Rough Idle and Misfire on a 2.4 L Four-Cylinder: Which Test Decides It

The leakdown test localizes the leak; the wet compression test only suggests it.

The progression of reasoning:

1. Compression spread. 150 psi against 194–202 psi is roughly 23–26 percent below the others - at or just past the usual 25 percent variance limit. That alone justifies suspecting a mechanical problem, but it does not say where.
2. Wet test. Jumping from 150 to 178 psi with oil added is consistent with oil temporarily sealing the ring/cylinder-wall interface. It is supporting evidence only, because oil can also raise a reading on a badly washed-down cylinder for reasons unrelated to ring seal.
3. Leakdown. 34 percent leakage is excessive (in practice anything over about 20 percent warrants repair on a production engine). The location of the escaping air is the decisive datum: air at the oil filler neck means it is going past the rings into the crankcase. A leaking intake valve would hiss at the throttle body, an exhaust valve at the tailpipe, and a head gasket at the coolant or an adjacent cylinder.
4. Vacuum gauge. 15 in-Hg is low (healthy engines typically pull 17–22 in-Hg at idle at sea level), and the rhythmic needle flutter corroborates a cylinder that is not sealing. It is consistent, not specific - a dead ignition cylinder produces a similar flutter.

Conclusion: the fault is in the cylinder 1 bore, piston ring package, or piston - not the valve train and not ignition. The earlier coil swap in the setup already excluded the coil as the cause of that cylinder's misfire.

Why component-swapping is the wrong move here:

• The coil was already moved to another cylinder by the previous shop and the misfire stayed on #1. That is a valid exclusion.
• 34 percent leakdown into the crankcase plus a 28 psi wet-test gain cannot be explained by a coil or injector. Replacing them would take the customer's money and leave the misfire in place.

Next steps, in order:

1. Borescope cylinder 1 through the spark plug hole to look for scoring, a scuffed piston skirt, or a washed cylinder wall. This converts a measured inference into an observed condition before quoting a teardown.
2. Leakdown the other three cylinders even though compression was good. If #2, #3, and #4 are all in the 12–18 percent range, that supports an isolated #1 failure. If the whole engine is 20 percent-plus, you may be looking at a broader wear or even cam-timing issue.
3. Verify cam timing against the crank before condemning the short block. A stretched chain or a jumped tooth will pull overall vacuum down and can concentrate the worst misfire on one cylinder; you do not want to sell a ring job on an engine that needs a chain.
4. Rule out the cheap mechanical causes that mimic ring leakage: stuck or broken ring land is unlikely to be fixable in-frame, but a broken valve spring or a wiped cam lobe on #1 is checked first because the repair is far less expensive.

Only after the borescope and timing check should you quote a piston/ring repair or a used engine.

Document catalyst risk in writing.

A cylinder that leaks 34 percent past the rings still fires intermittently, and on the cycles where it does not fire, the unburned air/fuel charge is pushed into the exhaust and burns in the converter. The converter is the only place in the system hot enough to oxidize it, and the resulting exotherm can exceed the substrate's thermal limit and melt or crack it.

Supporting evidence to cite to the customer:

• The MIL flashed, which on this vintage of OBD-II calibration is the manufacturer's catalyst-damage protection strategy, not a routine reminder.
• Misfire counter data concentrated on one cylinder means that cylinder's raw charge is being dumped every cycle it drops.
• A misfire severe enough to flash the lamp also typically costs 20–40 percent of that cylinder's power, which the customer may perceive as a general lack of power rather than a single-cylinder problem.

Practical guidance: recommend limiting driving to the trip to the shop, and after the mechanical repair, recheck converter efficiency (upstream/downstream O2 comparison and temperature rise across the brick) rather than assuming the catalyst survived.