03 Tahoe knock sensor code

[captainvi...@gmail.com]

We have a 2003 Chevy Tahoe with a 5.3 V8 engine. We recently got a check engine 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 like the harness inside the engine, rather than the sensor itself usually fails 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 that this sensor could be bypassed with a `100 ohm resistor. This would run the 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 we 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 metal 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 across them when they "hear" what is the beginning of a knock.The timing is then adjusted by the computer accordingly. This was the way I understood it anyway. I figured with such a small voltage, keeping the modification grounded 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 harness back into the engine to see if the power returns.

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

If anyone can please help me to get this thing to run half way decently without having to do this engine repair job, I would be very grateful. Lenny

[Scott Dorsey]

<captainvi...@gmail.com> wrote:
>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.=20

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

>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.=20

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.

>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.=20

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


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Geoff Welsh]

Scott Dorsey wrote:
> <captainvi...@gmail.com> wrote:
>> 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.=20
>
> This is why they call it "limp-home mode."
>

+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

[Vic Smith]

On Wed, 24 Jul 2013 10:03:34 -0700 (PDT), captainvi...@gmail.com
wrote:

>
>Is anyone familiar with this problem? Am I bypassing this nuisance sensor adequately? We're not going to take any trips with this car. It's just a second car and will be used by my wife to get the kids around town. As I've already said, it's just a second car and we'd really just like to be able to use it.
>
>If anyone can please help me to get this thing to run half way decently without having to do this engine repair job, I would be very grateful. Lenny

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.

[Geoff Welsh]

+1
GW

[ryan42]

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