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Very rough idle: spark plug wires?

My beloved 1999 RAV4 has 160K miles. A few years ago, it started sputtering while I was waiting at a red light. Within a few days, the engine felt like a bucking bronco and almost stalled when this happened. A friend did a tune up and that fixed the problem. About a year later, same thing started happening. My friend replaced the spark plug wires, then the same thing happened about a year later. So, every year or so, I’m replacing the spark plug wires. What could be causing this? Is it really the wires, or could we be masking the real problem with the new wires every year?

If the issue goes away each time the wires are changed then chances are it was the wires. Where are you buying the replacements from? Wire quality differs GREATLY…and the wires you buy from Auto-Zone or the like are usually crap. I’d price out some factory Toyotie wires.

As a test, go out and unplug and replug in the wires to each plug and see if the issue goes away. YOu can also open the hood and run the engine at night and look for sparks from the wires…they will be visible usually if a wires is leaking into another or leaking to ground.

My opinion is that a compression test should always be run when an engine performance problem exists, no matter if the engine has 160k miles on it or 16k.

These problems often work like a line of dominos. Low compression kills spark plug which misfires and kills the wire which then makes the coil work harder…
See where this goes?

If the wires are really going bad that often then they’re probably being killed rather than dying a natural death.
Weed out any compression issues first. With that mileage, ideally you should be looking for 170 and up, give or take.

Not directly addressing your problem, but If you would happen to be using those fat colorful fancy and expensive plug wires, they may well be the problem. Generally OEM plug wires are your best choice.

I have to agree with everything here except the one about the AutoZone wires. AutoZone does carry some, lets say less than desirable wiring, but it is mostly their real expensive “High Performance” wires. The lifetime Duralast are as good as the OEM.

I’m sure that is probably true for other house brands, the lifetime warrantee wires will be a good value, but avoid the “economy” and the “High Performance” brands.

I’m kinda surprised that the Toyota brand gave out so soon. I had an 86 Toyota Tercel 4wd whose wires showed the optimum 4kohm resistance after over 200 k miles in 2001. I replaced them, just for insurance, with the duralast and never had a bit of trouble.

I’m kinda surprised that the Toyota brand gave out so soon.

Really!? The wires are over 12+ yrs old? I think the 86 is an anomaly or you have decent weather conditions and likely don’t salt the roads.

Bad wires are usually first noted upon acceleration, not idle…Today, a rough idle is more likely a vacuum leak or a stuck-open EGR valve…