Knock sensor can be a costly, but necessary, pain to repair

Dear Car Talk:

I drive a 2005 Toyota 4Runner SR5 with 85,000 miles on it. Other than routine repairs, the vehicle has been trouble-free for all the years I've owned it. About two months ago, the check engine light came on. I took it to my regular shop to have it checked out, and was told that the service code indicated a problem with the "knock sensor." Not being a mechanic, I thought, "How bad could that be?" Wrong! Because of the sensor's location on the engine, I learned that it would involve considerable labor and cost roughly $2,200 to replace. I was stunned, and decided to wait on the repair until I could do some research. I found that having to replace the sensor is rare, but is indeed a costly repair. But here's the thing: The warning lights come on intermittently, shutting off for several days before reappearing. The engine appears to be running smoothly, no rough starting, idling or knocking. So, before spending a chunk of change, possibly needlessly, I thought I would get your input. Are there other possible causes for this problem, and can engine damage result from not replacing the sensor? I read your column weekly and trust your sage advice (your great sense of humor is an added bonus). Thanks for any advice you can provide. – William

RAY: Wow, you hit the bad-news jackpot, William. The knock sensor is a complete pain in the tailgate to replace. You have to remove the air plenum, the intake manifold, the timing belt and lots of other stuff to get at it.

The fact that Toyota buried it like that tells me that they never expected it to fail. And perhaps it hasn’t failed. It easily could be a broken or frayed wire leading to the knock sensor that’s causing the warning light to go on and off.

Once your mechanic removes the plenum, which is easy, he should see a wiring harness that leads to that sensor. Who knows? You might find a rodent nest in there and a chewed-up wire or two.

You really do need a working knock sensor. The sensor continuously gauges the timing of the explosions in your cylinders, and adjusts the spark timing if the explosions start happening too early (that’s called knocking, or pre-ignition, and it’s damaging to your engine – and in extreme cases, can even burn a hole in a piston).

Here’s my advice: Find yourself a mechanic who is willing to take it a step at a time. Have him start by removing the plenum and checking the wiring first. You can be standing next to him with a stack of $20s and dole one out to him every 15 minutes as he works.

If you’re lucky, you’ll be able to stop doling well short of $2,200. I sure hope so, William.

Stocking up on hard-to-find parts not a bad idea

Dear Car Talk:

I own a beautiful 1985 Mercedes 280SL. At my last hose changeover, I was informed that there were only 43 of the "molded" radiator hoses available, nationwide. The other hoses could be replaced off the shelf. My question is: Does it make sense to buy several of these hoses now, place them in zip bags and put them inside some sort of airtight plastic containers to prevent deterioration? Will they be useful in eight years, at the next changeover? If not, do you have any suggestions as to how to deal with this problem? I don't want to have to sell my beloved Benz to a collector. – Steven

RAY: I think you should corner the market, Steven. Buy all 43 of them and go into business. You'll be like the Hunt Brothers with silver in the 1970s.

I don’t see any real downside in buying a couple of the hard-to-find hoses for future use – especially if this is a vehicle you plan to keep forever.

If I were you, I’d put some Armor All on them to help keep them from drying out. Then I’d put them in Ziploc bags and put them in a box in the back of a closet somewhere.

By the way, I’ll be expecting my monthly checks from Armor All and Ziploc for those mentions, fellas.

Honestly, I think the rubber will be good forever. What degrades rubber is oxygen and heat. And in Ziploc bags in a closet, they won’t be exposed to much of either one. The bigger danger is that you’ll forget where you put them, and create a real head-scratcher for your heirs when they sort through your prized possessions.

And keep in mind, too, that there are plenty of other parts on this car that are going to become obsolete and hard to find. It’ll be a small miracle if the next hard-to-find part you need is that radiator hose in eight years.

So maybe you should pick up one of everything now. Instead of storing the parts in your closet, use them to build a second 1985 280SL in your garage. And then pluck it for parts.

Good luck, Steven.

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