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Wild Tachometer swings after Timing Belt Replacement

I took my 5-speed (manual) 99 Miata with 51,000 miles in for a whining noise last week. The dealership said it was the timing belt pretensioner. They also replaced the timing belt. The noise didn’t go away so they replaced the water pump. It was working ok when I picked it up Friday and drove it Sat. Today (Monday) tachometer needle swings from 1500 to 2000 rpms at idle while the motor revs up and down. The ride itself is somewhat jerky. Could this mean something wasn’t put back properly?

It is quite possible. Is a check engine light illuminated?

I would head back and ask what is going on.

Nope, no check engine light. Thanks for your reply.

It sounds like something wasn’t reconnected when everything was put back together. My guess would be a vacuum line or fitting. Take it back for a look-see.

Thanks, will do. Always helps to be armed with a little more information.

My car is back in my garage. Seems the timing was off… and no charge so I didn’t have to “fight” with them at all. They apologized for the inconvenience.

I think NYBo is on the mark with a disconected vacuum line diagnosis.

Ask the shop how they adjust the timing on a car that uses DIS instead of a distributor.