I have a 98 Ranger 4x2 Automatic 2.5L with 179,000 miles. Recently it has started to stumble whenever I start it up and when I back it up. It seems to smooth out when I take off and it's even smoothed out by the time I pull out of my neighborhood entrance. Then while I'm driving the same trip and stop again it idles nice and smooth like normal. At first I thought it was because the silicone on my Volant Air Intake air filter was cracked and air was escaping. So I bought one of the ghetto Autozone conical air filters as a replacement until I can get a new one from Volant. Well it didn't help much. So I figured maybe the Mass Air Flow Sensor has finally gotten too much filter oil shot into it. So I bought a remanufactured one since it was on sale. It actually helped performance but it still stumbles when starting and initially moving. I've run into similar situations before but it was much more dramatic and happened all the time when stopping and idling, but it turned out to be the coil packs. I've since replaced those with Accel EDIS coils and haven't had any problems. The Check Engine Light hasn't even flashed or come on. I hear no vacuum leaks when I have the engine idling. It's more dramatic with the A/C on. I was worried it might be the timing belt, but it's only got 60,000 miles on it. The original was changed at 120,000 miles and it was just because I wanted to change it. What are some other possibilites? The cat? Injectors? I always add fuel injector cleaner every time I change my oil, maintenance is done exactly on time. It is time for some more spark plugs, it's almost been 60,000 miles since the last time.

 

I would clean and or replace the idle air bypass valve. They can get dirty and sticky. It's worth a shot because it's free. Your engine basically idles with a closed throttle blade and relies on the bypass valve for idle variances, especially when the ac cyles on. You can also drill a small hole in the throttle blade but I would try the valve first.

Also, you can adjust the idle on most ford throttle bodies despite what the underhood stickers say by doing the following:

- disconnect the idle bypass valve
- run throttle stop screw in until throttle slightly opens
- start engine and adjust screw to desired rpm
- shut engine down
- disconnect battery to reset computer
- plug idle bypass valve back in and restart truck

Now, the idle is hard set and the bypass valve is just handling variances in idle to keep it smooth and allow for ac cycling etc....

 

There was a hole pre-drilled in my butterfly valve to allow air to pass and the engine to idle (albeit roughly) if the IAC bites the bullet.

Why don't you go down to your local Autozone and have the codes pulled. They'll usually do it for free. It may well have left a code for us to run with. Just bring back all the codes and let us know what they are. That'll give us a general idea of what's going on.

Look at your owner's manual and see how much the spark plugs should have left on them if anything. Might be time for a replacement. Wires as well, if you've never done them.

RP
Zach