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

[Mandrake]

One further piece of information regarding the low rpm pulling - the defining symptom when the car does not seem to want to pull well at low rpm is the behaviour as the torque convert clutch locks up.

Say you pull away moderately from stationary up to about 20 mph, going from 1st to 2nd, by the time you get there you'll be doing about 2400 rpm and the torque converter clutch will still be open. Around then the gearbox will decide that its time to lock up the torque converter. If I hold the throttle steady as the torque converter clutch locks (thus eliminating the slip) instead of the car accelerating due to the reduced slip, the engine rpm DROPS and the vehicle speed stays the same. So it will drop from maybe 2400rpm to 1800rpm with the same throttle opening while doing the same road speed, and you can hear from the exhaust note the increased load on the engine.

I'm pretty sure that when it was running perfectly that in the same driving conditions as the torque converter locks up the rpm does NOT drop, but rather the car continues to accelerate and the rpm stays much the same. This large drop in rpm as the converter locks only seems to happen when the car is in its "not pulling well low down" mood.

It seems at those low rpm that the engine is lacking torque so that when the load comes on (in the form of the torque converter locking up) the rpm just dies, instead of the engine producing more power to hold the rpm steady.

[addo]

Compressions are about where I'd expect to see them. Definitely no worries there. My PRV gauges (with my gauge) about 190PSI.

[CitroJim]

Yes, compressions are absolutely perfect. It's not the absolute value that's important Simon, but how close they are to each other and from that point of view they're very good...

I'm just thinking about the 1st 2nd change and converter lock-up you speak of Simon. I'll have to take a not of how mine performs in that respect tomorrow morning on the way to work...

What difference, if any, do you see now if you disconnect the oxygen sensor?

[citronut]

.,..They need to be warmed up slowly...

As in, applied suddenly to the midriff skin of one's nearest and dearest.

no Adam that warms you lug oles up rapidly

[Mandrake]

Yes, compressions are absolutely perfect. It's not the absolute value that's important Simon, but how close they are to each other and from that point of view they're very good...

Excellent, at least thats one possible cause (with expensive repairs) that has been ruled out.

I'm just thinking about the 1st 2nd change and converter lock-up you speak of Simon. I'll have to take a not of how mine performs in that respect tomorrow morning on the way to work...

Where I notice it is particularly driving quite slowly in busy city traffic up hills, it will pull away fine and change from 1st to 2nd but when you "hover" around 20mph or so especially if you're going up a hill it will then start to lock up the converter, as I say normally if the engine has plenty of torque the locking up of the converter accelerates the car even more as the slip is reduced... but when it is playing up the revs just drop like a stone and the road speed stays the same, as if there is no oomph behind it.

What difference, if any, do you see now if you disconnect the oxygen sensor?

I only tested it briefly by stopping to unplug it half way through my test drive, but it did seem to largely eliminate the "hesitation" I was seeing at slow speeds and low engine rpm where the engine wouldn't always pull away smoothly.

I would describe the hesitation as separate to the overall throttle lag I've noticed which seems to occur over a wide rpm range and somewhat intermittently.

Going out for a drive shortly to do a bit of house hunting so I will observe its behaviour carefully to see how consistent (or not) it is.

[Mandrake]

ARGHHHHH!! Bad news in this never ending saga I'm afraid chaps.

We went for a bit of a drive tonight both around suburban city streets and also on 70mph dual carriageways, and the performance of the car was absolutely AWFUL, worse than it has been for a long time. Nothing has been changed since I was driving it yesterday.

Completely gutless at all throttle openings and at anything other than high (>3000 or so) rpm. Plenty of exhaust boom and engine noise when putting your foot down, just no resulting power or acceleration! It was actually a struggle to keep up a decent speed on the motorway, it wasn't really until the revs were over about 3000 that I was getting anything approaching performance, so I was having to kick it down into 3rd to keep up with the traffic going up hills.

I am completely and utterly baffled by this, as far as I can tell its NOT misfiring, (performance is worse than with the misfire anyway) the engine note is smooth and quiet, not the noisy coarse engine note you get when one cylinder is misfiring. The gear changes are also nice and smooth with the gearbox seeming to behave itself.

It seems almost as if the engine is choked off and can't breathe, with a very spongy accelerator response. A thought that has occurred to me - is there any possibility that the air inlet between the grill and the air filter box could be crushed or blocked with something ? It's a bit hard to see the front of this tube without removing the grill but I'm starting to think I should have a look.

When thinking about the things I did to the car before I drove to David's, not only did I tape the ignition wire and test the fuel rail pressure, to test the fuel rail pressure I had to remove and refit the air filter box - could this be the connection to why it suddenly started running so well ?

I notice that its a bit fiddly to get the joint between the air filter box and the inlet hose below it to mate together well and its probably not a good seal.. is it possible that the hose is blocked with something, and that when the air filter box is removed and refitted I'm sometimes getting a good seal (thus choking it due to the blocked hose) and sometimes there is a lot of leakage around the junction of the box and the hose allowing it to breathe through there ??

The best way to test this I suppose would be to unclip the hose from the bracket and push it out of the way so its just drawing its air in directly at the bottom of the air filter box.

Anyone else have any thoughts, or are we just back to intermittently blocked exhaust again ?

I'm really starting to think I need to take it in to the "experts" to get it put on a full engine diagnostic system... if it keeps performing as badly as it was today, it surely shouldn't be hard for the problem to be found with the right diagnostic equipment.

One other possible fault that crossed my mind that hasn't been considered is a faulty or misaligned crank position sensor - missing pulses are going to completely mess up the ignition and also the injector pulses as well, and its the sort of fault that could be intermittent with heat and vibration. Diagnosing a crank sensor conclusively really needs a full engine diagnostic system though.

I know intermittent faults are difficult to find, but this one really takes the cake. The worst of it is, since it ran so incredibly well for a couple of weeks, whatever the problem is probably isn't serious... if I ever find it it will probably be one of those DOH!!! moments...

[Hell Razor5543]

As I said before Mandrake, DON'T give up. You are getting there. Sorting out the HT leads means one less problem to divert or confuse you. Once you get the last glitch solved you are going to have an AWESOME motor, and the reason for that is because you will be able to say "I got it all sorted out".

Where does the air intake start? IIRC from other posts it is in the space up in the nearside wheelwell mudguard. I would be inclined to get hold of a plumbers snake, and work it through from the filter box to the open end, and see what you get. I would do all I can to make sure the air intake is completely clear all throughout its length (right down to the engine) so that, if nothing else, you can eliminate that from the list of potential problems.

[CitroJim]

Experts? What a load of cobblers Simon. There is nobody more 'expert' than you are now... And precisely who do you think might be able to sort this problem? I'm stuck. I can thinkof nobody better than you actually..

Yep, check the intake and you know, there's not much left now bar the oxygen sensor and cat...

[addo]

Well, you could do a sonic test on the TPS by putting an audio signal through it and listening for excessive scratchiness as it's turned.

[Mandrake]

Jim:

I can't agree that I'm any sort of expert. My experience of diagnosing these types of problems is very limited. The fact that I've been struggling for months and gone down so many blind alleys and still don't have a final result shows just what an expert I'm not. I've managed to find the faults that I have more through trial and error and luck than any good diagnosis skills.

Experience counts for a lot for finding tricky problems like this, and so does having good test equipment and a good place to do the work. While browsing the 'net for troubleshooting help on these issues I've been amazed at just how advanced some of the engine diagnostic systems are these days and what they can do - and I don't mean dealer EODB tools like Lexia's that read fault codes, I mean ones that directly measure things like the waveforms for spark, crank shaft sensor output etc rather than relying on data from the ECU.

A really good example is this video diagnosing a misfire due to an erratic crankshaft sensor signal: " onclick="window.open(this.href);return false;

To be fair the guy knows what he is doing and many of the people that work at garages are muppets...but not all mechanics are muppets, and I just don't have access to that sort of equipment...and I do feel well out of my depth. Sometimes a fresh perspective is helpful too.

Addo: That's a cunning idea you have there, I can do one better than that though, I have a small battery powered analogue oscilloscope which I could directly measure the output of the TPS with which will show me not just whether the control is noisy but also whether it tracks properly in general.

A can't see how a noisy potentiometer could cause the massive loss of power that I'm seeing at the moment, but I can see that it could cause hesitation when accelerating as the movement of the TPS is the signal for the ECU to richen the mixture to avoid hesitation - duplicating the action of an accelerator pump in a carb...

On the other hand the TPS simply not working right at all - for example reading much lower than it should making the ECU think the throttle isn't open much would fit the symptoms perfectly - you'd get intake boom because the butterfly is opening, but the ECU thinks the throttle is closed so doesn't increase the injector times, hence lean mixture and no power, probably followed by the timing getting retarded due to the oxygen sensor reporting lean mixture.

Another way it fits the symptoms is when there is a lack of power in this way the gearbox is very reluctant to kick down - I can be doing 1500rpm in 3rd gear and depress the throttle 3/4 of the way down and it just won't kick down a gear, whereas when it is running properly it WILL kick down with the same amount of throttle. The gearbox also relies on the TPS to help control its downshift and upshift points, (throttle vs road speed etc) the TPS reporting a near closed throttle would prevent the kick down working properly.

The only problem with this theory is that every time I have checked the TPS reading on the Lexia it has been perfectly ok - with 0 to 100% seeming to be nicely proportional to the throttle depression. That's not to say that the TPS isn't intermittent though, and working ok when I test it but playing up when the car is performing badly... direct electrical measurement of it might be the next step and something I can easily test.

[addo]

It's unlikely to be at fault, but the last twelve months I have seen quite a few things I normally discount as "unlikely" in PSA cars, from a dead CAS to a sticky petrol injector. Nearly always, these parts - like coolant temp sensors - are replaced in the swap and pray capers.

That said, your symptoms, as you would have noticed, are becoming a steadily closer fit to my descriptions.

[Mandrake]

It's unlikely to be at fault, but the last twelve months I have seen quite a few things I normally discount as "unlikely" in PSA cars, from a dead CAS to a sticky petrol injector. Nearly always, these parts - like coolant temp sensors - are replaced in the swap and pray capers.

Well its easy to check the TPS electrically, and apart from checking it on the Lexia I have largely ignored it until now, so I need to rule it out.

Checking for obstruction in the air inlet pipe to the filter box is also something obvious that I haven't checked and ruled out either so that needs doing as well.

Swap and prey is exactly what I'm trying to avoid, there has been far too much of that already in this saga. Now that I have a few gauges etc I've been able rule a few things out - compressions were ok, fuel rail pressure was ok, at least during the one time I tested it, (I think I need to test it again though when the problem is evident rather than when the car is running well) I have a vacuum/pressure gauge that I can look for clues of exhaust blockage etc... more measuring and less swapping of parts is needed, and that goes for not swapping anything in the exhaust without conclusively proving excessive back pressure as well!

That said, your symptoms, as you would have noticed, are becoming a steadily closer fit to my descriptions.

I have to admit yesterdays symptoms did fit a blocked exhaust, that's the worst performance I've seen in a long time, and the engine did indeed feel like it was really choked and struggling to breathe at any rpm. The problem is that there have been at least two different faults, both intermittent.

I had two broken spark plug leads causing misfiring which was then replaced with one punctured spark plug lead which was also causing misfiring, that has been a big part of the problem, however it is apparently not the only problem, something else intermittent has been going on in parallel with that.

Until I see a pressure gauge connected to the exhaust I'm not going to assume that it is a blocked exhaust though. Still not enough data, which is why I think it could really do with being tested on a full engine diagnostic system.

[Hell Razor5543]

A dirty potentiometer can cause a heck of a lot of electrical noise. The volume control on my radio alarm gets dirty every so often, and then the crackling noise it makes as I adjust the volume is horrendous. When I get fed up with it (and I have time) I spray some contact cleaner onto it, and the noise goes away for a couple of years. I wonder if a dirty TPS would confuse the ECU, but if so, surely it not be an intermittant problem. Still, a shot of proper contact cleaner should take that possible problem off the list of things to try.

Mandrake, you may not believe you are an expert, but you certainly are learning (and are prepared to learn) about a Xantia V6. Yes, you are making occasional mistakes, but not twice. You are also helping others by posting regularly, and not editing to make yourself look good. You are getting there, making more progress than backward steps, and you have not given up. Experts don't give up (unless the problem is insoluable), although they know when to ask for help.

As I said on my previous posts, don't give up. You ARE getting there, and the feeling of satisfaction when the task is done (especially when it is a real sod) is worth the effort.

[Mandrake]

Hmm, starting the car yesterday I heard what sounded like the asymmetric beat of an obvious misfire for the first 30 seconds or so, despite there being no such sign of a misfire on startup just the day before that. Sigh.

So lets recap - compressions are fine, coil pack is new, spark plugs are new and still seem to be in good condition, spark plug leads are new and HOPEFULLY not damaged this time, fuel rail pressure was ok when I last measured it, although that was taken at a time when the car was running properly. Fuel filter is new as well.

What does that leave ? Sensor inputs like the crank sensor ? Far fetched ? Maybe not.

I don't know if any of you looked at that video I linked previously, if not skip straight to 12 minutes into the video to see a car with an intermittent misfire which turned out to be caused simply by the gap between the crank sensor and flywheel being adjusted too wide. You can clearly see the erratic pulses from the crank sensor which lead to erratic firing of the spark plugs despite the entire ignition system being fine. On that particular car it ran ok when cold but started playing up within 5 minutes or so of running. (The waveforms from the same car after the gap was fixed are shown at 22:40)

I don't have a fancy digital scope but I do have an analogue scope that I could "sniff" at the ignition wires where they leave the coil and look for any inconsistency in the timing or missing pulses. I could also make a makeshift inline spark tester as shown earlier in that video to check the basic "is it dropping sparks" premise.

Could I still have an ignition problem in the form of erratic crank sensor output ? Unfortunately I know its very hard to get at... can anyone tell me whether it is accessible enough to check and adjust the gap (if it comes to that) without major hassles ?

Something else for me to check anyway - its not difficult at all to lift the top plastic cover off the engine and connect an inline spark tester or sniff the leads with a scope probe.

[Ben82]

Semi Off Topic Didn't look at that video in particular Simon, but I've seen quite a few of ScannerDanner's videos, his equipment is amazing. To think they're not really an auto-workshop, but more for teaching about diagnostics etc. They (he) seems to love taking on cars that even dealers won't touch, or have been left unsolved by various shops.
Seems very common that the solution to the problem is really simple and not at all expected from the symptoms. e,g, there's a Toyota one where it has a new (scrappie) engine, and getting no comms with OBDII, dealer won't touch... actual problem? badly corroded ground on the new engine.

Obviously can't really help you with the crank sensor.