'94 4.0L Cold Rough Idle

[BHutson]

I have a 1994 4.0L Manual Trans.

When I start the truck (it fires right up every time) it idles fine for about 30 seconds. The check engine light comes on and it starts having a rough idle. While driving there is a lack in power and I can hear a difference in exhaust sound. Once I drive it (between 2-10 miles) all of the sudden it seems to pick power back and runs/sounds like normal.

Before checking the codes I replaced plugs and wires (it has 180K so I figured it wouldn't hurt). The plugs were fouled and that helped, but it still acts up.

I had the codes read and it says Mass Air Flow (I don't remember code number). I cleaned the MAF with MAF Cleaner and that helped, but didn't fix. I then replaced the MAF and it still is doing the same thing.

What should I look into next?

 

[Earl43P]

Welcome to The Ranger Station.

It's always nice to get the codes # when the CEL comes on.
This link to our Tech Library will give you a method to retrieve the Check Engine Light Codes to help your diagnosis: http://www.therangerstation.com/tech_library/OBD_I.shtml

Your symptoms remind me of a similar problem that I had with a 99 Explorer with the 4.0 SOHC. It would start and the idle would hunt around, the CEL would come on and it would run "OK" after a few miles until the next cold start. Mine was the Intake Manifold Lower gaskets (O-rings really). Once it heated up, the intake leak would seal!
Here is a link to help you with that job, if your diagnosis points to them (lean codes 171/174): http://www.therangerstation.com/Magazine/January2011/gaskets.shtml


http://www.therangerstation.com/tech_library/4_0-loose-intake-bolts.shtml
http://www.therangerstation.com/Magazine/Summer2003/VacuumLeaks.shtml

Good luck with your truck!