smelly 16valve exhaust

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Mike E (uk)
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smelly 16valve exhaust

Post by Mike E (uk) »

Dear All,

I am returning to a long time problem with my BX.

It runs too rich.

I have replaced the plugs, AFM, fuel regulator, ECU, and checked for air leaks. Adjusting the pot on the AFM has little or no effect.

The car drives fine, and idles quite well, but the exhaust is smelly.
So smelly that you want to wash your clothes after checking the rear of the car when it is idling, or you have been in a traffic jam with the windows open.

I suspect it is only too rich when at idle, mpg seems normal, as does power output. Is passes the MOT too. Plugs look like mixture too rich.

I have made a similar request in the past for info and carried out all the suggestions bar one. That is to replace the fuel pipes between the tank and the motor, in case these are corroded and letting in air.

I intend to do this soon, and I have the parts ready.

I have had this problem for several years now, and I want a non smelly car for the BXagon.

I promise a reward for the winning answer!

thanks,

Mike
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Post by Kitch »

Would pipes letting air in not make it run lean? :shock:

Both my 16v's stink to high heaven, and my red one has never had an MOT issue with emissions.
The one thats been off the road for a long time really really hums too!

Just something I live with, after all I don't have to sit behind it!
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Post by classic2cv »

We go to a few CCC rallies

The guys who own D series cars are careful to not leave them standing and idling next to the tents

(you can smell the petrol on start up even when warm)

I guess that modern cars are so much cleaner than cars made less than 20 years ago
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Mike E (uk)
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Post by Mike E (uk) »

I know modern cars have a much cleaner exhaust, but my 1990 Cavalier SRi never smelt bad like my 1992 BX does, even though it had done a higher mileage.

There is definately something wrong.

I went to a Bosch centre, but they were useless- they could only test engines with OBD. They had not the expertise - the computer has to tell them what to do.

Mike
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Post by Way2go »

:? You've already changed the regulator on the fuel rail then? For it to be rich then the injectors have to be passing too much fuel and they are commanded by the ECU which takes it's info from the AFM, tdc sensor etc which you've already paid attention to.

The fuel pump is also only supposed to operate under the control of the ecu so if this is working continuously then possibly you'll get excess pressure as maybe pumping harder than the regulator can relieve?

Easy to check this by removing the input hose off the injector rail and putting it into a collection jar. With the ignition on there should be no petrol pumped into the jar, turning to the starter position you should get petrol pumped which also should stop when you release the starter to normal ignition position.

(As you are aware this test should only be done with a cold engine and you need to depressurise the hose before removing)

Hope this is of some help.

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Re: smelly 16valve exhaust

Post by DavidRutherford »

Mike E (uk) wrote:Adjusting the pot on the AFM has little or no effect.
What type of AFM does a 16v have? I've little experience of them, but I do understand most Fi systems.

If it's the bosch flap valve type that I suspect it is, you have to twiddle the idle mixture screw a fair bit to make a difference. Regardless of the system, you should always be able to adjust the idle mixture to something sensible.

Also... what is your idle RPM? If it's too low, the only way the engine can be kept running will be with a rich(er) mixture.
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Re: smelly 16valve exhaust

Post by Kitch »

DavidRutherford wrote:
Mike E (uk) wrote:Adjusting the pot on the AFM has little or no effect.
What type of AFM does a 16v have? I've little experience of them, but I do understand most Fi systems.

If it's the bosch flap valve type that I suspect it is, you have to twiddle the idle mixture screw a fair bit to make a difference. Regardless of the system, you should always be able to adjust the idle mixture to something sensible.

Also... what is your idle RPM? If it's too low, the only way the engine can be kept running will be with a rich(er) mixture.
You're right with the guess...its the Bosch flap type with adjustment screw on it.
Even when my valver was running lean and reading very low on the probe, it still stank!

They just stink! :lol:
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Post by Mike E (uk) »

The idle speed is correct, and not adjustable on the Motronic system.

Perhaps the ECU thinks the engine is cold and is stuck in 'warm up'

I did check the temperature sensor ages ago, perhaps I should measure it again or replace it with a resistor to check.

At a BX16V meeting a couple of years ago, mine was definately the most smelly!

Mike
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Post by mat_fenwick »

It does sound like there is a real problem - as Mike said to start with the plugs show a rich mixture. Surely they can't all do that?
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Post by DavidRutherford »

If the emissions pass an MOT test, it can't be very rich at all, as that would be an instant fail.

I suspect it's running towards the rich end of the acceptable range, so leaning off the idle mixture will bring it in line with other 16v's, IE, similarly smelly, rather than moreso than others.
Last edited by DavidRutherford on Tue Mar 03, 2009 9:01 pm, edited 1 time in total.
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Post by Mike E (uk) »

Similarly smelly would be an real improvement.

If I adjust the mixture control until the engine idles 'lumpy' ie speeds up and slows down- does that mean it is too lean?

I need to MOT it soon, so I will have a chat to the guy with the exhaust gas analyser, perhaps I can set it up better with that.

Mike
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Post by Culasse »

Sounds an odd one Mike.
The exhaust smells but the CO is ok at tickover. That suggests that the mixture is in fact not too rich.
What value do you get for the CO. 1.5% is a good lean value. >4% is too rich really.
Do you have a pot to adjust the mixture in the AFM. If so then screwing it one turn in alters the CO by +0.66% on my 8vGTi.

But if the CO is correct and HCs within limits then the mixture at tickover is correct. It must be something else. Although the CO measurement is an average reading over time so it could be fluctuating between rich and lean (doubtfull)

A few possible candidates : pulsation damper, oil (valve guides and/or stem seals), carbon build up on inlet valves, no cat so it cant be that, petrol filter, a smell of petrol from a leak while the system is pressurised, a leak in return pipe to tank.
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Post by Mike E (uk) »

I need to get the CO re-measured, I will do that at the MOT.

The smell is not simply petrol fumes, rather unburnt petrol I suppose.

My 8V GTI never smelt this bad, and when its valve guide oil seals were shot, the blue smoke was very distinctive in appearance and smell, so it is not burning crankcase oil.
Also, the car uses very little oil between changes.

I removed the pulsation damper from my GTI when it leaked, and I never could tell the difference afterwards. Can the damper alter the emissions?

thanks,

Mike
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Post by prm »

Mike

A few pointers you could try that may solve the problem as youve replaced the obvious components. They can be, rather smelly.

The mixture control on the AFM should alter the CO reading by approx 0.5% per turn. If the fails to regulate whilst turning the adjustment screw, it may have become detached from the screw spindle inside the AFM.

Maybe the CTS or ATS are giving the ECU incorrect readings by increasing the fuel delivery with a higher (colder) resistance. Never had any really wayward units, only with the ATS solder connection on the printed circuit wiper board. Both temp switches have approximately the same resistance readings at a similar temperature. You can test these by removing the AFM and CTS. Place the CTS suspended across the outlet of the AFM, wedge open the main flap valve and direct a hair dryer at a reasonable distance into the inlet side. The ATS is more responsive, as this has an exposed element, both resistance readings should be similar after a short period of time.

If the rpm is oscillating on tick over you’ve most probably got a mixture misbalance between the ECU at the initial throttle stop settings. Usually this won’t show up on a quick (MOT) CO measurement as the ECU is constantly reducing the fuel supply to lower the rpm- 850-900, thus reducing the emissions.
Might be worth checking the TB stop setting.

I use a 2-3inch length of 5amp fuse wire super glued to a cocktail stick (eye sight is not that good!!!) to set the stop screw. Back off the screw, say half a turn, raise the main valve, insert the wire to the lower edge and gently lower the valve. Adjust the stop screw to very gently, just withdraw the wire. Add a ¼ turn at first, to obtain a engine start, then adjust to a max of 3/4 turn to obtain an even balanced tick over coupled with adjustments to the AFM mixture screw.

I’ve managed to achieve reasonably sweet smelling 16v’s, (I never had a good nose anyway).
Always put the smell problem down to the design of the inlet manifold and the crankcase ventilation system, tied into the early Motronic units, with the injectors all firing simultaneously.

Probe the plenum/inlet manifold via the fuel rail vacuum pipe, or the blanking plug to see if there is any large build-up of black gunk in the lower section, as this with effect the tick over mixture.

Change of engine oil brand???? – Perfumed!!!

Check and clean the oil filler tube top gauze, also the one under the filler neck, where it reduces down. Also the condition of all crankcase breather hoses. The section under the plastic filler neck to crankcase, collapse internally or flattens out.

Hope this may help with the problem??? And possibly??? claim your reward of --??? Mars bar and a packet of Walkers crisps!!!!!!!!!!!

Regards
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Post by Way2go »

prm wrote:
Change of engine oil brand???? – Perfumed!!!
Favourite of old motorcycle days was to use Castrol R. :)

(or cheat by putting a spoonful in with the petrol to give people the impression you were using it! :wink: )
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