By Jim Newkirk
Our problem car is a 1989 Isuzu Trooper with a 2.6L EFI engine. The customer complains of a hard surge on a warm engine, at idle only. The surge is between about 1700 and 1200 RPM and is constant until the throttle is opened.
The service manual shows that this vehicle has a fuel-cut mode to limit engine speed at idle. On this Trooper, fuel-cut mode will engage when idle speed reaches 1700 RPM, with the idle switch indicating closed throttle and the engine warmed up.
The vehicle computer responds by shutting the injectors off until engine RPM falls to 1,250 RPM. This could give us the hard surge the customer is complaining of. I mentioned to the technician that he will need to perform some testing on the idle controls and the throttle switch to determine the point of failure.
Testing the idle switch function is quite simple. The idle switch (located on the throttle body) has a blue wire with a yellow stripe on its connector. This wire is the signal return to the vehicle computer and should have 12 volts available at idle and 0 volts available off idle. This switch tells the vehicle computer if the car is at idle or off idle. By disconnecting the switch, the computer thinks it is off idle and this should eliminate the fuel cut function.
Sure enough, with the idle switch unplugged, the hard surge was gone, but the vehicle had an idle speed of 1800 RPM - on a warm engine. With no vacuum leaks found, the problem had to be in idle control. On this Trooper, idle speed is controlled in the following manner:
- Cold idle speed increase (or "fast" idle) is dependent on the amount of bypass air through the air regulator (also known as the auxiliary-air device). This air regulator is located on the intake manifold and has an electrical connector and two large vacuum hoses attached to it. The regulator is a bi-metallic valve with a heating element that opens and closes in response to temperature. On a cold engine, this regulator will allow air to bypass the throttle plates, increasing cold idle speed. As the engine warms, the heating element heats the bi-metallic spring and closes the valve - returning the vehicle to its normal idle speed of 850 to 950 RPM. A quick test of this valve can be made on a warm engine by pinching the vacuum lines. Engine speed should not change. If it does, the valve may be hanging open, causing a higher than normal idle. Cleaning the valve may help, but the usual fix is replacement.
- Hot engine idle increase and load idle increase are controlled through a thermal valve and a fast idle solenoid, respectively. The thermal valve will bypass air past the throttle plate if engine temperature is more than 198 degrees F. This valve is located on the thermostat housing. It has three vacuum lines running to it - one to the air intake tube near the center of the valve cover, one to the intake, and the final one to the fast idle solenoid. The usual failure here is with the thermal valve. There should be no bypass air through the thermal valve until the engine reaches more than 198 degrees F coolant temperature. To test the valve, pinch the vacuum line between the intake and the valve. If the engine speed changes and the vehicle's coolant temperature is below 198 degrees F, the thermal valve is faulty. The thermal valve's other vacuum line runs to the fast idle solenoid. This vacuum line is always a fresh air source unless the coolant temperature is above 198 degrees F. The fast idle solenoid is controlled by the air conditioning signal. With A/C on, bypass air will pass through the fast idle solenoid and increase idle speed. With A/C off, no bypass should occur.
- Base idle is adjusted with a simple air bypass screw located in the throttle body. If all the other systems are working correctly and there are no vacuum leaks, idle speed may be adjusted using this screw.
On this Trooper, pinching the vacuum line at the thermal valve brought idle back to its normal range and replacing the valve solved the problem.
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Jim Newkirk is the Identifix European team leader. He is an ASE master with L1 and Bosch BSC certifications.
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