Poor Acceleration, Lack of Power
POOR ACCELERATION, LACK OF POWERDescription
Engine performance is weak, acceleration is poor, and there are no other major symptoms.
Causes
Retarded Ignition Timing
If the ignition occurs after Top Dead Center (TDC) while the piston is moving downward, optimum combustion will not be achieved. Cylinder pressure at the time of combustion will be lower, resulting in more combustion energy being transformed into heat, rather than mechanical energy. Heat output of the cylinder increases while power output decreases.
Testing
- Check Ignition Timing and advance
Retarded Valve Timing
If the valve timing is retarded, the intake valve opens and closes late. Less air/fuel charge is admitted to the cylinder and some may be expelled back through the late closing intake valve during the compression stroke.
The exhaust valve also opens and closes late, with similar affects. Less exhaust is pushed out and some exhaust may be drawn back in during the intake stroke.
Testing
- Check engine vacuum. Vacuum will be lower than normal if the valve timing is retarded.
- Perform Compression Check. Compression will be lower than normal on ALL cylinders if the valve timing is incorrect.
- Check Timing Chain alignment. A timing chain which is off just one link will result in a large reduction in power. Timing Components
High Backpressure
A restriction in the exhaust system causes pressure to build up at the exhaust manifold. As a result of the the increased pressure, exhaust gasses are not fully discharged from the cylinder on each stroke. Engine power decrease dramatically as pressure begins to build up in the exhaust manifold.
Vacuum Indications:
Testing
- Check engine vacuum. High exhaust backpressure will cause a reduction in engine vacuum.
- While at idle, snap the throttle and observe engine vacuum.
- Normal - Vacuum drops to zero and quickly returns to original level.
- High Backpressure - Vacuum drops to zero, but slowly returns to original level.
- Examine Exhaust system for signs of damage (crushed or bent piping).
- Remove catalytic converter and inspect. Back pressure problems are often the result of a plugged/blocked catalytic converter.
Low Compression
low compression reduces peak cylinder pressure. Lower cylinder pressures reduce the amount of combustion energy which is converted from heat to mechanical energy, reducing engine power.
Testing
- Check engine vacuum.
- Perform Compression Check.
- If one or more cylinders indicate low compression, perform a cylinder leakdown check to determine the cause.
NOTE: A cylinder leakdown check utilizes regulated, compressed air to pressurize each cylinder (at Top Dead Center Compression). While performing this check, the cause of the compression leak may be determined by listening for escaping air:
- Air heard leaking from the exhaust indicates an exhaust valve problem
- Air heard leaking from the intake or throttle body indicates an intake valve problem.
- Air heard leaking from the PCV valve or PCV breather hose indicates a piston, ring, or cylinder wall problem.
- Air heard leaking from an adjacent cylinder indicates a head gasket failure between the two cylinders (both adjacent cylinders will have identically low compression).
- Air seen bubbling out of the radiator indicates a head gasket failure (or a cracked head/block).
Dead/Weak Cylinders
A single dead or weak cylinder has a significant affect on engine power. Not only does the engine loose the power output of the dead cylinder, but the remaining cylinders must continue to provide power to rotate the dead cylinder through the compression stroke.
Testing
- Perform Compression Check.
- Using an ignition analizer, perform a cylinder balance check.
NOTE: A cylinder balance individually grounds out the ignition spark for each cylinder and measures the amount of engine rpm which is lost. When a dead cylinder is grounded out there will be no rpm loss.
- Inspect all spark plugs for signs of misfire. Complete ignition misfire will result in a wet fuel fouled spark plug. Occasional or intermittent misfire may show no signs of problems on the spark plugs. Check porcelain insulators for evidence of cracking.
- Inspect ignition cables for signs for insulation damage or arcing.
- Inspect ignition cables for high resistance readings or corroded terminals.
Lean Air/Fuel Mixture
A lean air/fuel mixture provides too much air and not enough fuel. Incomplete combustion occurs and engine power is reduced.
Testing
- Inspect vacuum hoses and gasket for leaks.
- Check Fuel Pressure.
- Check fuel control system for proper operation. See Computers and Control systems. Computers and Control Systems
Poor Fuel Quality
Old fuel which has chemically broken down or is diluted with other non-combustable liquids will reduce engine power.
Testing
- Visually inspect a sample of the fuel. If the fuel appears cloudy, rusty, or dirty, it should be replaced.
Automatic Transmission Problems
A failure of the automatic transmision to smoothly transfer engine power to the wheels may feel very similar to a loss of engine power.
Testing
- Verify the transmission fluid is full.
- Verify the transmission is in low gear when starting off. If the transmission is starting off in second gear, acceleration will be weak.
- Verify the transmission is shifting at the appropriate speeds. If the transmission is shifting up too early, acceleration will be weak.