Ok, here is my latest problem:

On startup and only when it is very cold, it will fire, rev to about 1500rpm, and just die. You start it again and it does the same, and the same, and the same. If you just give it gas and hold it to about 2000rpms, it will finally catch and run fine. Sometimes it takes just a few seconds holding at 2000rpm until it catches the idle but sometimes it takes several minutes. It feels like I am trying to start an old 70s 351 carbed f150 in the middle of winter. Any ideas? I'm sick of two footing it(foot on gas, foot on brake) down the road every morning. Seems like I have a messed up sensor or vacuum leak but don't know where to start looking

Truck is a 99 f150 5.4, 4r70w. Has about 80k miles. Fuel filter was changed recently, new plugs, no check engine light (but haven't checked codes yet).


Thanks,
kev

 

I think you may be in need of a new IAT sensor. If it's bad it may be making your computer think it's warmer out than it is and not richening up the mixture enough to run nice.

 

What your missing is how the engine cold start stratigy works.
The ECT and ACT values are looked at as soon as the key is turned.
This tells the PCM how much fuel and air is needed.
The IAC is opened, the fuel injection goes rich and the PCM rasies the idle speed and ignition timing is set to some advance.
As soon as the Ox sensors are heated to about 650*f and begin to output, the system goes closed loop where the Ox sensors take over fuel control.
This is the point you see, after a couple minutes, the engine will begins to idle ok.
So with that said, the cold start hardware has a problem including possibly the IAC.
As a side note, the IAC is variable device to control air bypass around the throttle body plate and is actually "pulsed" by the PCM in order to control it's opening on a continious basis so near perfect air control is attained.

 

kev,

Start by cleaning the IAC, Idle Air Control, valve. On your engine it is located behind the throttle body elbow.
As the engine gets older oil deposits from the Crankcase ventilation system gets sucked into the IAC and coats it. Because the valve moves within a certain range during warm weather it cleans itself and functions quite well. When the season changes the range of valve movement changes because of the change of air density requires the PCM to adjust the volume of air entering the engine at idle. When this happens the valve is now trying to move over the area that has been clogged by the oil. It then gets stuck and the PCM can no longer properly adjust the amount of air entering the engine and it stalls or surges. Cleaning the valve should fix the problem. It is #10 in the picture below.

Regards

Jean Marc Chartier

 

I can't thank you guys enough. The explanations and graphic are excellent. I'll clean the IAC this weekend and let you know if it worked.

Thanks again,
kev

 

JMC,
Are you a Ford mechanic or engineer? Your knowledge and ability to express technical terminology in layman's terms is impressive!
Thank you for contributing to this website; I have been able to help other people because of your sharing with us.

Regards,
Curt

 

Curt,

None of the above. Thanks for the kind words. It is my hobby. I like improving things. To do that you need to know how things works. I took mechanics training but I never made a career of it. I don't like the stress....

JMC

 

Yeah JMC is very knowledgable on these trucks. He has helped me out several times prior to this issue as well. It helps when the f150 is your primary hobby though. Myself, I have two other project cars that are my main pieces of enjoyment. I love my truck but it is mainly for day to day commuting and occasional offroading. So I just try to keep it running good and only do modifications or research when parts wear out or break.

JMC,

I remember a year ago or so you were installing a remote locking front differential and lower gear ratios on your truck. Did you ever finish that? How does it perform now?


kev

 

I am still running the stock 3.55 gears. I am toying with going to a Turbo or a V10 so the gears are on hold. I came so close to swapping a V10 instead of the 5.4 that I did. The V10 motor deal fell through. The V10 was from a 2002 with less than 10K miles but it had sat outside in the weather and I would have had to tear it apart to be sure it was ok.

JMC

 

I have a similar idle problem, but not at all related to tempurature, more to humidity i think. It only seems to drop idle when it rains, or has just rained. I replaced the MAF, used 3 cans of TB Cleaner on the alum. parts of the intake system, cleaned the trottle plate front and back, no codes, no lights, just intermitant stubborness to idle. ANYONE?

2001 Screw 5.4

 

Next time the idle acts up tap the IAC with a screwdriver handle. If it fixes the idle you know it is the IAC.