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  #1  
Old 10-25-2020, 01:45 AM
volvo-express volvo-express is offline
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Default Loss of compression post head refurbishment.

Hi all,

I had reason to replace my cylinder head and I had a spare...so I had it sent off to local machine shop. Got it back and fitted it with a new MLS gasket and bolts. Set up timming as per green book perfectly and when started it ran like a dog.

I double checked all timming and ok....did a compression check and on average each cylinder is 17 bar.

As part of head refurbishment the shims were to be replaced with thinner ones. I've last night quickly "as it was raining" on cylinder 1 exhaust clearance and I was expecting 0.40 and it's more like 0.25.

Question is.....if the machine shop ground the valves and the seats but just used the old shims again would this cause the loss of compression?

I will check the other valve clearance and write down the results .
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  #2  
Old 10-25-2020, 08:58 AM
v8volvo v8volvo is offline
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Checking the rest of the valve clearances is a good idea, but as long as there is some amount of clearance, the exact amount should not affect the compression pressure (when measuring at cranking speed on a cold engine, at least).

With valve clearance, it's pretty much a black or white question: either the valve is closing all the way, or it isn't. There's not much middle ground to cause "low" compression -- typically you either have full compression with a sealing valve, or zero if a valve is not fully closing.

Also, it would be rare for valve clearance problems to affect the compression in a way that is highly consistent from cylinder to cylinder. You said an average of 17 bar (=250psi for those us reading in the US) -- were the cylinders all pretty close to 17 bar, or was that an average taken from a lot of variation (e.g. some cylinders very low, but others higher like close to normal spec?)

When you had the head off, did you spray any kind of solvent on the cylinder walls that could have washed them down and made it hard for the rings to seal?

Have you tried putting a little oil down the cylinders and seeing if that changes the compression readings? That is usually one easy test to show if the loss of compression is from the rings or the valve train. If the pressure increases significantly with oil added, then the ring to cylinder seal is what you're looking at. If the oil causes no change, then it's more likely the valve train.

A leakdown test, or even a simple seat-of-pants audible leakdown test involving just putting some air pressure to the cylinder and listening for where you hear it leaking out (listen to crankcase thru oil fill cap, and also listen at intake manifold and at the tailpipe) will tell you where the pressure is escaping. So that's another way to narrow it down.

I'd start with a "wet" compression test (with a few ounces of engine oil down each cylinder before testing) and see what that does. If you can report both your original dry compression test pressures and the new wet pressures for each individual cylinder here, not as an average of all 6, that will then help us get a lot closer to understanding what may be going on. Hopefully something simple.
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  #3  
Old 10-25-2020, 12:55 PM
volvo-express volvo-express is offline
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Default Wet test

Hi,

Thank you for such a detailed reply.

The compression average was fairly level rather than average of extremes.

I went out this evening and poured oil down the bore as suggested and did a compression check.....on cylinder 1 went upto 27 bar and cylinder 2 went upto 29 bar!
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Old 10-25-2020, 07:46 PM
ngoma ngoma is offline
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Adding a small amount of oil to the cylinders raised the compression appreciably? As v8volvo stated earlier, that can point to piston ring sealing issues.

Now come the questions:

Is there an actual problem with the ring sealing? Did the engine sit a long time inop? Sometimes that can cause the oil normally present around the rings to dry out. But you say it ran poorly in your test? They should easily have relubed and sealed up by then.

What is your compression testing method? Adaptors? Engine cranking speed?

Did they all consistently raise compression by the same amount?

Your measured 29 bar (421 PSI) is quite good! Close to factory spec new (455 PSI), well above min. (313 PSI).
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  #5  
Old 10-26-2020, 01:21 AM
volvo-express volvo-express is offline
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Default Compression test

After seeing the reply I went out to the car last night and did the oil / wet method just out of interest on no.1 and 2 pistons. Can imagen I'm feeling a bit hacked off at this as it's my daily driver.

Anyways the car as not been layed up for prolonged period...maybe 1 week without the head on it. When preparing the deck I used quite a bit of penetrating oil so maybe that washed the rings?

Is the best to soak the pistons.. ie drop oild down the bore and hopefully it will lube the rings again?

Later this morning it will be interesting to see if pistons 1 and 2 will return the same compression without putting oild down the bore.
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Old 10-26-2020, 01:50 AM
volvo-express volvo-express is offline
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Default Piston rings

Hi,

This is 2 pictures of the bottom end when I too off the head, I was quite pleased with how clean the pistons were and the cross hatch on bore.



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  #7  
Old 10-26-2020, 08:03 AM
v8volvo v8volvo is offline
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Quote:
Originally Posted by volvo-express View Post
... car as not been layed up for prolonged period...maybe 1 week without the head on it. When preparing the deck I used quite a bit of penetrating oil so maybe that washed the rings?

Is the best to soak the pistons.. ie drop oild down the bore and hopefully it will lube the rings again?

Later this morning it will be interesting to see if pistons 1 and 2 will return the same compression without putting oild down the bore.
Sounds like you are on the track now. Penetrating oil or solvent used for cleaning the block surface could well have made its way down into the bores and washed them down.

When I have a head off I'm in the habit of wiping a clean cloth soaked with clean engine oil around the cylinder bores just before reinstalling the head to make sure the rings are lubed when the engine starts again. Maybe lacking that let the ring seal suffer. The increase in compression with the wet test is a good sign that it'll come right back. It can be a little deceiving since the volume of the oil also increases compression ratio and can theoretically cause part of the boost in the compression readings with a wet test, but with such a large increase you saw, I think it is a good sign that loss of ring lube and seal was the main issue.

It probably would have come back in due time running the engine, as ngoma said, but that would probably take a while, maybe not until the oil warmed up and thinned out. Meantime the rings and bores might see excess wear. So the best play now, especially while you already have injectors out, is probably to manually add some oil to lube the rings, then retest, reassemble, and be good to go.

The one risk you want to be sure avoid when putting oil down the injector holes is making sure the engine can't end up in hydraulic lock. The preferred safest method is:

- put a several ounces of clean motor oil down each injector hole into all 6 cylinders, plenty of oil, not just a few drops.
- then, using a wrench on the front of the crankshaft, slowly turn the engine by hand through several full rotations. This ensures that the pistons have cycled in the bores a few times and get the oil spread around the perimeter of the cylinder bore.
- now, expel the excess oil from the cylinders (to prevent hydrolock) by cranking the engine on the starter with injectors out. This will blow any excess out of the injector holes. During this step, putting rags across the top of the engine (weighted down by a few pieces of wood if necessary) can help limit the mess, otherwise it'll shoot oil mist everywhere.
- finally, reinstall injectors, etc, and start it up. Or, if you wish for confirmation/recordkeeping purposes, perform a compression test first, then reassemble and run.

A method like this we have seen revive some long sitting engines that had almost been given up on for dead.

One other tip -- make sure the glow plugs are disabled while doing all of this, since the plugs wet with engine oil will make a lot of bad smoke if they become energized and get hot. And definitely make sure they are disabled if you do any compression tests with oil in the cylinders, since the hot plug plus the compression provided by the tester can cause the oil to ignite like fuel (diesel is fuel oil, remember!) and the engine try to start on that cylinder, which will blow up your compression tester!

Best way to disable them is to remove the fuse for injection/glow system, or just unplug the small black ground wire from the bottom of the glow relay under the hood.
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  #8  
Old 10-27-2020, 11:22 AM
volvo-express volvo-express is offline
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Default Running perfect again

Hi,

Thank you for the helpful tip regarding the oil - put it in yesterday and today put it back together and it works great again!!
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  #9  
Old 10-27-2020, 04:37 PM
v8volvo v8volvo is offline
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Great news

Coming back to your question from your first post -- now that you have the main issue squared away you still may want to go back and re-check valve clearances (cold engine) all the way around and adjust as necessary. Even though the valvetrain wasn't involved in the compression loss problem, you still will want to get those clearances set correctly to spec, especially if they are already a little tighter than they should be. For one thing, if the new head got a valve job, then the clearance may decrease faster than usual in the first few thousand miles as the seats and valves go through their initial wear-in process. You don't want to let it get away from you and tighten up to where the clearance goes to zero.

[That is, if your engine has mechanically adjusted solid lifters -- which we assume it does since you mentioned shims -- but are you sure that is the case? The late '80s or early '90s vehicles in your profile might have had hydraulic valvetrain from the factory, requiring no adjustment, unless the engine was at some point changed for an earlier one....]

Either way nice work, sounds like you did a lot of good refurbishment and gave this engine a new lease on life.
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