Leakdown - One Cylinder Really Low
 

Finally ran a leakdown on my '79 3.0

The numbers:

1 - 5%
2 - 4%
3 - 5%
4 - 98%
5 - 3%
6 - 4%

I was quite amazed at 5 out of the 6 numbers given the unknown condition of the motor.

The first clue that #4 had an issue was that the plug was black and oily. The rest of the plugs were nice and clean. The leakdown made it obvious. The exhaust valve isn't sealing. With the leakdown tester in place you can clearly hear air escaping through the exhaust. For all of the other numbers, air could be heard in the oil tank.

Looking inside the cylinder with my inspection camera, there doesn't appear to be any damage.

I adjusted all of the valves before installing the engine 2 years ago but never did a leakdown. I'll pull the valve cover and check to see if there is either a broken spring or if the valve adjuster is tight. Other than that, it'll be either a valve or seat issue.

The valve guides are worn so the heads should be rebuilt anyways as it does burn a fair amount of oil. I'll see what I find later!

Any recommendations for head rebuilders in the Northeast?

Where was it going? Case,int,exh?
Hard to beat Bob at Anchor Atlantic

Thanks Dan. As stated above, "With the leakdown tester in place you can clearly hear air escaping through the exhaust."

There might be a chip out of the valve..seen it before
Bruce

Valve cover pulled. No broken spring. Valve lash correct. Has to be some damage to the valve or seat. I'll post pictures later this week!

Sure sounds like a chip in the exhaust valve with that much leaking. Hard to imagine anything less would cause 98%! +1 on Anchor Atlantic. Quality, value and friendliness that are rare at any price.

Thanks for the referral! Here is another satisfied customer:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/548100-kudos-porsche-cylinder-head-repair-shop.html

Look inside with cheapo usb camera

Agreed. As stated above, "Looking inside the cylinder with my inspection camera, there doesn't appear to be any damage." Tough to see all facets of the exhaust valve with an inspection camera though.

It's coming apart regardless by the end of this week. I'll post decent pics of what I find.

Before you take the engine apart....try getting it very hot (running) and then dribble some cold water down the throat of the offending cylinder.
If there is a piece of carbon stuck on the valve or seat...it might break it off and send it out the pipe.
This is an old trick from the days of bad gas and dirt roads!!....LOL
Bob

Thanks Bob! Unfortunately, it's not that simple as I am running a 3.2 manifold. The valve lash checks out at .004". It's never idled right so I suspected at least one cylinder low. I just didn't realize it was THAT bad! It needs new guides for sure as well as some updates so it's coming out no matter what!

Well, not so fast...

I pulled the exhaust off to get a better look at the valve and found nothing wrong. Double checking my work, it seems that I crawled under the car and checked the wrong exhaust valve lash.

New numbers:

1 - 5%
2 - 4%
3 - 5%
4 - 3%
5 - 3%
6 - 4%

Doh! Crawling back into my hole now...

The joy of the savings will overtake the shame of a little mistake

So ... why is the spark plug black ... then?

...because cylinder #4 had little to no compression at idle due to the leak past the exhaust valve. Now with the valve lash properly adjusted, it won't be black anymore!

There must have been a piece of carbon holding the valve slightly open when I set the lash previously.

Glad to hear that your problem is not as bad as you thought.
The one black plug may be a bad plug!
I have encountered sets of plugs that have had one or two under-performing units.
Quality control back at the manufacturer was to blame...but testing at the factory is usually done in bulk...or perhaps only one out of a hundred is checked.
In any case...try a new plug...same heat range...and see if there is any difference.
If no difference...perhaps that one cylinder requires a slightly hotter plug to be happy.
My old drag engine used several different heat ranges...and manufacturers to get the best performance...probably had to do with torque and cylnder filling.
When dealing with a problem like this...I don't argue with the engine...I just go with what works.
Bob

moral of the story: adjust valves before changing plugs.