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December 11, 2009

Car Talk

Lean condition may have led to backfire that blew out manifold gasket

Syndicated columnists

Dear Tom and Ray:

My daughter was going to soccer practice in our 2001 Mercury Sable Wagon. When she turned the key, there was an explosion that apparently blew the intake manifold off the engine. Needless to say, it drove a bit rough after that. The mechanic could not offer any speculation as to why this happened. But I really need to be able to discuss this semi-intelligently with my father-in-law, who is a car expert. Otherwise, he'll think I'm not manly. Please help.
-- Brian

Tom: Well, you might want to stop wearing those flowered sundresses on New Year's Eve, Brian. That'll go a long way toward winning him over.

Ray: I'm wondering if it could be the manifold gasket that blew, rather than the manifold itself? I've never seen a manifold actually blow off an engine (although I'd like to!), but I can give you a semi-intelligent explanation for a blown manifold gasket -- which is a rubberized "seal" that goes between the manifold and the engine.

Tom: If the manifold gasket was already cracked or breached somehow, that would have allowed extra air to be sucked into one or more of the cylinders, creating what we call a "lean condition" -- that is, too much air, not enough gas.

Ray: My brother usually has too much gas, but that's a discussion for another day.

Tom: A lean condition also can be caused by a faulty fuel injector or a misfiring coil. But whatever the cause, a lean condition can lead to a backfire, which is an explosion in a cylinder that happens when it's not supposed to -- when the valves are open instead of closed.

Ray: And a backfire can go in one of two directions: It can either go through an open exhaust valve and come out the tailpipe, or it can go through an open intake valve and come out the air intake -- which is what happened on your car, Brian.

Tom: The backfire is most often recognized by the loud "ka-boom" it makes, and, occasionally, by the pieces of your former manifold or exhaust system clanging down the road behind you.

Ray: My guess is that a backfire blew out what was left of your already-compromised manifold gasket. That's what made the car run rough.

Tom: If it really was the manifold itself that blew off, it would have to have been a heck of a backfire -- like the ones they use in the "William Tell Overture." Or the manifold would have to have been cracked or loose before the backfire occurred.

Ray: In either case, now that you've replaced the gasket and resecured the manifold, you've probably also solved the backfire problem. So my guess is that you're good to go, Brian.

Tom: But if the car backfires again, you can impress your father-in-law by asking him what else -- other than a crack in the intake manifold or manifold gasket -- can cause a lean condition in an '01 Sable Wagon. Then just nod your head as he goes through the list and say, "Yeah, that's what I was thinking, too."

(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk Web site.)

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