Van stutters @ low rpm’s. Replaced literally everything except computer, manifold gasket & injectors. All emission related parts are new, air flow sensors, coils plugs O2 Senors ….etc pvc…everything…. Runs great for a day and goes right back to running lean and stutter at low rpm. I also took out the cats. Any ideas would be greatly appreciated! Codes say runs lean.

 

You changed practically everything, should have gone one step further and just change out the engine.

You provided very little information to go on.

Did you check and verify you have 30-45 psi on the fuel rail ? What exactly are the codes you are reading?

 

Fuel system operation is dependent on input from the O2 sensors after the cats.
Attempting to by-pass components creates more problems than it solves.

 

At this time don’t have the extra 7-10,000 for a new 5.4. The motor has 405,000 miles @ belonged to a diaper delivery service. Motor was was supposedly rebuilt 100,000 miles ago and it looks and runs like it, besides the stutter. Specifically I have no idea what that means other than the stutter the motor is strong. The codes are like I said P0171. Had a smoke vacuum test at the shop said they could find no leaks. I will check the fuel rail psi, I didn’t check it because it only happens at low rpm and when pulling a hill. When I purchased the van the cats were melted and plugged solid. Again didn’t have an extra 5,000 for 3 new cats. Wasn’t trying to bypass just trying to find out the problem before repairing them.

 

Would you know if anyone checked the throttle position sensor?
It can be checked with an analog meter, but a scope check would be more reliable.

As mentioned earlier, fuel system performance uses info from the after cat O2 sensor..
The reason the cats melted should have been determined and corrected. That is usually caused by an engine running too rich for a long time.
A good scan tool would provide some fuel trim info which would help a lot.

Something you might consider, a salvage yard may have what you need and it would cost a lot less.
If air was getting into the engine and not getting measured, the computer only goes by the info it receives and if faulty, so is the response.Check for air getting into the throttle body that is not getting measured by the MAF sensor and again if the info is faulty the response could be slow.

 

Make sure your fuel pressure regulator is not ruptured, dumping excess fuel into its vacuum line, greatly richening up the fuel mix. Pull the vacuum line off the FPR and cycle the key a few times. Replace the regulator if fuel squirts out its vacuum fitting. The melted CATS are a symptom of a poorly running engine. Fix the engine first, then addresss the converters or the new ones will get eventually ruined.

 

I’m going to try all the above. Thank you so much guys for your time to respond to the thread! Greatly appreciated!