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

[addo]

Keep up you two, the answers to both your questions are in my second to last post! (21:43)

I think (Lev) 25:46 is more the key; you are being kept a slave forever.

[Mandrake]

I think that the discolouration on the spark plug electrodes is ash, not carbon, likely to be from combustion of oil fumes, probably ingested via the crancase ventilation system, so no cause for concern.

Ok that's good I suppose. There is a noticeable amount of oil in the air intake manifold and piping so that would probably explain it. The car also is driven infrequently nowadays maybe only 2-3 times a week for mostly short trips so it hasn't had a really good run in a while. (I'm reluctant to drive long distances with a potential un-diagnosed misfire anyway as I don't want to cook my cat if its not already...)

Have you replaced the fuel pressure regulator yet? Low pressure with the engine stopped can't really be caused by anything other than a faulty regulator.

No, I haven't replaced it yet, before I did so I would want to do a proper injector balance test because low fuel pressure from the regulator might turn out to be a secondary issue. (It's only 10-12% lower than stated figures)

If an injector balance test comes out ok I'd then focus my attention on the pressure regulator.

Any further tests will have to wait for next month though...after a few unexpected expenses this month (of which a new tire was only a small proportion) I don't even have the money to buy a bottle of injector cleaner until next month and would probably be killed if I tried..

[Mandrake]

Forgot to mention this previously, but when I was working on the car on Sunday I noticed an odd thing - the IAC (idle air control valve) on the side of the throttle body makes quite a high pitched buzzing noise when the key is turned on but the engine isn't running.

Not just a noise either - if you touch the body of the valve you can feel it vibrating quite strongly....is this normal ? It's hard to tell if its doing it while the engine is running as there is too much other noise and vibration...

[CitroJim]

Simon, that's entirely normal...

The ICV contains a solenoid and a weighted piston. It's default position is closed with the piston resting under it's own weight at the bottom of the cylinder. The piston rises in the cylinder under the action of a PWM (Pulse Width Modulated) signal from the ECU which accounts for the hum and vibration...

Do a Lexis Actuator Test on it and you can hear it rising and falling...

[Mandrake]

Ah ok, so its a PWM solenoid then, much like those in an auto gearbox or even the hydractive electrovalves...

I thought it was a stepper motor which was why I was wondering why it was vibrating so vigorously at rest!

[Mandrake]

Silly question time again

Now that I'm suspicious of a misfire that could be due to a fuelling problem (either a blocked injector or low fuel rail pressure) one technique I've read about often for distinguishing between a spark related misfire and a fuel related misfire is to take the air filter box off and inject some propane gas into the inlet manifold while the engine is running - on the basis that a spark related misfire will not show any improvement, while a fuel related misfire like a faulty injector will show symptoms go away temporarily.

My actual question is whether a butane/propane mixture as used for a small gas torch is a suitable and safe alternative to pure propane for this test, or am I just going to set my eyebrows (or the entire engine bay) on fire.

I have a decent small portable gas torch that runs off a butane/propane mixture with an adjustable valve that would be ideal for injecting a controlled amount of the gas into the air intake which should in theory allow me to very quickly determine whether a misfire is fuel or spark related.

Anyone tried this ?

[CitroJim]

Interesting on using propane Simon. I can see how it might work but have never tried it I must confess...

Be careful though as the old LPG system using a single-point mixer that looked like a gas ring were prone to backfires and blowing plastic inlet manifolds and airboxes to bits. You're almost duplicating a single-point mixer in what you're doing...

I'd try a gas blowtorch to begin with but do take huge care!

[Mandrake]

It's a technique shown a few times on those diagnostic videos by "Scannerdanner" and I've seen it elsewhere quite a few times as well.

No need to worry about plastic intake manifolds being blown up in this case - I would remove the air filter box completely and the rest of the intake manifold on the V6 is alloy.

I'd inject it directly into the supplementary air hose that goes to the idle control valve since the main butterfly is completely closed at idle. The blowtorch has a progressive valve so I could carefully control it. (I don't want to blow myself up either!)

Worth a shot I think, and might be a very quick and easy test to help me tell if I'm going in the right direction suspecting fuelling.

I've found an exciter that I can use for doing a proper pressure drop balance test on the injectors but that will have to wait until next month when I'm paid...

[Ben82]

Simon, you piqued my interest to find one of those videos from ScannerDanner, and seemed to stumble across the answer to your question here (scroll down below the video for the "linked comment" thread).

[Mandrake]

Thanks for spotting that, looks like a propane/butane mixture should be fine then.

[Mandrake]

Ho hum.

Nearly a week after being told the wheel balancing machine wasn't working and not hearing anything further I chased them up today - now the story is that they don't have the necessary adaptor for their wheel balance machine for center-less rims, and that they have one on order, but don't know how long it will take! If they'd just told me this in the first place I would have gone elsewhere.

If they have genuinely ordered the adaptor where do I stand ? Should I sit tight a few more days give them a chance to rectify the situation ? Or should I try to get a partial refund equal to a wheel balance cost and try my luck elsewhere ? Bearing in mind that most other tyre shops won't even attempt to balance a center-less wheel (two others turned me away when I had the puncture) and National took 3 tries to get it right when they fitted the original tires...(and are a 20 mile round trip away)

Sigh. Luckily I do very little 60mph+ driving so in all honesty I can wait a bit longer, but its the principle of the thing...

[Mandrake]

A bit of an update on where I am with my engine diagnostics.

It's pay day in a couple of days so I intend to order a hand held injector exciter/pulser like this:

http://www.ebay.co.uk/itm/181022815272? ... 1438.l2648" onclick="window.open(this.href);return false;

This will allow me to do an accurate injector balance test with my fuel pressure gauge and once and for all identify or rule out an injector issue. I have a suspicion that the injector on cylinder one has a flow rate problem or may not even be injecting at all, given that the spark plug in cylinder 1 was unusually clean compared to the other two front ones. In the process I'll double check there are no faults in the wiring harness between the ECU and injectors by running the tester via the ECU socket as well as directly, and also check continuity while wiggling the loom. (Which I haven't done yet...)

During wide open throttle below 2500rpm the Lexia is still reporting lean from the O2 sensor which means either low fuel pressure under those conditions (difficult to measure while driving...) or a misfire, my current thinking is its a lean misfire in one cylinder due to an injector flow issue and not not a spark problem, as the spark seems fine with my in-line spark tester. (not a conclusive test though, as the spark to the front cylinders is hard to test)

A couple of days ago I put some Redex injector cleaner in half a tank of petrol and there is quite a noticeable difference within just a few miles, the car is a lot more zippy above 2500rpm or so but is still struggling and hesitating at lower rpm. Some change in the right direction but definitely not fixed.

I haven't had a chance to do my propane/butane test yet, I'll try that to help confirm whether its a fuelling problem, although if an injector balance test shows problems it may not be necessary to do this test. Another thing I could use it for is as a test to see how quickly the O2 sensor responds to a sudden mixture change.

Other things to check - voltage at the fuel pump terminals including when all the lights are on! (in case there are any bad earths causing reduced voltage to the pump when lights are on)

Finally one other promising avenue to follow up is to thoroughly test the throttle position sensor. Although I've checked it a few times with the Lexia I have only paid attention to the throttle opening percentage figure calculated by the ECU, NOT the voltage reading. I was watching the following video (guess who! ) and realised I had not tested the TPS thoroughly and that in fact some of my symptoms clearly point towards a dodgy TPS:

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The failure mode demonstrated in that video is the wiper of the potentiometer intermittently lifting off the track, mostly around the minimum throttle end of the range. Normal voltage range (for that car) is about 0.5v to 4.7v, when moved slowly it works normally but when the throttle snaps shut the wiper contact goes open circuit dropping the voltage to zero, which then remains at zero as the throttle is opened to about 30% or so. It's not enough to check the voltage with a digital reading, it really needs testing on a scope as the throttle is manually operated slowly and quickly snapped open and shut to reveal any intermittent problems.

On the Xantia the closed throttle reading from memory is around 1.2 volts. Lets think about what would happen if the wiper arm is going intermittently open circuit in the bottom 30% or so of its range, especially after the throttle snaps shut.

The ECU would over time "learn" that 1.2 volts is a closed throttle and ~4.7 volts corresponds to fully open throttle, and throttle opening percentage is then calculated based on that range. Throttle opening percentage is then used (among other things) to help calculate fuelling, and help the gearbox decide when to change down under throttle.

If the throttle snaps closed and the voltage suddenly drops to zero instead of 1.2v the ECU is now presented with a "less than idle" throttle opening voltage. I notice sometimes if I snap the throttle and then let it return to idle it tries to stall, then surges, tries to stall etc 3-4 times before it recovers and stabilizes, it was doing it tonight in fact.

What I believe is happening is that the TPS wiper has gone open circuit after the throttle snap signalling zero volts to the ECU, the ECU reduces the fuelling below the normal idle amount, the engine tries to stall, the ECU detects the unusually low rpm and commands the idle control valve to open to catch it just in time before it stalls causing a surge in the rpm to above normal idle. It then tries to close the idle valve back to normal which then tries to stall again etc.

I think what causes the idle to stabilize after a few attempts at stalling is the ECU re-learns the voltage range for the TPS - it now decides that zero volts is in fact closed throttle instead of 1.2 volts, and recalibrates itself based on this, thus the idle stabilizes. However now the calibration is wrong and it won't respond to the throttle correctly.

If I open the throttle a small amount in these conditions then instead of rising the rpm actually drops and tries to stall again, (and will keep surging if I hold it there) its not until I have the throttle open to about 20% or more that the engine suddenly revs up to 1500-2000rpm. What I think is happening here is that when I open the throttle a small amount the butterfly opens and starts to allow more air in but the TPS reading sits stubbornly on zero volts so the ECU doesn't increase the fuelling, at least at first - too much air, not enough fuel, the engine tries to stall. It's not until the throttle is opened beyond a certain point around 20% where the voltage reading suddenly jumps up to what it should be.

So you end up with an intermittent dead-zone at the bottom of the throttle range which could potentially be the cause of the intermittent hesitation I'm seeing, also if the TPS is going open circuit sometimes (a wiper which has lost spring tension for example) then it may also cause throttle lag as the fast response of the TPS to throttle opening is necessary to quickly increase the fuelling. (accelerator pump type action)

Another symptom that it fits is when there is a lack of power (exhaust boom but no omph when opening the throttle) the gearbox is also very reluctant to change down a gear despite a large throttle opening, an incorrect reading from the TPS would also explain that as that is how the gearbox knows how wide the throttle is. If the ECU has recalibrated itself to think zero volts is closed throttle instead of 1.2 volts, you'll have to push the throttle much further for it to register a certain throttle opening, (due to the dead-zone) thus the kick-down points will be too far down on the throttle opening.

On the occasions when the engine is running really well its quite eager to kick down with throttle opening and it most definitely is not when there is a lack of power.

I'd be very surprised if I don't find an intermittent problem with the TPS, now that I know what I'm looking for. I'm going to test it with both a scope and a DVM to see if I can find any signs of the wiper going intermittently open circuit... I don't think its the major loss of power fault, but I think its very likely a secondary fault that is affecting idle stability, throttle sensitivity, gearbox kick-down and engine hesitation, as those symptoms have been there intermittently long before the loss of power started, especially the surging idle.

Also related to the TPS another video I was watching showed how a dirty throttle plate can cause problems with idle stability due to the carbon build-up around the throttle plate acting as an air tight seal. Small openings of the throttle plate which should result in an increase in air flow don't because the carbon "gasket" around the edge blocks the air flow until there is a significant angle... so the TPS (if working properly) would report a small opening of the throttle yet no more air is admitted... so I think it would be prudent of me to clean the throttle body around the throttle plate as well...

[Mandrake]

RIGHT!! Some more good progress this weekend. More things ruled out, and one definite (IMHO) fault found.

Despite a faulty TPS fitting the symptoms perfectly it turns out its not faulty, so I'm glad I didn't go ordering one before testing it...I back probed the TPS with both a scope and a DVM and tested it very carefully and it did not misbehave in the slightest. No sign of any noise or intermittent behaviour on the scope while turning the throttle, the closed throttle reading is 588mv, open throttle 4606mv, I snapped the throttle open and shut many times, it always returned to identical minimum and maximum settings, the wiper never went open, and the Lexia always agreed with the DVM within just a few millivolts. So the TPS gets a clean bill of health.

Next up was the throttle body which was pretty dirty around the throttle plate with oily carbon build-up. I cleaned around the throttle plate (which is very sharp by the way, as the cuts on my fingers show) and although I wasn't expecting any change in lost power I was hoping that it would make the idle more stable and get rid of the hesitation and tendency to try to stall when opening the throttle slightly - which it did.

Previously a small throttle opening of a few degrees would cause the idle to actually drop and then become unstable hunting up and down, requiring a significant amount of throttle (maybe 10% or more) before the engine revs would suddenly zoom up to 1500rpm - in other words it was impossible to hold the engine at a steady speed between idle and 1500. After cleaning the throttle body the idle is absolutely stable when hot and a very small throttle opening results in a smooth increase in idle rather than a drop and attempt at stalling.

My understanding of what causes this is that the oily carbon forms a seal around the throttle plate that completely blocks the air flow through the main throttle body when closed - when you open the throttle just a few degrees the oily carbon maintains a seal thus no extra air flows despite the TPS telling the ECU that the throttle is beginning to open. So effectively you have a "dead zone" for air flow rate in the first few degrees of opening, which does not track with the TPS reading - the ECU increases the injector pulse expecting more air to flow but more air doesn't flow, so the engine tries to stall from being over rich.

From what I've read there should be a small amount of deliberate air leakage around the throttle plate when closed to help ensure that the increase in air flow with small openings is smooth and progressive. It's easy to prove that there is in fact more air flow through the closed throttle plate than before by looking at the OCR Idle valve reading - previously a hot idle with no load (in neutral, A/C off, radiator fans off, cabin blower off) the idle valve was sitting on 35-36% to maintain idle, now it sits on 29-30% under the same conditions. Because more air is leaking past the clean throttle plate the idle bypass valve does not have to open as wide to maintain the same idle speed...

When driving the car the difference is very obvious for small throttle openings. While small initial throttle openings before felt flat with no real urgency, it now responds very quickly and decisively to small openings of the throttle. No change to the overall loss of power with wide openings but definitely more responsive for light throttle. For the sake of 10 minutes work with an old tooth brush, some paper towels and a bit of petrol its something well worth doing. (watch your fingers though...)

Although I've checked it before I took a video of the fuel rail pressure as I needed the fuel gauge for the injector balance tests. See below:



As I've said before, its a bit lower than spec (2.5 bars idle, 3 bars under full load) but does anyone think its low enough that it could cause an issue ?

Next I did my injector balance tests. The basic process is to run the fuel pump to top up the fuel rail pressure, turn the pump off, give the pressure a few seconds to stabilize then use an injector tester to fire one individual injector - in this case I think my tester was firing 50 times for 7ms each. The measured pressure drop was from 38 psi to 31psi for all 6 injectors, all were within 1/4 of a PSI of each other (pretty much the limit of accuracy of my gauge, less than a needle thickness difference) which is well within the acceptable range of no more than about 1psi from highest to lowest. So the injectors seem to be fine.

You can get at the connectors for the front injectors directly with just the plastic top cover off, however the rear injectors are hidden under the intake manifold. Fear not though, because as it happens the injectors are wired in parallel in front/rear bank pairs which means that you can get an electrical connection to the rear injectors through the plugs that you just disconnected from the front injectors! As long as you unplug the engine ECU that is, so your not back feeding voltage into that...

Next I wanted to put the oxygen sensor under more scrutiny, on several occasions I've observed that it seems to be somewhat sluggish, (but hard to prove with the low sample rate numeric output of the Lexia) so I back probed the oxygen sensor connector and connected it to my scope. Unfortunately not a fancy digital scope that is more suited for slow changing voltages, but I think it amply proves the point.

There are some variations in opinion on what is considered good output from an oxygen sensor but after doing a lot of reading and watching videos of oxygen sensors being tested (especially those of ScannerDanner ) the general consensus is that for most engines the oxygen sensor voltage should show a centre line high-low or low-high transition about once per second at idle (about 0.5Hz) rising to about 2 transitions per second above 2000 rpm (eg 1Hz)

Here is a video of the oxygen sensor voltage at both idle and with the engine running faster. Of particular interest is the second half of the video:


(ps kudos if you recognise the model of scope )

It's quite clear that the oxygen sensor is REALLY slow, and its slowness varies from time to time as well. At the beginning of the video it is around 3 seconds per transition which is already 3 times slower than it should be however near the end of the video its as slow as 6 seconds per transition! Very very slow indeed. With a response that slow the mixture will be varying wildly. Instead of rapid small deviations from the correct mixture it will be slowly swinging back and forth over a wide range, and is very likely biased in one direction as well.

Another weird symptom I started to notice a few months back was a hunting engine speed in the 1500rpm range, the following video shows this engine rpm hunting with constant throttle tracking the oxygen sensor transitions: (you'll need sound to hear the rpm variation)



So it looks like a faulty oxygen sensor could be a big part of my problem. Not only does the very slow response time of the sensor prevent the ECU from regulating the mixture accurately, most failing oxygen sensors apparently give an over-rich reading meaning that its probably causing the engine to run lean.

If the oxygen sensor is reading wrong over time the long term fuel trim values for the engine will get all screwed up especially if the sensor is a bit intermittent...in that case temporarily disconnecting the oxygen sensor may not by itself make the car run any better as it is working based on incorrect long term fuel trim data. What I decided to try was disconnect the oxygen sensor, but also leave the ECU unplugged for 15 minutes to reset all long term fuel trim values to factory defaults.

The result is that together with the throttle body cleaning all the weird things that I was seeing with unstable idle, a tendency to try to stall with the throttle being opened etc is gone. You can see in the following video before the oxygen sensor was disconnected two occasions where the engine tried to stall after revving - this no longer happens:



The engine is significantly more responsive with the oxygen sensor disconnected and the ECU reset however the wide open throttle performance still isn't what it should be, and still varies, so I think I do still need to check for a restricted exhaust.

While the oxygen sensor is definitely faulty I'm not convinced that it explains all the power loss and I would be remiss to not take the opportunity to measure the exhaust back pressure while the oxygen sensor is removed, so the next step is to order a new oxygen sensor and also get an adaptor pipe to connect my pressure gauge to the oxygen sensor port...

If the back pressure tests OK I'll just fit the new oxygen sensor and see how it goes, if there is a problem with back pressure I'll try dropping the connection between the cat and the centre silencer to see if the restriction is in the cat or further back, and decide what to do from there.

[addo]

Your fuel pressure is fine; at idle the ECU will compensate for any leaning out with a stretch on the injector timing.

You're still confusing WOT with loading for purposes of assessment elsewhere.

[Mandrake]

Your fuel pressure is fine; at idle the ECU will compensate for any leaning out with a stretch on the injector timing.

I agree - although the pressure is slightly lower than spec we're only talking 10% lower, there is almost certainly a lot more than 10% margin available in injector pulse width even for full load scenario's. So I don't think fuel pressure is an issue.

You're still confusing WOT with loading for purposes of assessment elsewhere.

Not sure which comment you're referring to here, as I didn't mention wide open throttle in the last post except to say that the wide open throttle performance (while driving) still isn't anywhere near what it should be even after the oxygen sensor disconnection and throttle body cleaning... the implication being that the cause for overall loss of power is still elsewhere.

I know you'll say "I told you so" but exhaust restriction is next on my list of things to check along with replacing the oxygen sensor, as the potential symptoms of exhaust restriction have got steadily worse and more persistent over the last few months. I came across the following post today which got me thinking again:

http://club-xm.com/forum/index.php?showtopic=4730" onclick="window.open(this.href);return false;

I'd forgotten that some silencers have wadding in them and that its possible for it to break up and even get blown down the pipe into the back box blocking it... so it could be a broken cat, sure, but it could also be a disintegrated silencer that has has blocked the flow downstream of the centre silencer...

One thing I have noticed a few times recently including while doing work over the weekend is that when first starting the car there is a good strong exhaust flow at idle which you can feel more than a foot away from the tailpipes, but then maybe 15 minutes later after the engine has been run and/or revved for a while the exhaust flow drops right down so that at idle you only really feel it weakly from a few inches away... now maybe its the reduced idle speed when the engine is hot, maybe not.