Mechanics

Low Power


Testing Fuel System Pressure

Check Fuel Pump Pressure and Pressure Regulator - Fuel pump is worn providing low fuel pressure to the fuel injection system. A fuel pump that is faltering providing low system fuel pressure/volume will cause low power. Your car's engine depends on constant fuel under pressure to supply the fuel injection system properly. Your car's fuel pump performs this action and is located in or near the gas tank. The injection system meters fuel through the injectors and into the engine. Many engine run drivability problems can be related to a weak or failing fuel pump. Problems can range from hard starting in the morning, low power and stalling while driving. Most vehicles have a fuel pressure test port somewhere on the fuel rail, use a flashlight to locate it on your engine or consult a repair manual. Also test the fuel pressure regulator performance; if the regulator is stuck or leaking the engine will not receive the additional fuel needed for power. If the fuel system pressure is okay proceed to the next step.

Checking a Distributor Ignition Systems - If your car has a DIS (distributor-less ignition system) skip this step. Ignition timing is very important in achieving maximum power from your engine. If the timing is adjusted too far advanced or retarded (in front or behind the timing mark) in relationship to the engine crankshaft it will have low power output. To check your engines timing adjustment a timing light is needed. Once the timing light is hooked up to the battery locate the under hood emissions sticker that has the engine timing specification and instructions. If you can't find the under hood sticker or it is illegible consult a car repair manual. If the engine timing is ok proceed to the next step. If the engine timing is way off (20 degrees or more) check the timing belt or timing chain, the crankshaft camshaft correlation might be off.

Checking the MAF, (mass air flow) and MAP (manifold absolute pressure) Sensors - The MAF or MAP sensors help the PCM computer determine how much air the engine is consuming and distributes the proper amount of fuel. If these sensors become weak they can misread and cause the PCM to think the engine is turning slower than it is, in turn giving the engine less fuel and producing less power. You might say wouldn't the computer see a problem and illuminate the service engine soon light?- MIL (malfunction inductor lamp) and the answer is not always.  The PCM is designed to detect a shorted sensor or a sensor that is way out range. If the sensor is a little off but it is still within range the service engine soon MIL will not illuminate. This test procedure is simple but unique to each car, for the exact test procedure you will need a repair manual.

Check for Severely Worn or Misadjusted Spark Plugs - Engine timing is crucial to power output, if the spark plugs are severely worn or have a misadjusted air gap the timing of the spark can be delayed or ineffective causing power loss. To check your engine's spark plugs wait until the engine is cold, then remove any spark plug and inspect. You are looking for the correct air gap between the electrodes, if the gap is incorrect remove all spark plugs and recheck. If the spark plugs are ok proceed to next step.

Checking the Timing belt or Timing Chain Settings - The crankshaft and camshaft must stay in proper correlation. The procedure for checking crankshaft to camshaft correlation varies on each car. You will need to gain access to the timing belt or chain marks. Once you have gained access check your engine timing marks, timing belt or timing chain setting. Correct as needed and recheck engine, if the timing correlation is correct proceed to next step.

Checking the Exhaust System for Blockage - A plugged catalytic converter can cause low engine power by not allowing the engine exhaust to exit the engine properly. To check for this condition take your car for a 15 minute drive allowing the exhaust system to warm, park the car safely and use a pocket IR thermometer laser temperature reader to measure the temperature of the exhaust pipe entering the catalytic converter and the exhaust pipe leaving. The temperature should be lower going into the catalytic converter and hotter leaving the catalytic converter, if not there may be a restriction inside the converter.

Plugged Catalytic Converter from Broken Catalyst Material
Plugged Catalytic Converter from Broken Catalyst Material

Example: the temperature entering the catalytic converter is 300 degrees and exiting the catalytic converter is 450 degrees. This means the catalytic converter is working properly, if the temperature is the same or lower the catalytic converter is not working and is probably plugged.

CP7875-3 Pocket IR Thermometer
Pocket IR Thermometer Laser Temperature Reader

Check the Engine Compression - As the engine depends on compression to ignite the fuel air mixture. If an engine has major wear is will not produce the proper amount of power. A compression test can be performed to check for wear or internal damage. A compression gauge is needed to test the engine cylinders.  There are two types of gauge styles, one threads into the spark plug hole which is more accurate. The other style of gauge is designed with a rubber plug that is meant to be pressed against the spark plug hole, this style of gauge is difficult to use.

Checking the Transmission - Sometimes the engine will have good power but it is not getting to the drive wheels. If you are driving and the engine sounds like it is working hard but the car is not going anywhere it could be the transmission clutch or clutches are slipping. When a clutch or clutches in a manual (standard) or automatic transmission wear they can slip not allowing the power from the engine to be transferred to the drive wheels. In this case the only fix is to replace the clutch or clutches depending on automatic or standard transmission. If this is not the condition continue to next step.

Slipped Harmonic Balancer - Some cars have an ignition trigger system that bases the engine timing off of the crankshaft harmonic balancer. If this balancer comes loose it can cause the keyway in the crankshaft to dig its way into the balancer causing the timing to retard causing low power. To check for this condition remove the harmonic balancer and inspect, replace if needed and reassemble.

If further assistance is needed, our certified car repair technicians are ready to answer your car questions.

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-08-16)