It can be hard to tell an engine problem from a transmission problem, because both show up as shaking, power loss and odd noises. The key is whether the symptom follows engine speed or vehicle speed and gear changes. Engine problems usually affect running in neutral or park too: misfires, rough idle, knocking, smoke and warning lights. Transmission problems usually appear only when the vehicle is moving or shifting: slipping, harsh shifts, delayed engagement and noises that change with gear.
Engine or transmission problem?
| Symptom | More likely engine | More likely transmission |
|---|---|---|
| Shaking | Shakes at idle in park; misfire codes | Shudder at steady speed, on lockup or during shifts |
| Power loss | Engine bogs, no rev increase | Engine revs rise but speed does not |
| Noise | Knock or tick with engine speed, also in neutral | Whine or clunk that changes with gear or road speed |
| Warning lights and codes | Misfire, fuel, sensor codes | Ratio, solenoid, transmission temperature codes |
| Smell or fluid | Burning oil or coolant | Burnt transmission fluid, red leaks |
Simple checks
- Rev the engine gently in park or neutral: if the problem appears, it is likely the engine.
- Read codes; P03xx misfire codes point to the engine, P07xx codes to the transmission; see check engine light.
- Check both fluids: engine oil and coolant, and transmission fluid level and condition.
- Note whether symptoms change with gear or only with engine speed.
Use the tachometer
Watch the tachometer when the problem happens. If rpm jumps without the vehicle speeding up, the transmission or converter is slipping. If rpm and speed both struggle, the engine is short of power. If rpm stays steady while you feel a vibration, look at the converter, driveline or mounts; see car shakes when accelerating.
When it is both
Some problems involve both: a misfiring engine can make shifts feel rough, a slipping converter can look like engine power loss, and worn engine or transmission mounts can cause clunks that seem to come from either; see engine mount symptoms.
Check the mounts and driveline too
Before blaming either major component, rule out cheaper causes: a broken mount, a worn U-joint or CV axle, or a vacuum leak can all imitate engine or transmission trouble; see U-joint symptoms.
Which is more expensive?
Costs vary widely by vehicle, but both a replacement engine and a replacement transmission are major repairs. Compare with engine replacement cost and transmission replacement cost before deciding, and see is it worth replacing an engine?
Next steps
See signs of a bad transmission, how to tell if an engine is blown, our engines and our transmissions.
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