PistonHeads.com Forum

Low Compression

Author
Discussion

TooMany2cvs

19,608 posts

50 months

Tuesday 3rd October
quotequote all
klmorrell said:
I think you might of missed the bit about me towing a 1.4 tonne caravan from warrington to chesterfield over the peninnes without the expansion tank cap on.
No, didn't miss that. It doesn't change much, apart from the engine'd be working a bit harder - but you'd be bimbling along at lower speed.

klmorrell said:
There was no warning signs, ie the temp gauge sat on normal all the way, it didn't smell and there was no obvious steaming from the engine.
Bear in mind modern temp gauges lie. They tell you that the ECU thinks the temp's in the normal range, no more than that.

If you'd run out of coolant, then the sensor would not give a "normal-range" reading to the ECU, because it'd be sitting in air, not coolant.

Your coolant expansion tank would be running at atmospheric pressure, not at raised pressure, so the boiling point would be lower. There's a greater likelihood of the expansion tank slopping everywhere.

klmorrell said:
All I am trying to figure out is how likely this might have damaged more than just the head and head gasket.
If I was forced to guess what's happened, I'd be wondering if the cambelt's gone ping, purely coincidentally, and you've got a head full of bent valves.

klmorrell said:
bearing in mind the trade in value of £2000, , is there not an known issue with these engines and liners cracking ?
It's worth bk all because it's an ageing Mondeo.

souper

1,986 posts

135 months

Tuesday 3rd October
quotequote all
Surely the Mechanic should be able to do a leak down test, I'd imagine even small garages or mobile operators would have one they cost buttons.

Could always throw a bit of oil down each cylinder and retest compression that would show rings or head/valves are a problem.

Megaflow

6,474 posts

149 months

Tuesday 3rd October
quotequote all
Cambelt has gone.

Specifically, the aux belt went, causing the alternator light to come on, wrapped itself around the cambelt and then took that out.

Happened to somebody else on here not so long ago with the same engine.

klmorrell

Original Poster:

18 posts

62 months

Tuesday 3rd October
quotequote all
One of the things the mechanic noticed whilst doing the compression test was how the engine didn't seem to spin any quicker with all the spark plugs removed, the cambelt ect is about 12mths old and is still on and taught.

This is why for now we have not gone further, he seems to think because of that something bottom end might be damaged.

klmorrell

Original Poster:

18 posts

62 months

Tuesday 3rd October
quotequote all
just text the mechanic, he has already checked cambelt, and a leak down test was next on his list. but as I just said he expected the engine to spin faster without the plugs in and it didn't, hence him thinking it could be more than head related.

My problem is how far do I go trying to find out, this is what I meant when say stuck between a rock and a hard place. And why I thought I would ask here

Thanks again
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klmorrell

Original Poster:

18 posts

62 months

Tuesday 3rd October
quotequote all
Another update

mechanic been chatting to st specialist, this engine does have issues with cracked liners, but is rare for all to go at once.

however he said they could of ovaled due to heat



i

TooMany2cvs

19,608 posts

50 months

Tuesday 3rd October
quotequote all
klmorrell said:
just text the mechanic, he has already checked cambelt
OK, it's not snapped. It's jumped some teeth. Enough to bend valves.

klmorrell said:
and a leak down test was next on his list. but as I just said he expected the engine to spin faster without the plugs in and it didn't, hence him thinking it could be more than head related.
Well, of course it bloody doesn't - there's sod all compression with the plugs in, so not much difference with them out...

bearman68

2,023 posts

56 months

Tuesday 3rd October
quotequote all
Long shot, but I have seen it before where the piston bore looses lubrication (I guess), and it looses compression because the seal between the cylinder and the bore is lost. If the engine has been running hot, and working hard, maybe, just maybe, this has happened. Chuck a little drop of oil down the spark plug holes and crank it over a few times,and try the compression test again. Or chuck the plugs in and see if it will start in a cloud of smoke.

smile

Megaflow

6,474 posts

149 months

Tuesday 3rd October
quotequote all
TooMany2cvs said:
klmorrell said:
just text the mechanic, he has already checked cambelt
OK, it's not snapped. It's jumped some teeth. Enough to bend valves.

klmorrell said:
and a leak down test was next on his list. but as I just said he expected the engine to spin faster without the plugs in and it didn't, hence him thinking it could be more than head related.
Well, of course it bloody doesn't - there's sod all compression with the plugs in, so not much difference with them out...
Yep, timing is out and no compression means it doesn't know the plugs are in or out.

Have him check the condition of the aux belt, I've got a pint says it is gone.

souper

1,986 posts

135 months

Wednesday 4th October
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I'd be getting a second opinion from another mechanic.

liner33

6,332 posts

126 months

Wednesday 4th October
quotequote all
Leakdown test is pointless, head has to come off whatever to diagnose since the compression is so low across the board, fix that and the bottom end could still be goosed anyhow

Little Pete

295 posts

18 months

Wednesday 4th October
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liner33 said:
Leakdown test is pointless, head has to come off whatever to diagnose since the compression is so low across the board, fix that and the bottom end could still be goosed anyhow
It'll back up the compression test readings if nothing else. A pal of mine took the heads of a V6 Omega because a mobile mechanic told him the compressions were low on one bank. It turned out his gauge was faulty. The car wouldn't start because the crank sensor had failed!

Edited by Little Pete on Wednesday 4th October 13:26

liner33

6,332 posts

126 months

Wednesday 4th October
quotequote all
Little Pete said:
It'll back up the compression test readings if nothing else. A pal of mine took the heads of a V6 Omega because a mobile mechanic told him the compressions were low on one bank. It turned out his gauge was faulty. The car wouldn't start because the crank sensor had failed!

Edited by Little Pete on Wednesday 4th October 13:26
Anyone who after two attempts cant get a compression tester to give accurate data is best off staying away from a leakdown tester as they are significantly more complicated to operate

klmorrell

Original Poster:

18 posts

62 months

Wednesday 4th October
quotequote all
Why are people assuming the compression data is wrong ? the compression test data is correct, and it has nothing to do with cambelts, but then again why would it have anything to do with the cambelt, which was replaced 12mths ago and has been perfectly fine.

I mean come on think about it.

I have just dragged a 1.4 tonne caravan over the peninnes for 60 miles and 3hrs with no expansion cap on. then the engine lost power and stopped, followed by an low battery light

whats more likely to have happened in those circumstances ? the engine overheating or the cambelt snapping.

I am no mechanic but on opening the bonnet even I knew the engine had cooked itself

my question here was "how likely would there be serious bottom end damage based on what just happened. I had already assumed the head and headgasket were toast and just that would of been a best I could of hoped for.

after doing the compression test and noticing how laboured the engine spun my mechanic suggested bottom end, and low and behold he was right.


liner33

6,332 posts

126 months

Wednesday 4th October
quotequote all
klmorrell said:
my question here was "how likely would there be serious bottom end damage based on what just happened. I had already assumed the head and headgasket were toast and just that would of been a best I could of hoped for.
IMO , if there is water in the oil and you said that you suspect there is I would think bottom end/bearing surfaces damage extremely likely .

That's not saying its so damaged that it needs immediate rectification but that any damage would severely reduce the life of the engine

If it was a rare and/or expensive engine then it might warrant further investigation but its not

Boosted LS1

16,600 posts

184 months

Wednesday 4th October
quotequote all
If it's cooked itself then I'd expect the pistons to have partially seized. That'll definitely slow down cranking speeds. If so, then scrap it and start again.

stevesingo

2,863 posts

146 months

Wednesday 4th October
quotequote all
klmorrell said:
Why are people assuming the compression data is wrong ? the compression test data is correct, and it has nothing to do with cambelts, but then again why would it have anything to do with the cambelt, which was replaced 12mths ago and has been perfectly fine.

I mean come on think about it.

I have just dragged a 1.4 tonne caravan over the peninnes for 60 miles and 3hrs with no expansion cap on. then the engine lost power and stopped, followed by an low battery light

whats more likely to have happened in those circumstances ? the engine overheating or the cambelt snapping.

I am no mechanic but on opening the bonnet even I knew the engine had cooked itself

my question here was "how likely would there be serious bottom end damage based on what just happened. I had already assumed the head and headgasket were toast and just that would of been a best I could of hoped for.

after doing the compression test and noticing how laboured the engine spun my mechanic suggested bottom end, and low and behold he was right.
What is more likely,

1, All cylinders (head gasket/liners) failed causing all cylinders to lose almost all compression at the same time without warning?

2, Something which is common to enabling all cylinders to make compression failed without warning?

In an overheating situation, one would expect that one cylinder would fail first with a corresponding loss of performance, followed by another and another. You would probably notice 1 cylinder going down, 2 dead cylinders, almost certainly, particularly when towing 1400kgs!

Advice

1, Check the cam timing.
2, Check the function of the compression tester on a known good engine.




Little Pete

295 posts

18 months

Wednesday 4th October
quotequote all
I don't think people mean to be argumentative, it's just that some of what you've posted doesn't quite add up. However to answer your original question, if the engine has overheated to the extent that there is virtually no compression, it's highly likely that there will be considerable piston/piston ring damage. I'd be looking for another engine.

GreenV8S

24,916 posts

208 months

Wednesday 4th October
quotequote all
klmorrell said:
after doing the compression test and noticing how laboured the engine spun my mechanic suggested bottom end, and low and behold he was right.
I take that to mean he's looked at the bottom end. What damage did he find? Has he got a theory about how the damage occurred?

stevieturbo

12,711 posts

171 months

Wednesday 4th October
quotequote all
klmorrell said:
Just after leaving chesterfield the car died without warning.
klmorrell said:
I have just dragged a 1.4 tonne caravan over the peninnes for 60 miles and 3hrs with no expansion cap on. then the engine lost power and stopped, followed by an low battery light
And once again why we need clarity in order to believe anything that is being said including the test results.

Clearly losing power and stopping, are not the same as stopping without any warning.

And how there would be no signs of the engine ejecting water with no cap on.....that is just not likely. And when the engine is stopped, the battery light will normally be on. Or are you saying it came on prior to stopping ?

None of this really needs a specialist to diagnose, any halfbrain mechanic should be able to give a good shout at it with the car in front of them. With the compression test not being totally believable, what do the bores/pistons look like with a borescope ?
That should give an indication of whether you've really cooked it or not.