Many possible causes for engine skipping

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Last week, in response to a question from Stan Lanos of Augusta, S.C., we explored some possible causes of engine skipping on a S-15 GMC pickup. I focused on the probability of an ignition system fault and deferred some additional causes to this week’s column.

Intermittent skipping or stalling can be a frustrating situation, because the fault often won’t occur when the vehicle is brought into the shop for inspection. In many cases the cause is a poor electrical connection or an internal fault within a component. Heat, vibration, humidity, engine load and movement and other factors can bring on a fault. Then, in the blink of an eye, it can disappear.

The on-board diagnostics system used on newer vehicles often can help isolate the fault area, but most of the time old-fashioned detective work still will be needed to exorcise the demon.

The first step in tracking down an intermittent fault is to identify the driving conditions under which it occurs. Does it happen when it’s hot or cold, when you’re turning left, going over bumps, speeding up, idling or after a hot re-start?

Next, does the check-engine light come on while driving, especially when the skipping or stalling occurs? If so, a diagnostic trouble code(s) will be stored, greatly narrowing the search area.

An under-hood inspection also is helpful. Check for loose or disconnected hoses, wire connections, evidence of liquid leakage, odd noises or odors. Has routine maintenance been performed? Spark plugs might last much longer than they did in the past, but they also work harder due to the leaner air-fuel mixtures and higher compression ratios found in newer engines.

There’s one last step in our home diagnosis before surrendering the job to a pro: the wiggle test. Please don’t consider this unless you have an understanding of the engine’s moving parts (belts and pulleys), what is hot, what can shock you, and only after removing hand/wrist jewelry and securing loose clothing.

With the engine idling in park or neutral, parking brake fully set, and a helper to watch over the check-engine light (MIL) and you, try gently wiggling each under-hood electrical connector, noting any change in engine speed or smoothness. Move slowly from each connector to another. Should the engine skip or the MIL illuminate, gently wiggle it a second and third time, just to be sure. Aggressively moving a connector might temporarily fix the problem, only to have it resurface a month later. If an ill-fitting or corroded connector is identified and replacement terminals aren’t readily available, try putting a slight twist in a blade (flat) terminal with needle nose pliers so it scrapes/burrows its way into its mating socket when reconnected.

A variation of the wiggle test is to gently tap on components such as throttle position, mass airflow, vacuum, and temperature sensors, noting any changes in engine performance.

If you weren’t able to detect a fault using the wiggle test, but find the engine runs well for some time after, it’s likely you inadvertently restored a connection in one of the connectors. In this case, with the engine off, I’d unlatch, open and inspect each connector, then re-connect them firmly. This, as well as the terminal-twisting trick helps scrape away corrosion during reinsertion.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.

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