S1 4.2 . low engine pressure on one cylinder

[phoenix]

Hi the engine in question is a supposedly rebuilt unit and we have been having a problem getting it to tick over properly. When running it did not sound to bad but after checking ignition and fuel we decided to do a compression test . 5 cylinders about 155 psi but number 3 only reading 45 psi or with some oil down the plug hole about 75 psi . It has been stood for some years after rebuild ,probably about 10 years . any thoughts before we start stripping . (head off first)

[christopher storey]

First thing to do is to remove the cam covers and check valve clearances and at the same time check that a valve on the affected cylinder is not sticking . If you have an endoscope a look inside the cylinder also may help . If that reveals no obvious fault, then a leak down test is next, either a formal test using a leak down test kit, or an informal one ( often more revealing) introducing air pressure to the cylinder through the plug and listening to see where the air is going . A valve will show air going down exhaust or back through the carbs, and faulty rings will show a leak easily heard at the dipstick hole. Also check for bubbles in the radiator which will reveal a gasket failure, although this is less likely with only one cylinder affected

[phoenix]

Thanks for the advice I will try it tomorrow . Anything but strip it down .

[abowie]

Do a leak down test and listen for where the air's escaping.

If it's coming out the inlet or exhaust manifolds it's a valve. If it's coming out the crank case it's rings.

[phoenix]

Another question for you . If it comes to it is it easier to take sump off in situ or remove engine if we have to take piston out .

[Allrand]

I have taken the sump off insitu before, very fiddley but easier than removing the engine. Loosen the engine stabilser so you can lift the rear of the engine, helps clear the reaction plate and rear main bearing. When re-fitting make sure the cork/rubber seal on the main bearing cap is properly secured in place when offering the sump up.

[abowie]

Another question for you . If it comes to it is it easier to take sump off in situ or remove engine if we have to take piston out .

I haven't tried it personally but I suspect it would be very difficult to R&R a piston from below.

If you have a stuck ring you're best to take the engine out, put it on a stand and fix the problem that way.

[Allrand]

Another question for you . If it comes to it is it easier to take sump off in situ or remove engine if we have to take piston out .

I haven't tried it personally but I suspect it would be very difficult to R&R a piston from below.

If you have a stuck ring you're best to take the engine out, put it on a stand and fix the problem that way.

You can't take a piston out from below, the head needs to come off and piston removed from above, but you can remove the sump from below (with some difficulty).

[tinworm]

The sump will come out ok - make sure you have the crankshaft web pointing upwards on the rear cylinder so the sump will clear the reaction plate.

Barrie

[johnetype]

Based on my experience under similar circumstances I'd suggest you've got a bent valve, so I'd be doing the leak down tests suggested by others above and then removing the cylinder head before contemplating removing a piston.

[phoenix]

Got the head off and taken cams off poured fuel in combustion chamber of offending cylinder . Virtually no leakage . Çylinder bore has a small amount of scoring but as it is a rebuilt engine supposedly by a Jaguar expert and done zero miles it seems like a broken compression ring . So sump off tomorrow. Ug

[malcolm]

I assume valves were all ok then?

[christopher storey]

Is it scoring or is it just honing marks - and is it just this one cylinder or are they all similar ? If air on your leak down test didn't find its way past the rings I'm not sure why you conclude there is a broken compression ring . And as Malcolm says, what else was revealed by a leak down test ? And have you tried the liquid test on each combustion chamber on the upturned head ?

I wouldn't rush into this by pulling the sump off in situ, because if there is indeed piston/ring/bore damage then you are going to have to strip the block completely for the bore to be sleeved , so if you are certain there is such damage the balance tilts decisively in favour of pulling the engine out here and now

Perhaps a few pics of the bore for a collective opinion might be useful ?

[phoenix]

From what I can see in the bore there is some slight marking about 25 mm down .The engine has only been run for about 20 minutes .When I get the piston out I may just give it a light hone. (taking care not to introduce anything in engine)

[bitsobrits]

Really would encourage you to post some photos so help may be provided. You seem to be rapidly progressing down a path without logic to guide you. The odds of a broken ring are low, based on your reported pressure readings. Did you actually perform a leak down test, or just a compression test? You have not so far ruled out a sticking valve, as your disassembly process may have allowed the valve to fully seat, and once seated it obviously seals well. But it still could stick slightly open in operation due to a tight guide, for instance. Before I started to disassemble the bottom end, I would remove the valve springs for the offending cylinder and check the valve to guide clearance and free operation of each valve to rule out sticking. In other words, one step at a time, ruling out al given possibility before moving on the the next possibility.

[christopher storey]

-tight tappet clearance would have had the same effect

[phoenix]

We have put a measured amount of diesel in every cylinder and the suspect one empties quickest. there is also a slight score in the bore . Whoever built the engine supposedly a Jag expert has fitted new pistons (+20 low compression) but not honed the bores . We are trying to get the sump off but struggling . All bolts out ,lifted engine and box slightly but still fouls at the rear . Am I going to have to slacken or remove the reaction plate ?
By the way I have checked the valves on the offending cylinder an they are fine ,seating and movement .

[tinworm]

As I mentioned above - turn crank until web is in the right position...

Barrie