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Old 06-14-2011, 01:12 PM
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Please help me, 98 vortec 350

I have a 1998 gmc k2500 4x4 with a vortec 350 automatic tranny and 373 gears 100,700 miles. Problem is, it has great idle, and great wide open throttle, But has an irratic miss under light accelatation or constant speed, you can hear it out the ex pipe and just barley feel it, not a hard miss, very light feeling, from 30mph to 70mph speed doesnt matter, open loop or closed loop does it in both, seems a little more often in closed, just started doing it one day. It does not have any codes or miss fires when read with a obdII reader. Also checked live data, here is a list of the live data, first number is lowest ive seen second is the highest:
cacl load% 2.7 5.9
maf 0.78 2.16
map 8.9 18.3
iat 81 124
ing adv 19 29
stftrm 1% -3.1 -1.6
ltftrm 1% 4.7 5.5
stftrm 2% -2.3 0.0
ltftrm 2% -0.8 3.4
o2s12 .16 .85
stftrm 11% -.6 0.0
o2s12 .725
o2s21 .095 .8
stftrm 21% -.8 2.3
o2s22 0.16 .75
Now, what i have done to try and fix:
replaced all of this after problem started: cap, rotor, wires, plugs, fuel pump, tps, iac, egr, intake gaskets with the new felpro design, fuel inj to new updated multi port style, fuel filter, air cleaner.
I have also checked: compression, cleaned all carbon and deposits when i did intake, dist gear is in good shape,looks good and no slop when in place.
About the only thing I hav NOT replaced is; coil, ingntion moduel, map, maf, evap purge valve. Please help me, im so tired of throwing money at this thing.

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Old 06-14-2011, 04:22 PM
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Coil & Ignition Module would have been what I would have tried first. If you have access to a good junkyard locally I would get a different ignition module & try that.
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Old 06-15-2011, 09:06 AM
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I was kinda holding off on coil and module, i thought that they usually eather worked or didnt in most cases
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Old 06-15-2011, 09:54 AM
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There is a "kit" sold to replumb the injector. I don't know squat about the OEM EFI system on your truck, so this is just from what I've seen.
http://www.partstrain.com/ShopByDepa...GMC/K2500/1998
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Old 06-15-2011, 10:37 AM
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I have already replaced the old spyder style with the multi port style that has the control and nozzle at the ver end instead of the "poppet" style.
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Old 06-15-2011, 04:44 PM
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I don't know if this is your problem but a different distributor fixed a friend of mines truck that had a skip in it simular to yours.

He pretty much replaced everything you have.
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Old 06-15-2011, 05:03 PM
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Common failure is coil to start arcing externally under very light load/throttle. Usually the coil will show a white residue or degradation of the dielectric material around the magnet. This was common in the early 2000's when I worked on those year model trucks
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Old 06-15-2011, 05:46 PM
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Had a couple of bad dist caps from carquest , back to back . they had a miss worse than the old cap.
An old GM tech told me years ago , the best way to check coil and plug wires is to spray them down with salt water and then get someone to put a load on the engine too see the sparks jump from the wires or coil. Had alot of problems with aftermarket wires too, been using OEM for almost a year now.
On your live data your short term fuel trim all looks good .
MAF is it in volts or frequency . volts at idle should be around 5-6 volts , frequency ???????
the bushing would'nt be bad in your distributor ?? you usually have a cam senser code for that though, 4.3 l engine with high mileage are bad for this. if you change your distributor you have to use a scan tool to set the timing, o deg + or - 1 to 2 deg . warm the engine and then bring it up to 2000 rpm to set the timing.
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Old 06-16-2011, 07:22 AM
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The distributer gears are known to knifedge. the vehicle will feel like it has a small "buck" to it every few seconds while cruising.

peace
Hog
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Old 06-16-2011, 08:42 AM
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My guess would have been the OEM poppets!! They are notoriously bad!! Also check your fuel pressure....the fuel pumps in our trucks are also famous for checking out!! If you have goofy pressure readings, I would suspect the pump!! I have had 2 96 k2500's and have one currently. In the 650,000 combined miles between them I have replaced a half dozen pumps! I only use Delphi pumps now, due to junk aftermarket pumps not lasting! Not as many problems with the Delphi pumps! Also make sure you replace the fuel pump harness that comes with the replacement pump!!
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Old 06-16-2011, 09:58 AM
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ok, sounds like everone has some good ideas, I will try the water spray and see if that tells me anything, as far as fuel pump, it was doing this before I put the new pump in, but thanks for the info i will go ahead and check the pressure on the new pump as well.
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Old 06-16-2011, 09:27 PM
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I had the same kind of problem with my 96 vortec 350 after replacing the spider with an upgraded unit, it had a leaking line to one of the poppets. Replaced the fuel pump with an AC Delco unit. Did a complete tune up with aftermarket plugs, wires, and dist cap, and still had that strange miss while at cruise speed and showing no codes. I replaced the aftermarket spark plugs with OEM spark plugs and fixed the problem. Oh yeah, I had to replace the aftermarket dist cap with an oem cap also.
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Old 06-17-2011, 01:37 AM
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My Son's 98 has the exact same problem & has had the same parts replaced + all O2 Sensors. It does have 230,000mi on it tho'.
His has a Crank/Cam sensor sync problem Code. I'm thinking Timing Chain or Distributor gear.
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Old 06-18-2011, 09:01 AM
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Swing and a miss! New coil and ignition mod, did not fix it
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Old 06-19-2011, 07:15 AM
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Its fixed put new distributor gear, DIDNT FIX IT, new ac delco cap and rotor, DIDNT FIX IT, went down to KOI here in Richmond IN and got a whole new alum distributor, THAT FIXED IT, dont know what else it could have been in the distributor that was causing it, bushings maybe, but oh well it fixed it!!!
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