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Old 09-14-2012, 02:02 AM   #1
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Default No power under 4K rpms

I was hoping to get some suggestions as what could be wrong with my Miata.

It has about 202K miles on it. The Wideband O2 is telling me the car is very lean under 4k rpms. After 4k rpms the car behaves normal. The problem is intermittent and I am having problems diagnosing it. I suspect it may be the MAF sensor, but I would like to know for sure before spending the money on a new sensor.

Things I've tried so far:

Cleaned all major grounds.
New battery(it was dying).
Used engine start aerosol spray to check for vacuum leaks, none found.
Replaced leaking catalytic converter gaskets.

Tonight on my drive home from work, the car stalled at a stop sign. Started the car, problem went away. Until I was about 3 miles from my house.

Any help will be greatly appreciated.
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Old 09-14-2012, 02:05 AM   #2
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your timing belt
it slipped a tooth
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Old 09-14-2012, 08:00 AM   #3
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Or the CAS is way off.
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Old 09-14-2012, 08:55 AM   #4
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Stock tune? That would explain the WAY lean under 4k. Otherwise if this is a new condition the timing slipped.
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Old 09-14-2012, 05:58 PM   #5
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Thank you. I will check timing belt tonight. Will a skipped tooth cause an intermittent problem? I changed the timing belt around 175K miles.

The car has a stock tune as far as I know. This is a new condition.

Last edited by Gregor; 09-14-2012 at 06:11 PM.
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Old 09-14-2012, 06:04 PM   #6
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Carbon build up. The carbon soaks up gas vapor. Try an engine cleaning treatment or walnuts if you are fancy.
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Old 09-14-2012, 08:14 PM   #7
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Like seafoam?
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Old 09-15-2012, 05:11 PM   #8
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Checked my timing belt, all timing marks are lined up. The car also starts up on the first crank without hesitation. It will also hold idle just fine for about 30 seconds, then idles really rough.

I used this method to check the MAF:
How to Test a 1995 Mazda Miata Mass Air Flow Sensor | eHow.com

The MAF signal measured at .35V when the engine is off. And 1.23V when the engine is on. According to the article, I should be reading .6V and around 1.3V Will this difference in voltage be enough to warrant replacing the MAF?

And the car was idling fine when I tested it. I couldn't replicate the problem when I tested.
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Old 09-15-2012, 06:26 PM   #9
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How in-depth did you go with your vacuum leak testing? Still sounds like a unmetered air issue to me.
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Old 09-16-2012, 07:45 PM   #10
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I had the car idling, and I sprayed engine start on possible leaks. The idle never changed. I then sprayed on the air filter to be sure. The idle then dipped and returned to normal.
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Old 09-18-2012, 11:12 AM   #11
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Subscribed to this thread, having similar issues but don't know if 4k is the cutoff (tach isn't working)
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Old 09-18-2012, 12:07 PM   #12
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Well, I'm a boner. I inspected the timing belt again with an extra set of eyes, and the exhaust cam gear is off by one tooth. Then decided to replace the timing bet, tensioner, and pulley. Upon removing the accessory pulley, the key way in the crank is damaged. The timing gear is retarded one or 1/2 a tooth, the key is sheared in two, and the front main seal is leaking like its angry at me. I'm ordering new parts and plan on following this procedure:

Long Nose Crank Pulley Repair / Replacement
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Old 09-18-2012, 12:15 PM   #13
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Sorry for repost.
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Old 09-24-2012, 05:52 PM   #14
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Well I did the long nose crank repair and installed a new timing belt kit, and front main seal. Car started up on the first crank and started behaving exactly the same. But now when the car is over 4k it pulls harder than before. The timing gear on the crank was retarted previously.

My question now is, does this look right?

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Old 09-24-2012, 11:16 PM   #15
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Intake cam is off by one tooth. I advanced it, car runs like a champ now.

Now I can focus on my turbo build.
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Old 09-25-2012, 01:24 AM   #16
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Yay
It makes me warm and fuzzy that I was right as first response in this thread 3 minutes after you posted it <3
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Old 09-27-2012, 02:32 AM   #17
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Glad I could make you feel that way. Sorry I doubted you, the problem seemed intermittent. So had the notion in my head that this was an electrical problem(sensor or bad ground).

Thank you.
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