If an intake valve fails to open the symptom is the same as a bad
spark plug, as you describe. If an exhaust valve fails to open
symptoms are much worse. The cylinder is still filled with gas/air
mixture, it is still ignited by the spark plug, but because the
exhaust valve does not open the burned mixture has nowhere to go
until the intake valve opens again. At that point burned gas/air
mix is ejected back into the intake manifold causing backfires
and messing up the mixture to the rest of the cylinders. Does
not sound to me like your failed rocker was to an exhaust valve.
The loose adjustment was probably the cause of the premature
failure of the rocker arm. With hydraulic lifters there is no
"slack" in the valve system. Solid lifters (I doubt you are
using them) have a few thousandths of an inch of clearance.
In either case, too much clearance will cause failure of the
weakest part, probably either the rocker arm or the push rod.
Starting at the front of the block on a Chevy V8, the valve
sequence is exhaust, intake, intake, exhaust, exhaust, intake,
intake, exhaust. So if your bad rocker was the sixth one, it
was an intake.
If it were me, I'd replace the rocker arms as a set in my
driveway on a cool morning instead of one at a time on the
road dealing with a hot engine and a limited tool set.
Probably the pushrods too.
-----Original Message-----
From: GremlinGTs@aol.com [mailto:GremlinGTs@aol.com]
Sent: Tuesday, May 02, 2000 9:00 PM
To: oletrucks@autox.team.net
Subject: [oletrucks] broken stamped Chevy V-8 rocker arm...causes?
Ok, somewhat off-year ( 1974 Chevy 3/4 ton truck ), but applicable to all
vehicles....my truck has had the valves tapping for some time ( had the
heads
rebuilt 2-3 years ago, some 30k miles on it ), and coming back from a road
trip, it started running extremely rough. Drove another hour with it like
this and got it home ok, then it sat for 10 days before I could get to it.
Pulled the valve covers off, (because we had heard the rockers tapping at 60
mph, so I knew that they needed "some" adjustment...) and found a rocker arm
had cracked in half, on the # 5 cylinder. Rockers were extremely loose on #2
and #6 cylinders as well. I bought a replacement, did rough adjustments to
take the major play out of all the extremely loose ones, then put it all
back
together...bingo, runs great, better than before the "accident". Now, my
worry...I didn't readjust the valves after the initial adjustments right
after the rebuild, and it's been like that for 2-3 years. Would those loose
rockers also have received enough stress to start them to crack as well?
Should I be concerned about them? I"ve had many Chevies for years, ( '57,
'64, '66, '71, '73, etc ) and never seen a cracked rocker arm. The truck has
125k miles on it total, so are those arms due for replacement? I bought a
spare rocker arm to keep in the glovebox, in case it decides to break
another
one. Just wanted to know what some list members think about this.
Also, running the engine with a valve closed constantly, what might be
the
consequences of that? I think it was the 6th valve, don't know whether
that's
intake or exhaust side, but it might have been working some before the
rocker
totally sheared in half. My dad's Chevy truck had lost a pushrod before, and
it bucked something fierce after that, my truck just ran rough. maybe a
sticking valve, and the new rocker might start doing the same thing? just
hoping for the best right now! I'm gonna drive it short distances for
awhile,
and see how it does. Thanks for any input, Oletruckers....
Jerry Casper
'74 Chebbie running again, but worried about it...
55 Suburban
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
|