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1998 GMC K1500 +HELP+ It all started with a P0300 code…

953 Views 5 Replies 2 Participants Last post by  UpNorth23
Hello and thank you in advance for your insight into my challenge. I’m going to write a lengthy and chronological post in hopes of providing the best insight as to what I did or didn’t do or check.

Disclaimer: I’m obviously not a mechanic but I’m trying to do as much work on my truck, as I live out in the country and getting into town, for me, is more of a hassle than me just doing the most I can. Plus it’s fun learning and working with your hands! I will often google/youtube fixes and they usually work 99% of the time. I apologize in advance if I’m not using the proper auto mechanic terminology but I will do my best.

Last year I replaced my spark plugs (NGK’s) and distributor cap with no issues.

This winter has been slightly colder than normal, it dipping into the - temps at night. I’m not sure if this would cause some of my issues. And it obviously snows where I live and drive where “salt” is used.

Part I

Recently my truck gave me a P0300 code (random misfire). My understanding, I went down the laundry list of potential issues as they could be many, as several YouTube videos indicated. I disconnected my negative battery cable and got to work. I replaced both oxygen sensors (up stream) and I cleaned my EGR valve and reinstalled it with a new gasket seal. I also cleaned my Mass Air Flow sensor and reinstalled. I connected all associated electric cables. I went ahead and reconnected the battery cable and the truck ran fine for a bit but that pesky engine light came with with he same fault code.

Part II

I went ahead and continued on the list of potential issues. I installed new AC Delco spark plugs (I’m under the impression the OEM plugs work better than other plugs) I also made sure the proper air gap (.060) was achieved. I reinstalled the spark plugs wires and a new fuel filter, making sure I installed in the correct direction (arrow facing engine). I also installed a new ignition coil. I also added dialectic grease where appropriate. I started up the truck and I still had a fault code indicating my Mass Air Flow sensor cable was not reinstalled. I made the quick fix and re-connected it. I cleared the fault code and started up the truck fine after that initial priming of fuel through the new fuel filter.

I drove it home from a friends house about 1/2 a mile with no issues. I didn’t drive my truck for a couple of days. I then attempt to start my truck but no luck. The engine would crank but no start. It sounded like my battery was getting weaker and weaker. It’s a two year old battery which seemed odd. I decided to replace the battery after several attempt starts. After the install of a new battery, still no luck.

I made sure all my electric connections were properly installed, to include my ignition coil and wire leading to distributor cap. Still a no go.

Part III (spark/fuel/air)

I removed two (independently) spark plug wires and tested them both for a spark, which occurred. I also listened for the fuel pump to engage (both by drivers door and by gas fill). It “sounds” as if it’s working. I also checked the fuse box in the engine bay to make sure my 20 amp fuse (ECB-*) related to fuel was good, which is was. I also had someone turn on truck for me (position key 2) and I listened to fuel pump relay (boxey looking fuse) which made a sound kinda like a click.

Whew! So that’s what I’ve done thus far.

I recently watched a youtube video where the mechanic determined a ground cable (undercarriage of engine area) was corroded and broke off. I believe it was attached to frame of truck. After fixing that, the truck started right up. Crazy! Something so simple. Not to mention a horrible place to install it, unprotected and exposed to the elements.

Anyways, if you have some suggestions or know exactly where these ground wires are located, much appreciated and Happy New Year!

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Great detail, Mike.

The fuel tank ground is located on top of the frame rail at the filler neck. It‘s a 13mm screw on top of the frame that is a little hard to see. You locate it more by feel. The fill tube rests on it.

Other grounds:
Fender next to battery
Frame rail above the starter back to the firewall and forward to the back of passenger side head. Intake manifold should have at least three. Main negative, thermostat housing and at least one other spot on the intake.
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