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

[xantia_v6]

2 quick points...

Retarding the timing causes the combustion (and spark plug) to get hot, not cold, as the energy which would have got converted to pressure and power (with optimum timing) gets converted directly to heat (same amount of fuel and air, the energy has to go somewhere).

The torque produced by an engine is proportional to the fuel injector pulse width, assuming constant (relative to optimum) timing and constant volumetric efficiency (both of which should be true under normal conditions at low to moderate RPM), so the ECU could just be sending the injector pulse width to the transmission ECU (or could be fiddling it a bit).

[Mandrake]

2 quick points...

Retarding the timing causes the combustion (and spark plug) to get hot, not cold, as the energy which would have got converted to pressure and power (with optimum timing) gets converted directly to heat (same amount of fuel and air, the energy has to go somewhere).

Although I don't know for sure myself, that's not what I've read. A number of different references all say that that the spark plug tip (electrode and insulator) runs hottest when the timing is maximally advanced, and cooler when retarded, for example:

http://www.ngksparkplugs.com/tech_suppo ... timing.asp" onclick="window.open(this.href);return false;

Makes sense when you consider that part of the temperature rise is through compression, advancing the timing means the ignited mixture remains compressed longer, and is in a higher state of compression at the point where the flame is well under way burning. Also the centre of mass of the flame front is closer to the top of the cylinder since the piston is (still) higher than it would have been with less advance.

The torque produced by an engine is proportional to the fuel injector pulse width, assuming constant (relative to optimum) timing and constant volumetric efficiency (both of which should be true under normal conditions at low to moderate RPM), so the ECU could just be sending the injector pulse width to the transmission ECU (or could be fiddling it a bit).

It's not sending the injector pulse widths per se, from what I can see from the 4HP20 training manual page 119 onwards, it uses a cartographic map which contains maximum theoretical torque as a function of RPM and engine load (throttle/map sensor readings) from which it then subtracts power loss due to timing retard relative to the peak torque timing advance, passive losses from friction based on a cartographic map of engine RPM and temperature, and then subtracts losses from the A/C compressor if engaged.

So in other words the torque figure is based on ideal performance of the engine and probably wouldn't be representative of actual engine torque for some types of fault such as a misfire or faulty injector.

By the way when the engine was very lethargic at low RPM I've checked the injector pulse times and they have still increased with increasing throttle opening even though the engine didn't seem to be producing any increase in power or acceleration.

---

Unrelated but I noticed after a lot of driving tonight looking at houses, about 40 miles mostly in urban traffic and a bit on the motorway which probably got the gearbox nice and hot that the gearbox made its first "groan" at 1500rpm under load since the oil changes last year. (as well as the low rpm performance sagging a bit) It wasn't loud in fact I could hardly hear it, and it didn't recur later but I felt it clearly through my foot on the floor - like a shudder/vibration beyond a certain throttle opening at 1500rpm in 3rd with the TC locked up or in controlled slip. Exactly the same symptoms that later progressed into the full blown loud groaning noise last time...

The question is, why has it returned...oil overheating ? Friction surfaces on the clutch burnt ? Particles jamming one of the valves in the control block affecting pressure regulation to the clutch ? It might be that my oil changes have only put the problem into remission for a period of time. Sigh. I suppose I should be grateful that the oil changes helped the gearbox work relatively normally for a further 6 months past what I thought it would last.

[lexi]

Have you considered that sticky tappet and blocked oil feed to that cylinder, may cause ECU to have cut spark to it at times previous?

Why ain't you out changing oil in this lovely weather we are having?

[Mandrake]

Have you considered that sticky tappet and blocked oil feed to that cylinder, may cause ECU to have cut spark to it at times previous?

Don't know that I follow Alex ? I can't see any way that the ECU could be aware of a sticking tappet so that it could cut the spark ?

I think at least one valve may be sticking when first starting though, there has been what sounds like a single cylinder misfire when first starting from cold for the first 30 seconds to a minute which I had not been able to eliminate despite double checking the ignition system, new plugs etc, and confirming that all the injectors are balanced. It's also got a new inlet manifold gasket when I changed the plugs recently. Despite that the "misfire" for the first 30-60 seconds after a cold start persisted, after that it goes away.

Since adding the hydraulic tappet additive I noticed the cold start "misfire" goes away in about 5 seconds now instead of 30-60 seconds, could it be a valve sticking open causing compression loss on one cylinder until the oil starts flowing, therefore a low compression misfire ? I guess it really needs an oil change ASAP to be sure, seems like some of the small oil ways in the valve gear area are not happy campers.

Why ain't you out changing oil in this lovely weather we are having?

Haha very funny, we had hail predicted today but it didn't quite arrive as promised Trouble is we're in the middle of moving at the moment, we have to be out in 2 weeks and will be staying somewhere temporary for a month or so - somewhere without an off street driveway or even remotely level ground so I don't know how I'm going to do any oil changes. Both the engine and gearbox desperately need an oil change by the look of it.

By the way for those following the house move / garage story, the advertised "garage" of the house that we looked at today that I was suspicious of from google maps indeed turned out to be a garden shed that I could only just stand up in and certainly couldn't swing a cat in let alone a 4HP20. That was the least of the house's problems though, including an advertised but non-existent garden so we will keep looking...could take a while...

[addo]

Every real world engine event I've seen that specifically involves retarded (from optimal) timing, has made the motor run hotter. It may be more of an overall hotness <insert comment about preferred piece of fluff here> but definitely hotter.

[Ben82]

that I could only just stand up in and certainly couldn't swing a cat in let alone a 4HP20.

Feline or exhaust?

Sucks about lying estate agents, we had one over here saying that a house didn't need much work, but from closer reading of the inspection report, it would require tearing up floor boards to get rid of some serious mould.

[Mandrake]

Every real world engine event I've seen that specifically involves retarded (from optimal) timing, has made the motor run hotter. It may be more of an overall hotness <insert comment about preferred piece of fluff here> but definitely hotter.

It could well be that the overall temperature of the engine as seen by coolant temperature is higher when the timing is retarded as you're getting less mechanical power output from the same fuel when timing is retarded, (so the difference must go up in heat) plus in practice you'd be opening the throttle wider and burning more fuel in an attempt to make up the lost power which means even more heat.

However I'm referring to the tip temperature of the spark plug in relation to fouling or pre-ignition, which all the spark plug manufacturers and some other sources of information I've found are all unanimous on - more timing advance increases the spark plug tip temperature.

Another example here when talking about possible causes for carbon fouling:

http://www.ngksparkplugs.com/tech_suppo ... ugs/p2.asp" onclick="window.open(this.href);return false;

"Over-retarded ignition timing", as well as "Air-fuel mixture too rich". It's interesting to note that in response to knock sensor activity the ECU simultaneously retards the timing and richens the mixture, both of which promote colder spark plug tip temperatures, so excessive knock sensor activity is not going to allow the spark plugs to run at their optimal temperature.

Also under "Causes of overheating" (of the spark plug tips) "Over-advanced ignition timing" is one of the possible reasons listed.

My feeling is that the plugs got fouled due to a combination of the plugs being a colder heat range, short trips, and exacerbated by knock sensor activity due to low octane fuel, possibly even some spurious knock sensor activity due to noisy tappets. Time will tell if they foul again, they haven't so far.

[Mandrake]

that I could only just stand up in and certainly couldn't swing a cat in let alone a 4HP20.

Feline or exhaust?

Sucks about lying estate agents, we had one over here saying that a house didn't need much work, but from closer reading of the inspection report, it would require tearing up floor boards to get rid of some serious mould.

The feline variety.

[lexi]

Don't know that I follow Alex ? I can't see any way that the ECU could be aware of a sticking tappet so that it could cut the spark ?

Sludge can block feed to tappet and pressure is low. Some ECU pick on that and shut off spark IIRC. Maybe not on Xantia V6? maybe too early.

[Stempy]

FWIW I used several types of plug in my V6, including Bosch FR6DP, NGK BKR6EIX and Bosch Super 4 510, none of which ever fouled and all came out a lovely salmon pink colour when changed at around 20,000 mile intervals. I also had fitted a 3.5 bar fuel pressure regulator, a high flow air filter and carried some airflow smoothing mods to the intake and always used the highest octane fuel available at fill ups, pref V-Power. According to the MOT the cat was still working fine at 130,000 miles and I always used fully synth oil replaced every 6000 miles with filter and ran injector cleaner through at similar intervals. The fuel filter was also replaced every 25k miles. Tappets made a noise only for the first 5 seconds after start up and at 130k the engine would easily pull her up to an indicated 140mph on a private test track you understand, it ran like a dream and was the only part of the car that wasn't causing any problems.

During my ownership I had replaced the air temp sensor in the airbox (it was broken and kept falling out), the throttle position sensor (they wear out) and the ECU coolant temperature sensor (didn't seem to read resistance as per the manual). The thermostat jammed open at one point and this was also replaced. I recall some wierd and wonderful intermittent starting/running/gearbox faults occuring which were mostly caused by the electrics. In the end I replaced all the fuses and a couple of relays which appeared to have oxidized contacts and as a precaution separated all the under bonnet electrical connectors and gave them all a good clean and liberal dose of contact cleaner, noting that some were very oxidized. Never had another problem after that.

[Mandrake]

Don't know that I follow Alex ? I can't see any way that the ECU could be aware of a sticking tappet so that it could cut the spark ?

Sludge can block feed to tappet and pressure is low. Some ECU pick on that and shut off spark IIRC. Maybe not on Xantia V6? maybe too early.

Yep, too early. The diagnostic abilities and coherence checking of the MP7.0 are pretty primitive compared to later OBD-II ECU's. There's a lot of blatant faults it can't even pick up (such as a dead miss) that modern ECU's can. I joked earlier on the thread that it seems that the engine bay would have to be on fire before the check engine light would come on.

(I've never seen the check engine light despite an array of problems I've worked my way through and deliberately leaving sensors unplugged, and I've never seen any fault codes logged that weren't caused by my testing or disconnecting of sensors, as far as the ECU was concerned the engine has been fine the whole time even when it was running like a dog )

I've studied the ECU's for engine and gearbox very thoroughly while troubleshooting over the last few months, although there is an oil pressure switch and oil level sensor neither of them go to the ECU only to the dashboard warning lights. The ECU would be blissfully unaware of any blocked oil feeds. (The pressure switch is only an "oh s**t, there's no oil pressure, stop driving now!" warning anyway, it wouldn't pick up on slightly low oil pressure)

The only possible feedback I can think of that would allow the ECU to indirectly notice blocked oil feed to the tappets is that the knock sensor might be sensitive enough to pick up 24 clickety clackety tappets that should normally be silent and retard the timing thinking its picking up knock. I'm sure this is an accidental side effect if it does happen though, and not by design.

[Ben82]

(The pressure switch is only an "oh (-expletive removed-), there's no oil pressure, stop driving now!" warning anyway, it wouldn't pick up on slightly low oil pressure)

And don't I know it? Gladly it's just a warning, otherwise I probably wouldn't have been able to drive it ~60km home to replace the pressure switch myself.

Actually I think you're slightly wrong, I think the pressure switch has a specced range (0.40-0.60 bar) but not sure whether that's for off or on.

[Mandrake]

(The pressure switch is only an "oh (-expletive removed-), there's no oil pressure, stop driving now!" warning anyway, it wouldn't pick up on slightly low oil pressure)

And don't I know it? Gladly it's just a warning, otherwise I probably wouldn't have been able to drive it ~60km home to replace the pressure switch myself.

You were just lucky that it was the switch that was faulty not really a lack of oil pressure. If the oil pressure really had been that low the engine would most likely have been a write off after 60km... which is why we were so insistent you get it checked out locally first...(I ruined the turbo and damaged the big end bearings on a Daihatsu charade that I drove 40km with low oil...it always burnt a bit of oil which required regular top ups but I forgot that month and went on an unexpected long motorway trip, by the time I got home I knew all wasn't well, the turbo seized a couple of weeks later... )

Actually I think you're slightly wrong, I think the pressure switch has a specced range (0.40-0.60 bar) but not sure whether that's for off or on.

It will have a threshold that's well above zero, but it will be down in the "stop driving now" range, so it wouldn't give the ECU any advance warning of the pressure just being a bit low but still within the safe range.

[Ben82]

You were just lucky that it was the switch that was faulty not really a lack of oil pressure. If the oil pressure really had been that low the engine would most likely have been a write off after 60km... which is why we were so insistent you get it checked out locally first...

Indeed, well it was always going to get checked out, as by the time I got back to be able to post anything the car had already been towed to the nearest Citroen indy. Was no point in me going to collect it without them checking it out (and trying to missell me an engine flush!)

It will have a threshold that's well above zero, but it will be down in the "stop driving now" range, so it wouldn't give the ECU any advance warning of the pressure just being a bit low but still within the safe range.

Same with the coolant one, it goes from "everythings fine" to "stop driving and get some more coolant in".

[Mandrake]

FWIW I used several types of plug in my V6, including Bosch FR6DP, NGK BKR6EIX and Bosch Super 4 510, none of which ever fouled and all came out a lovely salmon pink colour when changed at around 20,000 mile intervals. I also had fitted a 3.5 bar fuel pressure regulator, a high flow air filter and carried some airflow smoothing mods to the intake and always used the highest octane fuel available at fill ups, pref V-Power. According to the MOT the cat was still working fine at 130,000 miles and I always used fully synth oil replaced every 6000 miles with filter and ran injector cleaner through at similar intervals. The fuel filter was also replaced every 25k miles. Tappets made a noise only for the first 5 seconds after start up and at 130k the engine would easily pull her up to an indicated 140mph on a private test track you understand, it ran like a dream and was the only part of the car that wasn't causing any problems.

Hardly a stock car you had there Stempy

I'm intrigued by you upping the fuel pressure to 3.5 bars... what encouraged you to do that and what was the result ? I'm assuming that's vacuum hose off pressure you're quoting. Mine is a bit on the low side - 2.7 bars with the vacuum hose off when it should be 3, but I didn't think it was low enough to cause serious problems.

In an ideal world I'd replace the regulator but I have to carefully prioritise what work I do so as to avoid this car becoming (even more of) a money pit... (If it was a more lowly Xantia I would have got rid of it after this amount of trouble but a V6 is something special that's worth persevering with a bit more IMO, especially when I had always wanted one)

FR6DP is two heat ranges cooler than standard, and the BKR6EIX is one heat range colder (NGK uses reverse numbering, NGK 5 is equivalent to Bosch 8, NGK 6 to Bosch 7) but if you gave it regular long or fast trips on high octane fuel it may not have given any problems. Sadly this car gets used a lot to do shopping runs and the like where it doesn't warm up and it doesn't often get a chance to really stretch its legs, so I can't afford to run colder than standard plugs in it.

I've used injector cleaner once but I didn't really notice any difference and the injector balance tests I did before and after were both perfectly ok anyway.

Do you notice any difference with the fully synth oil ? Because of the problems this one has had with very noisy tappets I'm strongly considering treating it to fully synth when I do the oil change, although some have pooh poohed the idea of "fancy" oil.

Since I put the tappet additive in my tappets are actually silent the moment the engine starts, not even after 5 seconds, however one does become a bit noisy after a few minutes. I'm tempted to put some of the same wynns additive in with the new oil when I do the oil/filter change to really try to clean the oil ways in the head out, as it does seem to be surprisingly effective.

During my ownership I had replaced the air temp sensor in the airbox (it was broken and kept falling out), the throttle position sensor (they wear out) and the ECU coolant temperature sensor (didn't seem to read resistance as per the manual). The thermostat jammed open at one point and this was also replaced. I recall some wierd and wonderful intermittent starting/running/gearbox faults occuring which were mostly caused by the electrics. In the end I replaced all the fuses and a couple of relays which appeared to have oxidized contacts and as a precaution separated all the under bonnet electrical connectors and gave them all a good clean and liberal dose of contact cleaner, noting that some were very oxidized. Never had another problem after that.

Interested if you can remember any more details on your gearbox problems... I think the root cause of trouble on mine is a worn out TC lockup clutch which unfortunately means a rebuild, but I will drop the gearbox oil when I do the engine oil to inspect and replace it, again, to see how it compares to the 4 changes I did to clean up the oil 6 months ago. I still have sample jars from last time... From the way its behaving lately I suspect there are significant clutch related containments in the oil again, causing mayhem...