Low compression 1990 300zx

[h.hoffman]

Does anyone know about or have experience with the V6-2960cc DOHC engine
(non-turbo) having valve problems resulting in low compression? I am being
told my valves are sinking into the cylinder head causing the seal in the
combustion chamber to be poor. This will result in me needing a new engine!
With only 66,000 miles, this seems unreasonable.
Help!

[Muppet (Z LOVER)]

well how often do you change your oil and what kind do you use?

[Olympian]

It's basically the same engine that has been in Z's since '84. You
should not have problems with a 66K engine. This is a crock. The
valve seats and valves are alloy steel and not aluminum like the
heads. They wear slowly and do not sink. In any event, you do not
need a new engine. Most likely, you need a valve job. You could get
an automotive machine shop to replace every valve, valve guide, and
valve seat in the heads for probably $400 parts and labor. They would
probably rebuild the entire engine for around $1200. In addition, if
you can't remove and replace yourself, it will probably cost you that
almost much to take the heads off and back on again or get the engine
out and back in.

I'd bet on the valves. These are the possibilities:

If only one cylinder has low compression, you have a broken valve or
valve seat, i.e. a chunk is missing from one or the other causing that
cylinder to leak. This is unlikely but you could also have worn or
broken piston rings on that cylinder. If all or most cylinders have
low compression your timing belt slipped and all of the valves are
slightly bent. I saw this happen to one other Z where the timing
slipped and somehow slipped back allowing the engine to run, but
poorly. Wierd but true. If two adjacent cylinders have low
compression you have a leaking head gasket.

To trouble shoot you need a compression tester. You can buy a good
one at Sears for $20 - $40. Remove all spark plugs. Follow the
directions that come with the gage. Check one cylinder at a time and
record the average reading for each. They should be at least 128 psi
each and not more than 14 psi difference between adjacent cylinders.
If you find one low cylinder squirt a little crankcase hole into the
plug hole and check it again. If the compression rises significantly
with the oil, it's the piston ring and you need the entire engine
overhauled. If not it's a valve and you need a valve job. If two
cylinders next to each other show low, it's the head gasket and you
only need to replace that.

On Thu, 20 May 1999 00:43:02 -0400, "h.hoffman" <h.ho...@erols.com>
wrote:

[Darryl Duncan]

The only thing that's going to be poor is you if you listen to this
line of horse-puckey. The engine is an interference fit engine. If you
had "drooping valves" you'd be seeing pieces of your cylinder heads
before your eyes. More than likely, one or more of your hydraulic
lifters may need replacing. This means head repairs, not a new engine.
Find a new mechanic- preferably one that knows Z32's. Better yet, find
the local Z car club and they'll point you to the proper people to take
care of your car.

Darryl Duncan
turbo...@yahoo.com
90 300ZXTT
s/n 00084

[Pony]

sounds to me like some dummy replaced a broken timing belt without checking
compression.

[Pony]

How can a double overhead cam engine starting from 1990 be the same as a
single overhead cam engine from 84 to 88..........duh