I'm having problems with my 94 ranger 4.0L with 95k miles.
Hopefully, you can help me out.

Symptoms:
Rough idle. Since I got this truck a year ago, the idle has always been a bit 'off.' It will start without a problem 99% of the time. The idle is very rough though, and in gear it stumbles. The computer seems to notice the problem and 'catch' it before I stall, but this leads to problems while sitting in traffic. The last time the computer 'caught' the problem, it caused the truck to lurch forward, even with my foot on the brake. This problem has gotten progressively worse over the past 2 weeks.

The check engine light works, but does not light up when I'm driving or when I have the rough idle.

I had the same problem last year during the winter, but my check engine light came on. I went to a shop and they diagnosed it as a lean air/fuel mixture. I changed the MAF sensor and O2 sensor, but it didn't completely solve the problem. Once the weather changed, the problem more or less cleared up, though the rough idle was still sometimes there.

What I've changed in an attempt to fix the problem:
-MAF sensor
-O2 sensor
-Air filter
-Fuel filter
-Plugs
-Plug wires
-Fuel pump relay
-Checked, double checked, and triple checked for vaccum line leaks

I'm at my wits end with this thing. Any advice will be much appreciated.

 

See if your year Ranger has the Idle Air Control (IAC). This seems to cause a ton of trouble, and controls and affects the idle. Its on the air intake plenum if the 94's had them. If you do, you can try cleaning the carbon and gunk deposits out of it with carb cleaner or replace it. NAPA has the best price at about $35. Get part number 229574.

 

I should have read all these other 'rough idle' posts before I went ahead with mine.

I'll give a look for that item and reply soon.

Any idea what it looks like? All those sensors look the same to me.

 

Ok I checked the IAC by unplugging it. The idle changed once it was unplugged, leading me to believe the IAC is working properly. When I plugged it back in, the idle jumped up a few hundred rpm and smoothed itself. I don't think the IAC is the problem.

There is a sensor right behind the IAC. When this was unplugged, there was no change of my rough idle, though my check engine light came on. During the rough idle, the check engine light will not turn on. I don't know if that sensor is working incorrectly. The vehicle runs pretty much the same with or without that sensor unplugged.

I also went ahead and changed a few vaccum lines hoping that I may have overlooked something. There was no difference after the vaccum line changes.

I went to a parts store today and was going to buy a new IAC anyway. The guy behind the counter asked me if the idle was surging or varying during idle. I told him it wasn't, so he advised me that he's 99% sure the IAC isn't the culprit.

Is there a way to check the coil pack while it's running to see if that is the problem? I'm really thinking there must be a fuel delivery problem or electrical (spark) problem.

I long for the days of carburated vehicles. It's SO much easier to troubleshoot them.

 

Try checking the lower intake bolts to see if they are loose. It is common for your vintage 4.0L.

 

The intake bolts weren't 'loose' but I was able to get a 1/2 turn on them without much effort. I didn't crank them down, just tightened them a little. You know the proper torque specs for the intake?

It SEEMS to have fixed the problem for the most part, though I still had one incident of a random stumble.

I also noticed that my temp gauge fluctuates between the 'n' in normal and straight up when driving. I haven't been able to find a consistent relationship between the stumble and the temp, so I don't know if that's where the bug lies.

The sensor next to the IAC on the uppen intake toward the cab- what's that called? I think it may have something to do with that sensor.

 

I think I've pinned down the problem as temperature related.

This morning (temp in mid 20's), after the truck was started, the temp was sitting on the 'm' in normal- where it should be.

After a 1 mile drive, I stopped at a light. The temp had dropped to the lowest part of the normal area. The stumble, coughing, and sputtering started. No check engine light came on, but I had to keep my foot on the gas to keep it from possibly stalling.

I drove about 3 more miles and then had to stop again. The temp gauge read at the 'n' in normal, and there wasn't any idle problem.

I changed the thermosthat last year because it was stuck open and sitting on cold. I don't know if the thermosthat continues to be the problem or not. What do you think? Is there a temp sensor that might be going?

 

open up the rad. cap adn run the truck that will get any air out of the systme and you gauge should level out other wise you could have a head gasket problem.