Random Misfire, 98 K1500 5.7

I have been having a random misfire with my 98 K1500 5.7. The O2 sensors have all been replaced within the past 2 years and fuel filter replaced every other oil change. Also have done a regular tune up of plugs, wires, cap, and rotor button. The code it is throwing is P0300 I don't know where to look for the cause. Most people say O2 sensors but I have also heard EGR valve. I have the OBDCOM software and interface cable to hook up a laptop to the OBD port and monitor various sensors. What should I look for as a clue of the cause. Any help would be greatly appreciated. Thanks- Ben

Do you actually feel the misfire and under what conditions?

I can feel the misfire very clearly. it starts out as a slight shudder and progresses until you can feel the entire truck shake. It usually occurs when it has a hard load such as a long steep climb but has happened on flat roads as well both while accelerating and decelerating.

The distributor drive gear can wear and cause a random misfire. Pull the cap and see if you have excessive movement.

Also check the fuel pressure.

//2000CAYukon

Most common failure for p-300 is wrapped plastic intake. On cold start there is gap for entering unmetered air into engine.You can spray carb cleaner and look at upstream o-2s for rich condition. 0.80-0.90 V, or use water and look for steam coming from exhaust, and rough running.Repair procedure here.

http://www.d-tips.com/General/Articles/article.aspx?id=/free/How%20to/GM%205.7L%20Intake%20Manifold%20repair/


Hope this helps.

Please post if this helps, for other members.

I can feel the misfire very clearly. it starts out as a slight shudder and progresses until you can feel the entire truck shake. It usually occurs when it has a hard load such as a long steep climb but has happened on flat roads as well both while accelerating and decelerating.


This kinda sounds like a fuel pump to me. I had same problem, where truck would slowly lose power as I climbed long hills, and it was the fuel pump. I tried cap, rotor, plugs, etc - just like you. I believe I had exact same code, but can't remember. If you replace the pump, make sure to use OEM pump. Some of the aftermarket brands just don't last at all.


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