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?

Exhaust seems fine when its running. I wouldnt be surprised if I had a bad cat though. One is original and then other about 5 years old

Bad 02 sensor would cause that one bank to run poorly and most of the time generate a code. Bad cat would have also result in symptoms on that one bank. Doesn't fit all the symptoms of a vacuum leak I would use propane or carb cleaner to check around the engine, especially the intake and check the air intake tube for a crack.

Can you monitor short and longterm fuel trim?

Which intake tube are you talking about? I should have mentioned I also replaced the intake tube off the throttle body because the vacuum hose that connects to it broke the nipple off the last time I pulled the hose off. Also when I did the valley pan I replaced intake manifold gaskets. Unfortunately the laptop I had dedicated to the bmw has crapped the bed so unable run the proper diagnosis on it.

Reference was intake tube connecting to throttle body that you have replaced.

Do I understand correctly that this problem started prior to the parts replacements you have done? If not, I suggest checking your work. The rule of thumb is there are no coincidences when working on a vehicle.

I suggest spraying carb cleaner, brake cleaner or spraying propane with the engine idling to see if you can locate a vacuum leak. Have you checked fuel pressure?

Yeah the problem started before I replaced any parts. I haven't checked fuel pressure yet. I have checked and tested all my fuel injectors before reinstalling. I will check fuel pressure and o2 sensors and see what comes up. I'll post the results afterwards. Thanks everyone so far for the tips. Since owning this vehicle I've done and replaced 90% of the repairs myself. Some jobs I've done a half dozen times. This one has me a lil stumped.