Hey Guys,

I have a 99 f150 5.4L that will not stay running. It starts and initially revs up and then when it comes down it goes way down. It will idle at 300-400 rpm and barely run, if i touch the gas pedal it dies. The fuel pump is working, it has new spark plugs and i have tested all the injectors. I have checked all vacuum lines even the one behind the plenum. I'm thinking it might be the IAC valve but i unplugged it and the engine would automatically die. There are no engine codes being thrown. If anyone has any suggestions it would be greatly appreciated.

Thanks,

Daniel

 

just checked the IAC and its reading 10 ohms.

 

The IAC is what controls idle speed.
It is either stuck closed or faulty in some other way.
Remove it and use throttle body spray cleaner inside to be sure the pintle is free to move so it can control air bypass.
Be sure the TPS is attached into the throttle plate end slot.
Measuring the IAC is either open or shorted.
It has a diode across the winding that is polarity sensitive when testing with an ohm meter. One direction the test leads will show nearly a short (reading diode conduction). The other way will read the winding resistance (the diode acts as an open).
I would also spray clean the throttle plate area the full diameter.
if your work clears the problem, the PCM still has to relearn the cold start under load and hot start under loads as worst case to cover all possible starting conditions.
The A/C also has an effect on idle speed so the IAC has to respond to small changes in load to keep the engine idle speed near constant under various loads.
The fact that the engine idle goes high then comes down tells me the IAC pintle is moving even if slowly.
Good luck.

 

ok so i just removed the plenum again (for the 3rd time today) and cleaned everything with cleaner. I also removed the iac and cleaned it with WD-40. This iac had already been changed 60K miles ago. Put it all back on and had the same problems, here is the weird part.

Start the engine and it does its initial high rev and comes down.

Engine is now idling around 500 consistently

give it any gas and it dies.

Spray started fluid into air box and it will run, idle and rev perfectly for 2-3 minutes.

Then all of a sudden it seems like something clicks and back to running like crap. I have full fuel pressure at the rails.

anybody ever had this problems?

On a side note none of this was happening before i removed my supercharger. Due to high mileage i removed the kit to keep the truck from blowing up (178,000 miles) I removed the diablo chip, changed from 42LB injectors back to stock. removed everything having to do with the kit and installe dall the stock parts back on. Could something be wrong with the computer. I contacted my dyno tuner and he said just remove the chip and disconnect the battery and the computer will reset. The truck has not moved since i have removed the kit and was running fine before it came off.

 

Now you tell us what really has been done.
I have no idea what is going on now.
Here is what normally would happen:
The IAC has to be controlled by the PCM for amount of air to bypass for idle control.
Acceleration is detected by the crank sensor, the mass air meter, the TPS where fuel is added per the logic requirement decision by the PCM from these input signals.
Seem now removing the larger injectors has resulted in little fuel for the engine to run on for some reason.
The injectors are pulse with varied to obtain the fuel for all run times. The higher the rpm and load the longer the pulse width is (on time).
If you have all non stock programming removed and the PCM is now totally stock with all stock hardware there should not be this issue apart from relearning the stock running routine and rebuilding the operating tables
Good luck.

 

How long does it usually take for the PCM to relearn this?

Do i just let the truck idle for awhile, because there is now way it can drive.

 

You have something wrong.
It needs to be able to start and run and be drivable before the full relearn become effective just like it did when it was stock.
As stock if you remove battery it reboots the computer and clears all tables back to their stock configuration.
The motor starts on these tables then learns as you drive.
My bet is the PCM software program is now broken.
If so a dealer needs to reflash it by accessing Fords data base with your VIN number and down load a copy of the original program. Also if you have Pats system, two keys need to be programmed at the same time.
I can't help you any further.
Good luck.