1990 w250 erratic idle

I have a 1990 w250 4x4 with a throttle body 318 that is being a little cranky. In the last couple weeks it hasn't wanted to idle at cold start. Starts like a carbed engine with no choke. I have to give it a little gas to keep it idling. It runs ok down the road but when you come to a stop sometimes it idles ok but other times it idles way to fast. I gave it a quick look the other day and all the vacuum lines look ok. Any body have any ideas what might cause this kind of problem?

Coolant temp sender could be culprit. It could be commanding too much fuel all the time, especially when it's cold. If it smells rich, that could be it. EGR is also suspect on these. Usually you can take that off and clean it up with some carb cleaner. I would verify the vacuum lines with some carb cleaner also.

I did check it for vacuum leaks and couldn't find anything with a can of carb cleaner. On a side note in the last day or so it has developed a surging idle. All this seems to happen when the engine is cold. Once she warms up it seems to idle ok.

I second the coolant temp sensor, playing arm-chair mechanic while fighting off a turkey induced coma......

Idle speed motor could be getting squirrely on you. One test is to have truck idling at warmed up temperature, then shut it off and watch what action the gas pedal takes. It should raise up immediately when you shut engine off, then once engine stops, it should go back down a bit all by itself thus opening throttle for next cold start. Mine went bad and was making alot of noise as a warning for a few days.

Two things that are common with those year dodge trucks to look for. First they are known for plenum gasket, under the intake, leaks. You can check this by using a vacuum gauge. Pull and plug the PVC hose to intake nipple and plug your gauge to the crank case hose end, where the crankcase vent usually attaches. Start the engine. You should only have a 1 or 2hg of positive pressure(if I remember correctly). This would confirm no plenum gasket issue. If it shows a significantly different reading, you have an issue. There is a nice video on YouTube about this. My next guess and likely your culprit is a gummed up/ carboned up IAC. They get dirty and this causes the throttle blade not to open and close properly causing the computer to try to compensate with adding fuel. This occurs at first start up when the truck is in closed loop or warm up mode as it has not gotten hot enough to start using input from the O2 sensors at that point and likely the reason it does not throw any codes. Clean or swap out the IAC motor. I think I paid $30 for mine. Took 15 mins to replace using a #25 torx bit on a small ratchet while intake was still on the car. Solved my problem, might solve yours too. Good luck.

You also said you checked for vacuum leaks. I would physically check all the hoses and connections. Mine had a couple frayed ends. I also did a smoke test after to confirm no leaks.