Hello Anchor,
Glad to hear that you found the previous efforts helpful. Common problems can often be fixed by throwing a
set of fresh plugs at the motor. Given the troubleshooting effort you have put in to date, this has the look &
feel of an Uncommon problem. :0) But with a disciplined troubleshooting approach we should be able to take
this problem pie and at the very least figure out what are the good 7 slices vs the remaining bad slice. (ie: I believe
that the SBC is a proven design, follows the laws of physics, and we just have to figure out where your specific
engine deviates from the formula that is known to work in millions and millions of engine bays in the real world.)
Disclaimer: I have troubleshot my entire career, but at the same time I'm still learning this remote troubleshooting
thing. I never realized just how much better it is to have your head under the hood as opposed to studying photos
and typing thoughts in coherently. Doing this remotely feels as clunky as trying to properly share the experience
of watching the movie '2001' using Morse Code. A lot seems to be lost in the translation. :0)
But if you are willing to sort this out in public, so am I. Fortunately for us, there's lots of talent in the audience
who can jump in and keep this moving in the right direction.
****
So with the disclaimer out of the way, let's step through the detailed current status you shared:
What you described (a steady 0° with the ECM wire disconnected) *is* expected behavior. I'll go
a step further, and tell you that if you didn't have a steady reading with the ECM disconnected then
we would have to stop here and figure out where the slop (spark scatter) was coming from.
(Bad timing chain, excess knife-edge wear in the cam/dizzy gears, excess vertical up/down travel
allowed in the dizzy shaft, or excess clearance/play in the dizzy bushings.)
In English, a steady 0° reading with the ECM disconnected is no small win. Let's keep that in the back
of our mind and proceed to the next step.
This is also expected behavior. IF the timing remained at a steady 0° when you
reconnected the wire we would have to stop and troubleshoot that. (Open wire
in the harness, ECM not responding, etc.)
Again, this is a win in my book. A feature, not a fault.
OK, now we are entering an illegal condition in the mechanical realm. A valve stem with enough friction
between itself and the surrounding guide that a stock SBC spring with the following specs (80 lbs
at installed height, ~200 lbs at full lobe opening) couldn't close it, then that's no bueno.
Given the spotty operational history since this engine was put together (plus the fact that at
least 2 other owners have tried to sort this out) ...I'm in agreement with the others that we
need to label this valve stem/valve guide area as suspected bad, and you have to do what it
takes to prove to yourself that the clearances meet factory specification in this area. (!)
(EDIT)
This is purely hypothetical, but if the exhaust valve had been (mis)adjusted at some point
to where the exhaust valve and piston tried to share the same space at the same time, then
it's entirely possible that the exhaust valve stem was bent/distorted just enough that
it was jammed into the valve guide? Remember the valve spring pressure against the
retainer is anywhere between 80-200 lbs, so finding the valve stuck open tells me that
there's a problem in this area...even if it did free up. No telling if the 'sticktion issue'
doesn't try and return once the valve heats up and expands during normal operation,
sticks in the process, and causes misfires in this suspect cylinder, leading to a false-lean
reading by the O2 sensor, which richens up the A/F mixture, giving you both rough running *and*
poor MPG to boot.
NOTE: The vast majority of valves that fail/drop their head into the cylinder were whacked
at some point prior to the failure. Stress fractures + thermal cycling = failure.
If you were my neighbor I would be starting the 'let's remove the cylinder heads as a safety
precaution and find out what's going on' sales job. I'd rather take the chance of removing
the heads and finding zero issues with the values as opposed to telling you that all is well,
only to have the suspect freed-up exhaust valve either continue to cause the driveability/MPG
issue at best, or worst-case, completely fail, possibly taking the piston/rod/block/head with it. (!)
OK. The Snap-On shows no knocks while driving but you can get them by tapping on the engine.
Excellent! This is a troubleshooting win, right up there with the steady 0° reading
with the wire disconnected and the ECM taking active control of the spark advance
once it's reconnected. We are tightening the problem description with proving what's
good.
Good question. The short answer is no, for properly operating hydraulic flat tappets and hydraulic roller lifters are the
same internally in the area where you are trying to get the lifter preload into the designated window. (1/2 to a full turn
past the start where a loosened/clattery lifter goes just quiet.)
Sounds to me that the valve preload is currently set so deep that any
further downward pressure at the rocker arm ball is enough to hold the valve
just enough off the valve seat that this cylinder starts to misfire. (Most people
don't realize that a 250hp V8 throttled down to only make ~5-7hp at a hot idle
is a pretty fragile internal combustion balancing act, easily disrupted.)
To me, both the sticky valve you discovered AND the too-easily disrupted
valvetrain both indicate to me that one of the POs must have had the valves
set way too tight. And when the valves in the head are off, they can be super
difficult to troubleshoot unless you have some hard-won experience in this area.
Good on you for being so observant, for I think this will be the key to eventual
success in this engine bay.
You are in good company, for I have also lost confidence in that valve that you discovered
hung/stuck open. Thinking about this, a valve that was properly adjusted could only
stick somewhere in the normal lift. Assuming for a second that the preload was set
properly but the valve stem guide was too tight, then the valve would have stuck
somewhere in the normal lift area, and the chances of piston/valve contact would be
slight.
But IF whoever assembled the engine way overtightened this particular exhaust valve, then
piston/valve contact would be a much higher probability, and I can see this being wounded
right out of the box, and never drove quite right from the first mile after the rebuild?
**** Where do we go from here?
1) If at all possible, I would recommend that you minimize how much this vehicle is driven in
order to avoid sudden failure of a previously overstressed part. (ie: The valve you found stuck open.)
2) Get the engine running, and since you are sitting at 0° base (initial) timing, add 4-5° degrees of
advance and see if this doesn't improve the drivability. If it does, let's remember this for after
you reinstall the heads.
2A) Or you decide to just go ahead & pull the heads at this point...and experiment with the ignition
timing after reassembly.
3) Time to stop poking around the edges. Given all that we know at this point, a normal tuneup or
sensor replacement isn't going to fix what you've observed. Pull the heads, and get them to a
SBC-savvy machinist. Explain the history to him. In an abundance of caution, be sure to identify
the valve that you discovered hung open, and ask him to retire it from service.
What you want from this machinist is all 16 valves moving freely in their guides, and providing a
positive seal when sitting on the seat. The better machinists are pretty exacting about all this,
hopefully you can find one of the better ones. (I've had my best luck with word of mouth. If you
know any local racers, they usually know which machinists are the wizards as well as the duds to avoid.)
4) Put it all back together, and methodically assemble the valvetrain and set the preload. Once the
engine is started, set the initial timing at 4° BTDC and let the functioning KS alert you if this is
a good or greedy setting. Since the combustion chambers are carbon free, I'll bet that it runs
smooth & sharp.
****
That's plenty enough for now. Like I said at the beginning, there are common problems that are
easily sorted out, and then there are the Uncommon problems that require us to dig a little deeper.
I'm assuming that you were able to buy this truck at a discount because the engine was part of the
problem instead of part of the solution. Even so, I am confident that this *is* fixable. The engine
isn't overheating, you didn't mention any noises, the ignition & ECM seem rational, etc.
But the engine runs rough and gets poor gas mileage. Both of which could be explained by the
valve getting whacked hard enough that an 80-200 lb. spring couldn't close it. (By the way, due to
hyperextension/misadjustment the interference fit might present at .600" lift, but since your cam only lifts it .480",
once you got it to close it hasn't been pushed all the way back out to the bad spot. This would help to
explain why the problem hasn't reoccurred since.)
Anyway, I share the above as food for thought, not a guaranteed diagnosis. I see that Schurkey has beat me
to this. It's my sincere hope is that we've independently come to the same general conclusion. :0)
Best of luck. And let's document this fix for those who come here troubleshooting similar GMT400 powerplants
that are resisting repair.
Cheers --
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Will post after my heads are checked out. With the valve train that turned out to be installed stock heads seem like a bad fit. Thanks
