piston ring de-tensioning

jgra1 · Unread post by **jgra1** » 20 Oct 2008, 11:33

Has anyone ever heard of this?

it was mentioned to me at the weekend, whilst talking about my dad's xud 306, which has cold compression of 90psi in all 4 cylinders..

Basically, after an overheat, the rings can contract permenantly and compression is forever lost...

John

TBONE · Unread post by **TBONE** » 20 Oct 2008, 12:21

never heard of that have heard of the sticking in the grooves due to excess heat like the inside of the cylinder catching fire briefly due to excess fueling

KP · Unread post by KP » 20 Oct 2008, 13:02

Is the crankcase gaining pressure at all or the cooling circuit as they are the first things i'd check?

jgra1 · Unread post by **jgra1** » 20 Oct 2008, 13:04

will try and check those remotely thanks KP...
all difficult with dad hanging on a phone a long way from here

jeremy · Unread post by **jeremy** » 20 Oct 2008, 18:29

Yes when the bores/rings wear there is increased blowby - and the rings get too hot - from the blast which wouldn't normally get to them if they sealed properly. To make things worse as their contact with the cylinder wall isn't good they don't cool by conduction as well as they should. There is possibly a build up of carbon in the ring grooves which will impede cooling that way as well.

The rings will loose their temper and the overheating will make them brittle as well - and eventually they break.

How was the compression measured? - measurement on a diesel isn't easy - which is why I ask.

jgra1 · Unread post by **jgra1** » 20 Oct 2008, 18:58

interesting Jeremy..

unlikely to suddenly go then?

Compression tested in injector holes with a diesel comp checker, made by laser.. (about £100)

Car wont run so hot-engine test not possible.. but if 350psi is needed for combustion, 90psi will never get there..

I want to check my engine (to wild tonight out there!) I have his tester so can do mine cold and hot and make a note ,,, but will have to wait till tomorrow eve

jeremy · Unread post by **jeremy** » 20 Oct 2008, 19:27

I should imagine that the engine has been cranked over for a while with the injectors in - so a fair amount of diesel has been injected - and so the whole thing is nice and oily - which means that there is some oil round the rings which in turn helps the rings seal.

jgra1 · Unread post by **jgra1** » 20 Oct 2008, 20:47

so why are we getting 90psi when cold...

I cant believe anything could cause all 4 pots to go low on compression.. unless the tester is mis reading.. I will definately try it on mine tomorrow to see what it thinks, if mine are all 90 I will be very happy, oddly

KP · Unread post by KP » 21 Oct 2008, 08:10

If all 4 have gone id say the HG is shot to burgery land...
Does it have the inter warmer on the top?
Have you checked it isnt one of those engines that got the wrong con rods when being made?

jgra1 · Unread post by **jgra1** » 21 Oct 2008, 10:02

Hi KP..
would think if the rods had all broken the compression would be less still

Think IC is on the top...

KP · Unread post by KP » 21 Oct 2008, 10:28

Dont need to break to cause issues, could place to much wear on the cran and cause issues that way or just bend a bit as well and then not stroke as much as they should?

jgra1 · Unread post by **jgra1** » 21 Oct 2008, 12:12

hmmm....

MikeT · Unread post by **MikeT** » 21 Oct 2008, 12:53

Hi John, I'm following this thread with interest, have you tested the pressure gauge yet? IMO, 60psi across the board sounds most unusual, not just the low reading but the uniformity of them all.

Unread post by **CitroJim** » 21 Oct 2008, 16:15

This is getting like a whodunnit

Given the gauge is OK John, those uniform reading are a little odd and now the conrods have been thrown into the equation.. I can't see it really but thinking aloud here. The car overheated and lost coolant. Towed to try to start it. Now suppose the head gasket failed and allowed water into the combustion chambers. The result would be a hydraulic lock and bent rods. Now the engine would still turn over but the pistons would not reach the top of the block resulting in a very, very much lower compression ratio, insufficient to start and consequent low cylinder pressures.

A good enough theory but it still does not adequately explain why they were all so low and uniform. There's only one thing for it John - off with its head! The last time I saw low and uniform pressures (20 psi) the problem was holes in the pistons. This was on a petrol engine and looking at the construction of a diesel piston, holing them is next to impossible.

Please do it sooner rather than later - there are hundreds of fourm members sitting here on the edges of their seats just waiting like coiled springs for the answer to this puzzler.

jgra1 · Unread post by **jgra1** » 21 Oct 2008, 17:05

sorry everyone

ok so now i am going home test the gauge on my car, hot and cold (later)

this will help in the whodunnit of my dads 306..

also doing my Glows as fed up of making more clouds than Autumn usually has