What I would PERSONALLY check is the timing chain. with that many miles,
when was the last time the timing chain was done?
The chain inside the motor which connects the cam to the crank may be worn,
and then the chain can get loosened by acceleration and deceleration and it
suddenly runs rough, then all of a sudden the slack comes out of the chain
and it runs a little better but not for long
Lots of ways to determine If this may be part or all of the problem some are
hard, some aren't:
Check it by pulling all the plugs and the distributor cap, put a big wrench
on the harmonic balancer bolt and move the engine back and forth with your
left hand and hold the distributor rotor button (or watch it carefully) as
you move the bolt back and forth, if every time you move the crank the
distributor does not instantly follow, that is the slop in the timing chain.
depends on how bad it is, but it really doesn't take very much slop to make
it run really rough and throw the whole core engine timing scheme off.
New timing chains have NYLON impregnated into the top sprocket which bolts
to the cam, the nylon gets brittle with age and starts to come off in pieces
this creates, (yep) timing chain SLOP or loosening. If you change your own
oil, run it down through a T Shirt or rag, if you find little chunks (or big
ones too) of nylon, plastic stuff, Time to replace the timing chain.
Another way to tell is put a timing light on the engine while its running,
if the timing mark seems to wiggle back and forth and everything else is all
tuned up, this can be your problem, you can be fooled by this one as it may
do that for a variety of reasons, but if you have checked absolutely all the
plug wires, cap, button, and fuel pressures and you still have a rough run,
might be time for a chain
You can buy aftermarket chains which do not have the nylon, but even those
eventually (about 125,000 mile ago for you) wear out.
So depending on how sloppy the crank to camshaft connection makes a huge
impact on how a Chevy runs.
Its fifty to 75 dollars worth of parts and will take you a day to do it, if
your a good mechanic. If this is your problem, and the chain fails the
engines exhaust values will be left to defend themselves against the
pistons, usually the pistons win this fight, but they do a lot of damage to
the pistons and can crack the head also.
So any time I find a Chevy that has a mysterious "run rough" and it has a
lot of miles on it, I like to figure out really quickly how good a shape the
timing chain is in.
If you try to get a mechanic to do this, it may cost you a lot more than the
truck is worth, so choose your mechanic carefully, if you have a lot of
deferred maintenance issues then you can expect them to want to fix all of
it before they warranty the timing chain, as to do it you have to pull every
belt and hose in it as well as the radiator, and move all that crap out of
the way to get to the chain, you can easily expect to replace all belts,
hoses, radiator cap or whatever else needs attention on the way in and out.
The other mysterious run rough is the Harmonic balancer, you didn't mention
whether or not you heard any noise, if you hear any clanking, especially on
gunning the motor, your harmonic balancer has a rubber insert in it, which
could be worn out. If the harmonic balancer appears to wobble that piece of
rubber on it may be worn out. (hope its this as this is a cheap fix.)
Hope this helps and Good luck figuring this out.
"pegreenb" < firstname.lastname@example.org> wrote in message
Click to see the full signature.