IDLING ISSUE
 

Ok guys im stumped right now on this one.:dunno: I have a 03 4.6is with just over 280,000. The vehicle fires up and runs at about 1000 rpm but once the secondary air pump shuts off the rpms drop down to 300 rpms. Itll run forever sitting still but once you give any gas and release off petal the rpms drop to 0 and no longer running. So far i replaced a few things and did a some routine maintenance on it. I did valve covers, valley pan, oil separator and hoses, CCV, replaced cracked vacuum lines, MAF, spark plugs, and lastly the throttle body. The engine runs very smooth and I do most of the work on it myself but this time I feel a little defeated lol so looking for some insight. Any suggestions???

Have you checked for error codes?

How did you determine that the items needed to be replaced? Was the car throwing codes for them? You really need to find out what codes are thrown currently otherwise you will be replacing stuff blindly and will be a total guesswork.

Good luck.

Yeah I checked for codes and weirdly and unfortunately nothing showed up. Im 2nd owner of this vehicle bought with 30,000 miles on it. The MAF and throttle body are original and having just having over 280,000 I figured those parts did their job and good time to replace. Could a oxygen sensor cause this? But that definitely throw a code I would think.

can you increase the rpms and it runs fine if for instance if you hold the throttle at 25-3000 ?rpms

Yeah I can step on the gas and run the rpms as high as I want and runs perfect until I let off the gas then the rpms drop to zero and engine dies.

Seems like an Air to Fuel ratio fault....and you've obviously replaced/checked those parts.

Try and diagnose O2 sensors and catalytic converters.......and lastly thermostat.

I think that's the next place ill start is o2 sensor. Maybe I got a lovely electrical gremlin and its not throwing a code for the o2 sensor lol

Yeah, do live readings while the engine is running. I've used INPA/ISTA or DIS. They do a pretty good job for that. You might notice something odd - observe every data point including coolant temp, etc.

All the best!

When it is in closed loop can you tell by exhaust if one bank is running smooth and the other is not?