I have a 1991 pickup - 3.0 engine - 4WD - automatic tranny - extended cab. I have owned her since new and have about 120 K miles.

I have already had the head gasket problem addressed by my dealer.

Several years ago she started idling very rough when starting up after sitting for a while (overnight). It would idle rough for about 15 to 30 seconds then would smooth out. It would smooth out faster if I applied a little throttle.

My dealer diagnosed it as a dirty cold start injector. It was cleaned and seemed to fix the problem.

About 6 months ago the problem started again. This time I replaced the cold start injector. Worked great until about 2 weeks ago. Now the problems back and worse than ever.

Idles VERY rough on cold start after sitting for a little as a few hours, takes at least 30 seconds to smooth out but will still smooth out faster if throttle is applied.

I have also noticed a slightly rough idle when warmed up, in gear and sitting still.

Any help would be appreciated.

I am assuming you ruled out sparkplugs and sparkplug wires, along the distributor cap. All get old and tend to give poor cold starting. The O2 sensor will be going south by now if it has not been replaced too.

Now it really seems like an injector is leaking. I would take out the injector, place it on top of the engine (do this when cold), on a rag and run the fuel pump. Then check for drips. You can run the fuel pump by turning the ignition on, then remove the air filter cover and moving the MAF flapper by hand. You will hear the fuel pump turn on and hiss.

The one of the normal fuel injectors could also be dripping. After you are done driving, with a rag under it, crack the fitting on the cold start injector. This will bled off the fuel pressure. Watch it because it will spray. Close it back up and let the truck sit overnight. If it starts easy in the morning, then you have a leaking main injector. A shop like RC Engineering can rebuild them better then new for about $25 bucks a injector, but you have to take them out and ship them.

Sounds more like ISC control. Intermittent speed control, causes rough idle, and occasional misses at 45mph, or there abouts. Gets worse over time, and is part of the throttle body assembly.