Can you have good compression and still have a cracked block?

[Baltimore Sailor]

I've replaced the head gasket on my A4, and after break-in and re-torquing the nuts I have good compression on all four cylinders: 105, 95, 95 and 100, going from #1 to #4. When I first ran the engine with the new gasket, I did get some oil into my raw water coolant -- which I could tell by my exhaust water having a milky color -- but it stopped after running for a few minutes and it's never done that since.

I have noticed some other odd behavior, however.

Today I changed the oil three times, per Don's instructions in the manual for what to do if you get oil contaminated with water. Since I was only going to run the engine for a minute or so to mix up the oil, I left the through-hull closed and kept an eye on the temp gauge, and shut it down when it hit 180.

(A bit of background: last season I installed a t-fitting with a ball valve just after the raw water input, so that I could close the raw water off and open the other valve to draw antifreeze into the engine through a garden hose fitting.) So, the first time I ran the engine to mix up the oil, I left the raw water valve closed and opened the bypass, just to allow any remaining water in the system to be blown out with no vacuum in the line. The engine ran very nice and smooth.

The second time, I closed the bypass valve as well. And this time, the engine did not run so well -- the rpms were lower, and when I looked over the stern at the exhaust I could clearly hear misfires.

The third and fourth times I ran the engine I re-opened the bypass valve and the engine again ran great. No sputtering or misses at all, just a smooth-running sound.

Now I'm wondering -- what would cause this behavior? I've checked the raw water valve to see if it leaks when closed, and it does not. When I looked at the exhaust during that test there was no water coming out, which would seem to validate that point. I almost seemed like having a partial vacuum in the cooling system was causing the problem.

I do have an old set of points in the distributor right now, as I haven't replaced the electronic ignition yet -- though I did receive the replacement set (thanks, Don!). I don't know if that could have any bearing on the behavior, considering that the engine ran great with the bypass valve open.

Finally, I read Don's article about getting water in the cylinders through a cracked block or leaking manifold, and this brings us to my question: if I have such good compression in all four cylinders, could the leaky manifold or cracked block even exist? Wouldn't such things keep the compression from being as good as it is?

Sorry for the long post, but I figure more details are better than less.

Thanks!

[Don Moyer]

If I read your question correctly, your concern all seems to revolve around a different sound from the exhaust when you have cooling water passing through the engine and when you don't. If this is the case, I don't think you have anything to worry about. The exhaust will sound much "throatier" when cooling water stops passing through the exhaust system which you may be hearing as sputtering and perhaps even misfires.

As long as your engine runs well when under load (with the cooling water turned on - which is the normal condition) and no water is getting into the oil (which you've never reported), I think your engine is probably OK.

Don

[Baltimore Sailor]

I didn't explain myself properly about the different sound and behavior from the engine during the two tests. At neither time did I actually have cooling water running through the system. The setup I have is as follows:

Immediately after the through-hull valve is a t-fitting with another shutoff valve that leads to a garden hose fitting so that I can shut off the raw water and allow anti-freeze to be drawn into the system. A rough sketch below:




At all times the valve from the raw water was closed; the difference was noticeable when having the bypass valve open or closed. That was what surprised me, as I would have thought there would be no difference.

My other question, in a nutshell, was "Can I have good compression with a cracked block?" I was also wondering about good compression with a bad manifold; but I figured out that even with a manifold that was leaking water into the cylinders through the exhaust valves, I could have good compression when all the valves were closed.

[Don Moyer]

A cracked block (normally in one of the cooling jackets) does not usually affect compression unless it enters one or more combustion chambers and causes one or more valves to stick, or compression rings to stick in their grooves.

Don

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