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zKars

Another Engine, another mystery. Another day in the life...

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So today I'm tearing down a very worn high mileage F54/P79 ZX motor.

It had low compression, and each cylinder was all over the place.  10 lbs, 50 lbs, 80lbs. Runs like crap. Anyway..

I take off the front cover and "clink" on the floor goes the chain tensioner piston. Pick it up and find about 1/2 the spring inside it.  This is not good....  The tensioner body is still nicely attached to the block. There is no way the piston can get back in the body, I tried. How did that get out? Well it did. I wonder how much oil comes out of the tensioner, and simultaneously NOT get where its supposed to go.  

I decided to do a leak down test before I rip the head off just to see if its the valves or rings. Well with pressure on every cylinder I have this nice little draft coming at me. Right from the sump with the front cover off.  Anybody wondering how the rings are now? Not me.

As a bonus, with air on #1, there is a really really nice draft out of its intake port. Oh goodie.....  And for you doubters, yes I did remove all the rocker arms before doing this to be sure they are all as closed as possible. 

Next we'll take the oil pan off and find that other 1/2 spring. Or its dust....  And check some rings.  The bores all look new, the usual no upper ridge and you can still see some cross hatch. Love these blocks.

 

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Pictures.

Not sure this resolves much. The #1 valves are different than the rest, but all pistons look pretty much the same.

Note: the scratch mark across the top of every piston top is a remenant of TDC finder probe, not an interesting artifact.

Not sure if I will have a chance to tear down the block this morning before the next Z shows up at my door. I'll try.

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Both #1 valves are black, while the exhaust valves of all the rest are nice and white. The carbs were running very rich in few hundred km prior to this.  Well #3 isn't much better... 

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Edited by zKars

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As to the mystery of the tensioner escape, I have a theory. If someone changed the head, but lost the tensioner in the process by not using a blocker, or just didn't know better and just put it back together anyway, I guess that might explain how it escaped it housing. The broken spring is just what happened later when it got caught in a nasty place. Good thing that tensioner found a snug home in a dark corner, so close to the chance of instananeous and total disaster

One thing I regret is not checking the cam timing when I took it apart. With no tensioner, makes you wonder if the chain ever skipped a tooth or two at some point. No indication of valve contact so things didn't get that out of hand. About the only clue to wrong valve timing is the fact that the motor didn't like to exceed 4 or 4.5k RPM. Never had a chance to determine if that lack of fuel or something more sinister...

With that loose chain, I would have expected more chain noise, but there was none. Maybe it was the fifteen exhaust leaks that masked the noise.

I do now that the block was rebuilt at some point in its life, and likely at a real engine shop. I noticed that the engine builder I use here put something I hadn't seen on a couple of frost plugs. And I see them on two frost plugs on this block as well.

I believe them to be a tell-tale to detect either excessive temperature or movement that might indicate a condition that would nullify the warrantee. Removing them is apparently a sin according to their inscription.  Any one see these before?

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Edited by zKars

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Yes, I have them on my Camry with a lower mileage motor from a wrecked car.  The place that sold me the motor has their name on there's too.  One is on the cylinder head another on the block.  They gave me a 90 day warranty but made clear if those were missing the warranty was voided.  He said the adhesive attaching them lets loose at a certain temp.

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Ok, I couldn't stand it. I tore the b down. Grab your popcorn.

First the debris from the tensioner spring migration. Found it! It made friends with #2 piston.

No this is not a piston return spring. Must have been a hell of a ride!

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After taking the pistons out and inspecting the rod bearings, well, there were a few extra oil flow guide grooves...

This is #2 and #1 rod bearings. Nice grooves. Some others have embedded bits...  Other than defects, the bearings are great. No hint of age or wear through. Other than maybe #1 there...

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Edited by zKars

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Now on to the pistons themselves. Rings should be dirty, gummy, nasty, worn, something.  Ah, no...

They look like they have about 10,000 miles on them. Clean, free, no obvious defects. At least not on first inspection. More in a bit.

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Now with #1 intake valve not sealing, very little in the way of serious combustion was happening there. I was able to clean off the black carbon with a quick solvent wipe. And what should reveal itself to me beadie little eyes?

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Two little eyebrows divits with the rear (intake) being a bit deeper and longer. I guess we know why that intake valve is not sealing very well. And maybe we understand a bit more about the chain timing accuracy. Me not think it "ideal"

 

Edited by zKars

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6 minutes ago, Zed Head said:

What are you planning for this old war horse?

Plans? Well I seem to have P79 head that needs some new valves. Looks like a recycling candidate. Now that nice flat top F54 block with the new piston reliefs, now that has potential!

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So now that I obvious valve / piston contact in #1, let's check the others.

All the others have normal combustion products on top and have to be cleaned with a wire brush (yey! Actual power stroke!), but the two I checked also have the new valve reliefs, though to a much lesser degree. They all had no audible leakage out the intake or exhaust ports during the leak down test (stethoscope used) so they might not be bent, they are just very very good friends with their pistons now.

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The last remaining mystery to me is the really obvious air leakage into the sump during the leak down test. Just how is that air getting down there?

So I see two clues. First the ring end gaps.

I popped #1 top ring off and stuffed into the bore, and measured the end gap.  0.020 ish. Hmmm, FSM suggests .010 to .015 is normal, with .040 max,. So .020 sounds ok, maybe a bit wide. 

With no obvious damage anywhere, about the only thing I see that is not "right" is a really random location of the three end gaps around the pistons. Some have the top ring gaps right next to each other, some are spaced roughly 1/3 of the way around. I likely moved them some while handling, I don't know how much weight to put on this. 

The walls were all well-oiled, how else does air get past the pistons? Usually I do this test with the motor buttoned up so I never get to hear whatever air escapes past the rings normally in a healthy engine? The leakdown tester was always saying the amount was well into the green/great/good region.  I don't think this means too much. The head gasket was great, no issues there. Oil was water-less, water was oil-less.

Finally I did a quick bore inspection. Only saw one scratch in #1 that can feel with a finger nail. Everybody else is nice and clean with clear hone marks.

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So we have a block with 250,000 + km. stock 86 mm pistons,  evidence of engine work at some point, rings that don't look the age of the block, poor erratic compression ( that BTW does improve a bit with oil added) only one clearly bad cylinder valve wise, a timing chain with no tensioner, that must have slipped or was put back together without it, that resulted in in-consistent piston smoochin'',  

The other clues I noticed:

-oil pan gasket is not a stock gasket. 

-chain guides were very worn. Not down to the steel, but grooved and very thin.

So fellow detectives? What say you? Not that it matters, she needs a full round of rebuild love.

 

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I agree with your theory...tensioner came apart and they just ignored the spring when returning the tensioner to its home.  Chain timing was off when they put the tensioner back or it skipped a  link or two due to the missing spring and caused the valve/piston contact.  The timing chain being off also contributed to the lower compression and the bent valves contributed to the variance in compression.

Edited by 240260280

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I really thought there would be more damage to the top of the pistons.  The tensioner is definitely the key here, like you said the spring was misaligned  or pinched, the driver over reved it, the spring popped allowing lots of slop, the intake smacked the piston, the driver may have heard or felt something and backed off but the damage was already done.  Tough engines.

5 hours ago, zKars said:

No this is not a piston return spring. Must have been a hell of a ride!

Ping ping ping Ricochet Rabbit

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I've wondered if a loose chain alone is enough to allow the valves to hit the pistons.  Doesn't seem like it should unless the camshaft can some how rotate faster than the crankshaft is pulling it, from inertia.  The inertia of the mass would have to overcome the valve springs and friction.  Maybe at high revs, on a downshift, or something similar.  With no tensioner on the loose side there would be a lot of slack to over-rotate on, if it's possible, even if the sprocket timing was right on the tight side.

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did you measure the pistons and bores? maybe someone bored-honed the cylinders without replacing the pistons, which would result in blow-by past the pistons. just an idea

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