Cold stumble

OK, my 93 Sport (HO) 4.0, stumbles at about 3800 -4000 rpm until it's fully warmed up and at full temp for 5 -10 minutes. It didn't start doing it until I bored out my throttle body and moved my AIS sensor to the air box.

I've moved the AIS back, and tried different (and known good) AIS, TPS, O2 sensors. Checked all plugs, wires, cap & rotor, which only have about 5k on them.

I've reset the ECM, but don't have 50 start cycles on it yet. It also acts like it doesn't know where to idle when you first start it up. Bounces from about 700 to 1000 rpm for the first 10 seconds or so.

Once it's warmed up good, it runs fine, mileage is acceptable at about 15 - 16 mpg with good power. Plugs look a little lean, but don't have much time on them since moving the AIS.

Any suggestions would be welcome.:doh:

What about IAC?

What spark plugs are you using?

Have not changed IAC, but idles fine after about 10 -15 seconds. Plugs are Autolite Platinums. Like I say everything ran fine until I bored the throttle body and moved the AIS. When I bored the TB, it was only to take out the taper in the lower portion of the bore, and I made sure I didn't get into the where the butterfly seats against the wall. I thought is acted like an O2 sensor that the heating circuit has failed because once warmed up really well, it runs fine, but I tried a spare one that I had with the same result. It was one I took out and replaced with a new one, but when I did, it ran no differently, so I think it's still good. This was before I started having problems and it ran well.

Mine was running rough at start up for a month or so and after checking everything multiple times I took my Autolite Platinums out and looked inside the threaded part at the porcelin and found two that had cracked and burned through the porcelin and had been arcing inside. A new set of NGKs and it is running like a champ again.

I originally thought it was a bad o2 because it ran exactly like it did when it was bad. 20 seconds of rough running then it was fine.

Check your coolant temperature sensor connector for grime & clean it. Make sure it is in spec electrically.

Also -
did your cold idle get rough immediately after you bored the TB or just a little while after or what?

Right after I bored it, but I was playing with the IAC just a little bit ago, and I can unplug the connector from it right after it's started and there's no change in idle. I have another I'll throw on to see if that fixes the funky idle. The coolant sensor might be the issue at the 3800 - 4000 rpm problem until it gets warmed up.

I pulled the plugs yesterday and eyeballed them, but I didn't check for cracked porcelain, just was checking color and such, I'll pull them again.

It might be coincidence that this started after boring the TB.

Whatever it is, it's kind of getting worse. If you give it too much throttle and get into the 4k rpm range, it'll stumble really bad and even slightly backfire.

Seems more like plugs or an ignition problem. Still goes away after really warmed up though.

These engines do not like platinum plugs.

That's good to know. I'll get some NGKs tomorrow.