Hey all. Ive come to a concensus that these vortecs are either really good or really bad. im at current wits end attempting to find any valuable solution to my problems. initially its always had rough idle. had a misfire code, was due for the generic tuneup, took care of the code. It doesnt get driven a ton but it has increasingly adapted a horrible idle, shakey, definitely feels like misses. I hook up HPT. cyl 2 and 3 have a large portion of misses, followed with a few other cylinders here and there. it doesnt trigger a CEL however. so down to diag mode i go. now last year i replaced fuel pump and filter, to be redundant pressures are fine, leak down is normal. i have no way to do an injector balance. theres a heavy maybe 500 miles on these items. had an issue with power and ground leads, all replaced. i have verified compression at one point to be around 5 psi of one another, 02s function in parralell, map/tps/iac/ etc all appear to react how id expect. head and intake gaskets newer prior to my ownership with new delco fuel spider. all the bases are covered unless im losing cylinders at a very rapid rate (hard to believe). vac leaks....doesnt appear to exist. for whatever god forsaken reason i have no explanation as to this aweful idle. today i reinspected FP, cap rotor, icm, coil, plugs and wires. no wierd resistance at wires, coil secondary was a bit weak i tossed ina known good unit, cap and rotor still look new. plugs are showing heat range right where id aniticpate. i said screw it and tossed in a good ICM off my other truck. misfires at wierd increments went away for now. the idle.........still atrocious. so do i molotov this thing or what? im starting to believe its haunted or something non logical.
sorry this is long winded but im at a loss. i think im quite intelligent with diagnostic efforts. i have access to hpt that provides me live data, no im not the smartest but i find nothing wrong with any function. I have a few .hpt idle files if anyones intrigued
Hello TylerZ281500,
Given that you have covered all the normal bases, assuming you aren't the victim of any bad new parts,
we need to look at a couple of possibilities of something specific to the Vortec implementation that that can cause
your symptoms.
****
I'd like to start with the cylinders that are showing the highest misfires. In terms of crankshaft rotation,
the firing order looks like this:
1st - 360°: 1 - 8 - 4 -
3
2nd- 360°: 6 - 5 - 7 -
2
.................0 90 180 270 {crank degrees}
Normally this troubleshooting theory would be a bit of a reach, but since all the 'normal maintenance' stuff
has already been verified by the OP, I'm trying to discern a possible failure pattern among the 8 individual
cylinders in your engine. These cylinders aren't physical neighbors on the same bank, and they also
aren't adjacent in the firing order. (Especially the double-whammy 5-7 pair.)
But since this is a Vortec, we have to keep in mind that our Initial (Base) timing is 100% derived from
the CKP sensor, so is it possible that there is something in the 270° area of crankshaft rotation where
the reluctor is now physically deviating from the rest of this tone ring? Maybe the PO got the gap wrong
and your sensor looks like the attached CKP sensor?
Or is the gap is wrong in the other direction, and the 'too big' gap is now causing a flaky/marginal signal?
(As a troubleshooter, we need to remember that *this* signal is what everything else is based on, both
firing the ignition system as well as the fuel injectors. (!)
If you want to see for yourself what a funky CKP sensor can cause, do a bit of research on the internet
for the following DTCs:
'96-'97 Vortec CKP DTCs:
*
P0336 "Crank Position Sensor Performance"
*
P0337 "Crank Position Sensor Low Duty Cycle"
*
P0338 "Crank Position Sensor High Duty Cycle"
*
P0339 "Crank Position Sensor Circuit Intermittent"
('98+ Vortec) As above, plus a new additional DTC:
*
P1336 - {FSM} System Variation Not Learned (See attached.)
NOTE:
commonly used generic term to refer to this: CASE - Crank Angle Sensor Error
And here's a little more detail about the P1336 DTC. (
Helpful details including diagnostic aids)
And for extra credit, here's a CKP misfire fix by credible scope dope Ivan at Pine Hollow Auto Diagnostics: (rough running newer GMC pickup)
****
There is one other Vortec-era sensor that causes similar rough running, and that's the MAF sensor. What's nice
about this is that you can disconnect the MAF and then engine will continue to run on the MAP input. And
there have been cases where the Vortec motor runs poorly on the MAF, but clears right up when just on the MAP.
However, I'm adding this in mostly for completeness, since I was having a hard time explaining how a bad MAF
could only adversely affect cylinders #2 & #3.
****
1 last thing to keep in mind is that the VCM is programmed to avoid crying Wolf by setting certain thresholds where
the errors aren't reported until they cross those thresholds. For example, emissions-related DTCs are not reported
until the misfire would causing the calculated emissions to exceed 150% of the factory calibration. And while 'A'
category DTCs are reported immediately, 'B' category DTCs must occur in 2 different driving cycles before the SES
bulb is illuminated. And the 'C' DTCs are recorded, but will not *NOT* set the SES light.
****
There's a ton more info waiting for you in the GM Factory Service Manuals. And since you are also driving a '99,
I happen to know that you will find those specific manuals in reply #77 of the FSM thread mentioned above.
Disclaimer: As always, this is just food for troubleshooting thought as opposed to a guaranteed diagnosis. But
I hope the above helps to get you into the Vortec-specific mindset. When they are running as advertised, they are a
sweet powerplant. But when they are off their feed, they can be ugly to live with. :0)
Best of luck with your Hunt for the fix. Hope this helps --
