P0302 and P0305 Misfire Diagnosis - 1998 Toyota Tacoma 6-Cylinder with 126K Miles

I'd recommend checking a few additional things. You've confirmed good compression, performed a full tune-up, and your spark plugs look clean and healthy. Injector resistance was also within normal range. Next, consider performing a fuel leakdown test—this requires specialized equipment but can identify whether injectors are leaking or clogged. If you're open to it, I can provide details on what tools to purchase. Otherwise, check if cylinders 2 and 5 share any common components—for example, could there be a vacuum leak affecting both? These misfire codes are non-specific, so systematic elimination of potential causes is key. A fuel leakdown test would help determine if there's an issue with fuel delivery in those two cylinders—this comes after ruling out vacuum leaks. What were your exact compression readings?

Compression readings were at the lower end of the range: 145 psi ±5 psi on all cylinders. I also want to note that due to my elevation (~5,500 feet above sea level), actual sea-level compression should be higher than this—so it's not a major concern. I'm curious about the fuel leakdown test—could you explain how it works? I know what cylinder leakdown is, but this one seems new. I'd love to learn more!

A fuel leakdown test helps identify whether injectors are clogged or leaking. You'll need a fuel injector actuator—a handheld tool that clips onto the battery and comes with adapters for your vehicle's injector connectors. The process involves installing a fuel pressure gauge, turning the key to 'on' (engine off) to pressurize the system, then one by one actuating each injector while monitoring pressure before and after activation. For example, on a healthy engine, cylinder 1 might show: before = 35 psi, after = 30 psi. All cylinders should behave similarly. If cylinder 5 shows before = 35 psi and after = 34 psi, that suggests a clogged injector. If it drops to 25 psi after activation, it likely indicates a leaky or faulty injector. Important note: Most fuel injector actuator tools supply power directly from the battery. You must verify 12V at the injector connector using a digital multimeter—using a test light could damage the PCM's injector driver circuitry. Avoid this at all costs. Also, avoid performing multiple injector tests before starting the engine to prevent residual unburnt fuel buildup. Always clear the system between tests. Double-check your firing order and ensure no wires are incorrectly connected—I’ve seen this happen before. Review every step carefully. Did the misfire develop suddenly or gradually? Note that valve adjustment is often overlooked on these engines, and improper adjustments can cause single-cylinder misfires—though it's possible to have two separate issues due to uneven valve clearances. If the problem persists only in cylinders 2 and 5, then the issue is likely specific to those cylinders. Proceed step-by-step with systematic checks.

Important reminder: when checking voltage at the injector connector, always use a digital multimeter—not a test light. Using a test light can permanently damage the PCM's injector driver circuitry. This is critical to avoid.

Thanks for the clarification—yes, that appears to be the tool I'm referring to: https://cgi.ebay.com/ebaymotors/Fuel-Inj ... enameZWDVW I confirmed the injector resistance measurement using a digital multimeter. Additional notes: if I had a sticky valve, would I expect lower compression in cylinder 2 or 5? And finally—this issue developed gradually. It started with an intermittent check engine light on highways, which eventually became constant and persistent. Thanks again, Rob

You'd typically see lower compression readings in affected cylinders if there's a valve problem—but the drop might be subtle. I’d recommend rechecking compression specifically on cylinders 2 and 5 to confirm. Valve adjustment issues are common on older Toyota Tacomas, especially at higher mileages, and could explain the gradual onset of misfires. This is particularly relevant given that the issue has been building over time.

Thanks for the help—after some troubleshooting, I finally found the root cause. I heard a ticking noise behind the timing cover, which led me to discover that the timing belt tensioner had disengaged and was no longer maintaining proper tension on the belt. This caused slight variations in engine timing, leading to misfires in cylinders 2 and 5. While it's still unclear why the ECU specifically flagged those two cylinders, fixing the timing tensioner resolved the issue completely.