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Low Compression! Piston Rings?


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#1 Vx220-blue

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Posted 15 April 2018 - 05:52 PM

Hi, After owning my Stage 4 Turbo for 3 years I've come across my first major problem. EML was coming on while driving and after removing plugs I found some oil around 3 of the plugs so assumed rocker cover gasket so ordered a new one. In the interim I tightened the bolts on the cover and reinspected the plugs after a couple of days. Only a very small amount of oil was present but after a compression test I believe the piston rings have gone on cylinder 3. (Figures without throttle open) 1 - 150psi 2 - 150psi 3 - 70psi 4 - 150psi After a wet compression test on cylinder 3 (soup spoon sized among of oil down hole 3) it read 100psi. Is the only cause of this worn piston rings?

#2 gaffer1986

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Posted 15 April 2018 - 06:03 PM

From my knowledge the air can seep through the valve seats also. A leak down test may be your next best option but you need an air compressor for this.

#3 vocky

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Posted 15 April 2018 - 06:05 PM

you should always check compression with the throttle open



#4 gaffer1986

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Posted 15 April 2018 - 06:07 PM

Is this so if the valve seats are worn you know.

#5 james4563

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Posted 15 April 2018 - 06:14 PM

Unfortunatly that is what another engine I worked on was like when the ringland cracked.

 

As above it could be the valves, so that is worth checking. The fact that the oil brings the pressure up though suggests that probably the rings are leaking.

 

Agree with the throttle open comment too, try it again with it open.


Edited by james4563, 15 April 2018 - 06:15 PM.


#6 gaffer1986

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Posted 15 April 2018 - 06:18 PM

Maybe throttle open so there is no vacuum before compression?

#7 vocky

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Posted 15 April 2018 - 06:21 PM

Maybe throttle open so there is no vacuum before compression?

plus engine usually cranks faster and thus a better reading



#8 Vx220-blue

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Posted 15 April 2018 - 06:33 PM

Just tested it again (cold engine & didn't reapply oil in cylinder). Showing 100psi. All other cylinders were stable so does appear to be something very wrong. Tested it three times when I discovered 70psi and all were 70-75psi until I did the wet test. So it's either rings or valve seats, and a leak down test is the only way of knowing?

#9 Vx220-blue

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Posted 15 April 2018 - 06:34 PM

Also forgot to say - thanks all for help so far :)

Edited by Vx220-blue, 15 April 2018 - 06:34 PM.


#10 Vx220-blue

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Posted 15 April 2018 - 08:15 PM

Before I look at replacement/ rebuild options- is this definitely necessary or could there potentially be a "quickish fix"? I'm thinking about buying a donor Astra (etc) that's had a recent rebuild

#11 james4563

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Posted 15 April 2018 - 08:26 PM

What does it run like?

#12 OL11TAL

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Posted 15 April 2018 - 08:54 PM

Having similar problem

Edited by OL11TAL, 15 April 2018 - 08:55 PM.


#13 Vx220-blue

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Posted 15 April 2018 - 08:59 PM

Runs alright tbh. EML comes on ocassionally with accompanying misfire sputtering however turning ignition off and on solves problem. Pops more frequently and slight reduction in performance. I thought it was the rocker gasket as plugs were covered in oil and carbon. I wish it was.

Edited by Vx220-blue, 15 April 2018 - 09:01 PM.


#14 gaffer1986

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Posted 15 April 2018 - 09:54 PM

Last time I did a compression test in a B207 with higher compression ratio I'm getting around 180psi. Didn't realise the turbos had such a low compression ratio.

#15 siztenboots

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Posted 16 April 2018 - 05:53 AM

Charge cooler fitted?

#16 Vx220-blue

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Posted 16 April 2018 - 06:02 AM

Charge cooler fitted?

Yup

#17 Nev

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Posted 16 April 2018 - 07:11 AM

1 - 150psi 2 - 150psi 3 - 70psi 4 - 150psi

 

I'm sorry to say that's a vast difference, either that cylinder's rings are broken/worn/bent or has an uneven/washed bore. :( I doubt its valve seat leak as that wouldn't make such a marked difference unless the valve stem has bent due to spring failure.

 

Who mapped it?


Edited by Nev, 16 April 2018 - 07:16 AM.


#18 vocky

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Posted 16 April 2018 - 07:30 AM

it could also be a burnt exhaust valve



#19 vocky

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Posted 16 April 2018 - 07:32 AM

a quick google says;

 

Normally this type of failure affects the exhaust valves only, but it can also damage the intake valves. A burnt valve will cause issues with your vehicle's performance and fuel consumption. Rough idle, reduced power, backfiring, and misfire are all symptoms of valve burn

 

 



#20 Nev

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Posted 16 April 2018 - 07:51 AM

I've never heard of a Z20LET burning it's valve seats TBH, but I suppose it's possible. I have however seen umpteen melted pistons and quite a few cracked rings though. If you go on the Z20LET lovers Farcebook forum there must be a new picture of a melted piston every week or so (here is one from 2 days ago: https://www.facebook...3&theater&ifg=1 ). :unsure:

 

I would imagine that the melting point of the pistons (which are only some sort of cast duraluminium) will be a good deal lower than the steel valve seats. Anywho, the resolution is engine out, take the head off and have a look, lots of work/expense I'm afraid. :(

 

If you fix/replace your engine, you should also review your map, IATs and existing sensors (so it doesn't happen to the new engine). Usually the cause is "over-optimistic" mapping, hence why I asked who mapped your car.

 

GL (and let us know what it was if you find out please).


Edited by Nev, 16 April 2018 - 08:14 AM.





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