Q: Q: Truck is back-firing.

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After changing the harmonic balancer my truck was running rough. I asked the friend that did the work what he suggested, he said a tune-up. He then performed a tune-up on the truck and now it is back-firing. He doesn't know why. I asked if he adjusted the timing (thought this was done after a tune up, not sure) he said he didn't touch the timing. Please help.
My car has an automatic transmission.

There are a few possible causes for an engine to backfire. Some of the more common causes are a vacuum leak, lean condition caused by a weak fuel pump, an exhaust manifold leak, or a faulty ignition system. With the information that you’ve provided, in that the engine was running rough before doing the tune up, I would focus on either the ignition system, or the fuel system. With the backfiring showing up after the tune up, I would suspect that there has been a pair of wires crossed when they were reinstalled after replacing the plugs. This is a very common occurrence, and even the best in the business do it from time to time. With the engine running rough before the tune up, I would look more at the ignition coil, the wires, and check the fuel pressure. If you feel you could use some help identifying and/or repairing the problem, contact a certified mechanic, like those available at YourMechanic.com.

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