No problem, new members do this all the time. These GMT400s look all the same on the outside,
but there are unique 'features' with each drivetrain variation. Us old guys are never offended if you
refresh our memory of what you're driving and/or what you are working on.
When troubleshooting an 'intermittent', you really can't truly fix it *until* you figure out how
to
break it at will. (!) So reading the above is good news, for you have figured out how to
stimulate the fault at will. For a lot of really tough problems, this is the hardest step of all.
By coincidence we've been huddling around another member's truck with a similar issue.
Because of the fuel trims the decision was made to take the running truck down and
replace the intake manifold gaskets. (Common Vortec issue.) Everything went to plan,
he stuffs the distributor back in, but now a hard no start?
What he did shouldn't have caused any starting issue? Sure enough, after some head
scratching & a hard look at all the bits, if you follow this link you can see a subtle-looking
failure that was the root cause: (
No start traced to dizzy cap failure)
And swapping in another cap cured the no-start, and he's back in the hunt. Don't know
what brand cap & rotor you are running, but there's lots of reported disappointment in this area.
(As in, high voltage stuff goes bad way too soon, Vortec dizzys cause weird problems and
are always suspect on the weird problems, etc.)
This issue crops up from time to time. The affordable stuff calls *all* misfires a P0300.
A true P0300 is a valid, global fail, and sends us down a specific path. At the same time,
an accurate P0301/302/303/304/305/306/307/308 allows us to really focus on what's right
vs what's wrong. At some point, the decision becomes whether to load the parts blunderbuss
for the all-inclusive scatter shot vs. the one-time investment in stepping up to a 'report a
misfire down to an individual cylinder' capable scan tool.
Solid diagnostic info - copy all.
OK, so I don't want to be guilty of leading the witness, but then again just to let you know
what is over the troubleshooting horizon, there is a very specific scenario where a SBC in
a GMT400 (among other GM lines) will misfire, but *only* under heavy loads, like climbing a
grade, pulling a trailer, etc. And this specific failure footprint only applies when every other
normal tune up part is as new, the engine never misfires at any other time, the compression
test and leakdown tests are good, and in short there's no reason for this to occur.
If this sounds like what you are up against, then the GM TSB PIP3081, dated 7/3/2004
**
would be worth a read for comprehension:
(
GM Technical Service Bulletin on valve stems sticking when SBCs are flogged, leading to misfire)
And for a hint as to how this could possibly happen, check out how the SBC cylinder heads have an
architectural hot spot designed into the center of the cylinder heads:
You must be registered for see images attach
(For comparison purposes, check out the BBC cylinder head symmetrical exhaust layout photo attached below.)
IMPORTANT NOTE: This is off the troubleshooting table until such time as every other possible
misfire under load possibility has been eliminated. But it's always good to know what the 'over the horizon'
worst-case scenario is out there. With any luck we will figure out a fix short of going there.
Excellent. If you could organize them exactly as they are arranged geographically in your
engine bay this would help us prove/disprove typical failure patterns. (
Example)
The FSMs are the definitive baseline for most everyone in here. Not perfect, but 99.9% better
than just winging it or using the off-brand manuals. Note: The '97 manual might be a few replies
in, but the '98 Manuals are there right at the beginning, and should prove to be helpful.
(
GMT400 FSM cache)
Just to give you a taste of how nice it is to have the shop manuals downloaded, here's what the (same) '99 FSM shows
for the electrical wiring between the VCM & the IAC. I would give every inch of those wires a careful visual inspection.
You must be registered for see images attach
This is definitely one of those cases where you always fix the obvious first, and oftentimes that will
also clear up the exotic failures. Even if we can't explain exactly why it happened. :0)
Summary: I'm throwing out all of the above purely as food for troubleshooting thought.
And we will need to verify that everything in the engine bay is good to excellent, including headroom
for insurance. (Think cooling system's ability to get rid of even more heat than you are generating
while flogging the old girl up the mountain pass with your trailer full of toys on the back.)
And at some point we may need to up our diagnostic toolage in order to crack this difficult nut.
Hope I was clear with what I'm trying to share. If you have any further questions, don't hesitate.
I'm off to read what popped up while I was writing my term paper.
Cheers --
(
**Located and shared by
@socal k1500 - tip of the hat in his general direction :0)
