When a Pup becomes a dog

Dear Car Talk

Dear Car Talk | Oct 01, 1990

Dear Tom and Ray:

I own a 1986 Isuzu Pup pickup that has given me a great ride for four years with the best maintenance record of any vehicle I've owned. It has 75,000 miles, and has only one problem that concerns me. It backfires out the tailpipe after every shift under acceleration. It also doesn't seem to have as much power as it used to. Is there a fix for this, or is it one of those things that has to be lived with?

TOM: What did you expect, Fred? This happens to all Pups. When they get older...they become dogs!

RAY: Actually, Fred, this is not a serious problem. You probably have a malfunctioning anti-backfire valve (no kidding). That's a valve that stops fresh air from flowing to the catalytic converter after you take your foot off the gas. When air continues to flow during deceleration, and combines with the unburned gasses in the converter, you get an explosion that sounds like it's coming out of the tailpipe.

TOM: This problem may have an indirect effect on the perceived power of your Pup. You may be subconsciously driving the car more gingerly than usual, because you anticipate a "ka-boom" every time you change gears. It may not have any less power, you may just be afraid to push it as much.

RAY: So don't call the pound just yet. Have your mechanic replace the anti-backfire valve, and I'll bet this Pup peps right up.

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