1999 Honda Passport P0300 Code: Multiple Misfires, No Improvement After Coil/Injector Swaps

Many older Honda models suffer from clogged EGR ports, which can cause misfires that appear unrelated to ignition or fuel delivery. Additionally, coil failures are common in these vehicles—your current tests may not be conclusive. Try installing all the coils from the misfiring bank onto the opposite side; if the misfire follows the coils, one or more are faulty. If the issue stays with the original bank, it could point to a vacuum leak, fuel delivery imbalance, exhaust restriction, or camshaft/timing problems.

I just wanted to share that I recently encountered similar issues with my 2000 Toyota 4Runner—six emission-related codes appeared within two days. It took a moment of frustration, but after some investigation, I found the root cause was related to hard-coded emissions faults.

This issue started after a timing belt replacement. A previous technician claimed no compression in cylinder #3 and suspected valve damage—however, when I rechecked the belt alignment (which was off by a few teeth), I found all cylinders had around 180 PSI on compression test. Using a Modis device to verify CMP and CKP signals confirmed proper timing. I manually rotated the engine to top dead center at #1 and verified the timing mark was correct. The EGR valve was removed and blocked—no improvement. The owner replaced spark plugs with Champion SPK, which were then swapped for Denso plugs (after a short run, they showed black deposits on the passenger side). No change. I’ve tested coil packs and injectors by swapping them—still no difference. Fuel pressure checks show 42 PSI at idle, rising to about 42 PSI when the vacuum regulator is disconnected. Intake manifold gasket issues are common in this model. I sprayed carb cleaner around the intake with no RPM increase. A vacuum gauge on the PCV port reads only 15 inches of mercury—very low and stable. Despite all these checks, nothing seems to resolve the issue. I’ve worked on helicopter EMS systems for over 25 years and consider myself a competent mechanic—but this engine is proving extremely difficult to diagnose. Thanks for any help!

The low vacuum reading at idle—only 15 inches—is highly suspicious. This suggests either an incorrect timing issue or valve train problems. Don’t rely solely on compression tests to confirm valve sealing; they can be misleading. If ignition and fuel delivery are confirmed good, and electrical components pass testing, the next step may require removing the cylinder heads to inspect for worn or damaged valves. A timing belt that was off by several teeth raises concerns about potential damage—especially if the tensioner was weak. I’d question whether any internal parts were compromised. Could you share your fuel trim readings at idle and wide-open throttle (WOT) in park? This would help assess fuel delivery consistency.

Never assume that the first or second technician correctly installed the timing belt. I removed the engine’s timing belt completely, reinstalled it according to factory specifications, and now the engine runs smoothly—like a kitten. The driver-side cams were properly timed (same side as cam sensor), which explained why timing readings appeared correct on initial tests. However, the passenger-side cams were misaligned. This is a critical lesson: always verify full alignment before concluding an issue lies elsewhere. Thanks to Jeff for pointing this out.

I have the same problem with my 1999 Honda Passport—only the left-side cylinder head appears faulty. I’m wondering if anyone has clear photos or diagrams showing proper camshaft positioning for both heads to confirm correct installation. Any resources would be extremely helpful!