenomenon can cause destruction of the mechanical parts by abnormal rise in temperature of the walls . This sensor provides a voltage corresponding to engine vibrations. After receiving this information , the computer performs a reduction of the advance to the ignition cylinder or 2 degrees concerned with a maximum decrease of 12 degrees .

The réincrémentation will gradually ( 0.5 degrees every 120 PMH vicinity)

Parallel to the withdrawal of advance , the computer applies a richer air / fuel mixture to prevent excessive temperature rise of the exhaust gas which could lead to the destruction of the catalyst."

On re-reading this the re-advancing in the absence of knock is actually a lot slower than I remembered - 0.5 degrees steps but only once every 120 revolutions. (We talked about this translation in my old thread but I remembered the numbers wrong) When I first discovered this document I was quite pleased to see that the 12 degree figure agreed with what I had already measured with the scope.

Is that unique to the MP7 or applicable to all engines with knock sensors?

The sensitivity of the knock sensor and the exact algorithm for retarding and re-advancing will vary from one engine design to another but the basic principles are always the same - if knock is detected retard the timing very quickly in relatively large jumps to stop any detonation in its tracks and only re-advance it very slowly and cautiously. Any knock retard learned is stored in a cell in a 2D table that represents RPM versus load, with load determined by MAP/MAF/TPS readings.

This table of knock retard values survive key off/on events (but not battery disconnection for 10 minutes on this particular ECU) and are constantly refined and adjusted as you drive. In the case of a cell that is heavily retarded you have to spend quite some time (10's of seconds) driving at that exact RPM and throttle load with no knock detected for it to gradually advance that cell again - since most driving involves constantly varying RPM and throttle load it can take a lot of mixed driving to gradually re-advance the timing across all the different cells in the table after the fault is removed.

Likewise if you go from high octane to low octane petrol it learns very quickly (knocking under load will happen almost immediately and very quickly retard all the timing cells) but going from low octane to high octane fuel is a very gradual re-learning process that can take days or weeks - better to disconnect the battery to reset it if you increase your fuel octane or you may not see any tangible difference for quite a while.

So what do you reckon was the root cause of your misfire then?

On the new V6 it was just an ignition misfire due to a secondary ignition problem.

On the old V6 it had ignition problems too but I got them sorted out by the time the car went, but there was something mechanical wrong as well that was causing an intermittent misfire. Even fairly early on that car started on 5 cylinders and misfired on one cylinder for up to 10 seconds on the first start in the morning then was fine for the rest of the day - later on that 10 seconds increased to 30 seconds...so even back before I seemed to be having any overt problems with it there was already a one cylinder misfire on cold startup...

I'd love to know precisely what it is but sadly I think we will never know. I think it has to be something that was causing a valve to intermittently not completely shut thus causing a misfire from lack of compression. It could have been as "simple" as a broken valve spring allowing the valve to float slightly at certain RPM...

[lexi]

Never dwell on what you can't change Simon.

Have you done this new car justice yet and got the bodywork sorted?

[Mandrake]

No, nothing done on the bodywork yet, (I've only had it coming up on 3 months!) and I doubt that I will get a chance to spend any money on bodywork until next spring at the earliest.

I already got away with murder buying the car in the first place at a critical point during saving for a house, and there are too many mechanical jobs that are more important than the look of the bodywork that need attending to first including spark plug leads, hydraulic pump seals, cracked auxiliary belt, blowing muffler, front suspension ball joint(s), handbrake cables, gearbox oil change/flush and a coolant and oil change before the winter to name a few...

Once I get on top of all the mechanical things and have a bit of a breather then I'll turn my attention to the bodywork. Surprisingly it's not nearly as bad as it appears in the photos - I thought it would look horribly disfigured in real life but I actually hardly notice it and I'm not bothered by it like I thought I would be.

[lexi]

Ah well. You have a few priorities then. At least you have done most of these jobs before. Here is hoping for a mild winter.

[CitroJim]

Thanks for all the references and sources Simon, I'm going to have a good read when I have some time....

I think you ave your priorities right over the work on the car... As long as things don't look too bad and rust can't get a toe-hold in the areas damaged then it's only cosmetic.

It's always been my way and I know yours is not bad at all. It's only when you're really up close to it you notice a few grazes...

[lexi]

Cosmetic? Rust never sleeps. If you can see primer, the primer 'aint waterproof. If metal is bashed, it is rusting.........on both sides.
I think both you and Simon have had rusty holes in cars before. There is much talk about saving Cits. Chassis and bodywork first is what saves them. Not 2 years down the line when it is much further gone.

Any classic car owner will tell you the same. Lest it is just bangers you want for a runabout