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Old 06-11-2011, 09:14 PM   #1
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Default Something to worry about?

Hey guys,

I took the Miata up island last weekend and back (250km each way), on the highway. I noticed something odd and just wanted to run it by everyone here to see if they thought it was something that I should worry about.

Cruising on the highway doing 130km/h @ 3800k RPM I noticed the odd RPM dip - the revs would, without rhyme or reason, dip from 3800k to 3000k for a split second before continuing on its merry way. On the entire trip, it did this 4 or 5 times that I noticed. I'm not really sure what it might have been, perhaps fuel cut?

Is this something that I should look into further or that I should be worried about? Or is this a common occurrence?

Thanks.
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Old 06-11-2011, 11:37 PM   #2
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changed the fuel filter?
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Old 06-11-2011, 11:46 PM   #3
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Something similar happened to me. I eventually realized I was in 4th instead of 5th and cruise control didn't like that. It was like the turbo kept wanting to spool but the engine was like "Hey we're going fast enough. Chill out."
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Old 06-11-2011, 11:49 PM   #4
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Quote:
Originally Posted by mx5autoxer View Post
I eventually realized I was in 4th instead of 5th and cruise control didn't like that.
Heh. I was definitely in 5th and, unfortunately, have no cruise control.
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Old 06-11-2011, 11:55 PM   #5
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Originally Posted by Cookiesoup View Post
changed the fuel filter?
Nope. Could that be what's causing fuel cut?
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Old 06-12-2011, 12:10 AM   #6
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Were the revs actually dipping, or was it just the tach dipping for a moment?
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Old 06-12-2011, 12:22 AM   #7
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Quote:
Originally Posted by Savington View Post
Were the revs actually dipping, or was it just the tach dipping for a moment?
It's hard to say. It kind of felt like the ignition was cut for a split second.
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Old 06-12-2011, 12:26 AM   #8
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A/C cutting in?
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Old 06-12-2011, 12:36 AM   #9
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An 800rpm drop is like a 12-15mph drop in speed. It's not "hard to say" whether that happened or not.

If you are getting an 800rpm tach drop and a small misfire, it's just that - a misfire. Change the plugs/wires and then look to ECU grounding or cam/crank sensor issues.
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Old 06-12-2011, 12:51 AM   #10
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Quote:
Originally Posted by rleete View Post
A/C cutting in?
No a/c.

Quote:
Originally Posted by Savington View Post
An 800rpm drop is like a 12-15mph drop in speed.
It not quite that dramatic. Its only over a fraction of a second and the speed loss isn't noticeable.

As far as changing the plugs/wires and all that other stuff... any recommendations for how-to's, tutorials, etc.? I don't exactly have much mechanical experience, or really any... at all
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Old 06-12-2011, 12:57 AM   #11
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any major altitude changes along the way?
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Old 06-12-2011, 01:33 AM   #12
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Does ~500m count as a major altitude change? Although it happened before I hit the altitude change...

Also, wouldn't this oddity happen when cruising around the city? The only time I noticed it was on the highway and only in 5th at constant RPM.

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Old 06-12-2011, 02:23 AM   #13
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im thinking its either only a few things, ill organize it from casual to worst case IMO:

-altitude change
-Bad fuel filter
-air in the fuel line
-dirty injectors
-pinched vacuum line to map sensor
-ECU fuel cut
-detonation.


Am I missing anything?
-Detonation
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Old 06-12-2011, 02:25 AM   #14
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My old 90 did this. Replaced plugs/wires and cleaned all the grounds and it went away. It was a small misfire under light cruise.
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Old 06-12-2011, 02:38 AM   #15
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Quote:
Originally Posted by falcon View Post
My old 90 did this. Replaced plugs/wires and cleaned all the grounds and it went away. It was a small misfire under light cruise.
lol I knew I was missing something
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Old 06-12-2011, 02:42 AM   #16
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Thanks guys, I'll try some of your suggestions and report back.

Quote:
Originally Posted by falcon View Post
My old 90 did this. Replaced plugs/wires and cleaned all the grounds and it went away. It was a small misfire under light cruise.
Wouldn't the 1997 BP swap rule that out?
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Old 06-12-2011, 02:44 AM   #17
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The shared similarity between all combustion engines is... well, ignition.
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Old 06-12-2011, 03:30 AM   #18
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Quote:
Originally Posted by falcon View Post
My old 90 did this. Replaced plugs/wires and cleaned all the grounds and it went away. It was a small misfire under light cruise.
Do exactly as Falcon says. I had this happen to me once after a heavy rain (not sure if related, to be honest) and all it took was a little cleaning/tightening to get things back to normal.
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Old 06-12-2011, 05:50 AM   #19
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Quote:
Originally Posted by viperormiata View Post
Do exactly as Falcon says. I had this happen to me once after a heavy rain (not sure if related, to be honest) and all it took was a little cleaning/tightening to get things back to normal.


SIG FOR THE MTFW!!!


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Old 06-13-2011, 07:51 PM   #20
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could be loose grounds? terminals?
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