MOT tomorrow (wish me luck!)

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Post by addo »

I think he'd die of naughtiness, if he ran it that long.

Much as I like the "extra free flow cat" presently on my estate, I'm hankering for that clean exhaust smell again.

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Re: Re:

Post by Northern_Mike »

CitroJim wrote:
addo wrote:I'd make it a decat test pipe. For the price of a gasket kit and two clamps you could run it for a few days to see how the car goes.
Or run it between MOT tests...
You don't have enough space on your server for the thread it would create :P
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Post by addo »

When I was buying some stuff on Fleabay the other day, at the "Checkout" stage its little charity top-up window at page bottom asked if I wanted to donate to a poorly Citroën in Scotland...
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Re: MOT tomorrow (wish me luck!)

Post by CitroJim »

:rofl: at all of the above...
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Re: MOT tomorrow (wish me luck!)

Post by Mandrake »

C'mon guys, knock it off! :mrgreen:

I've checked with KP and the whole exhaust system is already spoken for anyway :(

I have some thoughts about the CO levels that I'll post about later when I've had more time to research and think about it.

BTW I just filled right up from down at warning light level, drove a few miles, switched off for 15 minutes while waiting to pick someone up, started it up and its absolutely flying, responsive with no throttle lag, no lack of power, although I'm sure it won't last.

Anyone have any thoughts on why that might be ? The next person who says blocked cat gets to shake hands with one of the HT leads... :twisted:
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Re: MOT tomorrow (wish me luck!)

Post by CitroJim »

Mandrake wrote: I've checked with KP and the whole exhaust system is already spoken for anyway :(
By you I thought Simon :?
Mandrake wrote:The next person who says blocked cat gets to shake hands with one of the HT leads... :twisted:
Excellent. I LOL'd a lot at that...

I'll be intersted to hear what you think Simon, about the high CO. but do remember that a system managed for a cat is not running at optimum AFR. It's a tad on the rich side deliberately to ensure the cat wiorks at top efficiency. Therefore with no cat (or a dead cat) the CO will be a tadette high..

I hope that's not enough to force me into an HT Handshake :)

One thing though I think was raised earlier; a modern engine running catless stinks to high heaven. Can't recall they did when catless was the norm. Must be something different about modern petrol perhaps..
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Re: MOT tomorrow (wish me luck!)

Post by Hell Razor5543 »

I wonder if there was some crud in the fuel that has got burned out, with the remnants being diluted with new fuel. Does a V6 Xantia have a filter on the fuel sender, and, if it does, how clean is it?
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Re: MOT tomorrow (wish me luck!)

Post by CitroJim »

Hell Razor5543 wrote: Does a V6 Xantia have a filter on the fuel sender, and, if it does, how clean is it?
Yes it does James and I'm pretty certain Simon has replaced it..
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Re: MOT tomorrow (wish me luck!)

Post by Mandrake »

CitroJim wrote:
Hell Razor5543 wrote: Does a V6 Xantia have a filter on the fuel sender, and, if it does, how clean is it?
Yes it does James and I'm pretty certain Simon has replaced it..
The fuel filter underneath the car I've replaced some months ago yes - no difference. The strainer on the input of the pump has not been inspected or cleaned as yet.

I mentioned the observed change when going from a near empty tank to full in relation to possible partial blockage of the strainer or the pump itself being a bit sick, on the theory that it might struggle more when the tank is near empty than when its full, although that depends on the design and layout of the pump and its inlet which I'm not familiar with at the moment.

Other possible clues that all is not well with the pump is that measured delivery is 1.2 litres/min (under a 3 bar load) versus the service manual quoted 2 litres/min, the fact that it has been intermittently fairly noisy recently with a metallic whine especially in hot weather, and the fact that I'm often seeing a lean oxygen sensor reading under wide throttle, which could either be fuel starvation, (causing a lean misfire) or an ignition related misfire. There are enough symptoms to at least be suspicious of the fuel pump, even though I don't think its likely to be the only problem.

There is definitely a misfire under load occurring when performance is poor, I just don't know whether its a lean misfire due to fuelling or an ignition related misfire - both will cause a lean reading on the oxygen sensor, so are hard to tell apart with the limited test facilities I have.

In any case I have dibs on the fuel pump from KP's breaker which I intend to inspect/clean and then swap over, that at least should help me eliminate whether there are fuel delivery related problems as the rest of the fuel system has already been tested - pressure regulator and injectors.
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Re: MOT tomorrow (wish me luck!)

Post by Hell Razor5543 »

What I meant was, is there a mesh filter on the fuel pickup like the one in a diesel tank? I know that this one can get clogged with micro organisms that will limit the flow to the engine.
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Re: MOT tomorrow (wish me luck!)

Post by Mandrake »

CitroJim wrote:
Mandrake wrote:The next person who says blocked cat gets to shake hands with one of the HT leads... :twisted:
Excellent. I LOL'd a lot at that...
Glad you liked it. :-D It helps for me to keep a sense of humour when facing these types of problems...
I'll be intersted to hear what you think Simon, about the high CO. but do remember that a system managed for a cat is not running at optimum AFR. It's a tad on the rich side deliberately to ensure the cat wiorks at top efficiency. Therefore with no cat (or a dead cat) the CO will be a tadette high..
That's not quite true - from what I've read a catalytic converter actually needs to be cycled slightly to both sides of stoichiometric to function properly - constantly too rich and it (after a few seconds) runs out of stored oxygen so it can't convert CO to CO2 or combust excess HC, constantly too lean and it can't convert NOx to N2.

What I'm thinking about the CO is that although the lambda of the tailpipe reading is correct and would normally result in low CO, that assumes that all the cylinders have equal correct mixtures...

As soon as you get a misfire on one cylinder, either a lean misfire or ignition misfire you dump a massive load of unburnt HC's and unburnt oxygen into the exhaust. The oxygen sensor can only detect the oxygen not the HC so it thinks the mixture is now too lean so the ECU increases the fuel trim until the exhaust stream has a neutral lambda as measured by the oxygen sensor.

Due to one misfiring cylinder and the closed feedback loop (there is only one fuel trim figure for all cylinders) the other 5 cylinders end up running far too rich so will produce a lot of CO. Theoretically the cat can burn the unburnt HC in the exhaust from the misfiring cylinder AND also convert the high CO from the other 5 cylinders to CO2 using the excess oxygen from the misfiring cylinder, but if its swamped with more than it can deal with then it will get saturated and output CO will go up...(it's ability to catalyse is not infinite) Maybe a new cat can cope with this for a while (until it cooks) but an old cat with lower efficiency may not cope.

A waste spark system throws a spanner in the works for a misfiring cylinder - a second spark fires about 10 - 30 degrees BTDC on the exhaust stroke, if you have a weak spark (insufficient voltage) that fails to ignite the mixture at the top of the compression stroke it may still ignite the mixture at the top of the exhaust stroke (due to the lower pressure) as its entering the exhaust header - causing a flame to race down the exhaust pipe and burn some of the un-burnt HC before it gets to the cat, however I bet the combustion would be incomplete in the exhaust pipe and cause high CO levels.

Another possibility is an exhaust leak between engine manifold and oxygen sensor - extra oxygen gets into the exhaust stream giving a false lean reading causing the ECU to crank up the fuelling until the engine is running very rich and producing a lot of CO (and running poorly) and perhaps too much CO for the cat to cope. Yet the lambda reading at the oxygen sensor and the tailpipe will be perfect because of the feedback loop "correcting" the mixture.

Basically what I'm saying is that as long as there is an engine running problem especially a misfire the emissions readings can't necessarily be trusted to condemn the cat. (And even if it is faulty a new one would be damaged due to the same running fault unless the fault is fixed first) Let's get the engine running properly first then retest the emissions.

I've seen a video of a car on an exhaust gas analyser where it was temporarily forced into a rich mixture and the CO shot up from 0.02% to over 1.75% in a matter of 2-3 seconds despite the fact the car had a warmed up cat. Obviously the cat couldn't cope with the high CO from the engine with the very rich mixture. Although my lambda is correct that's only averaged across all cylinders, it doesn't preclude the possibility of one or two cylinders misfiring and the rest running rich.

A couple of things that could be done to test the cat to see if it is still working is an oxygen storage test (unfortunately that requires a downstream oxygen sensor...) or using an infra-red temperature gun to see whether the output is hotter than the input.

One thing I've always noticed about this car is the exhaust system gets REALLY hot, I'm not sure if its the exhaust header/downpipe before the cat, the cat itself or both, but even after a very short drive the moment you switch the engine off there's a loud "tick tick tick" of the exhaust cooling down that goes on for a few minutes...and it still does it. Could it be the sound of the cat getting really hot working really hard trying to clean up un-burnt HC ?
I hope that's not enough to force me into an HT Handshake :)

One thing though I think was raised earlier; a modern engine running catless stinks to high heaven. Can't recall they did when catless was the norm. Must be something different about modern petrol perhaps..
Could be - I still remember the days when petrol was leaded and dyed pink in colour :lol: This modern lead free yellowy/green petrol definitely smells different and more smelly to me...
Last edited by Mandrake on 25 Jul 2013, 21:38, edited 3 times in total.
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Re: MOT tomorrow (wish me luck!)

Post by Mandrake »

Hell Razor5543 wrote:What I meant was, is there a mesh filter on the fuel pickup like the one in a diesel tank? I know that this one can get clogged with micro organisms that will limit the flow to the engine.
I believe there is one yes, and it needs checking at the very least. I'm going to swap the whole pump over though, mesh and all for a known good one, and of course clean the "new" one before fitting, if the mesh looks cleanable.
Simon

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Re: MOT tomorrow (wish me luck!)

Post by Hell Razor5543 »

Simon, for what it is worth I would suggest you only work on one major problem at a time. If the cat is trying to clean up the exhaust, you will get weird emissions if (as you say) you have spark or fuelling problems. Some of the problems may well be symptoms, but are throwing you off track.

You ARE getting there. You have managed to get another years' grace, and (with the way you persevere) she will keep going for a lot longer than that.

I have learned that, if a problem is too big to solve, break it down into manageable chunks (but you already do that). I also believe that there are (basically) two types of problems; those you can solve and those you cannot. With this attitude I then get on with what I can solve, and pass those I cannot to somebody who can (while asking if I can learn).
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Post by addo »

The preserving additives to maintain fuel quality while unused, are probably what smell.

The in tank strainer is small; I am surprised nobody has posted images of one yet.

Happy to shake hands with an HT lead on the basis that if I'm right Simon has to hang on for three times as long.
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Re: MOT tomorrow (wish me luck!)

Post by CitroJim »

As far as I recall the in-tank strainer is like a fine mesh sock....
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