[Shotimes] What can destroy a cat?

[Kevin & Cheryl Airth]

Michael:
 A compression test is just the first test you need to do. You do NOT need
to remove the manifold, unless for access to the spark plug holes. Thread
the compression tester into the spark plug hole and crank the engine (just
use the key and starter and unplug the DIS) until the reading is steady. The
absolute number is less important than how even they are from cylinder to
cylinder. Adding a little oil can give you another indication of ring seal.
You really need to run a leak down test. This gives the actual % of leak
down. A good tight race motor might be at 1-2% a good street engine at
around 5% and over 10% things are starting to go south. More oil burning and
less power! This also can help define where your problem is. With a leak
down test make sure you listen carefully for air escaping at the tail pipes
and the intake manifold. This will tell you if it's an exhaust or intake
valve problem. So the compression test is mainly testing the ring seal and
the leak down test includes the rings and the valve seats.
 Valve guides can wear and cause oil consumption but more common is the
valve guide seals deteriorate from time and heat. I don't know how to
measure directly for worn guides without taking things apart? It's kind of a
logic problem. Check everything else for the cause of oil burning and if the
only cause left is the valve guides/seals? But guide wear causes bigger
problems than oil burning as it progresses. As the guides wear it lets the
valve start to "walk" a bit on the valve seat and this is what the problem
is. The valve seats get hammed by the valve seating a little off center
every time and don't seal as well and then you really get some loss of power
and very rapid valve seat  wear!
 Valve stem seals are installed on top of the head around the valve stem.
They are held stationary by friction. They are hammered on a metal stub that
is machined in the head/guide for them. They are hard to see because they
are hidden by the valve springs. The valve guide is just the round passage
way that the valve stem runs in. They can be machined directly into the head
or can be made of a different  material and installed. When you get the
guides replaced most shops put in a superior material called Manganese
Bronze. This is more self lubricating than cast iron, so has superior wear
properties. Most stock engines have cast iron guides. Here is a good picture
from Ebay that has the guides,seats and valves pictured. It's not a SHO but
a old FE Ford engine.
http://tinyurl.com/2g4vz

 Valve guide wear can also be found using some other tests. The seals are
under maximum stress when the engine is decelerating and in gear.
Compression braking. This applies a hard vacuum to the stem seals and if
they are leaking it draws oil into the combustion chamber. Then when you
step on the gas it should really smoke for a little bit then clear up. So if
you blow oil smoke on acceleration all the time that points to the rings. If
you have a blue cloud on startup, that points to guides and seals or a
sticking/broken piston ring.

Sorry that this is so long. Hope it helps!
.
.
PS: SHO's are NOT known for oil burning at that mileage but every car is
different. My 2 SHO's are very good on oil but they are both ATX's. The 160K
mile one still uses less than a quart in 5K mile oil changes with Mobil 1.
My 135K mile one uses about a quart in 3500 miles. How much oil are you
using?
.
.

> Thanks for the tip.  How is the test actually done?  I assume the car
isn't running since the only place to measure the pressure would be at the
sparkplug which means the manifold is off. Do I crank it manually or do I
crank with the key/battery (in which case I should probably disconnect all
the plug wires, right?)
>
> Btw, if you have a problem with bad valve stem seals (Is this the same as
valve guides?), firstly, how do you know this? And secondly, aren't you
concerned about it?
>
> Btw, btw, don't all SHO's smoke as hell when you're on the throttle and
between shifts?  Mine has always done this, I think, cause every time I do i
t, I can actually see the smoke in the rearview mirror.
>
> Michael Olsen
> 94MTX,green,BOS+