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What: Shares of auto parts-maker Tenneco (NYSE:TEN) were roaring higher today, gaining as much as 18% on better-than-expected earnings.

So what: Adjusted EPS for Tenneco, which makes emission control systems, came in at $0.85 a share, beating estimates of $0.75 and last year's per-share profit of $0.67. That total did not include a $74 million gain from the reversal of a tax valuation allowance. Revenue growth was less impressive, as it was essentially flat at $1.78 billion, short of estimates of $1.82 billion. Management expressed concerns about economic weakness in Europe, but counteracted that with increasing sales in North America and Europe.

Now what: Shares had come down about 50% from highs reached back in the spring before today's announcement so investors were happy to pounce on some good news. Tenneco seems to have overcome the cyclical weakness we've seen in many parts of the auto industry, but flat revenue growth should signal that it's not out of the woods yet. Keep an eye on that figure, as upward momentum will be limited without an increase in sales.

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