DIY Diagnosis: Causes of Low Engine Power
Low engine power usually means that your engine lacks power during acceleration, or that your engine suddenly fails to reach normal driving speed.
While low engine power could just mean that normal wear and tear has gradually taken away much of your car's horsepower, this guide is concerned with abnormal power loss: a failure caused by a system or component not working properly due to a fault or lack of adequate maintenance.
A low-engine-power condition can be caused by one or more of a long list of components in need of attention. Luckily, you can narrow down the list somewhat by knowing that some of the most common reasons for reduced engine power have to do with the fuel, ignition, or emission system.
Whether you are dealing with a faulty part or lack of proper maintenance, the tests and strategies suggested below will help you gain your horsepower back. The tests fall into different systems, specific faults, and conditions known to reduce engine power. Finally, you are reminded of some important diagnostics that may apply to your particular case. Each component or condition mentioned includes a "WHAT YOU CAN DO" section, so that you can take some action when you think is necessary.
Systems That Can Cause Low Engine Power
We'll go through the systems in order:
- Ignition System
- Fuel System
- Emission System
- Computer System
- Vacuum Leaks
- Transmission or Clutch
- Exhaust System
But first I'm going to list some very simple checks you should do before you get started testing.
Four Checks You Should Start Out With
These are some important but simple checks to consider first.
- If you notice sluggish acceleration right after some work was done on your car, make sure that everything was connected back properly. Check for loose hoses, unplugged electrical connectors, and loose bolts, and if fluids were changed, look to see whether the right engine or transmission oil was used.
- Check your tires' inflation pressure. If underinflated, your tires will drag on the road. Check inflation pressure with a tire pressure gauge when the tires are cold. Inflate tires 1 to 3 psi below the maximum pressure marked on the sidewall of your tires.
- Even if the check engine light (CEL) hasn't come on, scan your computer. You may have a pending code that can guide you to the source of the problem. A malfunctioning sensor or actuator may cause the car computer (or transmission) to receive signals with the wrong voltage, causing the computer to lean the air fuel mixture and rob your engine (or transmission) of power. Whatever codes you find, always test the circuit or components indicated in the trouble code. It is possible that a related fault may have the computer "think" that another circuit or component is failing. On some vehicle models, a failing camshaft position (CMP) sensor can cause your engine to suddenly lose power; the computer will most likely set a code if it detects a problem with this sensor.
- Many GM vehicle models come equipped with a Reduced Engine Power (REP) warning light, similar to the Check Engine Light (CEL). When this light (or both lights) comes on, you'll notice the engine barely responding to the accelerator. This is a scary situation if the problem happens while you are driving down the highway or in heavy traffic. The most common triggers of this warning light are the harness connecting to the throttle position sensor (TPS), or the TPS itself. Other problems that can trigger the REP light come from the throttle body (including wiring), oxygen sensor, accelerator pedal position (APP) sensor (or harness), low engine oil pressure, or coolant loss. Watch the video below for more information about this problem.
Testing Eight Systems That Can Cause Low Engine Power
Now here are the eight systems that commonly contribute to low power, and how you can test them yourself.
1. Ignition System
Sluggish engine behavior can very often be traced to worn or faulty parts in the ignition system. Several components in the system need service at regular intervals. For example, spark plugs and spark plug wires, but the ignition coil and ignition timing should be checked as well. If any of these components cause you are not getting a good spark, your engine will be sluggish
WHAT YOU CAN DO: When you feel your engine sluggish, one of your first checks should be to inspect the spark's strength. Use an adjustable spark tester ( is an acceptable brand) to check the spark's health. Check at 40KV and 30KV. If your spark can't bridge that gap at those settings, you may have worn out wires, a weak or failing distributor, a bad ignition coil or a bad ignition control module, depending on your particular module. Check the tests that follow, and consult your vehicle repair manual for the proper diagnostic for your particular model. If you don't have the manufacturer's service manual, I strongly recommend getting an aftermarket manual for your exact model ( Thexton is a good inexpensive brand). Haynes
When doing a visual inspection of the ignition system's components, like the distributor cap, rotor, ignition coil, or coil pack, look for carbon traces, carbon buildup, and damage as well. Carbon traces are like small lines of carbon that form around these components. They can short out the voltage going through the system, robbing the spark plugs of the necessary voltage to produce a good spark. Replace them if necessary.
After checking the spark strength, if necessary check the following individual system components.
1. Spark Plugs: The spark plugs may become fouled with carbon deposits and other chemical byproducts, especially if the vehicle hasn't been maintained on the suggested schedule.
Fouled plugs can't produce an adequate spark to ignite the air-fuel mixture. Also, after miles of service, the gap between the spark park electrodes will widen due to wear.
WHAT YOU CAN DO: Make a visual inspection of the spark plugs and check the gap between the electrodes using a wire feeler gauge and adjust it if necessary. Your car owner's manual or repair manual has the proper gap for your spark plugs. Your service manual can help you analyze your spark plugs, which can reveal much about the condition of your engine.
2. Spark Plug Wires: Just like spark plugs, spark plug wires wear out, and after miles of operation they can prevent the spark from reaching the spark plugs.
WHAT YOU CAN DO: Check each wire resistance using a digital multimeter (DMM) and compare your readings to the specifications in your repair manual. Usually, you need about 5000 Ohms per foot of wire. Otherwise, replace them with a quality set of wires.
3. Ignition Coil: The ignition coil produces the high voltage needed for the spark to jump the gap between the spark plug electrodes. That voltage is typically anywhere between 4000 and 30,000 volts, depending on your particular vehicle model.
Ignition coils also wear out or fail, resulting in a weak spark, an intermittent spark, or no spark at all.
WHAT YOU CAN DO: You may be able to test the ignition coil(s) or coil pack in your vehicle using a DMM, with the help of your vehicle repair manual.
4. Ignition Timing: Ignition timing refers to the relationship between the spark and piston position in the cylinder during the power stroke.
The ignition timing must be correct for the proper combustion of the air-fuel mixture. When ignition timing is retarded you may notice an increase in fuel consumption, a drop in engine power, and poor acceleration.
Timing problems can happen because of a worn out (overstretched) or damaged timing belt or chain. Even 2 or 3 degrees of difference from the correct timing can cause engine performance problems.
On most modern vehicles ignition timing can't be adjusted directly, but you still may be able to check timing yourself. On older models with a distributor, you can check and adjust the timing yourself.
For timing specifications, check the emissions control sticker located in the engine compartment, or your repair manual.
WHAT YOU CAN DO: Check ignition timing using a timing light and a tachometer. If your ignition system uses a distributor, you can adjust the timing yourself if necessary. Consult your vehicle repair manual. Your manual may also have a service schedule interval for the belt or chain.
2. Fuel System
Although modern fuel injection systems may come in different configurations, they all share many components like fuel injectors, control modules, and sensors. Any of these components can fail and cause your engine to lose power.
The fuel system can give you as much trouble as the ignition system. When the engine feels sluggish, there are some parts you need to check.
1. Fuel Filter: Over time, the fuel filter becomes clogged, reducing fuel flow and preventing your engine from accelerating properly.
WHAT YOU CAN DO: Check your car owner's manual or repair manual for the service schedule for the fuel filter. Even if your filter is not the root cause of the problem, replacing the filter at the manufacturer's suggested interval will relieve stress from the fuel pump and increase its service life.
2. Air Filter: Whenever your engine is running, the air filter in the air cleaning system traps dirt, dust, and other foreign particles and removes them from of the air stream flowing into the engine. Eventually, the filter clogs up. And a badly clogged air filter will make your engine work much harder. You'll notice it.
WHAT YOU CAN DO: Most car manufacturers recommend replacing the air filter every 12 months. So make sure to check your air filter, and replace it if necessary. Consult your car owner's manual or repair manual.
3. Fuel Injectors: The most common trouble with fuel injectors leading to engine power loss is clogging. But they can fail as well.
WHAT YOU CAN DO:
- On throttle body injectors (TBI), you can check the injector's fuel spray pattern by removing the lid off the air filter housing. The spray of fuel should be even and partially atomized, following an inverted V pattern. You can add fuel additive to clean a mildly clogged injector, or take it in to your shop for service. However, if the internal valve in the injector has failed, not just clogged, you'll need to replace it.
- On a multiport fuel injection system, dirty or clogged injectors are harder to detect. Depending on the configuration of your particular system, you may find it relatively easy to detach each of the injectors from their port to check their spraying pattern. On other systems, disassembly is a more involved process.
If you suspect dirty injectors, or you haven't serviced the injection system in a while, you can try adding a fuel additive to the fuel tank. Otherwise, you may have to take your car in for an injector balance test, which measures the amount of fuel each injector sprays when energized.
4. Throttle Valve: Throttle plate malfunctions are not common but they do happen.
WHAT YOU CAN DO: You can quickly check the throttle body to make sure the valve—the throttle plate—opens fully when the accelerator pedal is fully depressed.
- Remove the air ducts or air filter box lid to gain access to the throttle body.
- Ask an assistant to fully depress the accelerator pedal with the engine off.
- Verify the throttle valve responds accordingly to pedal operation.
- Otherwise, adjust or fix the throttle linkage, or remove carbon buildup from the valve and throttle bore. Buildup may also interfere with proper valve operation.
5. Fuel Pressure Regulator: A faulty fuel pressure regulator can allow either too much fuel to flow into the engine or too little.
WHAT YOU CAN DO: Test fuel pressure using a fuel pressure gauge. The test can point to problems with the fuel pump (low pressure or low volume), a clogged fuel filter, or a faulty fuel pressure regulator.
The exact procedure may vary from one engine model to the next, but the general steps are the same:
- Locate the Schrader valve on the fuel rail (this is a test fitting similar to the air valve on your tires). If your model doesn't come with this valve, you still can connect the gauge directly to the fuel line using adapters (consult your repair manual).
- Then, follow the steps described in your repair or service manual and compare your readings to the specifications.
3. Emission System
Another possible, though not common, cause for loss of engine power during acceleration is failure of the exhaust gas recirculation (EGR) valve.
The EGR valve allows a measured amount of exhaust gases at higher-than-idle engine speeds to re-enter the engine to reduce engine high temperature and harmful emissions.
When the EGR valve fails, it may stick open or closed. If the valve sticks (or intermittently sticks) open or fails to operate properly, the most common symptoms you'll notice are rough idle and stalling during acceleration; but other times you'll just notice a lack of engine power when depressing the accelerator.
WHAT YOU CAN DO: You can check the EGR valve at home, using a hand-held vacuum pump.
4. Computer System
As part of the computer system, both the manifold absolute pressure (MAP) sensor and manifold air flow (MAF) sensor influence on the air-fuel mixture as controlled by the computer. Usually, the car computer stores a trouble code in memory when detecting a fault with either sensor.
WHAT YOU CAN DO: Even if your Check Engine Light is not on, it's a good idea to scan the computer for pending trouble codes. The most common problem with MAF sensors is a dirty sensing element. You can clean it with electronic contact cleaner spray or MAF cleaner. Whether your car comes equipped with a MAP or MAF sensor, you may be able to test it at home. Consult your vehicle repair manual.
5. Vacuum Leaks
Leaks may happen because of a loose, deteriorated, or broken vacuum hose; a blown gasket, or a damaged throttle body gasket.
WHAT YOU CAN DO: A common technique to locate vacuum leaks is to use a length of rubber hose:
- Start the engine and let it idle.
- Using a length of rubber hose, put one end of the hose against your ear and use the other end to listen to the different vacuum hoses.
- Trace the hoses with your diagnostic hose
- Check around the edge of the intake manifold and throttle gaskets.
A leaking vacuum hose or gasket will produce a hissing sound, and you'll be able to hear it with your rubber hose. Just be careful around moving engine components as you troubleshoot vacuum hoses.
6. Transmission or Clutch
If you have an automatic transmission, and you haven't checked the transmission fluid lately, it's time to do it. If you have a manual transmission, it is possible the clutch is slipping.
A common symptom of low or contaminated automatic transmission fluid is slippage. Your engine races but your car doesn't move. Power is not being transferred to the wheels, giving the impression that your engine is lacking power. The same thing may happen in a manual transmission vehicle when the clutch is worn out; power doesn't get to the wheels.
WHAT YOU CAN DO:
- Check the transmission oil after the engine has reached operating temperature (drive or idle your vehicle for 20 minutes or more).
- Turn off the engine and allow the engine to sit for three or five minutes.
- Then pull the transmission oil dipstick.
- Use a rag to wipe the tip of the dipstick clean of oil.
- Fully replace the dipstick into its tube, and pull the dipstick out again.
- Allow the dipstick to rest horizontally on the rag.
- Oil level should be between the ADD and FULL marks toward the tip of the dipstick. Otherwise, add the necessary amount of the recommended fluid for your vehicle model.
- Inspect the fluid. It should have a clear reddish color. If the color is opaque and brownish or black, or has a burnt smell, replace it. Consult your car owner's manual or repair manual.
- Park in a safe spot, away from traffic and people
- Set the emergency brake
- Start the engine
- Set the transmission to a high gear
- Slowly, fully release the clutch pedal for two seconds (to avoid burning the clutch plate or flywheel) and reapply the clutch pedal
- If the clutch is good, engine should stall or stop, as soon as you release the clutch
- If the clutch is bad, your engine will keep running normally
Clutch slippage can be caused by a worn clutch disc, leaking oil seal, binding clutch linkage, clutch in need of adjustment, or even a bad motor mount.
7. Exhaust System
Just as your engine needs adequate air inflow, it also needs proper air outflow to push out all those gases produced during the combustion process. Still, a bit of back pressure is normal.
The exhaust pipes, the catalytic converter, and the muffler at the back of your car are all designed and sized with the goal of minimizing pressure.
Normally, water and acid are the most common enemies of a car exhaust system. But contamination, system overheating, and high mileage can result in restricted air flow.
The most common victim of exhaust system restriction is the catalytic converter. Besides failing or collapsing through normal wear, internal catalytic components can melt due to overheating and contamination.
Once the catalytic converter becomes plugged, you'll notice a decrease in engine performance, and, depending on the type of failure, a strong smell of rotten eggs coming out through the tailpipe.
But problems may not stop there. If the converter plugs up, it can overheat and burst into flames.
WHAT YOU CAN DO: Test your exhaust system for high back pressure.
The Temperature Test:
- After driving your vehicle for about 15 minutes, park your car in the garage and turn off the engine.
- Raise the vehicle and safely support it on jack stands.
- Using a kitchen thermometer, take the temperature reading of the inlet pipe at the catalytic converter (be careful, temperature there can reach over 1400F [760C]).
- Take the temperature reading of the outlet pipe at the catalytic converter.
- A clear difference in temperature means a plugged converter.
The Rattle Test:
Depending on the type of converter you have installed and the failure type, if the catalyst elements inside the converter have disintegrated, the converter will rattle when hit with a rubber mallet.
The Pressure Test:
- Remove a pre-converter oxygen sensor.
- Install a pressure gauge in the threaded hole.
- Start the engine.
- Take a pressure reading at idle and at higher speeds.
- High pressure readings point to a plugged converter or muffler.
- Disconnect the muffler and repeat the test to locate the obstruction.
The Vacuum Test:
- Connect a vacuum gauge to the vacuum hose going to the power brake booster.
- At idle, snap open and close the throttle valve allowing the engine to reach a speed of about 2500 rpm.
- You should see the gauge needle drop to almost zero, return past your previous reading about 5in-Hg (inches of mercury), and go back to your previous reading. If the needle returns too slowly to the previous reading, you might have a restricted exhaust system.
Poor acceleration can also have its source in engine compression problems. These problems are inevitable on high-mileage engines or those with a history of poor maintenance. And, as miles accumulate, more engine power is lost due to wear around cylinders, rings and pistons, and carbon buildup around valves. Rebuilding the engine might be necessary.
WHAT YOU CAN DO: Do a compression test using a compression gauge.
- Remove all the spark plugs.
- Block the throttle valve open.
- Disable the ignition system and the fuel system (those with electronic fuel injection).
- Connect the compression gauge to one of the spark plug holes.
- And crank the engine for about six compression strokes.
- Repeat the test on the rest of the cylinders.
- Then, compare needle movement patterns and pressure reading to specifications.
Your vehicle repair manual may guide you through this test.
You can avoid many low-engine-power problems, and many other issues, by following a proper regular maintenance schedule. The ignition and fuel system are often the primary culprits behind the problem, but a number of components in other systems can also cause the same issue. This guide helps you find the source of the problem, fix it, and save some money in the process.