DavidOfMA
Senior Member
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Getting back in gear after a long time away
Posts: 336
Beverly, MA
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One other method not yet mentioned: modifying the Transient Enrichment Valve (TEV), a small vacuum-operated valve on the right side of the carb.
I could not get the backfires on my 2007 S40 to stop by any of the standard methods: eliminating exhaust leaks (there weren't any), adjusting the idle mix screw, changing jets. What finally worked for me was taking a small amount off the spring that is part of the TEV. If the standard methods don't work for you, I suggest taking a very small amount off the spring (such as 1/4 turn), then test for backfires for a couple of days. If you still get backfiring, take a little more off and test again.
If you weaken the spring too much, the valve will open too easily and you'll get a surge at idle, but if you weaken it just a little, the time it stays open on decel will increase but idle will be unaffected. In my case, removing 1/2 round of the spring weakened the spring just enough to almost completely eliminate backfires. Just driving around town, it never backfires. On a recent 300+ mile trip, on a variety of roads and speeds, I had a total of 3 backfires. Before this mod, I'd have more backfires than that on my 6-mile trip to work.
My two cents.
David
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