KFtransam

Hey everyone,

Hoping someone can shed some light on me here. I've got a 96 k1500, owned it for about 4 years-50,000 miles now. Currently have 170k on the clock.

Since the day I picker her up I've had a nagging driveline vibe that's directly tied to rpm. 2200 to 3200 to be exact.

I've changed a large handful of things on this truck hoping that in fixing another problem the vibration would go away too.. Nope!

So I'm struggling with passing emissions and recently bought a cheap scanner that offers live data.

I've noticed the pcm is pulling timing like no other, idles in the -25 to -35 btdc and under various driving conditions the most advanced I've seen has been -12 btdc...

What's up here? What's my next move?

I know I could potentially be a bad knock sensor but I hate playing parts darts without knowing. Anyone know of a test procedure? My many many internet searches have lead me nowhere.

Any help is appreciated, thanks guys!

KFtransam

Bueller?

I'm curious if I temporarily unplugged the knock sensor would I see any change in timing.

MuhThugga

http://www.gmtruckcentral.com/articles/distributor.html

I'd make sure that everything is set up properly first. It is possible for the distributor to be out and things need to be adjusted with a good ol' timing light. I'm not too familiar with the Vortec set-up, but the older TBI/TPI computers required the engine to be set to 0* TDC. The computers didn't actually know where the timing was set and just swung the timing based on the engine sensors' inputs. So, if the engine's timing was not set to 0* as a base, the computer could potentially swing the timing way out into left field and be none the wiser.

1 FMF

iTrader: (1)

i have watched my handheld while driving my 2002 camaro [LS1] and under heavy load like pedal down going up hill in high gear i only see around -24° timing. not until you let off the gas at 2000+ rpm will timing jump up as far as -40°. general rule is heavier load = richer mixture = less timing advance = mixture burns faster. and light load = leaner mixture = more timing advance because mixture burns slower.
being a truck i'd expect your steady cruise where you think is light load is not really that light, the %load would be higher than expected resulting in less timing. how much and if timing should be as little as -12° i don't know.

question is, do you know the pcm is actually pulling timing? is there a KR value available?
what happens if you disconnect a knock sensor, how will the pcm react to what is thinks is a failed knock sensor? i would guess be conservative and prevent full timing advance which then doesn't help you.
with your handheld look at all other parameters and make sure they are all within expected range... coolant temp, air intake temp, manifold vacuum, etc.

cookseyb

Check the torque converter for balance. If the tranny was changed in a prior life and the shop added a aftermkt converter it can be out of balance. This happened to my Impala SS. New Converter and all is well...

VortecBlazer

iTrader: (1)

You don't set the timing on the 96-99 vortec motors with a timing light. They are easier than that because the computer will adjust it 4* one way or the other as long as it is in the ball park of TDC. With that said, check the timing and the distributor. Look on the crank balancer and you should see a timing notch, then look down at the timing chain cover between the water pump and A/C compressor. You should see a timing marker molded into the plastic cover. Now to set timing, turn the engine by hand until the notch on the balancer is lined up with the marker on the timing cover. With those lined up, make sure the #1 piston is up top. Now remove the distributor cap, look on the distributor housing and find a little number 8 (should be on the driver's side). Line the rotor point up with the molded #8 on the housing and you have now set TDC.

I had mine off by one tooth and it would run but threw it in limp mode which locked out timing to 22*

You need a scanner that will read knock retard to see if it's a knock issue. The vibration could cause knock if the sensor is picking it up.