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Old 07-15-2008, 07:59 AM   #1
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Question: Higher idle after spark plug replacement

I've searched for this info on his site and on Google, and I haven't had much luck.

I replaced the plugs, wires, and distributor cap on my car this past Sunday, and everything seems fine, except that the idle is a little higher and causes a bit more vibration than it did before. At least it seems to.

The only thing I found in my search was about vacuum leaks, but I've checked, and I didn't bump any vacuum hoses in the process.

Note that I used Bosch Platinum +2 plugs, because they were only a few dollars more than the regular plugs (for a set of 4, not each), and I thought it would be worth a try.

Does anyone have any idea why my idle would change like that, and if it is a problem I should worry about?

Thanks in advance for your help.
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Old 07-15-2008, 08:41 AM   #2
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Originally Posted by thornburg View Post
I've searched for this info on his site and on Google, and I haven't had much luck.

I replaced the plugs, wires, and distributor cap on my car this past Sunday, and everything seems fine, except that the idle is a little higher and causes a bit more vibration than it did before. At least it seems to.

The only thing I found in my search was about vacuum leaks, but I've checked, and I didn't bump any vacuum hoses in the process.

Note that I used Bosch Platinum +2 plugs, because they were only a few dollars more than the regular plugs (for a set of 4, not each), and I thought it would be worth a try.

Does anyone have any idea why my idle would change like that, and if it is a problem I should worry about?

Thanks in advance for your help.


If you were working in the area of any vacuum lines you should check them to make sure none of them are cracked and causing a vacuum leak. It could be they were brittle and cracked if you bumped it. Usually if you have a very bad vacuum leak it will make the idle much higher. I also wonder if it would help to disconnect the battery and let the computer reset, it may have something to do with the new type plugs. I have never used the +2 platinum plugs, so I don't know the effects of them.
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Old 07-15-2008, 10:34 AM   #3
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If you were working in the area of any vacuum lines you should check them to make sure none of them are cracked and causing a vacuum leak. It could be they were brittle and cracked if you bumped it. Usually if you have a very bad vacuum leak it will make the idle much higher. I also wonder if it would help to disconnect the battery and let the computer reset, it may have something to do with the new type plugs. I have never used the +2 platinum plugs, so I don't know the effects of them.
Thanks for the reply.

I had the battery disconnected when I did the job (and actually for about 36 hours beforehand as well), because it was recommended to disconnect it before doing the plug wires & cap, and it was necessary to remove it from the car to get the cap off anyway.

The idle isn't a lot higher, maybe 90RPM or so? I wish I had paid closer attention to what it was in various states before, so I would know exactly.

One thing I can say for sure is that idle in neutral with the AC on is almost exactly the same, before and after (about 1100RPM), it is the idle in neutral (no AC) and idle in gear w/foot on the brake that seems somewhat higher.

I'll try to remember to take a look at the vacuum lines for cracks before I leave work this afternoon. I'm not real engine-savvy (yet), so I've just been checking any small-ish hoses when checking for vacuum lines. That is what they are, small hoses, right?
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Old 07-15-2008, 10:47 AM   #4
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Thanks for the reply.

I had the battery disconnected when I did the job (and actually for about 36 hours beforehand as well), because it was recommended to disconnect it before doing the plug wires & cap, and it was necessary to remove it from the car to get the cap off anyway.

The idle isn't a lot higher, maybe 90RPM or so? I wish I had paid closer attention to what it was in various states before, so I would know exactly.

One thing I can say for sure is that idle in neutral with the AC on is almost exactly the same, before and after (about 1100RPM), it is the idle in neutral (no AC) and idle in gear w/foot on the brake that seems somewhat higher.

I'll try to remember to take a look at the vacuum lines for cracks before I leave work this afternoon. I'm not real engine-savvy (yet), so I've just been checking any small-ish hoses to when checking for vacuum lines. That is what they are, small hoses, right?

Vacuum hoses are usually no larger than 3/8" and most of the time they are even smaller. Sometimes after just having the battery disconnected the idle will go up until the ECU gets everything readjusted. It usually takes 50 to 100 miles for everything to be reset properly.
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Old 07-15-2008, 12:41 PM   #5
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Did you gap the plugs and adjust the timing when you were done?
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Old 07-15-2008, 12:52 PM   #6
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Did you gap the plugs and adjust the timing when you were done?
Bosch Platinum +2 are "gapless" and specifically say not to adjust the gap. I'm not even sure you could, those prongs look like they might break if you tried to bend them.

I did not adjust the timing. Is it necessary to adjust the timing when you replace the spark plugs & wires? I didn't see that in the Haynes manual or the Autozone online instructions...
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Old 07-15-2008, 01:27 PM   #7
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Bosch Platinum +2 are "gapless" and specifically say not to adjust the gap. I'm not even sure you could, those prongs look like they might break if you tried to bend them.

I did not adjust the timing. Is it necessary to adjust the timing when you replace the spark plugs & wires? I didn't see that in the Haynes manual or the Autozone online instructions...
Its not absolutely necessary, but its always a good idea to check the timing when replacing that stuff.

-Jay
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Old 07-15-2008, 01:42 PM   #8
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Its not absolutely necessary, but its always a good idea to check the timing when replacing that stuff.

-Jay
I suppose that in order to check the timing I need to take off the timing cover and have a timing light?

The only car I've even been remotely involved in the timing belt on was a VW TDI and we used a VAG-COM to monitor timing instead of a light...
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Old 07-15-2008, 02:22 PM   #9
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It's possible for this to happen. The more efficient combustion from the new parts can cause the engine to run a little higher at idle.

We get this at work all the time on older carbbed engines and notice that the IACV closes a little more at idle. If you have a EFI you should take a look at your IACV to make sure it's working correctly.
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Old 07-15-2008, 03:02 PM   #10
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I suppose that in order to check the timing I need to take off the timing cover and have a timing light?

The only car I've even been remotely involved in the timing belt on was a VW TDI and we used a VAG-COM to monitor timing instead of a light...
now I've only had american cars, but everything I've ever owned you did not have to remove the timing cover to check the timing. There was a mark on the harmonic balancer that you highlighted with a piece of white chaulk, then aimed a timing light at it while the engine was running. The mark would line up with a scale on the side of the timing cover and you could read what the timing was.

-Jay
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