Knocked back by “knock sensors”, if that’s a real thing

Should I be embarrassed to confess that I was nearly 45 years old before knowing what “knock sensors” are?

Okay. Show of hands. How many of you think I just made that name up, because this is the first time you’ve ever heard of knock sensors? Well, you’d be in good company. I thought the lady at the auto part store was just reading the scan tool wrong.

Now I’m a few weeks this side of that birthday, and now knock sensors are very real to me. Tiringly, painfully real.

One reason you may not believe in knock sensors is because you’ve never seen one, and fair enough: kind of like the ever-elusive snipe, they’re not easy to spot. In our 2001 Chevy Suburban, the twin knock sensors are mounted in round wells on top of the engine, but they are completely situated under the air intake manifold, which is basically the big, plastic thing on top of the actual, metal engine.

You must remove the air intake manifold before you can access the knock sensors, and you must remove about a dozen bolts, a few hoses and two fuel lines before you can remove the air intake manifold. Once you clap eyes on the knock sensors, they’re easy to replace. Then the fun begins with putting all that junk back together.

Of course, the apex of fun is taking the vehicle back to John the emissions guy and having him tell you the repair worked, and your SUV with 360K miles on it just passed inspection again. Applying the tag decal becomes like an Olympic medal ceremony.

No, I am not the Suburban’s original owner, or else I probably would have run into the knock sensor thing before now. I think we’re its third family.

For those of you still wondering what a knock sensor is, they’re basically microphone-type devices that help the engine detect the sound of knocking in the spark plug chambers. They help the vehicle computer keep spark plugs firing in the right timing to optimize fuel efficiency and to reduce carbon emissions. Something like that.

When the check-engine light piped up, that scan tool actually identified two problems with the engine: the knock sensors as well as fuel trim system leanness. What? Never heard of that before, either? Yeah, I had to Google that code, too.

Apparently, that latter fault is the trickier one to diagnose. It only means that too much air is mixing with the fuel going into the engine. Is the fuel supply lagging? Is there an air leak? And how can you tell?

I started with the no-brainers. The Suburban was overdue an oil change, spark plug replacement and fuel filter replacement. Maybe these would clear up the lean code, and for all I knew, that could cause the knock sensors to straighten up. After completing this tune up of sorts, I took the vehicle to another auto parts store for another scan.

The employee was nice, but he was no more a mechanic than I am. When he saw the lean code still there along with the knock sensor code, he said just buy some fuel treatment, and that should clear everything up. We were already well into the 30-day grace period, so I carried on with the plan to also change out the knock sensors.

We had our mechanic friend on standby just in case I disabled the Suburban beyond my ability to bring it back to life. I watched several videos on how to remove air intake manifolds and how to find the knock sensors. I was ready.

When that Saturday morning rolled around, I was nervous, but about not finding the knock sensors. I was nervous about all those hoses, wires and bolts, not to mention the fuel lines I would encounter. (Yes, you have to disconnect two fuel lines in this process.)

A few hours later, I had managed to remove the air intake manifold, locate the knock sensors and put news ones in their place. But then something weird happened when I tried to put everything back together. I was tightening up the eight main bolts when I realized a hose of some sort was trapped between the air intake manifold and the engine, so I loosened the bolts again, and I moved that hose out of the way.

And then a new fear entered the picture: I didn’t know what this hose was, nor did I know where it went. This particular hose was never mentioned in the videos, which had otherwise been very helpful. How could my Suburban have a hose unique to itself? Why wasn’t it mentioned in the video?

Gratefully, you can find pretty much anything on the Google search engine, so I dialed up a schematic of a 2001 Suburban engine. And there it was: a PCV valve vacuum tube. That metal thing shoved into the end of it was the PCV valve itself. I didn’t recognize it, even though a few years ago I replaced the PCV valve in our 2007 Town & Country minivan. The two look very different.

Eventually, I located the grommet where the PCV valve connects back to the engine, and I jammed it back in place. Then I reconnected the other hoses, reconnected the fuel lines and bolted everything else back down.

The moment of truth came when I cranked the engine. And the truth was that the engine was running rougher than ever before, and the check-engine light was still on. In fact, it was running so rough that it idled all the way up to 3K RPM and all the way down to about 400 RPM before conking out. It did this twice.

I walked back into the house, and my wife asked, “Did it work? Is the truck working?”

I forgot to mention that in the process of messing around with that PCV valve, I broke its vacuum tube. I also suspected that the PCV valve itself may be faulty based on a YouTube video that showed you how to check it.

The next day, a Sunday, I got back to work after church. I had bought a new PCV valve, and I had super-glued the vacuum tube, which was a temporary measure until the new, hard-to-find tube arrived Monday in the mail.

Still no joy.

“Call our mechanic friend,” I told my wife.

Our mechanic friend and his family were kind enough to make a house call that afternoon, and he reckoned it could be the air intake manifold gaskets. He said they were probably getting old and letting raw air seep into the cylinders. Before I knew what was happening, he was ripping out the air intake manifold. A step that had taken me two hours to accomplish the day before, he had done in 15 to 20 minutes. Not his first knock sensor rodeo.

Our mechanic friend showed me the gaskets, and he cleaned them off. He also cleaned the engine top around the cylinder holes in hopes it would help the gaskets make a better seal.

Still no joy.

“Buy new gaskets,” our mechanic friend told me. He also apologized for breaking my PCV valve vacuum tube again.

Gratefully, that Monday was the Martin Luther King, Jr. holiday, and I’m a fulltime government employee, so I had a whole day to take the air intake manifold out a third time. I’m pleased to report that this time it only took about 30 minutes.

Our mechanic friend was correct: It was the gaskets.

When the vacuum tube arrived on the porch the next day, but it was the wrong size, not the “exact fit” Amazon had promised. Never mind. I went to the auto parts store and spent $4 on two feet of sturdy hose that fit perfectly.

Here’s the really encouraging part: Using store coupons and online discounts, we managed to spend around $120 on everything I’ve mentioned in this story, including the oil change. Going to a regular mechanic would have cost us anywhere between $500 and $1,000 just to get the knock sensors and gaskets replaced.

My wife and I had prayed earnestly that God would help us keep our two high-mileage vehicles on the road at least another year, because we have some expensive home improvements we want to tackle in the coming months. $120? What’s that? Half a used car payment? We’re grateful.

What’s next on my DIY garage list? Replacing the suspension on my minivan. If you see me rollin’ in a low-ridin’ Town & Country, you’ll know something went wrong.

Don’t you worry, though. We have our mechanic friend on speed dial, at least until he changes his phone number.

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