Rover v8 3.5 no compression #5

[atkinsal]

Hi all

Wonder if someone can give me a bit of advice re: my Rover 3.5 v8 fitted to a Series 2.
Bit of background, the block was from an early range rover, I had it rebuilt (rebored, full Rimmer rebuild kit) by a friend about 6 years ago.
I originally had twin carbs fitted but had terrible problems getting it to start in cold so fitted an Edelbrock carb. Its always used too much oil and would puff a little blue smoke on start up when hot. No noticeable blue smoke when driving or overrun.

I’m now rebuilding the landy and so decided to look at the engine. Just completed a compression test (on a cold engine, as everything is stripped off the vehicle, with carb off so WOT). I’m getting 135-150 across all cylinders except #5. No compression registering on the gauge. With my finger over the spark plug hole I only feel faint pressure. I put 5ml of oil in the chamber and retested, again no compression registering on the gauge.
I’ve just pulled the head and as per photos there is significant carbon build up on piston on the inlet side. The chambers across all the heads look very black.

Grateful for thoughts on what to do next. Before I pulled the head I checked the valves were opening ok, couldn’t see and noticeable sticking in the valves and cam lobes seem ok. As I added oil and rechecked (with still no compression doesn’t this points to heads, but nothing obvious I can see.
I figured I should probably remove the piston and check the rings? I did wonder if I had glazed the bores but the other chambers have compression and figured if that was the case all would be low?

Thanks all

[stevieturbo]

There are no photos.

Are the valves sealing ? Generally speaking, NO compression will be leaks past the valves.

LOW compression more likely piston related. So the actual numbers, speed of any pressure build up etc etc matter.

[GDCobra]

Is the piston actually moving up/down the bore?

If I understood correctly you have the head of so presumably no big hole in the crown?
If the valves are not obviously open you should be getting some compression even at cranking speed, hard to imagine losing all your air via a partially open valve, cylinder head leak or piston blow by.
S'pose the other thing could be inlet valve not opening, no air in, nothing to compress.

[stevieturbo]

Is the piston actually moving up/down the bore?

If I understood correctly you have the head of so presumably no big hole in the crown?
If the valves are not obviously open you should be getting some compression even at cranking speed, hard to imagine losing all your air via a partially open valve, cylinder head leak or piston blow by.
S'pose the other thing could be inlet valve not opening, no air in, nothing to compress.

Very common and perfectly normal for no compression with even a very slightly open valve.

And even if the valves do not open, you will still register a reading from pressure changes from air already in the cylinder.

cranking or running test with a scope hooked up to a pressure sensor in cylinder can give good info.

But really, if the head is off, it should only take a few minutes to test if the valves are sealing, and have a quick look at the piston to see if there is anything catastrophically wrong with it

[atkinsal]

Thanks to all for the posts. I forgot to post the pics. Here we go

[scudderfish]

Lay the head on its back and fill the chamber with liquid. See if it drains out.

[GDCobra]

Is the piston actually moving up/down the bore?

If I understood correctly you have the head of so presumably no big hole in the crown?
If the valves are not obviously open you should be getting some compression even at cranking speed, hard to imagine losing all your air via a partially open valve, cylinder head leak or piston blow by.
S'pose the other thing could be inlet valve not opening, no air in, nothing to compress.

Very common and perfectly normal for no compression with even a very slightly open valve.

I can understand no reading on a gauge if it's farily low resolution but I'd expect more than "faint pressure" at the spark plugh hole unless there was significant air loss, of course 'faint pressure' is subjective.

And even if the valves do not open, you will still register a reading from pressure changes from air already in the cylinder.

Well if the inlet valve does not open any air entering the cylinder will be limited, I guess some could enter via the exhaust valve due to overlap but this would only be for of small amount of the piston's downward journey and hence only develop pressure when the piston goes above that point on it's upward journey unless I'm missing something and there is another way in? Point is though, don't ignore a blockage when target fixating on a leak.

cranking or running test with a scope hooked up to a pressure sensor in cylinder can give good info.

That's a good idea, most compression testers are looking for high pressure readings >150PSI but would be useful to know what value is actually being measured.
Even a piece of paper held over the spark plug hole can give a good impression of how much air is being moved.

But really, if the head is off, it should only take a few minutes to test if the valves are sealing, and have a quick look at the piston to see if there is anything catastrophically wrong with it

Yeah, although it's a shame the head's off now as nothing can be tested in the original condition.

[GDCobra]

Thanks to all for the posts. I forgot to post the pics. Here we go

Were the other 3 piston tops so clean when the head was removed or have they been cleaned?
Bit weird really, one seems too dirty but the others seem too clean particularly given the colour of the heads.

Looks a bit like the exhaust possibly leaking and letting burnt gass back into cylinder.

Has engine recently been rebuilt?