low compression on 1 cylinder

[BP10000]

I have just purchased a replacement engine with unknown kms from member Burn_your_money. He tested it and the compression was 420-250-380-420. He figured it was a bent valve caused by a crank nose beginning to fail. I have torn it down to discover that the crank nose and pulley are like new, and the valves all look fine. There seemed to be a lot of carbon build up in the combustion chambers though, and some white junk around the prechambers. There are the faintest signs of intake valve contact on the tops of the pistons but I think the impression is only in the carbon coating rather than the metal of the piston. Also there is some tar-like substance around the rims of the cylinders above the swept area of the rings.

Could the carbon/pitting on the valves be the cause of the low compression? In this case, would lapping them in fix everything?

Some pics:

[Rabbit TD]

If it were me I'd take the head to the shop and have it pressure tested and a valve job and cleaances adjusted at that point if they are mechanical.  ARP studs and a new set of rings and it'll probably go another 150,000.  If it's a hydraulic head somebody else will have to advise you on that.  I've never had one and really don't want one with their associated problems from what I've seen on here. 

[BP10000]

Here's a picture of one of the piston crowns, showing (poorly) some slight erosion near the prechamber, I suspect from a worn injector nozzle. I think the one with the lowest compression is slightly worse, but I don't have a picture handy.



does this look like cause for concern?

Could the same junk that is coating the piston have worked it's way into the rings and reduced their sealing ability?

[Baron VonZeppelin]

Its a hydraulic head.

In your last picture, it sort of looks like the head gasket might have been starting to let go just a little between the two cylinders. Could have been where some of the compression was being lost. And possibly some valve escape too.

Everything else looks fairly normal for wear/accumulation where an owner didn't use a fuel additive. And very acceptable for easy rebuild.

You got a nice easy engine to freshen up IMHO.

Could probably get by with lapping the valves. But looks like from the ridges at top of valve seating area that they should be cut for best/longest lasting duty. Might as well have the seats trued also. Check for straitness of the deck on head - which is probably fine.

Get Maxfax's attention and ask for an opinion.
He's torn down a lot of these and snapped them back together with his shoe strings and elmers glue in a pinch. He's familiar with allowable limits.

Would hone the cylinders too.

Looked like a 1 notch headgasket in one picture.
If you don't get pistons cleaned off, might wanna go to a 2 notch.

[burn_your_money]

Glad to see that the internals look ok on the engine.

I would agree with your assumption about the bad injector wearing the piston a bit. From that picture it doesn't look like anything to worry about.

[maxfax]

I'm also in agreement with an injector being naughty...

Also intersting that the center 2 cylinders were the weak ones....

I've lapped worse looking valves and gotten away with it..   But pending on what your plans are for the engine, and the size of your wallet, I think I would look into having the valves and seats ground... If there's any question on the head surface being straight, it would be a good time to take care of that as well...

 How do the bores look? Especially near the tops where the darker circle is??   They appear to be pretty good in the pics but it's hard to tell for sure.. A bore gauge woudl be best here, but crude redneck measuring consists of, is there enough of a ridge to catch a fingernail after all the gunk is cleaned off???

At this point I have to concur, valve job and rings and she oughtta be good for a while...

[BP10000]

Update: I lightly lapped all the valves and slapped her back together. I did find one lifter that felt stiff instead of springy, and it was on the low cylinder, so that might have been the problem all along. Got everything back together today and (after spending about 4 hours searching for a socket for the injectors) she started right up!

no smoke at all idling. lots of power, running great!

Thanks everyone. My first engine swap

[Baron VonZeppelin]

Cool !

[burn_your_money]

Right on. Did you recheck the compression?

[BP10000]

nope, don't have the tools for it. and I need to unbolt the IP to get my socket on injector 2. So I think I'll leave well enough alone for now.

[Baron VonZeppelin]

How much service did you do on the engine ?

Did you leave everything else alone and just lap the valves, replace a lifter, and new head gasket ?

[zukgod1]

Sounds like your good to go already..

I would have done what you did, lap and reassemble. The only other thing I would have done is quick hone and new rings. MLS head gasket and rock that baby!!!

Good job getting it back together..

[BP10000]

That's about all I did. I could just see some very faint diagonal hone marks still in the bores. New rings were not in the budget, but I did let it sit with some liquid wrench in the cylinders, so hopefully that broke down the tar on the rings if it was present.

[BP10000]

One more thing:

I've only turned the head bolts 1/4 turn past the initial torque. Should I do a certain number of km before finishing it off, or do it on a warm engine, or some other special circumstances?

Haynes says do it all at once, but I think it was different on my old jetta.

[burn_your_money]

looking in my mk3 bentley it says 33 then 44 then 1/4 then another 1/4. No mention of heating, warming, waiting etc.