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boycie

P38 compression test

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Posted (edited)

Hi Folks my 4.6 Thor engined p38 has started to tick over lumpy and is down on power ,if you pull the plug leads on cylinders 3 and 5 it makes very little difference to the way the engine runs ,I’ve changed the plug leads and the coilpacks with some secondhand ones I had in the garage I’ve changed the plugs around but it’s still missing on cylinders 3 and 5 .I have ordered some  new coilpacks,leads and plugs but in the meantime I want to carry out a compression test ,does anyone know what pressure I should expect to see  from a healthy cylinder ?.

Edited by boycie

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Not sure which of the 4 messages I should respond to 🙂 When I have done this in the past , I test all cylinders and look for a significant variation.

Cheers

Peter

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Four messages have I buggered it up .That makes sense I will check the compressions tomorrow and see  if 3 and 5 are low .

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The pressure should be around 150psi. I know cos mine has between 50 and 75psi and that is far too low to allow it to start.

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Posted (edited)

Thanks Simon ,all your cylinders were that low what caused that ?.

Edited by boycie

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Well I got back early today so I carried out a compression test on the P38 I think I may have found the cause of the problem the results are as follows cylinder 1. 10.5 bar, cylinder 2. 10 bar, cylinder 3. 3.75 bar ,cylinder 4.  10.25 bar ,5.  4 bar, cylinder 6. 10.5 bar ,cylinder 7. 10.5 bar ,cylinder 8. 11bar , I think I may have a problem with the head gasket what do you think ?.

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Yup, blown between 3 and 5.

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That’s what I am hoping,I will make a start on Saturday and let you know what I find .

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I managed to get time to strip the offending head off of the car yesterday I thought you might like to see what I found ,does it look like it’s had a top hat liner block fitted ?.

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Yup, that is a linered block for sure. Lucky boy.

And that gasket confirms the problem, glad you found it so easily! :)

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Thanks Bowie I thought it was ,I was like a dog with two tails when I saw those liners but I was not a hundred percent you have confirmed it for me .The heads look newish as well so I’ll take my time putting it back together, I have a Kent cam to go in it .

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Did the head bolt and head surface survive the heat?

The tops of the heads look pretty grubby, as does the Valley. My guess is that is that it has had the top hat liner block for a while, and not had that many oil changes since. Good job on changing the cam, should wake things up a bit :)

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What a lovely surprise! Have a good look for wear on the rockers and shafts, they're often worn with the cam. Worth checking if you've got a lovely new cam to go in it :)

Check the head for warpage with a straightedge too, just to be sure it's not that that caused the gasket failure. 

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Spent the day grinding valves and changing valve stem oil seals the head is back on I’ll strip the other one tomorrow ,I don’t mind telling you it’s a bit hairy to say the least torquing the head down I said to Jackie make sure there is a clean pair of pants on standby I’ve got to go around another 90 degrees .

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Hi Bowie every thing seems fine the head bolts seemed ok the head looks flat all the valves look good ,the only thing I could find not a 100% is the valve stem seal were hard ,so Iv replaced them  it’s a bit of a mystery to be honest .

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Sounds good :)

Stem seals go hard with age, another indicator that while it is top hat linered, it probably has done a fair few miles.

I'm still jealous though ;)

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On 5/21/2019 at 6:51 PM, boycie said:

Thanks Simon ,all your cylinders were that low what caused that ?.

Piston rings are stuck. I am going to strip the engine down do what you have to the top end and put new rings and hone the block. My block is a lovely golden colour in the valleys so I think it has been well looked after.

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Well I’ve done 30,000 miles in the car since I’ve had it Bowie ,I’m not disappointed with it really I only paid just over a grand for it 4 years ago and Jackie loves the thing which makes spending on it much less of a problem.

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Indeed, sounds like an excellent purchase :)

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