QUESTION: I have a 1990 Toyota Corolla LE, with automatic transmission, which I purchased new. It now has 101,000 miles on it. In all the years I've had it, there was never a major problem. All the service requirements were done by Toyota service departments.
In the past year or so, if I drive my Corolla at highway speeds of 55-65 mph for at least 10-15 miles, everything runs smoothly. If I exit the freeway, slow down and come to a stop, the engine runs roughly and many times it stalls. It will restart up again in a few seconds and run smoothly again. None of this happens during "city driving," no matter how many miles driven. Please, can you help?
--Lloyd Poleselli
ANSWER: Great job explaining the symptoms, Lloyd. You helped me narrow the search to one likely scenario. I believe your EGR valve is sticking open, and that's causing the intermittent rough running or stalling.
EGR, or exhaust-gas recirculation, is an emissions-reducing strategy that's been widely used on cars and light trucks since the mid-1970s. This system meters a small quantity of exhaust back through the engine a second time, reducing peak combustion temperature, and the formation of NOx, a toxic exhaust emission. The added exhaust, an inert gas, takes up space in the engine's cylinders that would have been air, altering the combustion process. EGR is used during part- to full-throttle operation, but never at idle, because it will cause severe engine roughness or stalling.
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Based on your symptom description, I believe your EGR valve, a flying-saucer-shaped metal gadget mounted atop the engine, is occasionally failing to close as the engine returns to idle. If your city driving consists of many starts and stops, without longer intervals at one speed, the EGR valve may not be as active or fully engaged. Perhaps that's why you haven't noticed symptoms then.
During steady driving at moderate to higher speeds, the EGR valve flows exhaust, becoming quite hot. That could result in the valve plunger failing to return to the closed position. It's also possible the vent filter on the EGR vacuum-switching valve has become dirty, making the EGR valve slow to close.
Troubleshooting the exact cause will require the services of a sharp technician at either the Toyota dealership or an independent repair shop. Here's how to confirm the fault is EGR-related: Locate the EGR valve and temporarily remove the vacuum hose that attaches to it. Block the loose vacuum hose end with an inserted pencil, and duplicate your typical freeway drive. If the Corolla runs smoothly when you stop and once again on the return trip, we've confirmed the fault to be within the EGR system.
If it should still run rough or stall when coming to a stop, another possibility is a lingering torque converter clutch, a transmission-related component. It should fully disengage, just like the EGR, when the vehicle slows to a stop.
