A car that loses power when accelerating is usually short of fuel, air or spark under load, is held back by a restricted exhaust, or has gone into limp mode to protect itself. Clogged fuel filters, weak fuel pumps, dirty MAF sensors, ignition faults, boost leaks on turbo engines and a slipping transmission are the common causes. A scan for codes is the fastest way to narrow it down.
Common causes
| Cause | Clues |
|---|---|
| Fuel delivery | Worse at high speed, uphill or under hard acceleration; see fuel pump symptoms, fuel filter and fuel pressure regulator |
| MAF sensor or air intake | Codes such as P0101; see MAF sensor |
| Ignition | Misfire codes and stumble under load; see engine misfire |
| Restricted exhaust | Power fades as rpm climbs; a clogged catalytic converter may glow or rattle |
| Turbo boost leak or turbo fault | Underboost code P0299; see turbo failure symptoms |
| Limp mode | Power capped and the transmission held in one gear; see limp mode |
| Slipping transmission | Rpm rises without matching speed; see transmission slipping |
Loses power at high speed
At highway speed the engine needs its maximum fuel flow, so a weak pump or clogged filter that keeps up around town runs out here. Low fuel pressure codes such as P0087 on direct-injection and diesel engines point the same way.
Loses power from a stop
From a stop, look at the throttle body, the throttle position sensor, vacuum leaks and the transmission. A torque converter or band that slips feels like the engine revs without pulling.
Loses power uphill
Climbing combines high load and high fuel demand, so fuel delivery, ignition and overheating protection all show up there. Watch the temperature gauge; an overheating engine or transmission may reduce power deliberately; see transmission overheating.
How to diagnose it
- Read the codes and freeze-frame data.
- Check fuel trims and MAF readings with a scan tool; see fuel trim.
- Test fuel pressure under load.
- Check for exhaust restriction with a back-pressure test.
- On turbo engines, pressure-test the intake for boost leaks.
Power loss with a check engine light
If the check engine light is on, read the code before replacing parts: misfire, lean, low fuel pressure and underboost codes each point to a different system. A flashing light means a severe misfire, so drive gently; see check engine light flashing. Power loss with no code points more often to a mechanical restriction, a slipping transmission or a fuel filter on the edge.
Diesels
Diesels add their own causes such as injectors, high-pressure pumps and turbo controls; see diesel losing power. If the engine has low compression or internal damage, compare repair with our replacement engines.
Trouble codes: P0171 System Too Lean Bank 1 · all OBD2 codes.
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