Twin Cylinder no pressure

  1. Steve_G New Member

    Messages:
    8
    Location:
    ALsace, France
    Hi All,
    Looking for some help on diagnosing a problem with my compressor not building pressure.
    It's a 2 cylinder V with 50L reservoir. Prodif V204705G.

    Ran fine until recently, but began to struggle to get up to full pressure and cut-out, now only just makes the gauge move. These are reed valve type compressors, so took the heads off but the reeds look OK. On searching around most pictures/videos show the reeds as broken. Bores do not show any scoring, not much oil around.

    Other symptoms: the air filter housings are plastic and one melted around the time the issue got worse.
    This cylinder is still sucking air in. Took the filter off the other and it's blowing air out.

    If I open the oil filler, it does blow oil out but given the speed of revving, I'd expect some. So inconclusive on the piston rings being dead.

    Given the above, do I just replace the gaskets and reed valves on one of the cylinders? If yes, I assume the one blowing air.

    Any ideas on further investigations appreciated!

    cheers
    Steve
     
  2. the snooper

    the snooper getting older by the day

    Messages:
    20,289
    Location:
    Hull UK
    Did you take the reed block off, some reeds on top some below, does sound like reeds if broken replace, otherwise clean up
     
  3. Steve_G New Member

    Messages:
    8
    Location:
    ALsace, France
    Yes, heads off both cylinders and separated all the pieces. Both reeds, both sides, no visible signs of damage. Just 2 per side. Did a clean up and reassemble ,no improvement.
    That's why I am slightly confused at to what can cause the symptoms if not the reeds or gaskets.
     
  4. Hopefuldave Intergalactic pot-mender

    Messages:
    1,300
    Location:
    The Shed of Danger, surrey, England
    Have you gone around the pipework with some bubbles to see whether there's a major leak?
    And checked there isn't a rag sucked into the inlet (had that after my old Passat had a service...)

    Dave H. (the other one)
     
  5. Wallace

    Wallace Member

    Messages:
    6,651
    Location:
    Staines, Middlesex, England.
    Follow the pipework from the cylinder heads and check the non return valve is working properly. This is usually coupled to the pressure switch and a relief valve with a small hose to unload the residual pressure in the pipework and cylinders once pressure is reached and the motor shuts down. If the valve is letting by and the cylinders are under pressure, the compressor will struggle to restart if the tank is pressurised or if empty not allow pressure to build enough to reset the relief valve.

    The brass tee on the tank is the NRV (non return valve).

    D61207BD-BE52-40F0-837F-2266F8F8430B.jpeg
     
  6. the snooper

    the snooper getting older by the day

    Messages:
    20,289
    Location:
    Hull UK
    If it was your non return valve I would expect it to build pressure but it would then escape back through the pump
     
  7. Steve_G New Member

    Messages:
    8
    Location:
    ALsace, France
    Thanks for the picture, very similar to my compressor and pump/motor/pipework is identical.
    The Cylinder that melted the air filter housing the one nearest in the photo, and the one blowing out of the filter/inlet is the furthest.

    No sign of leaks, and in any case don't think this would cause the cylinder to blow back out of the inlet.

    Non-return valve seems ok, and in any case the small amount of pressure the pump does put in the tank, stays there.

    Reaching the point of just replacing the gaskets and reeds on the cylinder blowing back. Not too expensive to get the bits. (€27 inc. delivery)

    After that it's pistons and rings, but the cost creeps up if I replace the bits that aren't broken!
     
  8. the snooper

    the snooper getting older by the day

    Messages:
    20,289
    Location:
    Hull UK
    Can't be piston rings if its coming out of inlet, are any gaskets damaged if so make some temp ones from cereal box cardboard, make sure no parts of gasket have disappeared
     
  9. Rig Pig

    Rig Pig Member

    Messages:
    3,653
    Location:
    Narrwich! U.K.
    Not familiar with this type of compressor but I wonder if there is a key or coupling that has failed between the motor and pump.
     
  10. slim_boy_fat

    slim_boy_fat Forum Supporter

    @Steve_G When did you last drain the tank?
     
  11. Steve_G New Member

    Messages:
    8
    Location:
    ALsace, France
    Gaskets look OK, but that probably means nothing! I guess if the gasket that separates the inlet/outlet valves is leaking it could behave as it is, even if the valve reed is ok. May try the temp option, even if it lasts only a few seconds it might be a good diagnostic!

    @Rig Pig : motor is turning the pump. Lots of noise and cylinder getting hot
    @slim_boy_fat : drained regularly (and yes I did tighten the plug!;))
     
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  12. Rig Pig

    Rig Pig Member

    Messages:
    3,653
    Location:
    Narrwich! U.K.
    It's got to be valves then, any external leaks would be obvious as your loosing the full output of the compressor air would be blasting out.
     
    Hopefuldave likes this.
  13. carbon

    carbon Member

    Messages:
    1,270
    Location:
    S. Glos. U.K
    If this has a separate plate for the reed valves then the lower gasket should be a different shape to the top gasket.

    Something similar to this, the one with the part in the middle stops compressed air going back to the inlet side of the pump, yours may be a different shape.

    gaskets.png
     
  14. Steve_G New Member

    Messages:
    8
    Location:
    ALsace, France
    Went to try the temp option which meant cleaning off the old gaskets.

    The reeds are held between 2 plates, and the gasket between them is metal. However between the top plate and the head is a fibre gasket (same shape). When I started to remove this, the part that separates the two chambers (thin) fell apart. Really just the jointing compound making it look good. So I think I have found the problem :thumbup:

    Thanks to all for the help in going through the options :clapping:

    I'll post back when I reassemble and hope fully fix it!
     
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  15. Steve_G New Member

    Messages:
    8
    Location:
    ALsace, France
    Took a while to get the new bits under the current circumstances, but all assembled and I have a compressor again!
    The picture is of the underside of the cylinder head, and you can see the wall that separates the 2 chambers; not very wide so really a weak point waiting to fail.

    cheers
     
    • IMG_20200327_113630.jpg
    Wallace likes this.
  16. 8ob

    8ob Forum Supporter

    Messages:
    4,891
    Location:
    moscow on thames
    Not really a weak point Steve, mine blew a gasket and when I took the head off I noticed the headbolts were loose. I replaced the gasket and checked the bolts after a few hours to find they needed retightening. We tend to ignore compressors until they stop, worth checking on occasion.

    Bob
     
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  17. Wallace

    Wallace Member

    Messages:
    6,651
    Location:
    Staines, Middlesex, England.
    It would be worthwhile having a spare set of gaskets for the next possible failure. I find that so long as you have a spare on the shelf for whatever it is, the original becomes so much more reliable. :laughing:
     
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  18. Steve_G New Member

    Messages:
    8
    Location:
    ALsace, France
    Not arguing with the idea of checking the bolt torque, but a long straight wall with a high pressure difference needs care and is a more likely point to fail. Ok, this time it lasted quite a few years (under lightish use) but making it a bit wider, maybe the same as the outside joining surface, would seem more sensible. But it is like it is.
     
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