Help - Losing Compression when engine warmed up

[Alistair Enser]

I'm currently restoring 837G which has been unused since 1990 . Chassis, body , all suspension , bushes , gearbox are now all pristine as I've sandblasted, powder coated the chassis and spent a small fortune at SJ's, plus a body off respray . The original interior is pretty good ( just a few trim bits to complete ) and so my attention turned to the engine. 

I have had the head off , 5thou skimmed and all the valves ( which were very clean ) lapped in. I didn't dismantle the bottom end, but the pistons looked very clean, as did the bores with no perceivable ridge at the top. I dropped some oil in the bores and 24hrs later when I went to reassemble it had not leaked past the rings . On reassembly valve shims and clearances have been checked and the engine starts fine . However, after running for about 5minutes it develops a misfire and the cylinders lose compression.

 

Hot compression test- 1,2,4 drop slightly to about 9 and cylinder 3 drops to 5 . 

Any ideas ? I'm guessing this could be sticky valve guide ?( I didn't change these ) , bad/stuck rings or perhaps a crack in the cylinder sleeve that I didnt see.  Any advice would be greatly appreciated 

 

 

 

 

[rgh0]

Yes maybe a valve sticking open when hot ( maybe a bent stem) also I would recheck the cold clearances on No3.  I assume no head gasket leak issue when hot with gases into the coolant system ?

 

cheers Rohan

[Alistair Enser]

Valves all true when removed and re-lapped in . It was doing exactly the same before I disassembled. I did a cold check on the valve movement in the guide and it didn't seem 'sticky' but doesn't mean it couldn't be an issue when hot - I suppose.

Head gasket could of course be a potential issue, but there doesn't appear to be any pressurisation or bubbles in the water system and the coolant stays quite cool ( I'd expect it to get hot very quickly if the gasket were leaking ) 

Oil pressure seems ok and not varying wildly - if I was getting blow-by on pistons would the increased crank pressure cause the pressure gauge to move ( I'm getting a very constant 50).

 

 

[rgh0]

I dont think you would see any change in oil pressure with changes is cylinder compression pressure

Its easy to get a valve clearance wrong or for a follower to jam or for a shim to get moved off the top of the valve stem or for a too thin shim to cause problems with clearance so I would double check all of those areas

[Alistair Enser]

Thanks for that . I will double check .I'd assume if the problem occurs when hot that valve clearance would be an issue if shim too big and clearance too small ie valve then not closing properly . Or can clearances which shrink with heat just be a problem for compression ?

 

I will check this week if the compression has returned now the engine is cold again ( ran out of time to retest )

 

Btw - same colour scheme as yours ( photo before restoration )

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[rgh0]

clearances especially on the exhaust will close up when the engine is hot

[Alistair Enser]

Thanks . 

I will check 

[viz139]

A leak down test would pin point a valve problem fairly quick. 

[rgh0]

Yes I would check clearances and then do a leakdown test when the engine is hot and lost compression to try to see where it is leaking to

[Alistair Enser]

OK, so leak down test showed poor results on cylinder 3 leaking into crank case. 30% loss which gets worse if warm. I have stripped the engine and the rings need doing, but also the bore is 7thousands out of round. What is the tolerance for the liners before I replace ?? Does anyone know 

 

thanks ALi

[drdoom]

The cylinder liners are spec'd and graded to 0.0005" tolerance within range and have no reason to be out of round other than wear. I'd regard your #3 liner as shot, and would proceed with a thorough refresh of the engine at this point.

 

[Alistair Enser]

Thanks Steve. That’s what I will do. Have already rebuilt the head completely, so bottom it is. It probably explains why it was in a barn and off the road since 1990 .

On a positive , I will have pretty much a brand new engine .

 

[Alistair Enser]

So , latest update . Engine apart . Crank being reground plus 0.001”

Complete new bearing  set, new pistons, new gudgeon pins, new liners . Top end already rebuilt . Oil pump and water pump cleaned and checked. New Crank Pulley as the old one had slight play.

When complete this will be pretty much a brand new engine.
 

Watch this space …. I will add photos .