Valve Lapping?

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french

MB Enthusiast
Joined
Dec 3, 2006
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1,361
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Audi TT RS 2.5 , Zanussi Wd12: - LG Combi Multiwave - Swan 9 slice toaster with removable crumb tray
Just thought I'd ask for your thoughts.
My sisters car has a burnt exhaust valve, the garage have told her everything is straight forwards ...but they say they have to take out all the valves in the head & Lap them or else there will be compression imbalance?
Before she took it to the garage a RAC breakdown mechanic had compression checked all cylinders , obviously the burnt valve cylinder was zero but he said all the others were factory spec?

I just want know if this standard procedure .
 
If all the cylinders but one are within spec on compression the thought would be just to repair the one with low compression.

But engineering sense would say if you have gone to all the trouble of removing the head get them all done , and maybe a little skim of a mil or so while its off.
 
The valves are cooled by conduction of their heat out into the cylinder head via the contact patch on the valve seat. If one valve has burned out it’s probable that the seat is pitted. It’s good working practice to remove all the valves, inspect the seats and lap them in if necessary. It’s possible others aren’t far away from the same fate. You wouldn’t want to be doing the job again in a month or two. Even compression is a bit of a stretch, but their heart is in the right place.
 
There is no end to this . What kind of job is needed .Fast one is to get a few more miles out of the car Or for the car to last a lot more years . Take it to engine shop for full service on the head and let them do whats needed,. Or grind the seat with a special tool .. , lap it in , and just replace the one valve. If the head is -> flat, <- then you might get away with that alone. You need to find the reason the valve gave up in the first place ,could have been a faulty valve spring so then that spring will need replacing also.
 
What engine?
 
Looking at French Profile it could be anything from a washing machine , to a microwave oven, or even a toaster or a vacuum sweeper or even a spin drier .... who knows.
 
Wow ... It's many years since I needed to lap-in a valve.
Never needed to do it on an MB.
I would buy a valve and a valve spring, inspect the seat carefully to check for any pitting, then lap in the new valve.
I would then inspect the valve seat carefully with a magnifying glass and good light to make sure that any pitting is removed.
Then re-assemble using the new spring.
As for compression unbalance .......
I wouldn't worry about it.
If all the other cylinder compressions were "factory spec" - Then the newly-repaired cylinder will also be "factory spec".
Job done.
 
Wow ... It's many years since I needed to lap-in a valve.
Never needed to do it on an MB.
If it is the engine I suspect it is not only will it have 'Engineered by Mercedes-Benz' emblazoned on it, it will also take a lot more work to rectify than has been thus far suggested.
 
I like to know why things have gone wrong before deciding on the course of action.

The burnt valve (and hence lack of compression) is a symptom, not a cause.

As said, what is the car? Approximate mileage/value and how long is it being kept for?

These things often dictate the best course of action :)
 
Thankyou ALL for your input, brilliant.
OK yes sorry should have put in engine & the cause, its not a mercedes & one of the reasons I did not say the engine etc as I wrongly thought not as much input etc.

So the car had a new set of Spark plugs fitted & a piece of the porcelain around the electrode broke away, this caused the plug to short out around the cylinder causing it to melt part of the plugs electrode & the Exhaust valve. This is the diagnosis from the garage.
The car has covered 45095 miles & its a 2012 Audi TT RS ,
I think I will agree with Jobsworth that there heart is in the right place & that its good working practice.
I feel awkward as it was mine ,I sold it to her but had it serviced at a different garage beforehand...
 
This is why I hate selling cars to people I know.

Even if you know the car and all is okay, you always feel bad when stuff like this happens, and/or the buyer feels awkward about it towards the seller too.

It’s good you know the cause. I take it the bore etc is all okay?
 
So the car had a new set of Spark plugs fitted & a piece of the porcelain around the electrode broke away, this caused the plug to short out around the cylinder causing it to melt part of the plugs electrode & the Exhaust valve. This is the diagnosis from the garage
An interesting diagnosis ?? !
 
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This is why I hate selling cars to people I know.

Even if you know the car and all is okay, you always feel bad when stuff like this happens, and/or the buyer feels awkward about it towards the seller too.

It’s good you know the cause. I take it the bore etc is all okay?
Yes but I will pay for the work, she's only had it a few months. I don't mind but I am a bit wary of the final amount.
 
An interesting diagnosis ?? !
Yes John I agree, but the injectors have passed the spray pattern test ?
There is a small burn on the piston but we will have to wait to see if ita gone down the piston, the bore seems ok & has the crosshatching still there. But a concern to.me is it looks as though the melted valve chip has melted in a few small beads onto the crown of the piston, it resembles drops of solder on the tip.
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If it is the engine I suspect it is not only will it have 'Engineered by Mercedes-Benz' emblazoned on it, it will also take a lot more work to rectify than has been thus far suggested.
What engine were you thinking of Bellow?
 
Engine fitted to first gen smarts. Poor oil control rings led to burned valves circa 50,000 miles.

A Rice Krispies re-map?
It was remapped by previous owner at quite some cost then, new HPFP & cam roller etc also the DSG mapped , its not so much the Bhp its the very low down torque from around 1400rpm. By 1750Rpm is shoving 663Nm which makes it really lift up its skirts... my sister loves it, she descibes it like fairground ride.
It does pop a bit but not as much as my Cerbera.
 

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