valve or cylinder problem

[rainagain]

I'm looking at a spare or repairs car that is running on three cylinders. It has an LPG conversion so I'm thinking it's suffering from valve seat recession. Is there a quick check I can do on the engine to determine if the lack of four cylinder running is down to a valve problem, which won't be too difficult to sort, just chuck on a new cylinder head from upullit for £60+vat or something more serious?

 

I was thinking of pulling off each injector connector until it makes no difference to the running to identify the duff cylinder, then doing a dry then wet (squirt some oil down the bore) compression test. If its the valves then there won't be much difference in the reading, if its the rings the wet reading should be higher.

 

What do you guys think?

[Rusty_Rocket]

That always was the standard way to check for ring/ bore or a head problem but I must say, I've never seen a used engine not improve its compression readings after adding a little oil.  Hence, I don't think that test is overly conclusive.

The best way is a leakdown test, using an attachment which supplies compressed air into the cylinder, and listening from where the air is escaping.  I can see how that would be awkward to carry out though on the owner;s driveway/ road outside of his house 

[lisbon_road]

What makes you think it is either?  So many reasons why it might run on three that are not connected with valves or rings.  If it is rings, and is really drastic, I'd expect to see a black plug and some smoke.  If it is valves, no smoke or black plug really.

[rainagain]

What makes you think it is either?  So many reasons why it might run on three that are not connected with valves or rings.  If it is rings, and is really drastic, I'd expect to see a black plug and some smoke.  If it is valves, no smoke or black plug really.

 

I've not seen it yet, I'm just speculating. Thanks for the pointers. 

[Albert Ross]

If it's LPG running on a supplementary loom for the injectors, try this area first. Does it run ok on petrol? If so, it's not a mechanical issue. If not, it might be a poor electrical connection. With power to the coil disconnected, when under initial cranking, does it "odd leg" or is it even sounding on each compression stroke? Once running, can you feel the "putput" at the exhaust?

[Mr_Bo11ox]

I don't know owt about the car but I doubt its VSR, I can't see the gradual effect of that suddenly knocking one cylinder right out yet the other 3 continue to work OK.

[castros_bro]

Run for 20 seconds when cold n feel ehaust manifold to find which 3 are firing.....swap a coiple of plugs n repeat..remove air filter .pour a bit of fuel in inlet n repeat. Analise the results over a pot of tea.