5.0l runs rough for 20 seconds

[Bronco2forlife]

5.0l runs fine other then cold start, after about 30 seconds it drops down into low idol and runs rough for about 15-20 seconds. After this is will correct itself and run fine, it only happens during a cold start. If I am driving and not parked it corrects itself faster. Have tried the coolant sensor already, wiring tested good as well. I have lived with this issue for a couple years.

 

[ba123]

Not really enough info there to help...is everything stock? Injectors stock? Drive it often or sitting mostly? Stock ECU?

No issues at all once warm, right?

Could be a sticky/dirty IAC valve. Could try removing it and cleaning it and the throttle body and see if it changes but just know if you start messing with something, you do risk making it worse until you figure it out.

 

[Bronco2forlife]

Its bone stock. No issue once warm, it is just the initial startup period after a cold start.

Goes into high idol for 1-2 minutes, drops down and runs rough for 20-30 seconds then reestablishes idol. Runs fine after that.

I'll check on the IAC.

 

[BroncoJimbo]

I feel your pain. I have a fuel injected Ranger 2.3L that does the exact same thing except it also does it when warm. It's very consistent but no one has been able to solve it.

 

[Broncobowsher]

I would be interested to see the fuel pressure during that episode.
If it is correct, there should be a couple PSI rise during the stumble (manifold vacuum drops, regulator regulates). If the pressure DROPS during the stumble, time to look at the fuel system.

Any codes?

If you leave the MAF unplugged, does it still happen?

 

[EPB72]

I too would look at fuel pressure ,, and look for codes,

But sounds also like when idle drops off this could also be when it transitions from open loop to closed loop, can you watch data? If obd2 should be able to see fuel trim , o2 data , ,injector pulse width , other data ,, but i'd be watching data and see if something isnt making sense.. If an older system data maybe less available,, ,, air leak,, PCV set up,, vaccum leak. ,, Keep alive memory issues unlikly, but could be trying to correct an underlying issue during every cold restart??? air charge temp sensor ? the older stuff they would get carboned up, used to clean them and help with drivabilty issus usually a surge but worth a look,,

 

[Galt72]

AI is your friend..lol

This 30-second delay on your 5.0L points directly to the transition when the engine management system leaves its initial startup mode and attempts to adjust its fuel and air mixtures as the 02 sensors start to heat up. [1, 2, 3, 4]
Common culprits for this specific cold-start stumble include:

• Mass Air Flow (MAF) Sensor: Carbon build-up or oil from an aftermarket air filter can contaminate the MAF sensor, making it misread the incoming air volume when the mixture is heavily dependent on it. [1, 2, 3, 4]
• Vacuum Leaks: Cracked hoses, aging intake manifold gaskets, or a failing PCV valve can leak unmetered air into the engine. This leans out the air-fuel mixture, causing the engine to sputter until the engine warms and the metal parts expand to seal the gap. [1, 2, 3, 4, 5]
• Throttle Body: Carbon deposits around the throttle plate restrict airflow at the lower idle positions, confusing the computer. [1, 2, 3, 4]

 

[rjrobin2002]

I put a new motorcraft IAC valve on my 96 5.0 f150 from rockauto that was doing the same thing with no codes and it fixed it. Verify fuel pressure like others have said.