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Old 02-09-2008, 07:50 PM   #1 (permalink)
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Default Bad Idle/MPG, Help Interpret Spark Plug color

I'm trying to work out the demons in the VR4 I got a little while ago. Symptoms:

-Generally gets 13-14 mpg combined
-Occasionally idles very rough/low idle when first started
-Boosting about 14.5 psi, even though no MBC/EBC installed (that I know of)
-Spark plugs are a strange color - white and black
-Occasionally the 'low coolant' light comes on (at rest) even though it is totally full
-narrowband a/f gauge shows it cycling almost totally from rich to lean to rich to lean on both front and back O2s under normal driving. This may be normal, I know it is supposed to cycle, I just didn't think it went from full rich to full lean


The idle issues only happen maybe every 15th time I start it. They go away if you give it some gas and let it warm up. Fuel mileage does not improve much (maybe 15mpg) even if I granny it.

Mods: bov, borla cat back, turbo timer, aftermarket boost and a/f gauges, intake

These plugs are 10k-20k miles old. When I pulled the front plugs, I found the electrode white, and the ring around it blackened with some white deposits.
I have looked at the pictures and charts online and stealth316, but they are very poor quality and I'm not sure which pic matches mine. Any ideas what this could mean?





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Old 02-09-2008, 08:34 PM   #2 (permalink)
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Default Re: Bad Idle/MPG, Help Interpret Spark Plug color

gray=rich

But you knew that by the mgp.

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Old 02-10-2008, 01:05 AM   #3 (permalink)
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Default Re: Bad Idle/MPG, Help Interpret Spark Plug color

rich = boost leak... pressure test your system first.
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Old 02-10-2008, 01:53 AM   #4 (permalink)
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Default Re: Bad Idle/MPG, Help Interpret Spark Plug color

Assuming I do find a vacuum leak, should I replace the plugs or just take them out and clean off the deposits?
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Old 02-10-2008, 05:44 AM   #5 (permalink)
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Default Re: Bad Idle/MPG, Help Interpret Spark Plug color

How do you know you are boosting 14.5psi? I take it you either have an aftermarket gauge, or are just reading off the stock gauge. The factory gauge does not measure boost, it measures engine load, and cannot be tied to your actual boost levels at all.

If you really are boosting that much, you probably have a hole in one of the lines running to the wastegates, or they are unplugged. Do you get any fuel cut or anything?

And as far as the plugs go, those don't look grey to me at all, they look pretty damn white to me. White is lean, black is rich, tan is normal. The deposits do appear to be carbon buildup, but the grounding strap and porcelain are white, indicating a lean condition...trust me, too rich usually leads to VERY black plugs.

Test for and fix any boost leaks, consider replacing O2 sensors (they can be had for $25 each on ebay).
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Old 02-10-2008, 06:39 AM   #6 (permalink)
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Default Re: Bad Idle/MPG, Help Interpret Spark Plug color

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Originally Posted by ZephTheChef View Post
How do you know you are boosting 14.5psi? I take it you either have an aftermarket gauge, or are just reading off the stock gauge. The factory gauge does not measure boost, it measures engine load, and cannot be tied to your actual boost levels at all.

If you really are boosting that much, you probably have a hole in one of the lines running to the wastegates, or they are unplugged. Do you get any fuel cut or anything?

And as far as the plugs go, those don't look grey to me at all, they look pretty damn white to me. White is lean, black is rich, tan is normal. The deposits do appear to be carbon buildup, but the grounding strap and porcelain are white, indicating a lean condition...trust me, too rich usually leads to VERY black plugs.

Test for and fix any boost leaks, consider replacing O2 sensors (they can be had for $25 each on ebay).

I do have an aftermarket boost gauge. And, you're correct, the plugs are more of a bright white than a grey. Coolant leaking into cylinders? My exhaust is clear when outside, but if I run it in the garage (door closed) for a short time, you can see there is a white/grey fog being produced by the exhaust that fills the garage. My other vehicle does not do this.
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Old 02-10-2008, 11:34 AM   #7 (permalink)
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Default Re: Bad Idle/MPG, Help Interpret Spark Plug color

Bright white like that with black scorches usually indicates too high of a heat range. What plugs are you using?
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Old 02-10-2008, 03:05 PM   #8 (permalink)
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Default Re: Bad Idle/MPG, Help Interpret Spark Plug color

These plugs are NGK PFR6J's, so just the stock ones.
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Old 02-10-2008, 07:18 PM   #9 (permalink)
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Default Re: Bad Idle/MPG, Help Interpret Spark Plug color

White smoke generally does indicate burning water/coolant, however my car that I just put back together is throwing out a LOT of white smoke. Considering the fact that I haven't put any coolant in yet, I'm going to say that mine is probably due to the fact that the gas that's in the tank has been at less than 1/4 tank for about 14 months just sitting.

White deposits can be formed by oil or coolant getting into the combustion chamber. It's kind of hard to tell from the picture if the plug is just generally white, or if it has any kind of build up on it.

These pictures are a little better, I think:

http://www.spark-plugs.co.uk/pages/t.../diagnosis.htm
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Old 02-11-2008, 11:23 PM   #10 (permalink)
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Default Re: Bad Idle/MPG, Help Interpret Spark Plug color

Quote:
Originally Posted by ZephTheChef View Post
White smoke generally does indicate burning water/coolant, however my car that I just put back together is throwing out a LOT of white smoke. Considering the fact that I haven't put any coolant in yet, I'm going to say that mine is probably due to the fact that the gas that's in the tank has been at less than 1/4 tank for about 14 months just sitting.

White deposits can be formed by oil or coolant getting into the combustion chamber. It's kind of hard to tell from the picture if the plug is just generally white, or if it has any kind of build up on it.

These pictures are a little better, I think:

http://www.spark-plugs.co.uk/pages/t.../diagnosis.htm


It is generally white, but with some flaky areas on it.
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