PO300 after cap and rotor change.

[OilBurner2003]

Hello everyone! I have the famous 1998 Yukon that developed a P0300 after the cap and rotor replacement, and new spark plugs. Of course, We didn't think it was the cap and rotor that caused this. I have spent over 6 months (That's when the P0300 appeared) Looking for vacuum line leaks, damaged crankshaft sensors, valve cover leaks, because oil can seep down onto the spark plugs, fuel pump, fuel filter, and more. Now this P0300 has gone away at random times, which led me to believe it was something not exactly mechanical, maybe computer? turns out I was wrong. Dad took the Yukon out, flashing check engine light. Really, really bad misfire. Yet again, P0300. Being a hundred miles from home, little more difficult to find the culprit, but I did find it. Before this point, I thought it was a bad sensor. It never gave a loss in power, Idle never changed, and sounded great. But when it got worked up hills all day, that's when it got really bad. Now somehow, I started looking at the cap and rotor. I figured that this problem can't be that complicated, and that I was just missing something. Put my hand on the cap while it was running and wiggled it ever so slightly, Check engine light went away, and misfires stopped. 50 miles from home, light came back and was misfiring, but wasn't nearly as bad until we made it all the way home. So, took the rotor out and inspected it, looked good! but the cap had so much corrosion on the metal points, just doesn't make sense to me. How could a cap only 6-7 months old already look like it came out of a stock Yukon that's never been changed?


Basically, I'm looking for confirmation that my problem is the cap. Obviously it's corroded, and I'm actually replacing it today, but is there anything I missed that could be staring at me in the face? Why did the engine only start to run rough 6 months later? I'm going to guess that's part of my cap problem too. Any advice is appreciated, the next reply I send will be with a new cap, and gap checked spark plugs. Now, we threw the plugs in, but we never 100% made sure that they were properly gapped from the company we bought them from lol. Just going to do it to make sure I'm not going crazy. Thanks guys! Will have a pic of the cap soon.

 

[iamdub]

Hello everyone! I have the famous 1998 Yukon that developed a P0300 after the cap and rotor replacement, and new spark plugs. Of course, We didn't think it was the cap and rotor that caused this. I have spent over 6 months (That's when the P0300 appeared) Looking for vacuum line leaks, damaged crankshaft sensors, valve cover leaks, because oil can seep down onto the spark plugs, fuel pump, fuel filter, and more. Now this P0300 has gone away at random times, which led me to believe it was something not exactly mechanical, maybe computer? turns out I was wrong. Dad took the Yukon out, flashing check engine light. Really, really bad misfire. Yet again, P0300. Being a hundred miles from home, little more difficult to find the culprit, but I did find it. Before this point, I thought it was a bad sensor. It never gave a loss in power, Idle never changed, and sounded great. But when it got worked up hills all day, that's when it got really bad. Now somehow, I started looking at the cap and rotor. I figured that this problem can't be that complicated, and that I was just missing something. Put my hand on the cap while it was running and wiggled it ever so slightly, Check engine light went away, and misfires stopped. 50 miles from home, light came back and was misfiring, but wasn't nearly as bad until we made it all the way home. So, took the rotor out and inspected it, looked good! but the cap had so much corrosion on the metal points, just doesn't make sense to me. How could a cap only 6-7 months old already look like it came out of a stock Yukon that's never been changed?


Basically, I'm looking for confirmation that my problem is the cap. Obviously it's corroded, and I'm actually replacing it today, but is there anything I missed that could be staring at me in the face? Why did the engine only start to run rough 6 months later? I'm going to guess that's part of my cap problem too. Any advice is appreciated, the next reply I send will be with a new cap, and gap checked spark plugs. Now, we threw the plugs in, but we never 100% made sure that they were properly gapped from the company we bought them from lol. Just going to do it to make sure I'm not going crazy. Thanks guys! Will have a pic of the cap soon.

Brand of cap and rotor? Are the plugs OEM or equivalent and where did you source those from? What about the wires? Sounds like the cap was loose and/or installed a little crooked and not sealing. This would create an excessive gap for the spark to jump as well as allowing moisture in. Both of which will cause weaker spark and corrosion. If the cap and rotor are parts store specials, they might just be crap quality. These engines are very picky about ignition components and electronics (sensors, etc.)

 

[OilBurner2003]

Brand of cap and rotor? Are the plugs OEM or equivalent and where did you source those from? What about the wires? Sounds like the cap was loose and/or installed a little crooked and not sealing. This would create an excessive gap for the spark to jump as well as allowing moisture in. Both of which will cause weaker spark and corrosion. If the cap and rotor are parts store specials, they might just be crap quality. These engines are very picky about ignition components and electronics (sensors, etc.)

Cap and rotor are AC Delco, wires too. I suppose maybe we got a bad pair, I did not know about that seal, which makes a very good point. Spark plugs are AC Delco as well, I think we got them from amazon.

 

[iamdub]

Cap and rotor are AC Delco, wires too. I suppose maybe we got a bad pair, I did not know about that seal, which makes a very good point. Spark plugs are AC Delco as well, I think we got them from amazon.

Even if the cap and rotor are OEM parts, they can still be ruined by improper installation (obviously). From your post, it sounds like that might have been what happened or a contributing factor.

Be VERY skeptical of spark plugs from Amazon and eBay. I'm not putting the blame on them as they're just the marketplace and not the manufacturer. But, spark plugs have been a hot item to counterfeit for a few years now and are sold on Amazon and eBay since a buyer is expecting a crazy low price in the first place. The counterfeits tend to work fine for a short while, but wear out very quickly. If you're lucky, this will be the worst of your experience. Others have had them fall apart and cause severe engine damage. A member here recently found his platinum plugs were worn worse than regular copper plugs at something like 40K miles. Definitely counterfeits. Platinum and Iridium plugs should easily eclipse 150K miles. I doubt you still have your boxes. But, there are indications on the plug itself if it's legit or not. Google around and you'll see it all.

 

[OilBurner2003]

Even if the cap and rotor are OEM parts, they can still be ruined by improper installation (obviously). From your post, it sounds like that might have been what happened or a contributing factor.

Be VERY skeptical of spark plugs from Amazon and eBay. I'm not putting the blame on them as they're just the marketplace and not the manufacturer. But, spark plugs have been a hot item to counterfeit for a few years now and are sold on Amazon and eBay since a buyer is expecting a crazy low price in the first place. The counterfeits tend to work fine for a short while, but wear out very quickly. If you're lucky, this will be the worst of your experience. Others have had them fall apart and cause severe engine damage. A member here recently found his platinum plugs were worn worse than regular copper plugs at something like 40K miles. Definitely counterfeits. Platinum and Iridium plugs should easily eclipse 150K miles. I doubt you still have your boxes. But, there are indications on the plug itself if it's legit or not. Google around and you'll see it all.

All that makes good sense to me. I suppose it is possible that we COULD have gotten some bad plugs, although it seems like I did the cap wrong somehow. I haven't started the engine with the new cap yet, waiting for some other parts to arrive before I do that. That is very helpful information, thank you! I will update when I start the engine.

 

[iamdub]

All that makes good sense to me. I suppose it is possible that we COULD have gotten some bad plugs, although it seems like I did the cap wrong somehow. I haven't started the engine with the new cap yet, waiting for some other parts to arrive before I do that. That is very helpful information, thank you! I will update when I start the engine.

The rotor is in a difficult location and them screws on the cap are a known PITA. You moving it and making a difference clearly says something alignment-wise was off.

While you're waiting for the new parts, Google some images and go pull a plug. That 5 minutes could save your engine.

 

[OilBurner2003]

The rotor is in a difficult location and them screws on the cap are a known PITA. You moving it and making a difference clearly says something alignment-wise was off.

While you're waiting for the new parts, Google some images and go pull a plug. That 5 minutes could save your engine.

Roger, will do. You've been a fantastic help! Yeah, in my eighteen years of life I've never had harder screws to put in.

 

[OR VietVet]

Even quality new parts can be bad at times. I have ordered Genuine GM and AC Delco parts at Amazon before and am always skeptical but have been lucky. RA is a great source as well.

 

[exp500]

Check the vent screen on bottom of Dist when cap/rotor off, may be plugged which may be why cap didn't last. NGK plugs a good buy.
If you drove it with a miss for several hundred miles, replace cap/rotor wires plugs or it will miss again in 10k or less.

 

[OilBurner2003]

Check the vent screen on bottom of Dist when cap/rotor off, may be plugged which may be why cap didn't last. NGK plugs a good buy.
If you drove it with a miss for several hundred miles, replace cap/rotor wires plugs or it will miss again in 10k or less.

I've never heard of the vent screen; I just checked the plugs, and they look fouled. Will be tearing it off and inspecting today!