SV21 Camry

[stephenc]

Car: 1987 SV21 Camry 2.0 Auto

When the car is coasting and my foot off the accelerator pedal, the engine starts surging around 1,000 rpm. It stops when the engine revs drop lower or if I hold the accelerator slightly depressed. If I adjust the throttle position sensor so that the IDL contacts are not closed, it will not surge. What is causing the surging. When it surges, it's as though I am tapping the accelerator on and off. Thanks.

[WPK919]

I have just purchased a used Camry (2 litre, twincam, auto, 12/1988 build) and noticed the same thing. Have you resolved the problem? Cheers, Ian

[Nigel Cox]

Sounds like the 'overrun fuel cut-off'. A standard feature on most cars since the mid eighties.

The principle is that when you let go of the throttle, like slowing for a queue or junction, or going downhill, you rely on some engine braking. The engine is not needed to drive the car, so any fuel used is wasted. As a fuel saving idea, generated by fuel shortages and high prices, vehicle manufacturers hit upon the idea of cutting the fuel in these circumstances.

There will be a switch activated when the throttle is fully released. this can be a simple switch at the pedal, or integrated into the throttle potentiometer (position sensor) on some carbs and most injection systems.

There will also be a fuel cut off valve on carb models.

When the throttle is fully released and the engine speed is above a threshold, usually 1000-1200 rpm, the fuel supply will be pulsed off and on, usually at half-second intervals for up to about 30 seconds. In most circumstances you don't feel this, as the car is still pushing the engine, but if it is out of gear you may notice the revs falling and rising. It is worse when out of gear, as the lack of fuel tries to stall the engine. As the revs fall below the idling speed, the control will reinstate the fuelling, which causes the revs to rise again. on carb models this is achieved by a fuel cut-off valve on the carb, injection models just don't open the injectors.

How smooth any vehicle is depends on the original design and application. Some are better than others.

The above applies to any car of any make since the mid eighties. You have to live with this as trying to by-pass it will confuse the control unit and upset the idle control.