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j3studio Update On Grace's Rough Idle
Elite Guru
Western Philadelphia Burbs
3018 Posts
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2007/9/2 0:00



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On June 30th, we took Grace out to celebrate the birthday of the Corvette itself. It was a hot day, but she performed well—until she didn’t. On the way home, I had to brake fairly suddenly when an old woman in a Camry pulled out in front of me. Immediately afterward (coincidence?), we started to get a rough idle and an occasional backfire.

The last time we had a similar issue (back in 2013), the reason was that the clamp following the MAF had come almost completely detached, allowing a stream of hot and uncontrolled air into an engine that was believing what the MAF was telling it. I pulled the car over and took a quick look. That wasn’t true this time—nothing was loose.

I nursed the car back home (about ten miles). Acceleration was fine, and the car was smoother at higher RPMs. However the rough idle and backfiring under load at low RPMs continued.

The first thing I did was check for codes—there were none. I then proceeded up the intake chain. I cleaned and re-oiled the Blackwing air filter (at 50,375 miles, it was due). The idle got a little bit better, but the car blew a code—the generic P0300 random/multiple cylinder misfire code.

Next, I cleaned the MAF (likely the first time that’s been done). The idle again got a little bit better, and the code went away, but the idle was still much rougher than expected. I cleaned the front of the throttle body. The idle once again got a little bit better, but the car gave a new code—the more specific P0305 cylinder 5 misfire code.

I carefully pulled the #5 wire, which disintegrated in my hands as I removed it. Hmmm … I ordered new plugs and wires. Next, I tried to remove the #5 spark plug. At this point, I discovered that my tools were striking sub-optimal for the task at hand, with my old Craftsman spark plug socket essentially useless and all my ratchet lengths incorrect. I ordered a modern swivel spark plug socket and a longer teardrop ratchet and asked for help from Guru member Rex Ruby. Before he came over, I sprayed a ton of PB Blaster on the plugs.

It took us about an hour to remove the plugs and wires, with Rex doing most of the hard work. As expected, #7 was the most challenging, but none were particularly easy. Despite all the PB Blaster, removing the plugs took all my strength. One of the other wires was also bad, but the rest looked pretty good. The plugs looked worn, but not too bad.

We checked the gap on the new plugs (they were all fine), put some anti-seize on, and re-installed the plugs and wires. With everything buttoned back, I started the car. After the PB Blaster had burnt off (a notable amount of smoke), the idle settled down to a much smoother note. It's still rougher than I remember, but now I'm starting to wonder if I remembered it as being smoother than it actually was.

Posted on: 7/21 19:37
Edited by j3studio on 2018/7/21 19:56:24
Edited by j3studio on 2018/7/21 20:22:06
Edited by j3studio on 2018/7/21 20:30:33
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Grace, 2003 50th Anniversary Convertible

Louis, 2012 Centennial Edition Coupe
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pianoguy Re: Update On Grace's Rough Idle
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Apple Valley, MN
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Nice of Rex to give you a hand - hope that does the trick! Sounds like you checked off some items that were due, anyway, and got a new tool to boot.
Posted on: 7/21 21:48
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j3studio Re: Update On Grace's Rough Idle
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Western Philadelphia Burbs
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Quote:

pianoguy wrote:

Nice of Rex to give you a hand - hope that does the trick! Sounds like you checked off some items that were due, anyway, and got a new tool to boot.

I am so lucky that Rex is a) close and b) willing. I did help him regain control of one of the family iPads, so I've started to pay him back.

You are absolutely correct about the items that were likely due. Moving on!

Posted on: 7/21 21:54
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teebee Re: Update On Grace's Rough Idle
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Lakin, Kansas 67860
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I thinking that I might ought to do that to 007! Glad you figured it out, but the hard braking makes you wonder doesn't it? Enough G-force to make that wire fail?
Posted on: 7/22 2:08
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TommyT-Bone Re: Update On Grace's Rough Idle
Chair-man of the bored
Homestead USA
30693 Posts
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2007/12/10 0:00



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Once your attention is drawn to a specific issue I believe there is some part of the human mind that recalls it as being better BEFORE the fix. I know exactly what you are writing about. Hopefully as you 'forget" about the new wires and plugs you will sit back as comfortably as you did before. Just to be safe, try to do some quick accelerations and and sharp stops. In an open area turn the wheel hard to the left and punch it. Turn it hard to the right and punch it. Now put it in reverse and accelerates sharply then jam on the brakes. Putting it in forward motion bring it up to around 33 mph on a gradual acceleration then punch it. Hopefully you have found you got good response to all the above. If so, take the car for a longer ride and go have some ice cream.
Posted on: 7/22 3:38
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Matatk Re: Update On Grace's Rough Idle
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Glad you got it sorted out!
Posted on: 7/22 6:08
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j3studio Re: Update On Grace's Rough Idle
Elite Guru
Western Philadelphia Burbs
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Quote:

TommyT-Bone wrote:

Once your attention is drawn to a specific issue I believe there is some part of the human mind that recalls it as being better BEFORE the fix. I know exactly what you are writing about. Hopefully as you 'forget" about the new wires and plugs you will sit back as comfortably as you did before. Just to be safe, try to do some quick accelerations and and sharp stops. In an open area turn the wheel hard to the left and punch it. Turn it hard to the right and punch it. Now put it in reverse and accelerates sharply then jam on the brakes. Putting it in forward motion bring it up to around 33 mph on a gradual acceleration then punch it. Hopefully you have found you got good response to all the above. If so, take the car for a longer ride and go have some ice cream.

All good advice, Tommy—thank you. This is, of course, the car we drove for 4,952 miles when we visited you last May.

Posted on: 7/22 16:41
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bogus Re: Update On Grace's Rough Idle
Grand Imperial Pooh-Bah
San Pedro, CA
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I did a tune up on an LS1 about a year ago... that was a learning experience... All of my muscle memory was useless... the plug orientation is just different enough... drat.

Then there is the relief... there is a machined surface that the plug rests in... the problem? it requires a thin walled socket to get in between the plug and the elevated portions of the head. A real bitch.

What plugs did you use? Wires? When I do mine, I will use KGK or Denso plugs and MSD wires.

As for the rough idle still being there, how old are the O2 sensors? I would replace with AC Delco, Denso or NTK (kin to NGK). I need to double check the lifespan of the newer O2s... the old single wire ones were 30k replacements.
Posted on: 7/28 20:22
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j3studio Re: Update On Grace's Rough Idle
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Western Philadelphia Burbs
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I started Grace again yesterday evening and let her idle for about twenty minutes. At this point, I'm going to declare victory—Ivelis agrees with Tommy that I may have halcyon memories of the idle's smoothness prior to the issues.

Posted on: 7/28 20:30
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bogus Re: Update On Grace's Rough Idle
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San Pedro, CA
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that makes sense. No reason to notice what appears correct.






Posted on: 7/28 21:02
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The single biggest problem with communication is the illusion that it has taken place. - George Bernard Shaw

Education is the best tool to overcome irrational fear. - me

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