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Offline Higher Caliber

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Need help!
« on: April 28, 2017, 04:04:59 PM »
1998 Chevy vortec 305

Was driving along thinking about how good my truck was running and all of the sudden it wasn't.

Missing consistently and backfiring. Put it on the code reader and all I have is the multiple misfire P0300 or whatever. Posted are the stats. I compared it to one of the others in the driveway and couldn't locate anything remarkable... what say you?




Here is the frozen data from when it tripped the code





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Offline cudakidd53

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Re: Need help!
« Reply #1 on: April 28, 2017, 04:42:24 PM »
Only thing I can think of is a loose wire somewhere in the system.  Had a dirt car that did similar and it turned out to be a loose ignition switch wire that rattled loose at speed- tightened it up and GTG.

Good luck!
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Offline EL TATE

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Re: Need help!
« Reply #2 on: April 28, 2017, 04:52:55 PM »
Same problem with a GM coil pack when the connection corroded. you, don't have coil packs, but I'm w/ Cudda on the electrical side. check the connections on your distributor and the spark plug connections. I think the dist runs off a little gear on the cam shaft and I've heard of those gears spinning on the shaft which would throw the timing way out. I'd like to hear from Ken here, but did you have specific cylinder misfire?
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Offline Higher Caliber

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Re: Need help!
« Reply #3 on: April 28, 2017, 05:03:21 PM »
No, multiple misfires. I checked all of the connections, pulled the cap. Stuff looks good. I'm reading the most common cause for the multiple misfire code is on the fuel side due to faulty O2 sensor readings


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Offline EL TATE

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Re: Need help!
« Reply #4 on: April 28, 2017, 05:56:45 PM »
you can check the O2 sensor w/ a live stream tester, should fluctuate rapidly from lean to rich. look at the MAF sensor too; unplug and rev and if stumble is gone you caught your perp. also, check the lock Bolt on the distributor; if it's loose and allowing the dist to move and change timing as you're driving I can't imagine that would make for smooth operation. air, fuel spark, has to be one of the three. I hated hearing that but it's true. fuel pump would most likely give you trouble at all speeds, crankshaft sensor would be either run or not run. The MAF test forces it to run in open loop and is the easiest to perform. ran with mine unplugged in the Jetta for over a year w/ no issues.
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Offline Sammconn

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Re: Need help!
« Reply #5 on: April 28, 2017, 07:26:59 PM »
What caught my eye was the 20* timing advance at idle by the looks of it.
Not sure if that is normal on these newer engines, yes I saw '98.
I know the older ones were in the 8-12 range at idle.

Multiple misfires leads me to think timing issue as well.
I know zero about them though, I'm a HEI, and carb guy on V-8 and such.
I just don't want to wind up missing a digit or limb.  I can sometimes get in a hurry to get results.
Sam

Offline Flyin6

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Re: Need help!
« Reply #6 on: April 28, 2017, 08:14:10 PM »
KEN??? Calling Ken...
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Offline KensAuto

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Re: Need help!
« Reply #7 on: April 28, 2017, 08:43:33 PM »
That truck has the "spider" injection, and it will not run well on less than spec fuel pressure... About 56 psi... it won't run at all with less than about 48 psi.
Also, after market caps tend to let spare jump through the plastic, between cylinders, and you won't be able to tell by inspecting it.
I would check fuel pressure, then replace cap, then possible crank position sensor and it's wiring (tends to lose insulation by oil pan)... This is all assuming you tightened plugs and snapped wires all the way on.


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Offline KensAuto

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Re: Need help!
« Reply #8 on: April 28, 2017, 08:45:37 PM »
*let spark jump through cap

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Offline Higher Caliber

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Re: Need help!
« Reply #9 on: April 28, 2017, 10:30:45 PM »
That truck has the "spider" injection, and it will not run well on less than spec fuel pressure... About 56 psi... it won't run at all with less than about 48 psi.
Also, after market caps tend to let spare jump through the plastic, between cylinders, and you won't be able to tell by inspecting it.
I would check fuel pressure, then replace cap, then possible crank position sensor and it's wiring (tends to lose insulation by oil pan)... This is all assuming you tightened plugs and snapped wires all the way on.


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Thank you sir! I'll get on all of these tasks in the am! What is the best way to check fuel pressure?

Would the cap just go toast all of the sudden? It was literally running perfectly! I drove it about an hour away to pick up some laying nests and about ten mins from the house she just crapped out?

What else do I need to change while I have her tore down again?
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Offline KensAuto

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Re: Need help!
« Reply #10 on: April 28, 2017, 10:39:13 PM »
There's a valve back by the distributor for a fuel pressure gauge...
The cap can do that all of a sudden. Did you look at the design of the old one? There's several places that the terminals cross over each other and they're only seperated by a bit of plastic (1/8" or so). The original cap was made out of a bake light type material.... Aftermarket ones are just plastic.  The spark will jump right through the plastic sometimes, and cross fire cylinders, and also backfire.

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Offline Bigdave_185

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Re: Need help!
« Reply #11 on: April 28, 2017, 11:01:59 PM »
I am of no value other than moral support, ah. Buy a duramax lol


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Offline Higher Caliber

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Re: Need help!
« Reply #12 on: April 29, 2017, 02:27:44 PM »
Fuel pressure checked at 56psi exactly. Swapped the BW cap for an AC Delco and she runs like a top again! Thanks Ken!


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Offline Higher Caliber

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Re: Need help!
« Reply #13 on: April 29, 2017, 04:09:26 PM »
I am of no value other than moral support, ah. Buy a duramax lol


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Oh yeah! I had one of those once!
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Offline cruizng

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Re: Need help!
« Reply #14 on: April 29, 2017, 07:37:25 PM »
Glad to hear HC.


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Offline stlaser

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Re: Need help!
« Reply #15 on: April 29, 2017, 08:09:50 PM »
Way to go Ken :likebutton:
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Offline TexasRedNeck

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Re: Need help!
« Reply #16 on: April 29, 2017, 10:51:35 PM »
Hillary said "it takes a village".


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Offline Flyin6

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Re: Need help!
« Reply #17 on: April 29, 2017, 11:34:25 PM »
KCTA!


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Offline KensAuto

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Re: Need help!
« Reply #18 on: April 30, 2017, 10:37:01 PM »
Awesome. Even a broken clock gets it right now and then.

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Offline Higher Caliber

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Re: Need help!
« Reply #19 on: May 04, 2017, 11:08:15 AM »
Awesome. Even a broken clock gets it right now and then.

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Offline JR

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Re: Need help!
« Reply #20 on: May 04, 2017, 06:21:03 PM »
Glad you got it going with a simple fix. I was going to add the pigtail plug to the coil will go bad in that series like the blazer I had. Only other issue was a fuel filter.
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