I have been researching your problem and found the following things;
There is a coolant sensor called a CHT, acronym. Don't know what it stands for, that can fail and cause idle problems.
The Ranger does not have a seperate FIT. It is part of the TPS and IACV working with the CHT.
Your truck has a reputation for motor mounts that get loose and cause a rough and/or shaky idle.
Since your truck is dropping in idle after warm, the."FIT Circuit" or choke circuit is working. That also means that the temp sensor is probably fine.
Since you reaplaced IACV, I found that a lot of Ranger owners helped idle issues by cleaning intake by either spraying cleaner down the TB when running, which I do not reccomend, or using Sea Foam or taking the TB and Intake apart and cleaning it.
How dirty were the small ports, or any ports that looked like they would connect with intake tract, as far as deposits?
From what I found you might check your motor mount with the standard, "Give it some gas with the hood open and the parking brake on", to see if motor moves more than it should.
That and the cleaning thing are the best options you have as the next moves to make. I don't think there is really anything, "wrong", other than possibly the motor mounts, with the motor or its idle unless its something we covered.
Those are all things you can take your time with. However, I would check into the motor mounts as that could cause you some problems.
If you find they are bad, check into Energy Suspension or other Ploy Eurethane motor mount inserts. They go into the motor mount and fill the gaps around the rubber to stiffent the mount. Some people say it is good enough to even repair a motor mount.
By the way, do you have the V-6 or the 4 cyclinder? Of course the V-6 is a little more prone to quicker mount failure but the mounts still fail on the 4 cylinder as rubber just does not hold up well with time and constant load shifting.
I hope the information helps. Definatly let me know if the mounts are bad as I always like to know if certain vehicles have issues with certain parts. The Poly inserts are not so hard that they will hurt anythng or feel rough, so don't worry about that. You can get shifter and suspension bushing replacements, practically all bushings, which I have done on my car and it does not make anything harsh or vibrate or add too much load transfer as they still give under load. I think sticking with just the mount inserts is fine but I heard a lot of the four wheel guys like them for the decrease in body roll. Especially if you have anti-roll bars and get the bushings for those.
Let me know how things pan out. I would not mess with the O2 sensor as of yet. But maybe plan on replacing it at 100,000+.
The mounts and cleaning intake really seem to be the most likely leads givent what I found and all that you have told me.
Tuesday, September 20th, 2011 AT 3:59 AM