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

[lexi]

The danger of a Spark Plug not sealing is that the cylinder will overheat. Have you ruled out the sticky valve or tappet now then?

I didn't know you had a "scope" Simon? In fact I didn't even know you were a Doctor , or I would have had you check this varicose vein on your last visit .
What fab weather today. Addo will laugh at us drooling over 12 deg. The weather we have had previous though?.......this is heaven! So much done.

[Mandrake]

The danger of a Spark Plug not sealing is that the cylinder will overheat. Have you ruled out the sticky valve or tappet now then?

Well I haven't changed the oil and filter yet if that's what you mean... Would a sticky valve or tappet not cause low compressions though ? Compressions were fine when I tested them, although I only tested them on a warmed up engine.

I didn't know you had a "scope" Simon? In fact I didn't even know you were a Doctor , or I would have had you check this varicose vein on your last visit .

Not that kind of scope, this kind:

What fab weather today. Addo will laugh at us drooling over 12 deg. The weather we have had previous though?.......this is heaven! So much done.

Yeah it was nice, the weather report for tomorrow is just as good too...

[DHallworth]

Simon,

Just a thought, did you change the plug lead that you find had a gash in it where it goes through the cable clips into the back bank?

David.

[Mandrake]

Just a thought, did you change the plug lead that you find had a gash in it where it goes through the cable clips into the back bank?

Hi David,

Yes I did - I replaced the whole spark plug lead cassette for the rear bank with another new one...(its impossible to change just one lead as they're moulded into the cassette)

On the old lead that was pierced and running poorly as soon as I taped it up (and did nothing else) the performance shot up back to normal performance - that was the weekend I visited you. It performed perfectly for about 2 weeks and several hundred miles without the slightest hint of symptoms or loss of power, then started to deteriorate again after that.

At that time I then found that a hole had melted through my tape repair (which looked like it had melted due to an arc) and assumed that was the issue so I ordered a new lead/cassette and fitted it. When I fitted the new cassette I also removed and refitted all the spark plugs to check their electrode condition. (Maybe not a good idea to do two things at once in hindsight...)

Heres the curious thing - the car never really performed any better after the second set of leads was fitted, performance has been mediocre and intermittent since then despite the new un-damaged leads, something I have not been able to explain so far.

To further throw doubt on the situation I later found when cutting it open that on the original "faulty" lead, only the black outer sheath was pierced - the inner white insulator which is the bulk of the insulation was not damaged, so in theory it can't have been arcing there in the first place... (And yet the rubber tape repair was melted with the pattern of an arc...)

It's unlikely that the 2nd set of leads is faulty, I was extra careful fitting it as well. Although I could have potentially mixed up two of the leads causing a spark timing error I think it would be running MUCH worse than it is now if that was the case...

As you can tell I'm quite baffled by what's going on and why it never worked as well with the 2nd set of leads... That's why I'm wondering if something is wrong with the spark plugs or maybe the spark plug boots.

[Mandrake]

FIXED!

Yes I know I've said this before, but I think I have found the problem at last, even if I'm not sure whether it's permanently fixed.

My gut feeling that it was ignition related seems to have been borne out, on a hunch I ordered some new spark plugs a couple of weeks ago, this time the OEM specified Bosch FR8KDC, not the FR7DC+ that I had put in last November.

I started off trying to do some under load voltage testing on the ignition, but I wasn't too successful as I couldn't really load the engine sufficiently at low RPM to boost the spark voltage much. At idle the voltage from the cylinder 1,5 coil on the top output did seem lower than the other two - it would only jump about 2mm (6000 volts) while the others would jump about 5mm, (15,000 volts) although it was inconsistent. This suggested maybe some fouling on plug number 5 causing leakage to ground. (which later proved to be correct)

With the coils disconnected from the ignition system I tested them with an adjustable gap style spark tester and all three coils could easily jump 20mm which is over 60,000 volts, so no problem with the coil pack.

So I forged ahead and took the intake manifold off and took all the plugs out to inspect them. As soon as I looked at the plugs from the rear bank I was pretty sure I'd found my problem:



It's not as easy to see in a photo as it is first hand but plug number 5 (middle in the rear bank) was not only badly fouled but there were 3 obvious white lines through the carbon on the insulator which will be the spark tracking through the carbon instead of firing at the gap...

Here's a zoomed in view of the middle one where 2 of the 3 tracking lines on the insulator are visible:



Carbon fouling was the worst on plug number 4, well and truly fouled, and although you cant see it in the image there was some evidence of tracking on it as well:



Plugs 1 to 3 on the front bank were not as bad but still less than ideal:



While I had the plugs out I cleaned the plug seats on the top of the head thoroughly, they were a bit mucky with a little bit of dirt/grit, fitted the new plugs, correctly torqued to the recommended 90 degrees this time...

I checked the plug wells for oil again and there was a little bit in two of the back ones but not much, (nothing inside the spark plug towers though) I soaked it up of course... this time I also thought to check the 8mm bolts that clamp the cam box covers on, and found all the ones on the front side of the rear exhaust cam box where its leaking were rather loose especially compared to the other cam boxes, so I tightened all the bolts on the rear two cam box covers up to the same torque as the front two cam boxes, which were a lot tighter by comparison... I doubt whether it will stop the leaks entirely but it may slow them down a bit not being loose!

I also carefully inspected the ignition leads and all the spark plug boots, all were in perfect condition with the boots clean and dry inside, and the leads measured the correct resistance.

The improvement in power and responsiveness when taking it for a drive was immediately obvious, I'm not sure if its quite 100% but its pretty close. As well as a lot more power its also quieter running and much smoother, its also ticking over perfectly smoothly at idle again and starting much quicker.

The question is, why were the rear plugs so badly fouled ? I do a lot of short trips that don't give time for the engine to warm up so that will certainly be a factor, but I've never had this problem on other cars I've owned that have also been subject to short trips. One factor is that I had the wrong heat range spark plug - the FR7DC+ is a colder plug than the OEM spec FR8KDC which I've now fitted, so it would have been running 70-100 degrees colder than the correct plug. If anything I should possibly have an even hotter FR9KDC in there if I'm doing a lot of short trips and it starts fouling again ?

Cylinder number 4 (right rear looking from the front of the car) I'm a little bit concerned about - does that look like just a particularly thick coating of carbon or is there actually a little bit of oil fouling as well ? I can't really tell...

Is there anything else other than short trips on a colder than correct plug that could be causing rapid carbon buildup ? Is it a sign of some oil consumption in the combustion chamber, perhaps from leaking valve stems ?

In the mean time its running great so I will observe it over the next few weeks and see if it stays that way or whether it deteriorates again. At least if it does I will know what's causing it...

[lexi]

I think super duper plugs might help...........the real spendy ones Better fuel might help. Working or hotter thermostat? Although the V6 generates lots of heat.
Lots of cars do short journeys mind. Can result in mayo under the rocker, but they don't usually misfire. Mind they aint complex V6's.
Hotter plug should do the trick.
Glad you got some answers and not costly..

[Mandrake]

Thermostat stuck partially open is something that crossed my mind too Alex, the heater used to warm up really quickly when I first got it, even in cold weather, now I think about it it does seem to take quite a while to warm up now, both heater and temp gauge...on the motorway it reaches full temp fairly quick but at 30mph I'm not sure if it quite reaches full operating temperature, or if it does it takes over 10 minutes to get there.

Perhaps a new thermostat is on the cards.... I wonder how much fun that job will be I've done thermostats before (dead easy to access on a 2 litre Xantia) but I'm not sure where it is or how hard it is to replace on the V6...

Let me guess, step one is remove the air filter box ?

[Ben82]

You already know step 1
http://www.406coupeclub.org/Files/HowTo ... engine.pdf" onclick="window.open(this.href);return false;

[Mandrake]

Thanks Ben, very useful info, saved for reference.

As I suspected, it looks like its a lot bigger job than my 2 litre Xantia, which was unbolt two bolts from the top house mount at the engine end, lift the hose off, swap the thermostat, tighten two bolts and bleed the system! About 5 minutes work not counting the bleeding...

I guess I should really test the thermostat first by checking the top hose temperature as the engine warms up - it should stay completely cold until nearly at operating temperature then suddenly get hot. If it warms up early and gradually its stuck open...

[addo]

Odd thermostat sensor readings can also be due to airlocks in the cooling system.

However I don't feel it's likely here.

Not sold on the latest analysis.

[Mandrake]

Not sold on the latest analysis.

In what way ? You don't agree that fouled plugs were really causing the loss of power even though replacing them has fixed the problem so far ?

Or you don't agree with the speculation on WHY the plugs got so fouled ? If so I agree, I'm not entirely sure how they got so bad.

A couple of other ideas on that front though - I did run with the oxygen sensor disconnected for a couple of months so that would tend to cause the engine to run permanently a bit rich thus hastening any fouling due to excess fuel and also causing the cylinder to run colder...also not forgetting that the oxygen sensor was actually faulty and not giving an accurate reading to the ECU...

Or there could have been a slight vacuum leak on some of the front cylinder inlet runners at the gasket making them run lean, which due to the feedback from the MAP and oxygen sensors would richen the overall mixture causing the remaining cylinders to run rich when the ECU thought the mixture was right...

I did find the inlet manifold gasket was torn into two pieces when I got the manifold off - of course I probably just tore it while trying to get the manifold out due to catching it on one of the locating pegs while moving it sideways, but there is a chance that I tore it last time I was putting it in...if it was damaged that could explain why some cylinders were fouled while others weren't nearly as bad.

I'm sure there are other possibilities like the thermostat running the engine cold for too long, and it certainly does need to remain under observation.

[lexi]

Sooting up and t/stat stories.

I bought a nice S/H Honda 8hp 2cyl 4stroke from a chap. Running fine. Took it first trip out. Checked oil........cleanish. I thought to use first day and drain oil while hot. Use of these engines for me is running at tickover all day for 9hrs in cold water. On draining oil ......mayo. Never on that model before had I seen it.

While thinking Head gasket, I checked T/stat first. Wasn't there! Cold running at slow tickover for trolling had cause mayo in one day. The PO had used engine hard on a rib in warm weather .......so no mayo. Engine still ran fine at tickover though without stat and the two plugs did not foul bad enough to misfire. Runs great now.

On another Honda model , a 5hp single. The engines ran fine in tickover for about 1 season. Then the plug fouled and engine cut out. Clean plug, and away you went.......for another hour. I had Honda tech out in boat with scopes, mixture adjustment fancy plugs, hotter T/Stats different jetting the lot. I had sold about 30 engines for that dealer through personal recommendations to fishing buddies throught Scotland. We got no answers from Honda. They said that worldwide they had no real problems. That was because they were used hard and not low speed lugging in cold water.



All those engines were sold and replaced with a similar spec Yamaha. Perfect runners for 7 seasons now. Modern engines should run decent in all conditions. I would think probably more so with an ECU system that is functioning at factory level.....there lies the rub I suppose

[addo]

Another one of the weird stories that defied analysis until forensic stripdown.

Scooby, non turbo four. Would run well, then ragged, gutless and knocky, then perfectly well again. Fluctuation between states was unpredictable; it could be seconds or weeks. Cause? Damaged cam timing key that allowed maybe 15° of slop. So, it would time up OK, compression test reasonably, spark fine but just run horribly when it felt like it.

Can you get it on a dyno? Two flying runs would show a bunch of info you're not getting at the moment - lambda, HP, torque. I realise this is maybe £100 of spend but you'll have some good hard data at the end of it.

[Mandrake]

A £100 dyno run just ain't happening I'm afraid, especially when the problem seems to be gone for now. There is no way that would fly with the other half after the money and time that's already gone into the car, even if I had a spare £100.

I am going to check the thermostat operation ASAP though, I have been slightly suspicious of it for a while as the maximum under load temperature is a bit higher than it should be - peaking around 98 degrees on the Lexia instead of 90 (?) with heavy use, while on the other hand it does anecdotally seem to take longer than it should to initially warm up, and drops a bit below fully warmed up temperature with a return to slow driving... All easily explained by a thermostat that is stuck half way open or doesn't open and close through its full range of movement.

Once I've either confirmed the thermostat is working ok or replaced it, I'll just play it by ear and see whether it remains in good running condition or deteriorates again.

[Mandrake]

Went for a drive today, no misfires that I could notice, engine nice and smooth, tons of power above about 4000 rpm, good pickup with light to moderate throttle at low rpm however intermittently lacking in "oomph" and urgency in the midrange from around 2500-4000. When accelerating in sport mode reaching about 4000rpm it would suddenly roar away like a switch had been pressed. (Something I've noticed before, a sudden increase in power above a threshold rpm around 4000)

I didn't have the Lexia on the car but it felt like intermittently retarded timing - very much like it felt when I had the knock sensor unplugged for a test drive recently. (Which did log a fault code by the way)

Still definitely a lot better than before I did the plugs, but maybe its not all its woes...I'm back to thinking about noisy hydraulic tappet(s) falsely triggering the knock sensor. On doing a bit of research I've definitely found instances where noisy valve gear can be enough to trigger the knock sensor at certain engine rpm ranges.

Also from my reading many engine ECU's only pay attention to the knock sensor below a certain rpm - above a threshold rpm (in the vicinity of 4000-5000 rpm for some of the ones I was reading about including Honda and Subaru) it becomes too difficult for the ECU to distinguish between actual knocking and overall mechanical noise of the engine, so the knock sensor is ignored and more conservative timing is used. This could well explain the very abrupt increase in power that occurs around 4000 rpm when accelerating hard, which always occurs at the exact same rpm. (The knock sensor is no longer heeded)

Before my test drive yesterday I had had the battery off for several hours while I did the plugs etc which will have reset any memory the ECU had of the knock characteristics of the fuel forcing it to relearn the fuel characteristics - perhaps it ran better yesterday because it had not yet "learnt" to retard the timing due to excessive knock sensor activity but by the time I drove it today it had memorised what it thought was low octane fuel due to feedback from the knock sensor...(just speculating)

Another possible connection is that when the ECU detects knocking it will richen the mixture as well as retard the timing, which could tie into accelerated spark plug fouling...

Perhaps I should hurry up and do the oil and filter change which is due.

Are these type of additives useful for helping to clean out sticking hydraulic tappets, or are they just snake oil ?

http://www.halfords.com/webapp/wcs/stor ... yId_255221" onclick="window.open(this.href);return false;

Regarding coolant temperature it took around 8 minutes to reach an indicated 75 degrees on the temperature gauge driving on a windy country road around 50-60mph and it then stabilised at around 80 degrees after reaching a motorway and for most of the rest of the journey which was a mix of motorway and 30mph.

After getting home and letting it idle it crept up from 80 degrees to an indicated 100 degrees or just below in about 2 minutes, the fans came on to the normal low speed but it then took a long time to come back below 90 degrees - around 5 minutes of idling with the fans running before the fans cut out and the gauge dropped below 90, so something doesn't seem right with the coolant system... are these results consistent with a partially stuck open thermostat ?