Your Corner Wrench: What can you miss about a misfire?

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Many DIY techs’ least favourite symptom to resolve has to be the engine miss; that 2-word description referring to a rough running motor that just doesn’t seem to be firing on all cylinders. If it’s on an injected engine equipped with an onboard computer, at least you have a good chance at learning which cylinder has the problem or if it’s system-wide. Most tackle this problem by swapping in a new set of spark plugs, and maybe a wire-set or ignition coil before grumbling in frustration when their efforts and dollars spent bring no improvements. Let’s look at the less obvious reasons an engine might develop a miss.

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Your Corner Wrench: What can you miss about a misfire? Back to video

Air problems

Remember the magic trio of things needed to combust gasoline in an engine: air, fuel, and spark. It’s not just the absence of any one of those substances that can cause a miss, but so too can too much at the wrong time and 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 see if this might be a cause 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.

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Bad fuel

This can be more common than you think for vehicles that are stored for winter. As we discussed in a previous column, fuel can start to degrade in a matter of a few months. And while water content has been addressed by fuel distributors with seasonal blends, heavy rains on stations with low lying lot set-ups can see water seeping 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, don’t head straight out to the parts store for a replacement. First, 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 and throwing coils at it won’t bring any results. Control problems stemming from a bad crank or cam sensor — or any number of components — should also not be handled by dumping a pile of new parts under the hood. If you haven’t got a scanner or access to wiring diagrams, you might save money by taking it to the pros.

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Emissions

Older emissions systems can provide symptoms that feel like a miss, but are usually related to vacuum leaks or mis-fueling injectors. For example on some EGR valve equipped vehicles, if the valve doesn’t close completely at idle (due to a rusted activator) it can cause an extremely rough idle. And then there’s the infamous oxygen sensors, which, when they malfunction can lean out the fuel delivery to 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. Might be time to dig out the compression gauge or pull the cylinder head covers to take a peak.