Okay, this is a 1999 Blazer with the 4.3 vortec. It's the 4x4. I've had this thing about a month and it has had P0300 coming on constantly.

The idle is low when the car warms up, it is sluggish when accelerating, its sucking gas like crazy (about 9 mpg) and the thing shakes like crazy when it is stopped.

When I did plugs and wires on it I noticed this little guy dangling next to the engine between wire 3 and 5 on the drivers side. There is a hole there it goes into but obviously it is broken.

My question is, what is it? I brought it up into the daylight today and started the car. I didn't recognize it because it ran so smooth. I unplugged the connector from the broken piece and it started its old tricks again. Plugged it back in, the engine corrected itself.

I thought the cool air may have had something to do with it (its about 65 degrees today) so I tucked it back where it was but couldn't get the engine to act up. Maybe it needs to hang out down there for a while, I don't really know.

I've read it could be a knock sensor or maybe its an 02 sensor, I really don't know but once I find out what exactly it is I can get it replaced and have a reliable car again.

Any help would be greatly appreciated!

 

That looks to be what is left of the IAT (intake air temperature) sensor. It is supposed to be mounted into the intake duct work. If there is an open hole in the intake duct between the MAF sensor and the throttle body, it could explain the poor fuel mileage as the PCM is not getting a full air flow signal from the MAF sensor.

 

No, it definitely came out around the spark plugs right around the steering column.

 

Ok, I went more off of the picture than the description of the location.

The ECT (engine coolant temperature) sensor is located between the spark plugs for cylinders 3 & 5. That can also cause excessive fuel consumption as the PCM basically doesn't ever see the engine coming up to proper operating temperature. THIS THREAD has an illustration showing the ECT location.

 

It's definitely the right shape per the stock photo on autozone. It screws in. Gotta get the rest of it out of there now.

Would that cause the low/rough idle as well?

 

I was having the exact same trouble as you, and throwing p0300. It ended up being the ignition coil along with bad plug wires. As far as the ECT causing low/rough idle...i've been told it does.