P0303 Misfire only under load

[Schurkey]

I'm going to start trying to track this down myself, I'll post updates as I go.

Still waiting on those updates...

Same! Bump

Did you perform the diagnostic steps already outlined in this thread? What are the results?

 

[Road Trip]

98 K1500. Stock original motor. Have had truck for 15 years. Had this problem for a couple years now but just recently started digging into this to fix it.

So here we go- Truck runs smooth everywhere EXCEPT going up hill(cresting a hill), going to pass on the freeway, merging, anything above 55mph that has any load on the engine. I have to be really ginger and watch how long I hold the gas pedal so it doesn’t misfire. Misfire is strong engine will shake and won’t stop until I let off. I have to wait about 5 seconds otherwise if I get back on the gas it will start shaking again. WOT is ok from O to 45mph. Any longer misfire will occur. Can’t really go 25% Throttle for more than 5 seconds. Cruising at 70mph is great. If I push it to 75-80mph misfire will occur.

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My 1998 Yukon just started doing almost the exact same thing. P0303 only under load up hills. No other codes showing. Any luck with the diagnosis? I'm going to start trying to track this down myself, I'll post updates as I go.

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Same! Bump

Greetings SableSlayer,

Looks like both @Jacobchevys & @98yuk haven't been in here for awhile, so although my comments
pertain to all 3 forum members, I am going to focus on your situation, since you are currently active.

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The SBC in our GMT400s can falter in one of 2 ways. The first, and most prevalent way, is
simply a need to refresh all the maintenance items/consumables. The engines tend to miss the
driveability/performance/economy goal in about as many nonspecific ways as there are engine bays.

The other way is when after a lot of time/miles accumulate they fail in a very specific failure pattern
despite having all the maintenance items up to date & in a serviceable condition.

A) For example, the engine isn't running quite right, but not using coolant. Nevertheless 2 adjacent cylinders
fail a compression test. Especially if it's the 5-7 neighbors where the PO had the spark plug wires run adjacent
to each other for most of the run. And after disassembly, it's discovered that the head gasket failed between
those 2 cylinders. And the smart money is that there was a 5-7 misfire going on for some time, with detonation
stretching the head bolts/lifting the head just enough to cause the resulting underclamped head gasket span
to fail?

B) Another specific failure pattern is the constant consumption of engine coolant over time, but there's
no signs of external leakage? The compression test doesn't really show anything all that far from
normal, but during a follow-on leakdown test when (only) cylinder #4 is pressurized that air bubbles are
showing in the radiator cap opening? And when a family photo of all the spark plugs are taken, the
#4 plug is way cleaner than the others...as if it was steamed cleaned? Removing the cylinder head,
it's discovered that the head gasket failed between the cylinder and one of the water jacket openings?

C) Or in Vortec-land, the SBC runs fine under load, but misfires under light load/high intake manifold
vacuum? And Schurkey asks for and gets the fuel trim numbers, and they are showing 10-15+% additional
fuel being requested? And as it turns out, the intake manifold gasketing was marginal, allowing 'false air'
to be added to the measured air under light load conditions, causing too-lean misfires > false lean reporting
by the O2 sensors, twisting up the feedback loop, and causing the VCM to be further misled?

D) You buy an old GMT400 powered by a SBC with an unknown history. Seems to drive OK, but you
decide to do the right thing and change the oil. You drop the oil pan plug, and the oil drains out a little
more like honey than water? And once you replace the oil filter and refill with an oil that's always worked
for you, the oil pressure seems a bit low and there seems to be a rod knock near the front?

Wanting to know more, you use the Schurkey shorting method to individually short out each cylinder's spark,
one at a time...and you find out that most of the time it's the #1 or #2 cylinder that has the rod knock? Why
is it always in the front? (A: Read around and you will find that L31MaxExpress has a pretty solid theory why. :0)

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Q: Where am I going with all this?

A: What I'm trying to illustrate is that due to architectural strengths and weaknesses, the SBC is going to end up
at the far end of the bathtub curve wearing out primarily in a few specific ways. And if you look closely at the failure
pattern of the powerplant in your engine bay and are familiar with these patterns, then it can help you figure out
how best to respond to the failure scenario. And there will be variations on the theme according to which of the
3 general groupings for a street-driven SBC: Pre-Computer, TBI, and Vortec. (GMT400s came from the factory
with the latter 2, but there will be the occasional retro carb motor observed here or there.)

Now that I have supplied some SBC context, let's discuss the "Only misfires under heavy load, otherwise it drives fine" scenario.
(Again, the following only applies after ALL conventional tune up, lubrication, and cooling systems are 100% known-good.)

This is a really specific failure footprint. And this TSB (tip of the hat to @socal k1500) is how the General describes the DTC(s)
occurring during very specific driver demands, the root cause, and the repair:

You must be registered for see images attach

And here's a photo to help explain why the center cylinders are the most likely to suffer from the
SBC's architectural hot spot that shows up only during extended high duty cycle driver demands:

You must be registered for see images attach

(credit: unknown Vortec head gasket failure photo from the interwebs)

NOTE: When the BBC was redesigned in the mid-'60s from the Mark II 348-409 and re-released
as the 'Mark IV' 396-427, note that the exhaust valves were now laid out symmetrically. (See attached.)
And also note that the (post-1st gen SBCs) LS motors did the same thing with the exhaust valve layout. (!)

But to put all this into proper perspective, for 99+% of the SBC owners out there, this issue never
showed up, especially on the lower HP/lower duty cycle installations.

But this reply is focused on the GMT400 SBC and the misfires that only occur while towing, passing,
climbing a grade, or sustained heavy demand. And everything else has already been tried.

By the way, when we were all researching this 'difficult to diagnose' misfire for socal K1500, I found
literally dozens of abandoned threads across different forums. But just about the time I was going
to give up on finding physical confirmation of this TSB, I finally stumble across someone closing the
loop on this with a real-world report from ~18 years ago:
(Misfire while towing uphill, heads removed, machine shop discovers too-tight exhaust guides)

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That's pretty much all I got for now. But hopefully this provides those of you who are suffering from
this specific failure footprint a direction to at least explore the possibility that this is your root cause.

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And for those of you who want to learn more about all this, I would highly recommend that you
read through socal K1500's thread on all this. While troubleshooting all this, there are several
twists & turns, including retrieving FOD in cylinder #1, and mysterious engine roughness after it
was all said & done that had a surprise ending. And thanks to his live data we were able to
quantify the improvement he made, and he came up with a measured response to the problem.
(Socal K1500's informational misfire while during uphill saga.)

Good stuff. Recommended reading for any troubleshooter/troubleshooter in training.

Hope this proves helpful. Happy hunting for the fix!