Greetings socal k1500,
You have certainly done your homework and made (nearly) every reasonable effort to cure
this mystery #4 misfire when climbing hills. Plus you are sharing live data on the issue. And
you dug up that GM TSB (PIP3081 dated 7/23/2004) having to do with these come & go
misfires (particularly from the center cylinders #3/#4/#5/#6) under heavy load.
From my perspective all this shows that you are serious & have what it takes for us to
sort this out once & for all. BTW, you are fortunate in the fact that someone who's already
slogged through a near identical issue has been generously sharing his hard-won first-hand fix.
(@SableSlayer
) And being somewhat new you may not realize the proven troubleshootingexperience that @Schurkey brings to the table -- his comments are well worth reading for
comprehension.
****
Given the above, it we were neighbors I'd be telling you that for the expenditure of time, effort,
and cost of a full intake manifold gasket set, for relatively short money (DIY) we could find out
IF curing the underlying 'lean from a leak' issue that the fuel trims are indicating might be just
enough of a fix in order to avoid stimulating the fault you are experiencing.
Why? Because lean = more heat.
And the SBC has an architectural hot spot that only a very small % of the owners stimulate via
sustained high duty cycle operation. But at the same time which happens to be even easier to hit
when the cooling system is no longer able to move the necessary quantity of BTUs from the coolant
to the air passing through the radiator...which may explain why this is happening to you. (!)
Of course while I'm trying to be a good neighbor & sell you on replacing both upper & lower intake
manifold gaskets and fixing the neglected cooling system, I would at the same time be setting
expectations that this last-gasp reasonable repair effort may or may not be the final fix. But spending
$100 in order to really prove/disprove the need for a "$3800" cylinder head replacement makes
sense to me.
And before I write any more, I'd like to share this photo that helps to explain why that TSB
could explain how an architectural hot spot in the SBC (side by side exhaust valves in the center cylinders)
could cause these exhaust valve stems to grow enough under sustained hot, heavy load to stick in
the corresponding exhaust valve guides:
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(credit: Unknown. Vortec head gasket failure photo between center cylinders)
After looking at this Vortec cylinder head photo, please re-read reply #9 to this thread. (#4 cylinder missing, but only under heavy load)
And keep in mind that SableSlayer was able to stop the madness by simply fixing the intake manifold gasket. In English,
he cleared the fault before permanent damage requiring $$$$ parts replacement was done. (!)
And also reread reply #14, and try to visualize how it might cure the issue of a cylinder misfire due to a sticky exhaust
valve...especially if the problem disappears once the heavy pulling is over. (TSB PIP3081)
Last but not least, after a fair amount of rabbit holing I came across an 18 year old thread over in the gm-trucks
forum where the problem was observed, the TSB was read, the cylinder heads were removed, and they found
physical proof the exhaust valve/guide clearance was the root cause of his issue: (Cylinder #4 misfire when towing up a grade)
{And to the author's credit he closed the loop by sharing the results...}
PS: Just to show that I'm not part of the anti-Bowtie brigade, I've attached a photo
of a BBC cylinder head showing the symmetrical exhaust port layout that helps prevent
the same kind of hot spot from occurring in 454 engine bays. (Hint: Look up a Mark I 348/409
cylinder head -- were the exhaust valves siamese in the center or symmetrical like the
Mark IV?)
Food for thought. Hoping you are able to address this before the mystery irritating
intermittent issue degrades to constant 7-cylinder sadness. :0)
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