I have a 98 Ranger XLT 4x4 4.0 and it idles a little rough in park (around 900-1100rpm) but when I reach a constant speed, be it 35 mph or 70 mph, it starts to get rough again, and a little jumpy. When I'm accelerating or decelerating it's fine, and when I'm just coasting there is no problem, but at speed it's rough. Any ideas on what could be causing it? I cleaned the MAF recently, but I still need to clean the IAC.

 

IAC only effects idle.

Idle with engine warmed up should be 750-800 on an automatic.

With engine warmed up unplug the IAC connector, idle should drop to 500rpms or it may stall, either is OK, it mean IAC was working.
If idle doesn't drop then you have a vacuum leak.

Vacuum leak causes Lean fuel/air mix at idle.
When accelerating computer adds more fuel so mix is richer, when cruising computer tries to save fuel so mixture gets lean again.

 

I'm a noob when it comes to these things. I thought it was idling high to begin with. So if it's the IAC, does it need to be cleaned or replaced? The truck has around 156000 miles. Are there any common places for vacuum leaks that cause idle issues? And if the computer controls the mixture at cruising speed, could it be a computer problem? Sorry for all the questions, I just want to get this stuff taken care of.

 

How old are the wires/plugs/coil pack ? Pop of one wire at a time and see just how different it runs. If you take one off and it doesn't seem to change how it runs then the coil is failing. Try a timing light on each wire, if one is dimmer then the other then the sprark from the coil isn't making it to the plug and the plug wire is allowing the current to escape to the nearest ground. Did you cross any wires???

 

The IAC Valve is a Control not a sensor, the computer uses the IAC Valve to raise or lower the idle to preset RPMs it has in memory, i.e. cold idle 1,000, warm idle 750(auto), 700(manual).

In the throttle body is an air passage that by-passes the throttle plate, the IAC valve is on that passage, by allowing more air to by-pass the throttle plate the idle will go up, less air and idle will go down.
So the IAC valve is a "controlled" vacuum leak, but this air does come thru the MAF sensor so computer sees increase in air flow as IAC valve opens.

A dirty IAC Valve will stick so idle won't change or will not change much.
A high idle could be a dirty IAC Valve, but it could also be an external vacuum leak, i.e. computer has closed IAC Valve all the way but idle is still 900rpms.

If idle is higher when cold and then gets lower as engine warms up I would think IAC and computer are working OK.

If you unplug the IAC Valve on warm engine and idle doesn't drop then that usually would mean it was already closed all the way so computer has done it's job, but can not set a lower idle because of a vacuum leak elsewhere.

PCV valve and it's hose is a common leak area, but you can find most vacuum lines on the intake's vacuum manifold; with IAC unplugged, remove them one at a time and plug intake opening with your finger, if idle drops then that line/hose has the leak.


Also does your CEL(check engine light) work?
It should come on when key is turned on and then go off after computer runs a self test.

 

As far as the wires go I'm fairly sure everything is fine. I didn't install them, but the wires look fairly new, so I assume they were put on not too long before I bought the truck. The check engine light wasn't even in the cluster when I took it out, I had to buy a new light to put in the socket. That's how I found out it was on. This weekend when I get the chance I'm going to check all the plugs and wires, plus check the IAC and look for a vacuum leak. When I had the code checked (I can't remember the number) it said something along the lines of evaporative canister something something. The gas cap is solid, so I have to work from there.

 

Also, when the RPM idle at cold is around 1500 when I first start it up, but it never goes below 900 when it's warm.

 

1,500 cold idle is high as well, unless your outside temp is well below freezing right now.

What does your dash board temp gauge show when engine warms up, about 5 minutes after starting?
Should be just below 1/2, 200degF/93c
If it's down at a 1/4 then thermostat is stuck open or the wrong one, and that will keep idle high.

 

The thermostat is a possibility. It takes a while to warm up and the operating temp drops almost back to cold after sitting for just a short amount of time.

 

I drive a lot as well and I've only reached operating temp maybe 5 times. So I'll definitely check the thermostat.

 

Use a 192degF or 195degF thermostat, get one with a "jiggle valve" if available, that's just a t-stat with a hole in the plate so air can get out, install t-stat with jiggle valve at 12:00.

It is often easier to get all the air out when refilling by removing one heater hose at the firewall, or if you have a by-pass temp control valve then remove one there, these are "high points" where air gets trapped.

 

I just drove around 30 miles at 55mph and the temp never went above 1/4 of the gauge. I got a 192degF thermostat tonight. I also got new plugs and wires, so hopefully I'll find out if anything of this helps by the time the weekend is over. Thanks again for all the advice everybody!

 

Yes, t-stat is stuck open, it's suppose to do that when it fails, stuck closed would, of course, cause over heating and possible engine damage.

After install of new t-stat engine should heat up to just below 1/2 within 5 minutes.

Another test if t-stat is closed or stuck open is to start cold engine and then feel the upper rad hose, it should stay cold, not heat up as engine heats up, when temp gauge in dash gets to just below 1/2, t-stat should open a bit and upper rad hose will start to heat up.

 

I changed the thermostat today and now the idle is lower, around 900, but it still sounds like it has a miss in it. I got new plugs and wires for tomorrow, so we'll see how that works out.

 

For anybody that is interested I made a short video to show you what the miss in my truck sounds like. The exhaust is where I hear it the most. The engine isn't as bad, but it sounds like I can hear a faint but audible miss under the hood as well.