What's behind this 4-Runner's red hot exhaust manifold?

Dear Car Talk

Dear Car Talk | Oct 01, 2000

Dear Tom and Ray:

My '86 Toyota 4Runner's exhaust manifold turns cherry red every time I drive it. I know this is not normal, but I haven't found a mechanic who can diagnose the problem. First, I brought it to the exhaust shop because I figured my catalytic converter was plugged. They said everything in the exhaust system was free and clear. Next, a mechanic hooked it up to his machines for a diagnosis. He said the fuel mixture was fine. It sounds like nothing is wrong, but something is wrong. Any ideas? -- Chris

TOM: I think your timing is off, Chris.

RAY: Yeah ... we're just heading out on vacation, so we won't be able to answer your question for another two weeks. Bad timing, bud.

TOM: I mean your ignition timing, Chris. There's a very precise dance that goes on in the cylinders. The piston goes down, sucking in the mixture of gasoline and air, then the piston goes up, compressing that mixture. Then the spark plug fires, igniting the mixture, and the pressure from that combustion pushes the piston back down (that sends power to the wheels). And then the piston comes back up, pushing the exhaust gases out of the cylinder and into the exhaust system.

RAY: What's happening in your case, I believe, is that the ignition timing is off. That means the spark is "sparking" much too late. When it sparks too late, you can have combustion that's still taking place while the piston is pushing out the exhaust gases. So you'll essentially be sending "fire" into the exhaust system. And that's why your converter is red hot.

TOM: Ask a mechanic to check your ignition timing and reset it to specs. And if you haven't ruined your converter already, you should be all set, Chris.

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