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polo 1.2 9N3 BZG cylinder compression figures


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gerards19

Hi, I think I may have an issue with the dredged exhaust vales leaking on my Polo 1.2 petrol  9N3, 1ith 186,000 on the clock.

I have just completed a compression test and the figures obtained are as follows:

 

 

Cylinder 1 - 230 & 243 psi

Cylinder 2 - 210 & 220 psi

Cylinder 3 - 200 & 205 psi

 

 

I understand that there should be only a 10% difference between the highest and lowest figures and mine has 14%  to 17% difference.

The engine is running fine with the only ECU error of P0171 lean fuel trim, but only at idle with all sensors testing OK and sometime a bit rough starting form cold

 

 

Is it time to remove the head and get the vales replaced?

 

 

Any thoughts please

 

 

 

 

 

 

 

 

 

 

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The time for major work like that is when it's unavoidable, which doesn't sound like now.

I bet the lean fault code is an air leak, and probably therefore a cheap or even free fix.

Misfires that just can't be attributed to anything else are the usual 'calling card' of exhaust valve troubles on these.

 

 

 

 

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gerards19

Are you saying that figures for compression look OK for now.

 

The lean fault code does not appear to be an air leak, I have checked and replaced the MAP sensor, smoke tested for leaks, check the timing for a jumped cog, all to no avail. Run all the checks with VCDS, oxygen sensors are working as they should be, but found the short-term fuel running high, but only at idle and get worse under load with the AC switched on, as higher revs the fuel trim is spot on.

Any other ideas as it what could be causing the lean fuel trim

 

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I think the compression figures are probably  not indicating a major/significant problem.  

 

For air leak, try this: find your PCV valve. On the top cover you'll see a small hole on the circumference at one point. With engine idling, try putting a finger over this hole. See what happens.

 

If you're not sure where your PCV valve is, post a photo of the top of your engine, minus any cover, and I'll point it out.

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gerards19

Many thanks, this was changed a while back when we got the car as it had a crack in the flexi pipe, but will give it a go.

 

What should happen if you cover the hole ?

 

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Nothing if all is well.

The other common spot for air leaks is the brake servo vac hose, has that been thoroughly examined?

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gerards19

Just tried the covering the hole on the PCV valve and no change.

For information the following items have been replaced when we bought the car which improved the long term fuel trim a bit.

1. Break servo line, had a tiny crack.

2. Throttle body O ring, perished

3. MAP sensor, showing faulty

4. pre CAT oxygen sensor, showing faulty

Spark plugs have been replaced at the recient service and they looked fine, but changed anyway

 

Can you explain why long term fuel trim drops at idle (below 10%) when the engine is under load from the AC being switched on and when the engine revs at over 1500 rpm.

 

Many thanks

 

 

 

 

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gerards19

I know frustrating isn’t it.

All the symptoms say vacuum leak but can’t find any and the engine runs just fine and no other error codes.

 

Many Thanks for your help.

 

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You're welcome.

Some more distant possibilities for air leaks are the purge valve pipework, and inlet manifold o-rings where it meets the head face. Your smoke testing might possibly have missed the latter if the leak were on the lower side in a difficult to see place?

Not a hideous job to get the manifold off and clean up that face and replace the o-rings?

Clutching at straws though, really.

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gerards19

Worth a go, but as you say clutching at straws, if it there is a leak I have missed it must be tiny as I did a very thorough smoke test.

Thanks once again.

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