You are having cold start and idle problems, from what I get. Your engine needs a smidge more fuel when cold to run properly, until the engine gets warm enough to vaporize the fuel better. If the ECM doesn't know it is cold, it won't send the extra fuel. Check the resistance on the ECT and ACT sensors. They should change as the engine warms up. You might find the numbers on this site. Additionally, a leak that the ECM doesn't know about, such as a perforated tube connecting the air cleaner to the throttle body, would allow extra air that the MAF didn't signal to the ECM. That would lead to a lean condition, aggravated when the engine is cold. It would, probably, also lead to a higher than normal idle speed when warmed up.
The O2 sensors do not come into play until the engine has warmed up. They send a signal back to the ECM telling it to 'trim' the amount of fuel squirted from the injectors. They will cause a loss in MPG if they are dis-functional, as they will not be sending a signal, which is interpreted by the ECM as a 'lean' condition. The ECM will try to compensate for the lean condition by spritzing more fuel... thus worse MPG. Try to remember "low is lean" as that is what the O2 sends... from tenths of a volt to about 1.0 volts, with the higher number indicating 'rich". So if it croaks, it will send too much fuel, as it will see the 'low is lean' condition.
Humidity sensitivity generally points to ignition problems, as the water can act as a conductor when it is on plugs, wires, or other ignition parts. It will, however, give your engine more power, as the air charge inducted is more dense.
One last shot is to remove the throttle body from the upper intake, and squirt O2 safe carb cleaner all over the inside of the upper intake. You will be surprised how much gunk is inside the intake. I suspect that EGR and PCV stuff being thrown in there contribute a whole lot to it. I'd suggest that it needs to be cleaned when your idle becomes flaky when cold, as that is my experience. Cleaning it will also help you to pass emissions tests, as I found out when the vehicle failed its first test at age 12 months.
tom

 

Although people say there is no such thing as a stupid question, I think I have one...

I removed and cleaned the IAC Valve a few months ago when I was trying to fix this problem. It wasn't that dirty.

Is this the same procedure that you recommend? Do you suggest I clean more than just that valve? What do you mean when you say clean inside the upper intake? (I am kind of dumb when it comes to the lingo. I can do the work if I know what to do though!)

Thanks!

 

I am assuming that this is a 4 cylinder. They have a curved aluminum piece that goes from the horizontal above the cam cover to the vertical leading into the intake manifold. It makes a 90 degree turn. At the end closest to the air filter, the butterfly or throttle plate is mounted with 4 bolts. It can be removed after removing the air intake tube and the throttle linkage. It probably is covered with gunk on the inside, as is the upper intake. If your IAC is mounted on the upper intake, as most if not all late models are, make sure that the passage on the OUTER side of the throttle plate (air cleaner side) that leads to the IAC is clean also, as that it the passage that supplies air to the IAC.
Does this help?
tom

 

Yes. 4 cylinder.

Sort of...

Is the air intake tube the large hose that connects the air filter to the engine?

You mentioned to use "carb cleaner". Will throttle body cleaner work the same? Will I actually be spraying the cleaner into the engine itself to get the intake? Maybe I'll find a picture somewhere on the site...

How big is the butterfly/throttle plate?

Thanks for your patience!

 

What about a relay problem? FP or EEC relay?

I read another post that seemed to match my symptoms...

 

Hi All -

The truck acted up again this morning. (It was very humid and foggy again). The guy at NAPA gave me an interesting diagnosis. He told me to check the electrical connection to the fuel pump.

Would a dirty/corroded/loose connection cause the symptoms I've experienced?

It seems to make sense...but then again, so does every other suggestion! Any thoughts?

 

This acts like a bad Iac I have had them on other Fords that would have the same symptoms on a cool damp day. They warm up a little and work fine. If the hose between the maf and the throttle body is bad they don't run right at any time. (been there fought that one.) If it has a vacume leak the idle is usually high if it is a bad vacume leak it will miss on one or more clyinders and will idle poorly all the time.(if it is that bad you can usually hear the leak.) I think we all know that the idle on these is nothing more than a controlled vacume leak(via iac) around the throttle plate and that the EEC IV V systems will compensate for other vacume leaks up to a point. A which time it will post a lean code. If you will notice on a normal start up the engine idles higher for a few seconds and settles down this is because the iac is set at startup to be open a gven amount then when the system goes to closed loop it makes its adjustments.

 

Thanks.

I recently cleaned the IAC and it wasn't that dirty... How else should I go about testing your theory?

Also, I've never tested for any vacuum leaks. What is the best way to do that?

Thanks again!

 

I use a spray of some kind usually carb cleaner wd-40 works also and start spraying gasket areas vacume lines etc etc. And see if some spot changes the idle. When it changes you have found your leak. Here lately I have been using a modified stethscope with a piece of small copper tubing. connected to it and listening for the leaks and they really make a rackit when you come past them. It also doesn't make a mess on your engine.

 

I also had an iac on a 302 that would freeze shut (my theory) when it was cold and the engine would not idle until it warmed for a few minutes at which time it would then run fine. It too was cleaned (I cleaned it) several times. I finally replaced it with a new IAc and problem went away.