BMW 850 V12 – Emission Test Fail / Valve Timing or Ignition Timing?

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The clam of an early morning in the workshop, where the quiet of the day is only punctuated by crisp rustling of a newspaper and the polite munching of toast, suddenly changes with a call form an agitated potential client – can we help, his BMW 850 has failed its MOT (the UK car mechanical worthiness licence) and he is concerned because the British Police are cracking down on expired MOT’s.

Time to stop drinking my tea and instead look sharp – this 850 V12 needs our help (maybe I should have been wearing my ‘Y’ fronts over my overalls at this time).

When the car came in, it was evident by just listening to the back exits from the exhausts she was in trouble and some way off form getting any sort of MOT.

I wouldn’t call myself a musician but I can tell when something is out of tune – popping sounds from both banks with a strange ‘tangled up’ noise. Unfortunately, the best description I can manage at the moment, sorry (Pete Waterman I am not)!

Even more proof of the bad state of affairs came when we gas testing her – very high levels of hydrocarbons and Co2.

Better put the kettle on and adopt Michael Angelo’s “The Thinker” pose!

The obvious thought is the Lambda Sensors which should automatically check the cars emissions and make adjustments through the cars brain or DME / ECU.

The high hydrocarbons are the biggest concern as this is simply unburnt fuel – which will naturally effect the cars performance and economy – apologies, I used the “economy” word!!!!!!!!!!

This, like almost all of our work, is going to take some ‘elbow grease’ and potentially my circus contortionist skills.’

First thing, check spark plugs – yep even the hard ones to get out especially No. 6 & 12!!

Had a good look at the condition – they should be a nice light brown with the central electrode possibly being a little lighter.  The key thing I am looking for is no heavy black sooting.

On this patient, she had black sooting on all twelve – some very bad on cylinders 7 & 12.  In addition, they were wet.

All of our findings are looking bad so definitely time to change all spark plugs and start looking at all the spark leads, ensuring all caps to the plugs are in good shape. Next place to visit is the distributor caps and have a look inside at the rota arms.

More tea and a few chocolate biscuits required as we contemplate the next stage!

A quick look round the workshop shows that there are not too many tools out of my tool box – this is how I gauge how involved I am getting in the car!!! – Will have a look round later to assess – ‘toolidge’ use!!

Uuummmm  – The distributor caps, rotas and leads are all good and now put back together  – Sparks changed.  Time to restart the engine and listen (and ask one of the guys lift the engine revs up to about 2500 rpm)

‘DANGER Will Robinson’. The Health and Safety man would be having kittens by now, as lots of spinning things going on in the engine compartment and the opportunity to knock the car into gear go into the stratosphere!! – well most damage would be done to the WC’s and that tape has been dripping for years, it would be a good excuse to fix it.

That non musical ear of mine again listens out for that “down” pulse – the out of balance audible tone change.  Its still there! On this lady our down beat pulse seemed to be coming from both banks at the exits .

Ok – we will need more tools and more milk, this time with coffee plus cake. All in preparation for a bigger strip down as we extend our range of things to take off.

As we have dealt with the plugs, leads, distributor caps and rota arms, its time to change or swap the MAS air flow meters and the exhaust Lambdas – (just had a quick check of the fresh air intakes to ensure nothing is blocking them).

We checked the Lambdas on our diagnostic kit first and they showed all functioning correctly – as taking them out can mean removing the exhaust if they are stubborn.

The MAS air flow’s now changed with a set of second hand ones. Time to start the engine again – no change to exhaust pulse.

Like a doctor with his stethoscope, another quick exhaust emissions test – hydrocarbons were becoming worse on both banks – aaahhh.

Let’s start her up again and have another listen at idle revs only, this time focusing to try and find an air leak, listening for that sucking sound.

Decided that it was time for the vacuum tester. Plug this in to a bank at a time to establish if there is an imbalance with engine vacuum. Guess what!  There is our girl!!??x x

The vacuum drop was about 20% on the gauge between each bank – will now need to explore intake manifold gaskets.

Started her up again. When she was at idle speed, we squirted WD40 or carburettor cleaner around each intake mounting to the head and listened for any tone change (if there was we would have to explore taking the intake manifolds off and have a look at these gaskets).

Also checked associated gaskets to the intakes.  However, no change!!

So at this stage I decided not to remove the intakes as experience was telling me something else was happening.

Now this was becoming a really interesting challenge – “we are going to need a bigger cake”.

Time for some more quick checks – tested the Ohms for the top & bottom crank sensors (both were within correct range) and did a quick compression test – all pots good which was a relief for the customer!

After all of that excursion, time to sit down with a nice cup of tea and the cake of course.

As the cake took effect, I was I thinking to myself that we may have a valve timing issue or that the ignition timing could be out.

In order to check this out, we would have to roll the crank to the top dead centre position (TDC) and remove the distributor caps, rotas cam shaft rota arm carrier (both banks) and just have a look at the positioning of the drill holes in the carrier – they should be facing each other with about a 30% off set.

If my theory does not work out, we will need to remove intakes, rocker covers, fuel rails, injectors, engine wiring loom, then check the position of the camshafts at the TDC position.

Did all of the above and oh glory – both cams were off their marks. Now its time to set the valve timing.

Using our specially fabricated tool to lock the cams we were able to set her up correctly and then came the job of putting her back together and putting the tools away (a quick look around earlier had unsurprisingly shown high ‘toolidge’ use).

Another test before the customer is back and gone is the unburnt fuel, she is running fully balanced again, back to full performance and improved economy as she passes the emissions tests on our rig.

As soon as the client hears that exhaust note, he knows that she is back to full health.

Another happy customer and another beautiful V12 running smooth as the day she was born!

Kettle back on and where is that newspaper!

Chris. B

Latest Blog July 20th 2010

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