Low compression, but valves don't look burnt :-(

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flanagaj
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Low compression, but valves don't look burnt :-(

Post by flanagaj » Sat Apr 13, 2019 6:38 pm

Hi all.

2006 Polo started misfiring when idling and the engine management light came on. I took it to a local garage who did a compression test and told me cylinder 3 was low on compression and that they had swapped the coils around as well, but it made no difference. They then told me that it was more than likely a burnt valve and it would be a minimum of £500 to replace the valve.

So to cut a long story short, I have stripped the engine down, but upon turning the head over and expecting to see said burnt valve they all look in perfect condition with no visible signs of damage :-( which has left me feeling rather depressed as I am now at a loss and with only 55k on the clock and full service I can't see it is a ring that has gone. Does anyone know whether there is a way to test whether it is a ring or a valve which has gone with the head off as I don't want to send the head off if there is nothing wrong with it?

For example, could i put oil into the cylinders and into the upturned head or is oil too viscous and would not detect an air leak issue?

Thanks in advance.

RUM4MO
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Re: Low compression, but valves don't look burnt :-(

Post by RUM4MO » Sun Apr 14, 2019 10:19 am

Now that you have the head off, why not remove all of the valves, making sure that you know which one came from where (a piece of cardboard with numbered holes punched in it) and check every valve/guide for slop/slack/wear when compared with others and what you consider is acceptable?

Also that would let you examine the valve/head touching areas.

Edit:- also check the static timing.

flanagaj
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Re: Low compression, but valves don't look burnt :-(

Post by flanagaj » Sun Apr 14, 2019 10:01 pm

Thanks for your post. I did a water test by turning the head over and filling the valve area of each cylinder with water and it became apparent that both 2 and 3 have a leak as water has drained through and was coming out through the exhaust manifold. Number 1 cylinder was fine.

Going to drop head off at local machine shop to sort the valve out as it is not a job I want to attempt myself.

One other question. I have cleaned the block using acetone and it has come up clean, but would you recommend sanding the block with wet and dry using an oil?

RUM4MO
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Re: Low compression, but valves don't look burnt :-(

Post by RUM4MO » Sun Apr 14, 2019 10:13 pm

I’d roughly check the block for flatness or lack of any high spots using something flat like a straight edge or steel rule in good condition - really just a rough quick check before fitting the head back on.

As you are taking that head to a machine shop for refurb, would it cost a huge extra lump of money to get all the exhaust valves/seats/guides checked fixed - that would save any future trouble.

flanagaj
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Re: Low compression, but valves don't look burnt :-(

Post by flanagaj » Tue Apr 16, 2019 6:52 am

Thanks for the tip regarding checking the block for flatness. I wasn't going to as it was a valve issue that seems to be the cause of the compression issue, but for the sake of 10 minutes it is a good idea.

Dropped head off at machine shop and he said the valve guides go on them so is skimming faces and doing the valve work too. £220 + VAT and that includes the head gasket, exhaust gasket, inlet manifold gasket and head bolts. Not sure if that is cheap or expensive, but the machine shop must buy a full kit when refurbing the head. Iv'e not heard of that before, but daughter needs her car back ready for school next week so a bit up against it.

RUM4MO
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Re: Low compression, but valves don't look burnt :-(

Post by RUM4MO » Tue Apr 16, 2019 10:14 am

I'd just be telling lies if I commented on the cost to refurb these heads, so it looks like that machine shop knows about these engines, which can only be good news, onwards and upwards!

flanagaj
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Re: Low compression, but valves don't look burnt :-(

Post by flanagaj » Tue Apr 16, 2019 10:56 am

The only nagging concern I have regarding refitting is whether an alloy block should have the threads chased or whether I can just oil up an old head bolt and make sure that it can be threaded in without resistance.

Just paranoid about refitting the new head only for a head gasket issue to emerge soon after.

RUM4MO
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Re: Low compression, but valves don't look burnt :-(

Post by RUM4MO » Tue Apr 16, 2019 2:36 pm

Now that you have mentioned it, it would be a very smart thing to do to clean up and check the threads a bit beyond where they have been down to in depth, but I'd not use an oiled old bolt, only a dry tap - I can't say what the thread is though.

If these bolts are the socket headed maybe female spline ones, 8-off, then they are M9 X 1.25 - so maybe buy a tap of that size and a tap wrench.

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