My 98 ram 1500 3.9 has started idling rough expecially noticeable when the engines cold, so far I've replaced to plugs, wires, ignition coil, distributer cap and rotor, idle air control valve, throttle position sensor , sea foamed the motor and cleaned the throttle body, I checked for vacuum leaks and didn't find any, I only run mid grade fuel in it, it got better when I replaced said parts but still is doing it, am I missing something stupid? I'm not throwing any codes so that's no help, It also just started spiking the temp gauge on start up then goes down to normal, I'm unsure if this is related or not.

 

I don't see distributor cap and rotor on that list.

 

My mistake, they've been replaced as well.

 

Ooops.

Time for a compression test methinks.

 

Even with 104,000 miles? If it's a compression problem with that few of miles on it me thinks it'll be my last dodge truck after my 03 blew a headgasket with out getting anywhere close to over heated.

 

Spiking the temp gauge? (Just noticed that part.....) is it just a momentary thing while you are cranking the engine? Or does the gauge actually stay over on the right for a little while, before coming back down?

 

It stays over the 260 degree mark roughly 10-15 seconds long enough to set off the check gauges dummy light and then goes to normal and shows actual temp.

 

Does that rough idle come in about a minute after starting the engine cold, then get progressively less ugly as the engine warms up? If so, try disconnecting the front oxygen sensor to see if that symptom goes away. If it does, replace that front sensor.

 

That's exactly what it does, wouldn't an 02 sensor throw an engine light though?

 

nope not always :P unplug the front sensor and see what happens, check engine light will come on of course due to this.