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Suburban Update: Replacing the O2 Sensor

I love my 1988 Suburban. With a Throttle Body Injection (TBI) 350 and 700r4 transmission, it can get half decent mileage for a full size SUV and still get up and go if you mash the go pedal. There’s no speed parts installed, but it’s an upgraded GM Goodwrench motor, according the the previous owner. But that doesn’t mean it isn’t a 31 year old vehicle with maintenance issues.

What TBI means

Throttle Body Injection means that there are electronically actuated fuel injectors where a carburetor would otherwise be. A carburetor has to be tuned to give the right amount of gas at the right time for the current throttle position and engine vacuum. They’ve been described as “well tuned fuel leaks” for this reason.

In the 1980’s, automotive manufacturers transitioned from carburetors to fuel injection. For a few years there were some abominable crossovers between the two: cars that had the computer control of fuel injection with the cheapness of a carburetor. They were bad, and we try not to think about them too much.

Where a carburetor just makes educated guesses as to how much fuel to leak into the engine, a fuel injection system doesn’t have to guess. It has sensors that tell it the current state of the engine. It knows RPM, how much air is going in, what the throttle position is, engine temperature, and how much fuel is being burned by measuring how much oxygen is in the exhaust. The computer takes all of this, looks up these conditions in a lookup table in memory, and fires the injectors to put in just the right amount of fuel. This is called a closed-loop system because the sensors give feedback to the computer. But if a sensor fails…

When Sensors Fail

GM’s solution is to intentionally blind people who don’t fix their engine.

If any of these sensors quit working, the the computer has to do what a carburetor has to do: make an educated guess based on the information it has. This is called open-loop. In some cases, this isn’t enough and the truck won’t run or will run erratically. What’s amazing though is that the computer knows what kind of input to expect from its sensors, and if any of those sensors is out of its expected range, the computer tells you which sensor is out of range. On a first generation Chevy TBI setup, you just get a blindingly bright orange Check Engine light. A little fiddling gets you some codes that you can look up and find out which sensor bit the dust.

The Throttle Position Sensor went out right after I bought this truck and I got the code for that. That was a cheap fix and took 10 minutes with a screwdriver to replace. The Oxygen sensor (aka
O2 sensor) also failed a little later, and I got the code for that too. It took me a lot longer to replace this one, because I feared it fairly badly- and irrationally, I might add. Let me explain why.

Pointless Fear

I heard that O2 sensors were hard to replace. Somewhere rattling around in my brain are horror stories of having to pull exhaust systems off to have the sensor removed, cutting the sensor to get it out, and more. A mechanic friend of mine recommended spraying it with penetrating oil daily for a week, and then using a torch to heat up the metal around it. I don’t have a torch or penetrating oil. I just kind of ignored the problem.

Biting the Bullet

I decided that I’d quit worrying and just do the job. The truck had been running worse and worse to the point where it would stall and surge at lights, and it got pretty bad mileage. It was time to Just Do It. I ordered up the O2 sensor on Amazon (for WAY less money than I expected!) and then it sat on my desk for a month. Finally the day arrived.

I got under my truck and looked up at the sensor. It screws into the drivers side exhaust manifold and points toward the back of the truck. There’s an exhaust crossover pipe in the way but not horribly so. I unplugged the connector and got my 7/8″ end wrench. It didn’t budge. After fiddling with it for a few minutes I realized I’d need a breaker bar. I don’t have a breaker bar. But I do have some pipe. But for that I’d need a socket and a driver for it. Out came the 1/2″ drive ratchet and the hunt began for a socket. The 21mm was too small, the 23mm too big. Where on earth was my 22mm?

It was at the bottom of my tool box with some booger welds on it. Years ago I’d hacked the only socket I never used into a disk brake tool by welding some flat iron to it and shaping it into a disk brake tool. I wasn’t smart enough to just weld a big nut instead. Oh well. Hindsight. I ground down the welds and reclaimed my 22mm socket and then went to work.

Ryan:1 O2 Sensor: ZERO

To get to it the hex head, I had to use my angle grinder to cut off the body of the O2 Sensor that was in the way. Then I was able to put on the 22mm socket, ratchet, and cheater pipe. It very slowly started to turn. The crossover pipe was in the way and so it took some creativity to move the ratchet enough, but with some force I was able to get it to turn more and more, and in just a few minutes I had the old O2 sensor in hand.

The threads were a little boogered up, but after carefully threading in the new sensor and cranking it down with the 7/8″ end wrench the job was done. I plugged in the O2 sensor, cleaned up my tools, and then started the truck. It fired right up as usual, but this time it idled perfectly! A test drive confirmed it: No Check Engine Light, and running like a top. Now I need to put a timing light on it to check the timing and take care of some leaks I noticed while under the truck. But that’ll have to happen another time. Thanks for reading!

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