Ok. so here is my problem. I go to start my truck up in the morning. Or any cold start. 2000 1500 4x4 5.2 it will, after a min or 2, start idleing down reaally low but not die. If i hit the gas a bit, it will idle super high for a sec, go back down to reg idle and then back to low low idle. When it warms up, it is fine. I've changed the idle air control valve and cleaned the throttle body. oh ya..also when the idle is low, i hear a loud hissing noise coming from the engine somewhere. Any ideas??

 

Maybe next time I should read the whole post before giving a suggestion... Sorry guys its the cold medicine I'm out of it...

 

You've just described the most common symptoms of a failing pre-cat oxygen sensor. Swap it out for either an NTK brand or a replacement from the dealership -- the rest are all garbage.

 

Thanks a lot. I will try that. Does it matter if i dont have cats on it? And also there are no codes coming up.

 

Even without the cats you've got to have the oxygen sensors, or at least the upstream sensor and a simulator for the downstream. Without the upstream sensor your PCM will stay in open loop mode all the time, eating fuel and not making the power it should. If you just unplug the downstream sensor the engine will run normally but you'll have a perpetual CEL.

Because simulators are hit and miss solutions, most folks either install the downstream with a couple of spark plug non-foulers so it doesn't see the full exhaust stream, or just hang the thing on the frame rail where it sees atmospheric air all the time. Either way, you head off the perpetual CEL.

In the vast majority of cases, there will be no CEL or codes set until long after you already know you've got a flaky oxygen sensor -- or at least know you've got the symptoms of it.

 

Would you consider Denso a good brand for O2s too? Or not for our trucks?


-JT

 

Dealership or NTK for our trucks. 'Nuff sed!

 

Welp. Thanks a lot. I really appreciate it. I'm gonna try it today and see. Keep ya Posted

 

Well nobody in Indianapolis has a NTK o2 sensor, so i ordered it and will be here tuesday. Can you explain more about the simulator? and spark plugs for the downstream sensor? and also are you saying that if i just unplug the downstream sensor, it will make my gas mileage better until tuesday?

 

It's an aftermarket black box that you plug into the harness where the downstream sensor ordinarily goes, that allegedly looks to the PCM like an oxygen sensor that's seeing a good catalytic converter ahead of it. Its entire purpose is to prevent the perpetual CEL that comes if you just unplug the downstream sensor.

I've never used one, but have heard that they sometimes don't work.

Not spark plugs, but one or two spark plug non-foulers. If you have a downstream oxygen sensor bung in your catless system, you'd install one of these non-foulers into it, then screw the downstream oxygen sensor into that. What this does is keep the probe of the sensor out of the exhaust stream so it indicates as it would if there were a functioning catalytic converter ahead of it. Sometimes one non-fouler doesn't quite do the trick, and a second one is required to get the sensor even further out of the exhaust stream.

This setup would be in place of a simulator. Alternatively, if you've got the downstream sensor, you can plug any downstream sensor bung you've got and tie the sensor to the frame rail with wire so it reads atmospheric air, which will also work just fine. You just have to be careful that the sensor end is securely positioned where it won't damage anything or start a fire, because the sensor has a heater in it that will bring it up to about 600 degrees.

Nope. The only purpose of the downstream sensor is to detect a failed catalytic converter. It is not used in engine management at all. Engine management uses the upstream sensor.

The upstream sensor you've got, unless the symptom has changed, is in fact working, though imperfectly, most of the time. Leave it in place until you get the new sensor installed, unless the symptom progresses to the point of disabling the vehicle. If it gets unbearably bad, unplug the sensor and you'll be able to run in open loop mode -- and chew through fuel like crazy.

What's happening is this: Oxygen sensors don't read correctly until the elements within them have reached some very high temperature, right around 600 degrees for the ones in our trucks. This is why we have an open loop mode programmed into our PCM's, and also why we have sensors with heaters in them. Before the heated sensor came along, the controllers in automobiles were in open loop for several minutes while they watched for the oxygen sensor signal to settle into what looked like a reasonable range; now that we have heated sensors, the programming is a bit different and time since electrical power application is factored in. If it's been long enough that the heater should have done its job, and the sensor isn't obviously totally broke, the PCM switches to closed loop.

The rub: A failing sensor doesn't heat as quickly as a new one, and requires a bit more heat to provide an accurate reading, too. So the time the PCM waits isn't quite long enough, and as a result it starts controlling the engine using a false reading from the sensor.

Between now and the time you get your new sensor installed, you can shorten the duration of the ugly spot by running the engine up to 1200-1500 RPM for around 30 seconds or so after the PCM goes closed loop and starts starving the engine. The increased exhaust flow will heat the sensor enough to make it work. But don't count on doing this for any length of time, because as the sensor continues to deteriorate it'll start falling out more and more often -- you'll find that the symptoms crop up when you're sitting at a traffic signal, or even when cruising lightly loaded.

Replacing the sensor will clean up your idle, reduce fuel consumption, and restore the performance that's been lost to the failing sensor. Once you've got a new, high quality part in there, you shouldn't have this problem again for right around 100,000 miles. If you've still got the truck under your butt in 85,000 miles or so, order up another sensor to keep on hand just in case, and to replace whether it seems to need it or not at 100,000.

These sensors tend to seize in the bung, so it wouldn't hurt to spray the threads of the old one with penetrating oil once a day between now and the time you go to replace the thing. Then use a spark plug thread chaser (NOT a tap!) to clean up the threads of the bung, and be sure that there's anti-seize compound on the threads of the new one before it goes in.