MX5 Miata Tech Forum

99 Miata - Rough Idle / Poor Acceleration - No Codes

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Drareg B. M.
Looking for some insight from the collective...

I picked up a '99 NB automatic with 125K miles as a project car. The car is fine once up to speed but starts off painfully slow and idles rough at stops. I've lurked the message boards and based on similar posts have cleaned (throttle body, manifold, MAF, etc...) have replaced (air filter, plugs and wires, fuel filter, valve cover gasket, etc...) and have drained/added (oil, transmission fluid, seafoam, etc...) to no avail. I've tested for vacuum leaks (negative) and the car isn't throwing any codes.

I read some posts that these cars are infamous for developing a wobble in the crank shaft pulley. Mine is wobbling ever so slightly but not like I have seen in various videos.

Based on the aforementioned, would you consider the wobble the likely culprit or is there something blatantly obvious I'm missing? With a slight wobble, is there hope of tightening the pulley or otherwise resolving it or will it worsen in time?

Thanks for your insight!

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tanseyj Avatar
tanseyj James Tansey
Sierra Vista, AZ, USA   USA
Fuel Pump? Check pressure if about 20 and then climbs up to about 60 after warmed up. could be Pump. Does Check Engine Light go on?

gravymeister Dwight S.
San Tan Vallwy, AZ, USA   USA
I don't know if you have fixed the problem yet, but my 2006 did have a loose crankshaft pulley. The symptoms were rough idle and loss of power. I verified it with a timing light.
The keyway in the pulley and crankshaft were hogged out into vee shapes. The fix was a new pulley and crankshaft, or a good used engine, or an experimental repair.
I bought a new timing gear and vibration dampner, crankshaft bolt, also a tensioner, and all front radial seals. I replaced the reluctor and the lower timing cover because they had been in contact and rubbed through. I bought an oversized key, and filed the top half to a press fit in the new timing gear. I filed the bottom half to a wedge shape to exactly fit the crankshaft keyway. After many cut and try tests, I was ready for the final assembly. I used a liberal application of Loctite 660, quickly assembled the parts (with a new timing belt, of course) and ran it home with an impact gun. I then torqued it to spec.
That was three years ago and the problem has not recurred.
Loctite 660 is a special product designed specifically for loose keyway repair.

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adavecone Dave Cone
Bend, OR, USA   USA
Check the flexible tube between the air filter housing and throttle body for cracks. On the NA (and NB as well, I believe), there is a sensor on the airbox that measures airflow to the throttle. If air is leaking in between this sensor and the throttle body, the fuel mixture will be too lean, resulting in your symptoms.

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