Re: What does "No compression" mean?

Programbo wrote:

> My 1994 Jeep Cherokee (Inline 6 4.0) started running really rough last
> week as if one of the cylinders was missing. I changed all the plugs
> and wires and cap/rotor and that fixed nothing. I was thinking maybe
> an injector wasn`t working and turned the Jeep on and unplugged each
> injector one at a time. Unplugging 4 of them made the engine start to
> die off right away but the #2 and #3 injectors didn`t seem to change
> anything when I unplugged them. So I figured those injectors were dead
> and replaced them. No change again. Still running really rough. (I
> know it was stupid wasting money but I really needed to try and get it
> running as I had a trip down the highway I had to take on the
> weekend.) I finally got tired of messing with it and took it to a
> repair shop and the guy there called and told me, "There`s no
> compression in those two cylinders." To make a long story short I just
> told him to leave it alone and I`d come pick it up. But today I was
> off work and went out to mess with it and I unplugged and removed the
> 2 spark plugs for those 2 cylinders (They were black with soot) and
> when I fired the Jeep up there was a lot of air being pushed out of
> the plug holes as the engine stroked. So I guess my question is: Isn`t
> that compression? I`m trying to eliminate possible simple fixes before
> junking this thing. Thanks for any input.

Even a cylinder with visible air gaps between the piston and the cylinder
wall and valves stuck open will push a pretty fair amount of air out the
plug hole - but their may be no significant compression.  The only way to
get meaningful data is to use a compression tester.  You can probably
borrow one from a local parts house but they only cost something like $20.
 

Did they just give you a "no compression" or did they specify a number
like
0 PSI?  Having 2 adjacent cylinders go tits up at once while the others
are
OK suggests the likelihood of a leak between the two cylinders or
something
common like a leak to the outside from a common part.  The head gasket
comes to mind - but that's going to be a sizable leak for a head gasket. 
Get a compression test and re****t the actual numbers for each cylinder so
we can take a better WAG.

One thing to note: since the plugs are fouled black in the two bad
cylinders, we know they are actually firing and burning the fuel but they
are not burning it completely, which would be a fair indication of low
compression as the main problem.  The fact that two adjacent cylinders are
affected would also tend to make me think that it may not be a
catastrophic
problem - unless you've manages to crack a head or cylinder wall.

-- 
Will Honea
** Posted from http://www.teranews.com
**