03 Tahoe knock sensor code

We have a 2003 Chevy Tahoe with a 5.3 V8 engine. We recently got a check en gine light for a knock sensor. The message on the scanner was "Knock sensor reading low voltage". I have read about this problem a bit and it seems li ke the harness inside the engine, rather than the sensor itself usually fai ls and causes this problem.

This repair apparently involves removing the intake manifold, and this is a job we'd rather not have to do on an occasional vehicle. So I also read th at this sensor could be bypassed with a `100 ohm resistor. This would run t he car in "limp" mode. I don't really know what that is but if it is just a small loss of power I didn't think that would be a problem for something w e would just use to bomb around town in.

So we unplugged the harness from the engine and plugged a 100 ohm resistor into it. We taped it up well and pushed this wire into a tight grounded met al can. I read that the sensor, or rather I think there are two of them in there are piezoelectric sensors that will develop a very small voltage acro ss them when they "hear" what is the beginning of a knock.The timing is the n adjusted by the computer accordingly. This was the way I understood it an yway. I figured with such a small voltage, keeping the modification grounde d would shield it from the electrical "noise" of the engine.

We reset the code and started the car. The check engine light came back on and a message I've never seen on the dash appeared: "reduced engine power". The car had no power, would not accelerate, and we had all we could do to just get it back in the driveway.

Now up until the time we made this modification even though we had the code there was no loss of engine performance noted. We just had this code that prevented us from getting a sticker. We haven't yet tried to plug the harn ess back into the engine to see if the power returns.

Is anyone familiar with this problem? Am I bypassing this nuisance sensor a dequately? We're not going to take any trips with this car. It's just a sec ond car and will be used by my wife to get the kids around town. As I've a lready said, it's just a second car and we'd really just like to be able t o use it.

If anyone can please help me to get this thing to run half way decently wit hout having to do this engine repair job, I would be very grateful. Lenny

Reply to
captainvideo462009
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This is why they call it "limp-home mode."

The sensor is giving an error. As long as the engine never knocks, it doesn't matter. But, the engine may be damaged if it ever does, because the ECU might not know it, and won't be able to do anything about it.

Putting it in limp home mode will probably prevent engine damage, but as you noted, it won't be very drivable.

If it's a disposable car and you have plenty of money to buy a new car when the engine is wrecked, there's no reason not to keep driving it as-is with the check engine light on. It'll probably be fine, but then again it might not be.

If you would worry if the car just up and died all of a sudden, then fix it.

--scott

Reply to
Scott Dorsey

+1

when the effort to get around a problem exceeds the effort to just fix the problem, it's illogical to keep going in that direction.

GW

Reply to
Geoff Welsh

There is no engine repair job, just a sensor replacement. A new intake gasket is a bonus. A quick google on this says about $300 to get it done , but I don't know. I'd guess $500 is more like it. But that's a guess. You could DIY. Plenty of instructions out there. Try putting the wiring back how it was. You gave no reason to think it's the sensor wiring harness. They're commonly replaced when replacing the sensors. $35 part. You have to decide if you want to spend the money to fix it, that's all. You're going nowhere doing what you've been doing. Don't know why you call it a "nuisance." A code indicates something needs to be replaced or repaired. That's life, not nuisance.

Reply to
Vic Smith

+1 GW
Reply to
Geoff Welsh

I've repaired this on a friend's Sierra 2500HD. The problem in this case was that water entered the knock sensor recess and caused it to rust. From my research I found that to be the most common failure cause.

I had never worked on a GM V8 before but managed to change the sensor in a parking lot in about an hour and a half. Its not that big of a challenge and its the right thing to do. The knock sensors control the spark timing. When you unplug them the computer is firing the plugs blind and likely with severely retarded timing, robbing you of performance and fuel economy.

There are two knock sensors, front and rear. The rear one is probably the damaged sensor. By unplugging the whole knock sensor harness you took away the remaining sensor that the car had to go off of. Don't do that.

Just fix it right. If it's a spare car you can afford to have it down for a day to repair it. Buy a new knock sensor and intake manifold gasket and search for a guide on doing the repair. Here are a few pointers:

- Before removing the intake manifold, clean the area where the manifold attaches to the heads to remove any leaves and loose dirt. You don't need it to shine like it just rolled off the assembly line but you do need to remove any debris that could fall into the intake port of the engine when the manifold is removed. Also make sure to clean the cowl below the windshield as debris hanging around that area could fall onto the engine while you work. You must not allow any debris to enter the intake ports.

- There is a TSB for the water damage issue. The TSB advises that you build a "dam" of RTV around the grommet that the sensor wire passes through in the intake valley, however you should not fully seal the grommet with RTV. There are photos and diagrams demonstrating this online.

- The intake manifold attaching bolts have a tightening order and torque spec which needs to be followed! Use a torque wrench if at all possible. Don't overtighten.

- The intake manifold gasket should not need any sealant. It is installed dry.

The total I spent on this fix was probably around $60 to $75. The ACDelco knock sensor was purchased on eBay and I got an intake manifold gasket from RockAuto.com.

Regards, Ryan

Reply to
ryan42

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