What you can miss about a misfire
The least favourite symptom to resolve for many DIY techs has to be the engine miss — a rough-running engine that just doesn't seem to be firing on all cylinders.
Most swap in a new set of spark plugs, and maybe a wire set or ignition coil, before grumbling in frustration when there's no improvement. Let's look at the less obvious reasons an engine might develop a miss.
Air problems: It's not just the absence of air, fuel or spark that can cause a miss, but also too much or at the wrong time or location. A leaking intake can cause a lean engine miss as one or more cylinders get too much air, thus throwing off the mix.
An easy way to check is to introduce another fuel source around the manifold with the engine running at idle.
Try spraying small amounts of easy-start (ether) around the manifold in a well-ventilated area to see if the idle suddenly picks up speed when that fuel gets sucked in past a leaking gasket or cracked manifold. Make sure you've checked the ignition wires first, to ensure there are no stray sparks that might light up your experiment in a not-so-nice way.
Bad fuel: This can be common in vehicles that are stored for winter, because fuel can start to degrade in that time. Although water content is addressed by fuel distributors through seasonal blends, heavy rains at low-lying stations can sometimes cause water to seep into underground holding tanks. Put a sample in a clear glass container and let it sit for a few minutes to see if the fuel separates from any water.
Coils: If you're dealing with a miss on an engine with individual coils mounted on each spark plug, try switching the coil to another cylinder to see if the miss moves with it. If it doesn't, you may be dealing with a control or wiring problem. If you haven't got a scanner or access to wiring diagrams, you might save money by taking it to the pros.
Emissions: Older emissions systems can cause symptoms that feel like a miss, but are actually related to vacuum leaks or mis-fuelling injectors. For example, on some EGR valve-equipped vehicles, if the valve doesn't close completely at idle (because of a rusted activator), it can cause an extremely rough idle.
And there's the infamous oxygen sensors, which, when they malfunction, can lean out the fuel delivery and cause a miss.
Finally, don't rule out a mechanical problem in the valve train or pistons. A broken rocker arm or worn camshaft lobe can give you exactly the same feeling as an ignition miss. It might be time to dig out the compression gauge or pull the cylinder head covers to take a peek.