Having a BAD day and may need an engine rebuild.

[Shawn W.]

I recently noticed exhaust fumes coming from my oil breather cap on the driver side. I have been doing a number of simple checks for temperature, firing, etc. and decided to do a compression test. The test showed my #3 cylinder at ~50 psi and all other cylinders were around 185 psi. The engine is a 427/390 HP and had some work done on it ~ 40 years ago (cam, forged pistons, etc.). I have a friend with a bore camera and the cylinder wall look OK. Unless I am mistaken the compression test results indicates a bad piston ring. Any other thoughts? Assuming the worst I am guessing I will need to have the engine tore apart and rebuilt. Any idea how much I could expect to spend on something like this?

[Terry M.]

Re: Having a BAD day and may need an engine rebuild.

While a compression test can be useful, a cylinder leak down test will tell you more. Using the cylinder leak down test one can determine if the issue is valves or rings rather easily, but like so much else there is a minor learning curve to it. If you have an adapter to introduce air into the cylinder through the spark plug hole (You may be able to get one from any of the large parts stores that "rent" tools.) you can set #3 to top dead center (valves closed and piston at the top of the bore) and listen for where the air comes out. Listen at the intake (carburetor), exhaust (muffler) or the oil breather. You will get your answer.

[Joe L.]

Re: Having a BAD day and may need an engine rebuild.

While a compression test can be useful, a cylinder leak down test will tell you more. Using the cylinder leak down test one can determine if the issue is valves or rings rather easily, but like so much else there is a minor learning curve to it. If you have an adapter to introduce air into the cylinder through the spark plug hole (You may be able to get one from any of the large parts stores that "rent" tools.) you can set #3 to top dead center (valves closed and piston at the top of the bore) and listen for where the air comes out. Listen at the intake (carburetor), exhaust (muffler) or the oil breather. You will get your answer.

Terry and Shawn-----


I agree. A cylinder leak-down test should be the next step here. A decent leak down tester kit can be purchased at Harbor Freight Tools.

Assuming that the problem does turn out to be the rings in the #3 cylinder and assuming that there is no damage or significant wear in that bore and assuming that the rest of the engine is in good condition, you might get by with removing the left side cylinder head, removing the oil pan, and removing the #3 piston for inspection and re-ringing. Even if the piston needs to be replaced, this could be done as long as you could obtain a replacement piston of the same design and weight as the piston that's in there now. If the bore turns out to be damaged or worn, then you'll need to remove and rebuild the engine.

[Jim T.]

Re: Having a BAD day and may need an engine rebuild.

I had a engine that on start up one day did not run right. Compression test showed no pressure on one cylinder. Problem discovered to be a pressed in valve seat had dislodged and holding open a valve. New valve seat in the head fixed it.

[Michael W.]

Re: Having a BAD day and may need an engine rebuild.

I had a engine that on start up one day did not run right. Compression test showed no pressure on one cylinder. Problem discovered to be a pressed in valve seat had dislodged and holding open a valve. New valve seat in the head fixed it.

Another strike AGAINST hardened valve seats.

[Duke W.]

Re: Having a BAD day and may need an engine rebuild.

You didn't say if you did a "wet" compression test. Pump a little oil into the cylinder. If compression comes up, it's likely a ring issue. If not, it's likely a leaky valve.

If it's just a valve you can repair the one head. If it's rings and the engine has to come apart, it's a good opportunity to return it to OE condition, and you have the option of massaging the heads for more power.

This OE configuration will be much better for normal driving than a high overlap cam, which is probably what was installed.

Cost can only be estimated once you know what has to be done to solve the problem, and even then it can vary widely depending on what else you decide to do, how much you can do yourself, and how much you have to farm out to an engine shop.

Duke

[Joe L.]

Re: Having a BAD day and may need an engine rebuild.

Another strike AGAINST hardened valve seats.

Mike-----

Yes, as I've mentioned previously this sort of eventuality is absolutely the biggest concern when hardened valve seats are installed. It has to be weighed against the benefit of the hardened seats (which is virtually nil).

Keep in mind, though, that all modern engines with aluminum heads use valve seat inserts for BOTH the intake and exhaust valves. However, these are usually of an INTER-LOCKING design which precludes their dislodgement.

[Larry B.]

Re: Having a BAD day and may need an engine rebuild.

I agree that a leak down should be done next but also check cam.If valve doesn't open there won't be compression.Another would be to fill radiator start it and look for bubbles.They can blow head gasket to water.Normal is to next cylinder,but not always.Valve seats don't fall out if put in right. Let me know what you find.