What could cause an engine to turn rough after about an hour of highway driving?

Dear Car Talk

Dear Car Talk | Feb 01, 1997

Dear Tom and Ray:

Help! My wife drives an '85 Nissan Sentra station wagon, one of the few cars available she can comfortably see over the hood of with the aid of a pillow. This car does not have a powerhouse of an engine, but it does perform as expected and has been good, reliable transportation. It has just over 60,000 miles and has been well-maintained. But when this car is driven for more than an hour at highway speed, the engine will idle extremely rough and almost die when you come to a stop. If you go back on the highway, it runs fine, until you stop again. If you turn it off for an hour, it starts up and runs perfectly. I have spoken with Nissan, consulted with both professional and weekend mechanics, researched in the library, and even considered prayer. Outside of this little problem, the car is perfect for my wife. Do you guys have any ideas? -- Dan

TOM: I have three ideas, Dan. One is that the exhaust gas recirculation (EGR) valve is sticking. The EGR valve is an emissions device that sends exhaust gas back into the cylinders to cool the burning fuel/air mixture. This process reduces nitrous oxide emissions. The EGR is supposed to be wide open at highway speed, but if it sticks in the open position, it could certainly make the engine run rough at low speeds.

RAY: But more likely, what's sticking is the secondary throttle of the carburetor. When you run the engine at high speed, the secondary throttle opens up to let in more air and fuel. And if it's lazy, and doesn't close all the way when you stop, it creates the equivalent of a vacuum leak at idle, which causes the engine to run very poorly.

TOM: After a while (you say an hour or so), the throttle slowly shuts and the car is fine again ... until you go back out on the highway. That would be my bet.

RAY: But there is a third possibility. Your wife may be making all of this up, Henry. As much as YOU think this is the perfect car for her, she may be trying to convince you that it's a hopeless wreck, and it's time to get rid of it. Have you noticed any Lexus brochures lying around the house?

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