Is there an emission cleanup sequence that occurs about 30 seconds after startup and lasts for about 30 seconds?

During that time, if I don't touch the throttle, it begins to run rough and I can hear a vacuum leak at my evap canister. Then shortly it will go away and run fine.

This is a '96 5.2.

Thanks

 

Mine acted up like that in cold weather when I had a dying battery.... But if your hearing something sounds like a vacuum leak

 

Pre-cat oxygen sensor, almost guaranteed. Get Denso or NTK brand -- dealership parts are Denso, I believe. Don't fool around with any others or you may very well be yet another who's sorry for doing so.

 

Is the pre-cat o2 referenced in OL?

 

Have you done anything to the truck just before this started?

You may want to disconnect battery for a few minutes and reconnect (reset PCM).

 

It seems that it's always performed this way. A buddy's 97 ram 5.2 does the same thing.

It only happens if I start it without touching the throttle. It seems that the evap vacuum valve near the fuel rail is opening up for this short time, then closing.

 

Mine does the same thing when it is cold, unless its cold enough that it high idles at 1000 for a while. I'm pretty sure it is the IAC making the noise (whistle), because after the noise I get a slight rpm increase. Quite frankly... i've seen plenty of threads over the years, one of which I started, and never found an answer. I've tossed plenty of parts at it, never changed a thing. It doesn't affect anything that I have found, it just seems like the truck wants to idle down to target idle faster than the truck can stably run, and then realizes it can't just yet. With the remote start I put in I don't notice it at all anymore :P

 

Ditto here.

 

"OL"??? Original Literature? Old Lady?

I should have qualified and clarified my earlier statement: If the temperature is above freezing, that stumbling, low idle with a big sucking noise is an indication of oxygen sensor failure. If the temperature is well below freezing, it might not be and probably isn't if the thing runs normally when cold started at above freezing temperatures.

What's going on: The first most important thing to remember is that from the PCM's perspective the engine's only purpose is to regulate the oxygen sensor's output. The second most important thing to remember is that oxygen sensors must reach 600 degrees Fahrenheit or thereabout to deliver accurate readings, somewhat higher than that as they age, and quite a bit higher than that at end of service life. If not run hot enough to become accurate, they indicate a false rich condition.

Stumbling on transition to closed loop is a sign that the oxygen sensor is not hot enough to read the exhaust gas composition correctly. The PCM thinks the engine is running a mite rich so it shortens up the injector pulses, resulting in a lean condition that necessarily lowers the engine speed. Seeing this, the PCM says "WTF?" and cracks open the IAC to compensate -- and after it goes far enough you can hear the air whistling through the IAC. If the oxygen sensor is far enough out of range, the PCM will crank the IAC to wide open and it still won't be enough to get normal idle speed. (The sensible thing to do in that case would be to go back to closed loop for a while, but the PCM isn't programmed that way.)

When it's quite cold out, well below freezing, the cold metal of the exhaust system sinks energy from the sensor heater and exhaust gas that would otherwise be available to heat the sensor to get it into its happy range before the transition to closed loop. So all of the above stuff happens. The above will also happen at above freezing temperatures if the sensor is winding down for retirement, but in this case because it must run hotter to provide accurate readings.

Older designs with two wire, unheated oxygen sensors don't suffer this malady. Their ECU's watch the sensor signal and wait for it to come into the normal range before transitioning to closed loop. And they also turn on the check engine light at 60,000 miles to convince you to change the sensor, too. Modern designs with four wire heated sensors instead (generally) use a timer, either X seconds after startup or X seconds after coolant temperature Y is reached, then go closed loop regardless of any obvious bogosity in the sensor output. The newer design reduces cold start emissions but is a bit more troublesome when the sensors are cold and/or are approaching end of service life.

So dat's da schkoop, based upon my work on a standalone mixture controller design I worked on for an outfit that does CNG conversions for municipalities, utilities, and a handful of large corporations like oil companies trying to clean up their public images. Oh, and my design worked fabulously, too.

 

UU, that makes sense. Thanks for the clarification.