I have a 2000 c3500 with the 7.4 vortec engine. The issue I'm having is a little weird but I'll try to explain. If I start the truck and leave it in park it will warm up and idle down to 750rpm. If I then put it in drive it will stay at 750. If I put it back to park it will remain at 750. This is good. The problem comes up after driving the truck for 10 minutes or so. If I come to a complete stop it will idle at 800 in gear, and then climb up to 1000 in park. In both scenarios the truck is up to temp coolant wise. I've replaced the thermostat and coolant temp sensor. I have noticed the truck will sometimes have a little difficulty starting when warm which I've gathered is a sign of bad injectors. Could this be the issue? Thanks you in advance to anyone who can give some guidance. I'm new to GMT400s.
I had planned to check the manifold for vacuum leaks as I know that's a common failure point. It's just weird that it is only after actually driving.
Greetings zdangerh,
Welcome to the GMT400 forum. The crowd in here is adverse to guessing with your wallet as to
why something is malfunctioning, but they will join in if there's some data from your engine bay to
kick around. So let's figure out what we need to see and how to go about getting that.
I'd like to start first with your second comment. Way back when they made big blocks with cast iron
everything (blocks, heads, and intake manifolds), there were far less issues with leaky intake gaskets,
since with shared metallurgy all the parts had the same coefficient of thermal expansion. (
Rabbit hole)
So no relative movement. Intake manifold gasket leaks on stock (all cast iron) motors were infrequent.
On the other hand, by the time GM made your '00 C3500, the intake manifold is cast aluminum in order
to save some weight. Imagine a big chunk of aluminum sitting inside of a cast iron Vee from the cylinder
heads bolted to the engine block -and- the intake manifold. With every thermal cycle (trip) the gaskets
have to allow this dissimilar expansion from one surface to the other while maintaining an air-tight (and
water tight) seal.
I can personally vouch for this, having troubleshot & fixed aluminum intake manifold leaks bolted to a
cast iron V8, both air or water leaks.
****
But I am *not* going to guess with your money that a fresh set of intake manifold gaskets will fix your 7.4.
As Schurkey said in reply #9, we need to look at the live data and see if what is being reported supports that
theory. Or not.
Even before I am thinking about the intake manifold gasket, I first want to try to figure out if A) the computer
is getting good data & trying to compensate for something like a leak, -or- is the computer being
misled by a
'drifted out of spec' sensor and therefore is trying to compensate for something that
isn't happening?
For example, the following live data would be very helpful:
* MAF reading
* MAP reading
* Fuel trims, short & long term
* O2 sensor activity
In addition to the above, I would be interested as to why your VCM (aka: Black Box) isn't kicking the
following DTC?
You must be registered for see images attach
(credit: '99 FSM, Vol. 3 & 4, p.6-2274. NOTE: Both 1999 & 2000 models are carryover/unchanged, so '99 manuals work on the '00 GMT400)
A brief glance at this first page for the P0507 DTC makes it look like there are a lot of
scenarios where this test is blocked from running. Q: Is your SES light constantly on?
Are there any other codes being kicked?
And has this truck ever idled correctly, both cold & hot, since you've owned it? Or is this
a condition that was present when your purchased the C3500?
****
Another variable that we have to keep in mind is that cold we are in Open Loop operation,
and once the engine is at operational temperature the system goes into Closed Loop operation.
But as of yet we don't have enough data to decide whether or not to go down that path.
****
Since you are new to the forum, you may not be aware of the download links for the original
factory service manuals that are available for you to take advantage off. There's a sticky thread
where all the years are listed, so here's a link to there: (
LINK)
Once you have the books, now it's time for you to decide in what you want to obtain in
the way of live data capable test equipment. (Not just a code reader.) Future-proofing
this a bit, will you ever want to use the bi-directional controls needed to do stuff like
CKP relearning, etc?
FWIW, some favor used period-correct professional strength scan tools, while others
get the job done with a smart phone, app, and ODB II adapter. (Hopefully others will
chime in here with what's worked for them on a Vortec OBD II GMT400.)
Apologies for the length of this reply, but since you are suffering from a higher than
expected idle, but at the same time you didn't mention any DTCs, this will take some
live data...so that the folks in here have something to kick around in order to give you
some troubleshooting steps to try to narrow down the possibilities.
Hope this helps. And let us know what you discover --
