My '76 2.0L is putting out a little smoke on hard acceleration. Is there a way to tell if the oil is leaking past valves vs. piston rings ? The old trick I always heard was to test compression, put a little oil in the spark plug hole, and retest. But that won't work on a horizontal cylinder.

My hunch is that it's the valves, but only because its easier for someone (PO, not me !!!) to screw them up by not doing regular valve adjustments. My records are a little patchy, but I think the engine was rebuilt less than 50k mi. ago. Any consensus on what tends to go sooner ?

Thx

Not valves, but valve guides (which have nothing to do with adjustment), can cause smoking. However, the usual rule of thumb is that a car will smoke on acceleration if it's rings, and smoke on deceleration if it's guides. In either case, I'd do a full top-end rebuild (new P&Cs, new guides). The oil in the cylinder trick works fine on a horizontal cylinder, btw. You just have to crank it a bit to get the oil splashed around some. You only want a few drops of oil to help seal the rings.

Never recommend a top end only rebuild...

The engine ages evenly, if the top end is worn, so is the bottom end.. Strip it down and do it rigt, or not at all....


Top end rebuilds tighten up the top end and create more of a challenge for the bottom end, new found top end power will show you a weakness in the bottom end very quickly! Don't learn that the way I did.

The usual way do diagnose a compression problem is to do a leak-down test. Put the compression fixture on your air compressor. Insert the other end in your spark plug hole. Rotate the engine to TDC of the compression stroke for that cylinder and turn on the air. The differential compression reading indicates how much you're leaking. You can then listen to hear where the air is coming out. If its coming out the crancase breather its rings. If its coming out the intake or exhaust manifold its valves (or guides).

If you have it on TDC it can't leak past the guides.
Both valves would be closed.
Leakdown will tell you if you have bad rings or bad valves, but not valve guides.

Yes. The only way to tell if you have bad guides is to pull the heads off and find out. Compression and leakdown tests only check ring and valve sealing.

Bad valve guides will show up on start up. If you see a cloud of smoke when the engine starts up it is usually attributed to sloppy guides or in real engines, bad valve stem seals.

If the engine is tired usually you will get blow by at higher rpms and you will see it in the rear view mirror. A compression check won't necessarily show bad rings unless there is a mjor problem. Leak down is a much better indicator of engine health. But you can look at plugs and exhaust pipes as well to see signs of sealing issues.

Wow,

You folks have chops. I never knew why leakdown was better than simple compression. Also, I would never have guessed that the "oil trick" worked on horizontal cylinders (I was kind of dubious about the slant of a V-8...) And Jake, man, you shattered my hopes of an easy fix...

Anyway, I'm having probelms passing emissions (see new thread) and I'll have the leakdown test done as part of that adventure...

Thx,
Jeff

Get it waived. Adjust your valves, buy a couple case of dino motor oil (keep one in your front trunk) and drive the piss out of it.

In the meantime save up for a new motor and replace the old one when it blows.

QUOTE (BMartin914 @ Sep 30 2005, 11:57 AM)

In the meantime save up for a new motor and replace the old one when it blows.

Hey Ben,

That's kind of where my thoughts have been drifting... Been reading threads about Subaru installs, and the NA 2.5 is sounding really tempting. Of course, I do need to keep the $ rolling in. Kinda hard w/ 914world.com back up and running....