Engine Surges

ENGINE SURGES

Description
With a constant throttle position (at idle or at cruise), the power output of the engine decreases and increases alternately with no apparent reason.

Explanation
Surging usually results from either an Air/Fuel Mixture problem or an Idle Speed control problem.

Above Idle Speed Surging
Surging above idle speed is often the result of a condition which causes an incorrect air/fuel mixture at a certain power level. As the engine power output begins to drop (due to the incorrect air/fuel mixture), the mixture corrects itself and the process repeats.

Causes

Low Fuel Pressure
The fuel pressure is to low to sustain engine operation at a certain power level. The engine output then drops to level at which the fuel demand equals the fuel supply and the process repeats itself

Testing
- Perform Fuel Pressure test.
- If fuel pressure is OK at idle, check fuel pressure under engine load.

Sluggish Oxygen Sensor
An oxygen sensor which reacts slowly may result in a slight surging as the fuel command switches back and forth between rich and lean.

Testing
- With the engine warmed up and operating in closed loop, verify the oxygen sensor is switching properly. Switching time should be less than one second. Oxygen Sensor

Defective or Out of Calibration Sensors
Sensors in the Engine Control system may have glitches or calibration errors at specific engine speeds or power levels. When these speeds or power levels are reached the engine control system is unable to calculate the correct air/fuel mixture or timing advance and a corresponding reduction in engine power occurs. This is followed by an increase in power as the defective sensor reaches a point where its output is within specifications.

Testing
- See Computers and Control Systems. Computers and Control Systems

Idle Speed Surging
Surging at idle can be caused by all of the above conditions in addition to problems caused by the idle speed control system.

If a condition occurs which causes idle speed control to operate at the limits of its control range, the idle speed may begin to surge between the upper and lower limits.

Causes

Throttle Body Sludge
Oil contamination and sludge from the PCV breather hose tends to build up in the throttle body just in front of the throttle plates. This build up blocks the airflow around the throttle plates and may cause the idle speed to surge between the upper and lower limits.

Testing
- Check for related Technical Service Bulletins (TSB).
- Inspect the throttle body and clean as necessary.

NOTE: Some vehicles are equipped with "Sludge Tolerant" throttle bodies and should not be cleaned. Check for TSBs and see Fuel Delivery and Air Induction before cleaning. Fuel Delivery and Air Induction

Incorrectly Adjusted Minimum Idle Speed
If the minimum idle speed is adjusted too low, it will have the same affect as sludge build up in the throttle body. The idle speed control system will be operating at it's upper limit and any variation in idle speed may cause it to begin surging between the upper and lower limits.

Testing
- Check Minimum Idle Speed and adjust as necessary.