I have a 1998 Ranger 4x2 3.0L. For the last 6 years, I've had a problem with the IAC getting stuck and the engine revving fast. I would just open the hood, tap it with a 13/16 wrench and it would unstick, I guess, and idle down. I finally replaced the IAC. It continued to have the problem. It would be running fine, out of nowhere, it starts idling really fast. I got another IAC. Same thing. By the way, it always seems to do it when I drive to a specific gas station. So, I just continued to deal with the situation by tapping the valve, or, just waiting for it to go back to normal after stopping and restarting the engine. This week, I had the valve cover gaskets replaced, plenum gasket replaced, new plugs, pcv valve and fuel system cleaned. Runs like a top. I think they also did something to the idle, as I don't have the fan clutch, or ac compressor, don't know which, clicking on and off, constantly, messing with the idle. So, it's now idling fine. For now. It's been one day. I didn't specifically take it there for that. Mostly the valve cover gaskets. The tune up was just something I wanted. The check engine light comes on and off at times. In the fall, it was off for 3 months after mostly being on for two years. It's still on. I'm hoping maybe some of the work they did will stop the idling problems. The codes I got back earlier, before I had work done.

P0153 oxygen sensor circuit slow response (bank 2 sensor 1)
P0171 system too lean (bank 1)
P0174 DTC description not found
P0500 vehicle speed sensor circuit
They gave me all kinds of possibilities, but it's $99 to get a full diagnostic test. If anyone has experienced this and fixed it, I would like to hear about your experience. Thank you

 

Welcome to the forum

You should always look up codes on a full code list never "google" a code until you do that, list here: https://www.therangerstation.com/tec...II_codes.shtml
Bookmark it

The reason for this is that the computer is not "smart", it just uses on/off logic and "picks" a code that fits the best on/off logic, so the codes it does NOT USE are as important as the one it did use
You will see that there are MANY codes for one sensor, each means something slightly different, but lucky for us they are often grouped together, lol

P0153 oxygen sensor circuit slow response (bank 2 sensor 1)
P0171 system too lean (bank 1)
P0174 system too lean (bank 2)

On a V6(or V8) engine there are 2 "banks", passenger side is bank 1, drivers side is bank 2
On a 4cyl there is just bank 1

Gasoline engines use 14.7:1 air:fuel ratio
The computer "knows" its running a 3 LITER engine, so it knows how much air should be coming in at any RPM, it's just math
Then it uses throttle position(TPS) and MAF(mass air flow) sensor to adjust that air flow measurement
Computer then calculates how much gasoline to add to that air using the 14.7:1 ratio, this calculation is the length of time it needs to open each injector
After this gasoline is added the engine burns it and exhaust comes out, the upstream O2 sensors read the AIR(Oxygen) left in the exhaust, too much Oxygen means Lean burn, too little Oxygen means Rich burn
Computer then adjusts the OPEN TIME for each injector to get the O2 sensors to read the correct amount of Oxygen in exhaust
This OPEN TIME is call Short Term Fuel Trims(STFT)

In the case of a Lean code that means the computers calculation was off by at least 15% low, so computer is having to open the injectors 15% longer than calculated, STFT +15% or higher
This means something is wrong with the calculations
So computer sets the Lean codes and turns on CEL to notify driver
At no time was the engine actually running Lean, well maybe 1 or 2 seconds, lol, this all happens very fast in real time

O2 sensors have a lifespan
100k miles or 10 years, they use chemicals to "see" Oxygen and they run out of chemicals, they get used up
And they show LEAN as they run out of chemicals
However BOTH upstream O2 sensors going lean at the same time are a real long shot, so doubtful in your case, UNLESS you have had 1 lean code for a year or so and just got the other one now


If BOTH sides of a V6 engine are showing Lean codes(like yours), 95% of the time its a vacuum leak, a larger air leak in upper intake, because it effects BOTH sides
4.99% can be MAF sensor or low fuel pressure, low fuel pressure would effect BOTH sides of an engine
0.1% computer problem, no way to test a Ford computer, you just exhaust all other possibilities, and then swap out computer

P0500 means computer is not getting a speed signal, in a 1998 Ranger that comes from the GEM module behind the radio, it sends out speed signal to speedometer, cruise and computer on ONE wire, just splits to each unit
So if speedometer is working then could be wiring issue or...........................computer issue


Just a heads up on IAC Valves
You can only use Ford(motorcraft) IAC Valve, OR Hitachi brand IAC Valve
Any other brand will give you problems
Rangers used a solenoid only type IAC Valve, other brands used a solenoid/STEPPER type valve, and they will work just not 100% of the time

You have a fan clutch in a 1998 3.0l, its a mechanical device and won't effect engine operation but can effect engine temperature

 

Thank you so much! I had trouble logging in, so, sorry for late response. This tells me so much, now. I have looked at a couple other forums. I wonder if my original equipment fuel filter is causing a problem? LOL! I went to have it changed, the guys said it was basically rusted on. Have to cut it off. The forum I read said that a dirty filter wouldn't cause a pressure drop, just restricted flow, which may throw a lean code. I have some other things on that thread I was looking at. Wondering if it's not a leak in that elbow they speak of, which causes a lot of P0171 and P0174 lean codes. The speed sensor you speak of, behind the radio, would that maybe be causing my once in awhile, idle problem? I was reading the code chart that you left a link to. Anyway, gonna get back to it, soon. It's running great. For 189,000 miles, I can't complain.

 

Yes, I used a Motorcraft IAC. Like I said, when it acts up, starts revving high, I tap it, and it stops. Since tune up, hasn't happened.