Car acted pretty sick on the last pass of the season. Great, up til then. Cylinder leakage on 6 of the cylinders ranged from 5% to 7%. However, #5 leaked 20% and #6 leaked 50%. For you Ford guys, no, that is not indicative of a head gasket. They are in opposite banks on a Chevrolet. Think there is anything else I should do/check (besides crossing my fingers and praying) BEFORE pulling the heads off? Take care. Tom Worthington

During the leak down are you able to detect if the leaking sound is coming from the carb area, the headers or valve cover? Remove your rockers and check all the spring heights to make sure they are all the same too. It is possible to narrow it down to broken springs, bent valves or rings without pulling the heads. Good luck!

The leakage was audible from the crankcase (oil fill cap), but it was no more on the bad cylinders than the good ones. I MAY have been able the hear some leakage at the tailpipe on the 50% cylinder. Nothing from the carb on any of them. Jeez I really hope it isn't broken rings. If the issue is at least isolated to the head(s), I will feel as though I got lucky. Take care. Tom Worthington

Squirt a little oil into the cylinder and see if it changes any. Heard this will help determine if it’s the rings as the oil will help seal the rings some for leak down testing.

I've always heard that one associated with compression tests. Might be worth trying on another leakdown. Take care. Tom Worthington

Ok Tom maybe my thinking is wrong as it’s been a while since I heard it.
The way you describe it as opposing cylinders has me thinking something went done the intake and bounced between cylinders. I had this happen when I broke part of a valve off, it visited three other cylinders before I got motor shut down.

In my opinion you already know what you need to do. The car acted sick, and it’s giving you bad numbers on the leak down. Just go ahead and dive in, gaskets are relatively cheap if you happen to find that something isn’t amiss inside the engine.

How old is the motor? Might be time to just pull it and freshen it with new rings and bearings too.

quote:

Just go ahead and dive in...

If he pulls (ie peels off) the heads and they are OK, how is he to access ring wear or broken rings short of damaged cyl. walls? I second the oil test.

Cant hurt to try the oil test, may tell you something. But I bet it's a valve / valvetrain issue if it all went bad on one run. Ring seal would go bad slowly over time I would think ??

quote:

Originally posted by Eman:
How old is the motor? Might be time to just pull it and freshen it with new rings and bearings too.

This....

My opinion is if it leaks that bad it would be time to go through the engine.. They do not fix themselves..

Dave

Tear into it! This is the only time of the racing season when you have 3 months to get it back together. Time and gaskets. Hopefully, it's nothing serious.

It was fresh in March. I destroyed (threw a rod) the last one in January (which had been in there for around 35 years).

Oh, I know it has to come apart. As someone mentioned, I just don't want to give up a good diagnostic test while it's still in one piece. Take care. Tom Worthington

I'm with you, I'd rather know than guess.
Did you do the leakdown cold? Many times rings won't seal good on a motor that hasn't been run. Did you attempt leakdown at TDC on each cylinder? Since you are looking at pulling the heads regardless can you pull the headers and get a better idea if it's coming out the exhaust. Pulling the carb will give you a better idea if it's leaking in the intake.
With the oil deal I'd squirt a little in and turn the motor over and then check it. Did you do a compression test? What did the spark plugs that came out of those cylinders look like? Did you have the radiator cap off when you leak checked it to see if it might leak to the cooling system?

quote:

Originally posted by Canted Valve:

quote:

Just go ahead and dive in...

If he pulls (ie peels off) the heads and they are OK, how is he to access ring wear or broken rings short of damaged cyl. walls? I second the oil test.

I would normally agree except he said it acted sick the last run which is why he was looking for issues in the first place. I personally do every diagnostic check I can to try and talk myself out of pulling an engine apart but usually it ends up coming apart anyway.

quote:

Originally posted by Eman:
I'm with you, I'd rather know than guess.
Did you do the leakdown cold? Many times rings won't seal good on a motor that hasn't been run. Did you attempt leakdown at TDC on each cylinder? Since you are looking at pulling the heads regardless can you pull the headers and get a better idea if it's coming out the exhaust. Pulling the carb will give you a better idea if it's leaking in the intake.
With the oil deal I'd squirt a little in and turn the motor over and then check it. Did you do a compression test? What did the spark plugs that came out of those cylinders look like? Did you have the radiator cap off when you leak checked it to see if it might leak to the cooling system?

Yes, I did the leakdown cold, each piston at TDC. I think I may try that pulling the headers first trick. Didn't do a compression test. The plugs all looked uniform. Yes, I was watching the coolant very closely while the air was applied. I had sort of convinced myself it was a headgasket, before I even started. I only managed to confirm that wasn't it. Darn (unless it was both gaskets and it only impacted one cylinder on each side...highly unlikely).

I am thinking I might be able to confirm a broken top ring once the head is off. With a fair amount of piston to bore clearance, I can usually look all the way around the top ring and determine if I now have more than one end gap. At this point, I am leaning toward a leaking exhaust valve, but we'll see.

I appreciate all these suggestions. I sure hadn't considered many of them. The heads were gonna come off, regardless of what I thought the issue was, beforehand. 50% leakage isn't from a bad sparkplug. Take care. Tom Worthington

I referred to spark plug condition looking for an indication of what's going on in the cylinder. Color of plug can help greatly in diagnostics.
Since you're going to pull the heads pulling the intake will give you a good view of the intake valves and like pulling the header it will allow you to feel the port for leakage.
The joy of pulling BBC heads off with the motor in the car is not fun especially if they are iron!

I second the compression test idea. And repeat it 3 times on each cylinder, watching the gauge as the engine is being cranked. Worked for me, after the leak-down test had me ready to pull a head. It can help you find a broken inner spring, because the pressures will bounce around and not be consistent. And even with the most thorough visual inspection of installed springs there is no way a normal person can see 100% of every coil in every spring.

If you think it’s a valve or spring then pull valve covers off and do leaked down test and tap valve to see if it changes while air is applied. I found a bent valve this way that barely tapped the piston.

Doing what Green1 says will also point out a worn guide that allows the valve to not seat properly.