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

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

Came across this interesting article:

http://www.picoauto.com/tutorials/knock-sensor.html" onclick="window.open(this.href);return false;

Obviously it only applies to some engines and quite possibly not something as old as the ES9J4 but in the example not only does the knock sensor have an internal 560k resistor, the ECU does a power on diagnostic to test the sensor that is a bit more clever than just checking a bias voltage - it drives the sensor at the sensors resonant frequency (about 5.6Khz) in short bursts and measures the results.

A working piezo element would continue to resonate for a moment after the drive signal stops and I assume this is how the ECU decides the piezo is ok. So its mechanically testing that the piezo is working not just confirming wiring continuity via the shunt resistor. Clever :)

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

For what its worth I've measured my knock sensor at the ECU terminals (11 and 30) and I got 5 Meg Ohms.

Can't say I'm happy with that reading, if its supposed to have a resistor internally I'd be expecting it to be more like 560k, that's what other makes seem to use, or if it doesn't have a resistor I shouldn't be able to measure anything... :?

For example on most Subaru's the normal value is around 560k and anything over 1M is considered by the troubleshooting charts to be faulty.

I'd definitely be interested to see what reading others get. :)

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Post by addo »

5M is effectively OC; it's well possible the sensor has no shunt.

Have you tapped into the MAP feed with the pocket scope to graph its data when the car is running poorly?

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Re:

Post by Mandrake »

addo wrote:5M is effectively OC; it's well possible the sensor has no shunt.
If it was just a piezo with no shunt resistor it should be much higher than 5M as ceramic crystal is an excellent insulator. A known good reading from someone would definately help here. I might whip the air filter box out and measure it again at the green knock sensor connector to rule out a poor connection there.
Have you tapped into the MAP feed with the pocket scope to graph its data when the car is running poorly?
No I haven't, what would I be looking for ? A lower than usual reading or higher ? I have looked at the MAP sensor millibar figure on the Lexia under dynamic driving conditions however I really have no idea what reading to expect for a given rpm and throttle opening...it seems to behave in the way that I would guess it would (higher pressure at wider throttle and lower rpm) but without knowing specific figures I can't tell if its within spec or not.

I've checked the oxygen sensor reading under cruising and wide open throttle under load a few times recently and it suggests that there is no problem with fuel starvation, which is what I'd expect from poor MAP data.

I'm almost certain that the intermitent low power is ignition related, and fairly sure its rertarded timing related, however I have noticed in the last couple of months the odd random miss when idling, maybe one slight stumble every one to two seconds in an otherwise smooth idle, and it doesn't always do it depending on temperature.

As the coil pack, spark plug leads and spark plugs have all been replaced recently and there are no problems with oil in the spark plug wells or compression problems and injector balance checked out ok I'm at a bit of a loss as to where this random miss is coming from, especially at idle, when the idle was absolutely perfect and miss free a few months ago...I'm starting to wonder if my crank sensor waveform is slightly ropey...another thing to check on the scope.

Whatever the cause, everything is pointing to an ignition related problem, whether its on the sensor input side or on the ignition output side I'm not certain.

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Post by addo »

Vacuum should diminish as the motor struggles, the reading will thus rise. Would be interesting to see if it's a sudden flatline or a ragged pattern.

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

Are you suggesting that actual vacuum could be changing when the performance goes bad, or just that the MAP sensor reading is going bad ? It's a new MAP sensor remember...

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Post by addo »

I'm suggesting the vacuum reading will change, if the sensor is working correctly.

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CitroJim
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by CitroJim »

Simon, I measured the knock sensor on my XM and I get the same as you - 5megs...

Chances are then that this is normal...

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

CitroJim wrote:Simon, I measured the knock sensor on my XM and I get the same as you - 5megs...

Chances are then that this is normal...
Bother! :twisted:

Thanks for checking Jim, that gives me something to think about...

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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by larppaxyz »

Mandrake wrote:
CitroJim wrote:Simon, just one question. I know you have added a can of snake oil to the oil but have you actually changed the oil and filter yet?

If you haven't then do as soon as possible and then report back :wink:
No I haven't changed the oil and filter yet but will shortly. :wink:
My engine has 155000 miles on it and there is absolutely no noise coming from valves. Like said here before, you should change your oil. I do it when i can hear my valves when starting up cold, around once a year.

I'm pretty sure now that all issues i had with my car (for two years) were/are knock sensor related.

edit : i see you did your oil already :)

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

Yes I've changed the oil and filter a few days ago :) There is still a small amount of valve lifter noise on one or two valves though. There is also a very faint noise that I can only describe as a "metallic whine" at higher rpm around 2500-3500 rpm which I didn't hear before the oil change. So far I have been unable to identify the source of the noise. :?

So are you saying that your engine has performed flawlessly since you checked the knock sensor connector ?

Did you see my recent posts about measuring the resistance of the knock sensor from the main engine ECU connector ? Are you able to measure yours and see if you also measure 5 Meg Ohms ?

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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

Change of diagnostic direction guys! :) Well not exactly a change of direction, but revisiting something that I thought I had already ruled out a while ago...

Today has been sunny so being off work I was out tinkering on the car and made a bit of progress. I was trying to eliminate as much as possible the knock sensor so even though the ohm reading at the ECU connector was the same as Jim's out came the air box so I could inspect the green knock sensor connector again. Last time I looked at it I just gave it a squirt of contact cleaner and that was all...

This time I was more thorough. I checked the continuity of all three wires from the harness side connector back to the ECU connector - all below 0.5 ohms and nothing intermittent about them when wiggling the plugs/cabling etc. I then checked the reading of the knock sensor at the connector - slightly less than 5 Meg Ohm, and it too didn't change when wiggling the cable and connector. I sanded the male contacts just slightly, tensioned the female contacts slightly, used some more contact cleaner and reassembled it. Of course none of this would test for a broken shield wire on the knock sensor cable but short of stripping down the engine there is no way to test that...

I then went for a test drive and....nothing. No change at all. Still intermittent hesitation, the performance sometimes bad, sometimes good, but never great. Changing from good to bad and back again in just seconds as the driving conditions, selected gear, rpm and throttle opening changed. Feeling a bit deflated I glanced at some of the live sensor data and noticed something - during the lack of power and hesitation, under heavy throttle below about 2500-3000 rpm the oxygen sensor was reading constant lean for as long as I kept the throttle down. Wait, what ? :shock:

Those of you who have studied engine management systems in some detail will know that while the ECU will try to maintain a stoichiometric mixture in closed loop mode, which occurs during idle, cruise, and light acceleration, on a narrowband oxygen sensor system as soon as you go into heavy acceleration at any RPM it switches to open loop where it will deliberately fuel the engine rich (based on pre-computed map tables with corrections from long term fuel trim added) to maximize performance and minimize risk of knocking.

The exhaust should never be lean under heavy acceleration it should stay rich, the only time it should stay lean is during closed throttle overrun while coasting when the injectors cut off completely. The thing is I had observed this happening before a few months ago but thought I had resolved it, apparently not, or it has always been intermitent.

This got me thinking about the knock activity in a new light - lean under heavy throttle can only be due to fuel starvation, such as a faulty fuel pump etc... Lets say that there is a fuel delivery problem, the pump can achieve the regulated pressure with low delivery but not high, and lets say its a bit intermitent as well.

You put your foot down to accelerate, the vacuum diaphgram on the fuel pressure regulator should cause an instant increase in fuel rail pressure to match the increase in mainfold pressure but if the delivery of the pump is low the pressure will droop and cause a flat spot while the pressure recovers. If the delivery is really low then maybe the pressure doesn't recover at all under load and the engine then runs lean as long as the demand remains.

What does an engine under heavy load do when it runs lean, and its timed as far advanced as it can be ? It knocks of course! The lean mixture CAUSES the engine to knock/ping under conditions where it otherwise wouldn't. Even though I'm running on 99 octane a lean mixture can still cause it to knock, although not as badly as it would on 95. Running on 99 is allowing it to run on a leaner mixture without complaining as much.

Not only do you loose power from the lean mixture and potentially lean misfires occuring, when the ECU hears the knocking it retards the timing which reduces the knocking but causes further power loss! It takes a second or two for the timing to be retarded to the maximum possible and 5-10 seconds for it to be advanced again after the knocking subsides.

These time delays roughly match the delays I'm seeing where the performance sags under load and then recovers over a few seconds under a lighter load.

In other words yes the ECU is retarding the timing due to knock sensor activity, (I saw some of that happening today on the Lexia too) but its real knocking caused by running lean under load not a faulty sensor! #-o

Ok what to do about it then...One thing I haven't checked before is the voltage at the fuel pump, so I ripped up the seat cover and pulled out the bung for the pump. At the top side of the connector there is 12.75 volts while idling (seems a bit low but not excessively) however there could still be a voltage drop across the connector so I unplugged it and gave it the same contact tweaking and cleaning treatment as the knock sensor and went for a drive.

Was there an improvement ? YES, there was! Not cured because it still swings lean and stays lean under wide throttle below 2500 rpm... but acceleration off the mark was a lot better, it even spun the front wheels on take off in 1st in the dry for the first time in a long time.

The connector didn't look bad so I suspect that its not the real problem, but that what I did may have reduced a small voltage drop allowing the pump to eek out slightly more delivery.

So what could my fuel starvation be ? It isn't the fuel filter because that's already been replaced and didn't make any difference. I think I've ruled out the voltage to the pump - 12.75v is a wee bit lower than I'd like but the wiring is quite thin so it could well be the normal voltage drop for that wiring. (Someone want to measure theirs ?)

Could there be a blocked fuel strainer on the bottom of the pump ? I've read about them getting badly blocked on diesels due to poor biodiesel, but I'm not sure whether its something that would happen with petrol ? The pump doesn't look easy to remove, how hard is it to lift it out and clean the filter ?

Faulty pump maybe ? The trouble is they're not cheap, and I can't see a good way to conclusively prove that the pump has poor or intermittent delivery with the facilities I have, and I'm not willing to replace it on a hunch.

Intermittent fuel pressure regulator ? Normally I couldn't see how the regulator could cause poor delivery as such, but it could certainly cause intermittent fuel pressure if there was a tiny crack in the diaphragm, and that problem may come and go intermittently with throttle changes due to the change in vacuum flexing the diaphragm...the pressure on mine is also a bit low - 2.7 bars instead of 3.0.

I DO actually have another fuel pressure regulator on hand, Stempy very kindly donated a spare 2nd hand but known good regulator he had off his V6 which he no longer has need for (much appreciated!) so I guess the logical thing to do would be to replace the fuel pressure regulator first, now that I have one ?

Obviously its a manifold out job, I still have one spare 2nd hand good manifold gasket (that was only on for a few weeks) however once the top manifold is out, how is the regulator replaced, does it require further disassembly ?

By the way the gearbox has settled down nicely now, I think it was a mistake to do the auto-adaptive reset, I should have just left it alone after putting in the additive. It's not as-new by any means, but there are no obvious signs of trouble at the moment, its quiet, doesn't shudder, gear changes are smooth, no banging into 1st from 2nd, kickdown changes are very quick but smooth, no sign of slipping or rpm flare up during up-shifts, basically it seems to be working normally. :)

So I'm very happy with the improvement in the gearbox, now if only I can nail this engine problem...

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CitroJim
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by CitroJim »

Simon, that is very interesting indeed...

The pump should be able to deliver vastly more fuel than needed. generally pumps are eitehr good or not good...

Time now try to prove things by rigging up a fuel pressure gauge. Easily teed off the fuel rail at the Schrader valve under the blue cap.

It's not good to see when in motion but an electronic pressure sensor and a voltmeter might be quite easy to rig up so you can see dynamic pressure as you drive along...

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

So does the pump have a strainer on its inlet that might get clogged ? And can it be removed from the tank and refitted without a huge amount of trouble ?

I do have a proper (mechanical) fuel pressure gauge but as you imply its not possible to use it while the car is being driven...so while the fuel pressure seemed ok while stationary, snapping the throttle etc, I couldn't test it under prolonged wide throttle conditions. I had thought about some sort of electronic pressure gauge with a remote readout last time I was investigating the fuel pressure, but didn't get around to finding something suitable.

I was just out again in the car picking someone up and at least half the time it was going like a rocket, very zippy, the other half of the time it was its usual mediocre self, sometimes with only 10 seconds separating the good from the bad.

A second thing I checked after the fuel pump connector was on a hunch I blocked off the connection between the canister purge solenoid and the throttle body, just in case there was an air leak on one of the lines to the purge solenoid. I tried this because according to the Lexia the solenoid fully opens with more than about 1/3 throttle, normally it draws in a little bit of fuel vapour from the canister which richens the mixture (which the ECU compensates for with shorter injector times) but if there was a vacuum leak between the canister and the purge solenoid it would actually cause a lean condition when the solenoid opens instead...

Hard to say if that has made a difference as the performance still comes and goes, but the peak performance when its good is definitely a lot higher since cleaning the fuel pump connector and blocking the purge solenoid line...

If its not too difficult to change I'm tempted to just go ahead and swap the fuel pressure regulator over, a cracked diaphragm could certainly explain intermittent symptoms, it may not necessarily be causing low fuel pressure but sometimes a cracked diaphragm will let fuel suck through the crack into the intake manifold intermittently causing a rich condition. If the ECU compensates for the rich condition while the crack is leaking fuel then it stops leaking then you end up with lean... and a confused ECU that doesn't understand why it has to keep going between adding and removing extra fuel in the fuel trim!

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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by CitroJim »

Simon, yes there is a strainer on the pump intake. It's plastic and very fine - like a plastic sock...

Wrong time now but if you can wait a day or so I'll take a pic of one.