Over the last few years, I swapped out a 4.5L with a 4.7L in my 81. Used 4.7 short block, cleaned up, valve job, new gaskets and seals.
Initial startup went fine, first 50 miles were good but with rough idle and power loss. A week ago, emissions passed with very good values.

Since, I've been troubleshooting the rough idle and power loss.
Today I did compression checks. All cylinders are 155-160 except #7 (CORRECTION: #4 last cylinder passenger side) which is 40lbs!!! I put some oil in the cylinder and only 45lbs...

If it is rings, I would think it would burn oil / smoke and emissions would have failed.
That said, if I remove the oil fill cap and filter and place a paper towel over the top of the oil fill, it pops up when the engine is cranked w/o plugs.
Which could mean air getting past rings into the crankcase.

I removed the intake tube for #7 and the intake value looks good (no discoloration) and it opens and closes when turned.
Exhaust valve could be the issue, not sure how to check without pulling the engine?
I've put about an eye dropper full of seafoam into the cylinder and have been hand turning it.
All the bores were inspected, cleaned and oiled before assembly but the pistons were not removed.

I'm not sure from here.. any suggestions or ideas would be appreciated.

Thanks for the help

 

Do a leakdown test.

 

Quit wondering why....you will find out when you get the driver's head off.

 

This morning when I found this, I knew I was pulling the engine again...

 

a borescope will also provide you with info before the pull

 

Yes, however it won't change what needs to happen....

 

It happens to the very best mechanics.

Things go wrong.

Parts fail, even new ones.

I personally dislike pulling these engines....it just sucks up so many hours.

I'd pull the cam carrier, with the engine in the car, and hope that it is a problem in the valve train.

Once the cam carrier is off, it's a really quick job to pull that one head, if needed.

With everything clean and fresh you could start at 8am and have the head off by lunchtime, without breaking a sweat.

 

I had a similar dilemma, so I pressurised the suspect cylinder at TDC, with compressed air via a spark plug adapter, to determine if I needed to pull the head or pull the motor. I reasoned that if you get major air flow via intake, out the exhaust, or water jacket ... then I could pull the cam box/head ... and it was a gasket or valve problem. If I got major blow by out the oil fill throat ... then I knew I'd have to pull the motor.

Using a tyre pressure gauge on a compressor, but with a spark plug adapter fitted at the end of the hose, I identified rings as the culprit on a single cylinder ... and knew I had to pull the motor. All other cylinders would take about 6 - 7 seconds to leak down from 100 psi to 10 psi on the gauge .... the one with a broken compression ring lost pressure in 1.3 seconds, with enough blow by to spin the basket in the oil fill throat.

 

Now that's brilliant. Nice diagnosis. Thanks for sharing, Dave !

 

I corrected the origiinal post. It is cylinder #4 last one on the passenger side.

 

This was my reasoning when I took the oil fill cap off, removed the basket and put the paper towel over it. If air is getting past the rings into the crankcase then this would be it's path of least resistance.
As I cranked the engine with the compression gauge in #4 the paper towel popped up on each cycle. Because of this I'm thinking it's rings.
I did not however, do the same on a different cylinder with #4 open.

 

Here's all I used ... took off the tyre pressure hose, changed it to the other one in the pic, to screw into the spark plug port. Results were absolutely obvious and conclusive. Even though it was only one broken ring, the difference was massive. Rather than a using a percentage leak down tester (which compares leak down to a measured orifice leak down), I've always used this basic identification method.

Throttle wide open and no air cleaner so you can hear it, exhaust easy to hear, coolant tank top off to let air escape, oil filler cap off to hear and see blowby. Lock crank at TDC of each cylinder tested, so motor won't spin. Raise to 100 psi ... start stopwatch ... see how long it takes to leak down to 10 psi... simple and cheap.

Paper towel would respond to normal blowby, compressed air pressure was massively different on a leaking cylinder, and actually spun the basket around. With 100 psi, air from other areas is noisy, and just as easy to identify.

 

Thanks for the setup pic and the procedure.
I was wondering if normal blowby could be the towel movement.
I do need to do the test and see.
What kind of compression values did you have with the broken ring?
Did you just replace rings in the one cylinder or do all 8?

Thanks for the help

 

Just looked at my records and coincidentally mine was #4 too !! I've altered the leakdown times to what my records show as accurate, and not what I recalled. Results were still massively different.
(6 - 7 seconds compared to 1.3 seconds)

I had a compression on that cylinder of 110, compared to all the others at 165, and it was just down on power. 10ml of oil in each cylinder raised normal compression cylinders to 185, and the one with the broken ring to 175. As yours is way lower, and hardly responds to oil, it's still a strong possibility its top end ... test will tell.

I did a complete strip down and replaced all rings.

No matter what the outcome you're still pulling something apart ... hopefully it's only top end ... fingers crossed.

 

Id be thinking about swapping in ladybugs engine.