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Discussion Starter #1
Hi,

I just did a compression test on my 2002 2.0 Hdi 110bhp.

Starting at the timing belt end...

1st cylinder - 380psi
2nd cylinder - 360psi
3rd cylinder - 350 psi
4th cylinder - 20psi

My thought is that the 4th cylinder has a valve stuck open as I suspect worn rings would result in a higher pressure building up very slowly.
Does this sound reasonable?

Also, a mechanic neighbour suggested I remove the rocker cover and check for snapped rocker/followers before removing the head as it is a common problem.
I've also read it is a common problem but usually associated with a timing belt slip or snap. As far as I know the timing is ok. If the belt had slipped a little and caused rocker damage wouldn't I also see the same damage/symptoms on the other cylinders?

Can a broken rocker/follower cause low compression? Wouldn't it cause the valve to snap shut rather than stay open? Or could it be that a closed inlet valve would cause the same as it could not draw in air from anywhere.

Finally, if it turns out to be a rocker/follower problem how much work is involved in replacing them (myself). I assume the cam is overhead and would need removing to fit the new parts, hence timing belt would have to come off. Is this correct?
Is there anything else I should check at the same time?
Should I remove the head anyway to check the valves?

Thanks for your help :)

Steve
 

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Rockers could be broken on that cylinder meaning no air is drawn in so no compression is developed but it does seem a bit low

Rockers are not too bad to do if you can do a head swap you can manage rockers no problem last set i done was due to belt jumping teeth and it was running again in an hour no bent valves

Check rockers first you only need the black cover on the head off to see them then you can decide whats next
 

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Discussion Starter #3
Thanks for the reply reliable406.

Rockers could be broken on that cylinder meaning no air is drawn in so no compression is developed but it does seem a bit low

I wondered the same but I also don't know the position of the piston when I started the test. If it was high up I suppose the compression would be very little, but again this is just a guess.

Rockers are not too bad to do if you can do a head swap you can manage rockers no problem last set i done was due to belt jumping teeth and it was running again in an hour no bent valves

I'm no mechanic but am competent at diy car jobs. Biggest I did several years ago was to remove the head and pistons from a petrol Fiesta and replace the rings. I think this job will be far more awkward though, so little room in the 307, the engine is half under the bulkhead.
Good to know it could just be the rockers, it would be nice to not have to remove the head. Am I right in assuming timing belt has to be removed either way though?


Check rockers first you only need the black cover on the head off to see them then you can decide whats next

Its on my todo list for tomorrow :)
Here's hoping the outside temp gets several degrees above zero as I have no garage ;)

Thanks again
 

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Belt will need to come off either way but thats no big deal you pin the cam pulley and crankshaft no need to time the fuel pump on hdis

You might need the starter out to pin the crank if you have not done it before as its hard to see i use an old junior hacksaw straigtened out a little if i dont have the locking tool handy :)
 

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Figure is so low i would expect it to be a bent/burnt out valve crack in head, headgasket blown or holed piston. If valve rocker broken i would still expect much better compression figures as the cylinder will still pump air and draw air past rings if valves stay shut.
 

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Figure is so low i would expect it to be a bent/burnt out valve crack in head, headgasket blown or holed piston. If valve rocker broken i would still expect much better compression figures as the cylinder will still pump air and draw air past rings if valves stay shut.
The cylinder will draw air past the rings which are designed to seal air leakage :confused: Are you sure :confused:
 

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They will leak because there is vacuum in cylinder if the valves don't open. The positive pressure a cylinder has on a power stoke tries to escape/blow by the rings when this happens some of this gas will try to leak behind the rings which will then press the rings tighter against the cylinder walls improving ring sealing.

Another hopefully cheaper fault for low compression has known to be stuck hydraulic lifter or part of a broken rocker jamming valve open
 

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Discussion Starter #11
Just a quick update...

I got the rocker cover off today and although I haven't yet removed the camshaft it appears that all rockers are in the correct place, no excessive movement or evidence of anything being snapped. The camshaft lobes also appear to be in good shape.

Next step is to remove the camshaft and then the head. I'm at the mercy of the awesome British weather so exactly when this will happen I cannot say.

If in the meantime any of you good chaps have any more tips for when removing the head/timing belt please don't be shy :)
 

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they'll pass some, yes.They're designed to seal in the opposite direction after all.
They are designed to seal in both directions otherwise there would be insufficient vacuum to ensure a full cylinder on the next stroke :nod:

They will leak because there is vacuum in cylinder if the valves don't open. The positive pressure a cylinder has on a power stoke tries to escape/blow by the rings when this happens some of this gas will try to leak behind the rings which will then press the rings tighter against the cylinder walls improving ring sealing.

Another hopefully cheaper fault for low compression has known to be stuck hydraulic lifter or part of a broken rocker jamming valve open
The blowby gases will not force the rings tighter into the bores as they are vented out of the crankcase . I agree about a partially jammed valve tho :thumb:

Dack , If you don't see anything with the rocker cover removed then it's time for a cylinder leakage test , Basically you pump air into the relevant cylinder with the piston at tdc on the power stroke (cam lobes pointing upwards) and listen for air leakage at 3 areas , The back end of the exhaust indicates an exhaust valve problem , The air inlet (manifold) indicates an inlet valve problem and the oil filler cap indicates ring/piston nightmares :eek:
If any of the above is unclear PM me your mobi number and I will explain further :thumb:
 

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Discussion Starter #13
Thanks for the info Chris, I know exactly what you mean regards the leakage test, watched several videos of it being done lately.

Problem is I don't have anything to provide compressed air, at least nothing other than a spare tyre but I'm guessing I would need greater pressure/air volume than that would give me?

If I went ahead and stripped the head would it likely be obvious where the problem lies? Or could I be in a situation where it all looks OK whereas something is clearly not right?


Thanks
 

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Thanks for the info Chris, I know exactly what you mean regards the leakage test, watched several videos of it being done lately.

Problem is I don't have anything to provide compressed air, at least nothing other than a spare tyre but I'm guessing I would need greater pressure/air volume than that would give me?

If I went ahead and stripped the head would it likely be obvious where the problem lies? Or could I be in a situation where it all looks OK whereas something is clearly not right?


Thanks
Your cylinder is only maintaining 20 psi of pressure so the spare wheel at 30 psi should be sufficient but you'll have to be quick

Failing that I should have a large pressure tank in work that will hold 150 psi and a large volume and could pop over (after work) from sunny Merthyr Tydfil (as long as the weather improves slightly) to take a look for you :thumb:
 

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They are designed to seal in both directions otherwise there would be insufficient vacuum to ensure a full cylinder on the next stroke :nod:

The blowby gases will not force the rings tighter into the bores as they are vented out of the crankcase . I agree about a partially jammed valve tho :thumb:
you have miss understood the expanding gases on power stroke will push rings not blow by gasses. Piston and Piston Rings:thumb:
 

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Discussion Starter #16
Chris, that is a very generous offer, and if I get nowhere I may just take you up on it, thank you!

I have a Gunson Eezi-bleed kit which fits onto a tyre valve but I'm doubtful that the other end will fit onto the glowplug adapter from my compression tester but I may be able to fashion a rubber bung of some sort that can be held in place to form a seal.

I'll let you know how I get on :thumb:
 

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Chris, that is a very generous offer, and if I get nowhere I may just take you up on it, thank you!

I have a Gunson Eezi-bleed kit which fits onto a tyre valve but I'm doubtful that the other end will fit onto the glowplug adapter from my compression tester but I may be able to fashion a rubber bung of some sort that can be held in place to form a seal.

I'll let you know how I get on :thumb:
A tubeless tyre valve would be my suggestion for a bung or in valleys talk bodge it butty :thumb:
 

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in the old days just used to put some engine oil down the injector hole to seal rings a bit better
IE inject oil do compression test if compression increase a lot good chance piston rings shot
Don
 
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