Won't stay running

[rex450se]

I have a 1998 with the 5.4L that has been running great until today. It ran fine this morning and then this afternoon it started and then died. Since then, it will start and then die. You can keep your foot on the gas to keep the rpms up but as soon as you let off it will die. I checked the vacuum assembly from the PCV valve. I think it may be the IAC valve or the MAF. To check the MAF I removed the one off of my 2003 4.6L and it did not fix the problem. But, though they have the same plug, they are not identical so I don't know if that proved anything or not. I don't want to purchase parts that I don't need to so are there any other ways to try to test these items to narrow it down? Are there any other suggestions? Thanks in advance.

Victor

 

[gaping46and2]

Is it throwing any codes? Mine had the same symptoms and I sprayed some carb cleaner through the throttle body and it helped for a while, but then it threw a code related to the IAC. I'm now in the process of cleaning the entire upper intake manifold elbow including the IAC. It's really nasty and gunky.

 

[rex450se]

It is not throwing any codes.

 

[qcksnake]

If it will run by holding the pedal open its certainly the IAC. you can remove it and try cleaning it. Most likely just need one. If you had acces to a scan tool you can log IAC duty cycle and it will tell you if its trying to add air(which you already know it isnt if you are opening the tb blades manually)

Or you could use an SCT xcal to do it if you have one. Those of us that use this stuff to tune with have to fiddle with this stuff all the time working with big camshafts getting them to idle. Or large modified intakes, CAI, big throttle bodies etc.

Only other thing it could possibly be is if the TPS took a dump.

Take a voltage meter and check your tps. SHould be at .95-1.04 volts key on engine off, foot off accelerator.

 

[rex450se]

It was the Idle Air Control valve. About $45 and 10 minutes and it's running as good as new. Thanks

Victor