2004 Kia Sorento P0300 misfire on 2 and 5—vacuum or ignition issue?

Kia Models Forum Sorento
1

I’ve got a 2004 Kia Sorento 3.5L with 116,000 miles throwing a P0300 code, specifically misfires on cylinders 2 and 5. Here’s what I’ve done and noticed so far:

  1. Found a vacuum port on the upper intake manifold without a hose attached and another hose plugged with a bolt. I’ve capped off the unused ports.
  2. Replaced all six spark plugs with NGK plugs (pre-gapped, but I didn’t confirm the gap).
  3. Cylinder 4’s coil broke during removal, so I swapped it with cylinder 2 and installed a new coil on 4.
  4. Checked all plugs—none showed signs of unburnt fuel, oil, or excessive carbon buildup.

After replacing the plugs and addressing the vacuum ports, I cleared the codes. The engine idled fine for 20 minutes and revved to 2,000 rpm without throwing a code, but I’m not confident it’s resolved.

Any advice on whether this might be a vacuum issue, ignition coil problem, or something else (like injectors or compression)? What’s the best way to test the coils for spark? Appreciate any help!

2

Capping the vacuum ports was a good move—unmetered air can definitely cause misfires. Keep an eye on the codes, but if it comes back, I’d look at the ignition coils next.

3

Juneta said:
Capping the vacuum ports was a good move—unmetered air can definitely cause misfires. Keep an eye on the codes, but if it comes back, I’d look at the ignition coils next.

Agreed. I’d also double-check the plug gaps. They should be set to .039-.043 for this engine.

4

For testing the coils, connect a plug to the coil and ground the plug to the engine. If you get a strong spark, the coil is likely fine.

5

If the misfire comes back on cylinder 2, you might want to test the injector next. Swap it with another cylinder and see if the code follows.

6

It’s good that you capped the vacuum ports, but keep an eye on the idle. A vacuum leak could still show up under load if it’s small.

7

If the misfire persists and you’ve ruled out spark and fuel, a compression test on cylinder 2 would be the next step. Low compression could point to a mechanical issue.