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Discussion Starter #1
I've been chasing down a misfire and I performed a compression test while the plugs were out. Six cylinders are giving me 145 PSI but two of them are way up at 175 PSI.
What would be the likely cause of this high compression? Carbon buildup? Why would it be just in two cylinders? Leaky injectors maybe?
 

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I've been chasing down a misfire and I performed a compression test while the plugs were out. Six cylinders are giving me 145 PSI but two of them are way up at 175 PSI.
What would be the likely cause of this high compression? Carbon buildup? Why would it be just in two cylinders? Leaky injectors maybe?
Did you turn the engine over by the same number of revolutions for each cylinder? If you did then there's a possibility that you have 2 leaking valve stem seals which are allowing oil to enter the 2 cylinders with the higher compression. Did you take note of the condition of the spark plugs as you removed them, did 2 oily plugs match the 2 higher compression cylinders? Perhaps leaky injectors could cause a similar high reading, if you disconnect the fuel pump it would eliminate or confirm that theory.
 

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Discussion Starter #3
Hi Shadow11, I turned the engine over the same amount of times for each cylinder and all of the plugs are equally black and oily, they're also fairly new. I put a good squirt of clean oil into one of the cylinders with a lower reading and it made no difference.
The fuel pump was disconnected, I was more wondering would a leaky injector lead to higher carbon buildup in one cylinder over time. Having two cylinders 20% higher than the rest seems odd to me.
 

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I cannot explain why you have the different pressures but I recently did a compression test on a late SY engine and the results where very very similar, two cylinders much higher than the others. From memory, I think it was the front two cylinders that were higher compression.
It was compression tested because the engine ran very rough but the fault was in the fuel system, carburettor fault.
 

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Discussion Starter #5
Glad to hear I'm not alone :)
The two high cylinders are A2 and A4, not even beside each other. The car was stood for a a decade and was fairly neglected before that so it's full of little surprises.
 

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From memory I think that the normal compression rate on these V8s should be roughly 140 PSI (don't take that as gospel, you should verify further) , they're a relatively low compression engine. Your readings probably do indicate a carbon build up on A2 and A4 ( caused by either bad injectors or leaking valve seals or both) but I think that a leak down test might help to narrow the problem a little more.
 

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Discussion Starter #7
Just looked up the specs and 145 for a 9:1 engine is factory standard. Would a leak down test not just identify problems with lower compression? Or am I missing something? I'm thinking I might pick up a cheap borescope and have a look, if anyone has any recommendations?
If it is indeed carbon buildup, is there any way to remove it other than pulling the head?
 

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I would not jump to conclusions at this point. Your car needs to be driven ( put some miles on this) and then do another test. Two cylinders that are a little off will not change much, only your perception that something is wrong.
 

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Borescopes are readily available on eBay etc., for a few quid, I can't really say which manufacturer is best, I reckon they all do the job adequately. It would be helpful to scope and compare cylinders for carbon build-up which might help to confirm your suspicions. I suggested the leak down test just in case the 2 cylinders with the high compression readings were actually the good cylinders but since you have confirmed that 145PSI is normal then a leak down test probably wouldn't help in this situation. I've used the trickle of water into the air intake (after the MAF sensor) with a warmed up engine running at 2000 RPM to steam clean the cylinders and I've detected a slight improvement in performance afterwards but that could be merely my perception based on optimistic expectations, I've never done a borescope check beforehand and afterwards. Wraithman's suggestion to drive the car for a while and see if there's an improvement in performance following a good long drive is good advice. I think I would fit new spark plugs and inspect their condition every few hundred miles to compare how efficient the combustion is between cylinders and if there's a lot of smoke at cold start I would have a look at the inlet valve seals which could be contributing to carbon build-up if leaking. Cylinder liners can warp when corrosion builds up between the liners and the block as a result of lack of attention to coolant changes or of not using the correct concentration of coolant but that's a worst case scenario that hopefully won't be the problem in your situation, the fact ha the car was lying up for so long is, however, slightly concerning.
 

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Like Shadow II suggested, a fresh set of plugs will never hurt and it will establish a baseline for further diagnostics. Plug readings can tell a lot.
Borescpes are cool. I use one that has attachments such as a hook that unables the camera to look backward which is helpful when trying to look upward at valves. Mine is LED illuminated, has focus control, and you can snapshot along the way. It will work with a smartphone, tablet, etc. You have to download an app.
 

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Discussion Starter #11
Thanks for the common sense guys. Yeah drive the thing and check back :)

Still have a few more jobs to get done before she goes back on the road for the first time in 13 years! Hopefully soon.
 

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Hi!
I agree with driving the car first, and worrying about the compression later.
I read up on your story - very brave of you to tackle a free Bentley that's been standing so long!

Cheers

Es
 
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