Rough idle, P0300, Help!

[wiscomick]

I mean, maybe. But this thing ran absolutely perfectly, and I mean perfectly, up until last Wednesday night. No codes, no hesitiation, no vibration at idle, nothing. Then one 8 mile 70mph trip to the hardware store and has run like a bag of **** since. Like a light switch. Don't know that valve seals would go like that. I feel like that'd be a slow descent into suckery.

 

[wiscomick]

More info.

Checked distributor cap and rotor at lunch. Still clean as a whistle. No corrosion whatsoever on any terminal, no burning, no evidence of any goofiness whatsoever. But - like I said earlier - this distributor does seem to have a bit of play. I've attached a YouTube video link to show what I'm talking about below:

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Seems sloppier than it should be? Idk.

Also, another fun fact: The misfire goes away at 3000 RPM. Here's the datalogs (including the requested fuel trims that I found PIDs for in the app) at 3000 RPM and Idle.

Idle:

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3000 RPM:

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Spark issue? Thoughts?

 

[Erik the Awful]

If you remove the nuts on the A/C compressor bracket, you can pull the belt and slide it forward enough to remove the valve cover without too much fuss. IIRC, the EGR tube is a pain to weasel around. I'd look at the rockers, studs, and valve seals.

Question - when you did the compression test, you had all the spark plugs out and not just an individual plug out at a time, right?

 

[wiscomick]

No, I did one cylinder at a time. May have been a mistake. Not sure.

I did find a known-good distributor last night to try today. It's noticeably tighter than mine and the gear is in much better shape. I'll be installing it this afternoon and will report back.

 

[wiscomick]

Well, swapped the distributor out. Much less play than my old one. Before I put it in, I put the new ACDelco cap and rotor I purchased in March on the new distributor. Plugged it in, set timing, no difference. Same misfire.

At this point I was absolutely furious. Only thing I could possibly do that wasn't 'go buy head gaskets' was put the old GM distributor cap and rotor that came with this junkyard distributor on and see if these 'new parts' were already trash. The OEM one on the replacement distributor looked fine when I pulled it off but I figured why not put the actual new Delco stuff on that I just bought.

Put the old OEM stuff on..... Misfire completely gone. It was the new ACDelco cap and rotor the whole time.

Don't regret putting the new spider on even though it was expensive, was clearly leaking and was super brittle when I pulled it, but wasn't the problem.

Thanks for all the help guys. This one is solved.

 

[RawbDidIt]

Well, swapped the distributor out. Much less play than my old one. Before I put it in, I put the new ACDelco cap and rotor I purchased in March on the new distributor. Plugged it in, set timing, no difference. Same misfire.

At this point I was absolutely furious. Only thing I could possibly do that wasn't 'go buy head gaskets' was put the old GM distributor cap and rotor that came with this junkyard distributor on and see if these 'new parts' were already trash. The OEM one on the replacement distributor looked fine when I pulled it off but I figured why not put the actual new Delco stuff on that I just bought.

Put the old OEM stuff on..... Misfire completely gone. It was the new ACDelco cap and rotor the whole time.

Don't regret putting the new spider on even though it was expensive, was clearly leaking and was super brittle when I pulled it, but wasn't the problem.

Thanks for all the help guys. This one is solved.

Wow. That's crazy. Good to know

Sent from my SM-N976V using Tapatalk

 

[1997]

Good to hear you got it sorted.
ACDelco now isn't like the old ACDELCO imo, now made in China too.

 

[HotWheelsBurban]

That's why my cousin says (he probably learned this lesson from his grandfather, who was a lifelong mechanic and ran a Texaco station in Austin, Texas for decades) N-E-W does not spell G-O-O-D. Brand new parts,even OEM, even back in the day, aren't guaranteed to be perfect. Hope you are able to get a refund on the new Delco parts.
On the other hand, at least you have known good replacement parts on the stuff you replaced. Thanks for the updates!

 

[Erik the Awful]

Never underestimate the power of a well-written letter. I'd write up a letter to AC-Delco about my tribulations and ask how they're going to restore their reputation with you. They at least owe you an apology.

I know I felt disheartened after recommending the MPFI and it didn't fix it. I'm relieved that it truly needed it as well.

 

[OutlawDrifter]

Thank you for reporting the solution to the problem. Most threads never have an "end."

Glad you got your issue sorted out!