I have a 1991 4x4 TBI with a rebuilt engine and plenty of new components under the hood. Also new fuel pump and sock. During cold starts it takes what seems like a bit of cranking than normal, but it runs and drives normal with fairly smooth idle. After warm up is where it presents the problem. The idle starts to get a little shaky almost like a misfire/lope and sometimes will stall. There is also a bit surge (if that's what you call it). Like it's trying to keep itself from dying. The motor will raise up as if it wants to take off for a second making you apply brakes just a touch harder at a stop. When it does stall it starts right up with less cranking like it should and runs down the road fine. Pull up to a light the problem comes back but not always with a stall.

I've checked for vacuum leaks.... found none. Checked for codes constantly even right after it happens.. none except for the usual 12.. I'm stumped!!!

 

Driving home from work today all was fine until about halfway (approximately 4-6 miles) coming to a stop I apply brakes and the engine immediately starts to stumble as if it was going to shutoff. I quickly put it in neutral to see if it would continue and try to pinpoint if it was transmission related. It stumbled a little less but didn't cut off. I still applied the brakes like normal.

Wish I could have been alone in traffic to see if I ease off of the brakes and relate it to the brake booster, but when I had put it in neutral kinda eliminates that theory. Almost as if the torque converter lock up solenoid is failing. But would that cause the erratic idle at a light even if it didn't shut down the engine? Getting really stumped! Possible there's several problems that exist I take it... Any help please....

 

Okay nobody has answered this thread for obvious reasons that this may have already been covered. After much refurbishing to the body and interior I decided to work again on the motor today.

After a pain staking valve lash adjustment (valve covers are a pita to get on with gaskets!!!!!) using a vacuum gauge to monitor progress afterwards I decided to recheck the timing. Vacuum was a bit better but still fluctuating 2-4" from 18" with an occasional 6-7" drop. Timing was way off by 4-6 degrees so I back it down to zero as best as possible using the normal procedure. After that idle was watched to see for any changes.... none. I still wanted it right so I left it at 0 deg and on to the next clue.

Lately when i did drive it (it's been sitting several months while working on it) I've been noticing a loud sucking noise coming from the breather. Take the breather off and it's coming from the IAC port. My buddy suggested that we replace it after noticing the tiny scrape mark on one side of the pintle hinting that the shaft is bent and maybe it's not working. I decided to test it by removing it. While still connected, I grounded the ALDL A & B with key on. Sure enough it extended all the way with no glitch so that killed the idea just simply replacing it. I also put an ohm meter on the terminals and got consistent readings well over 20 ohms. My next hunch that since it was working properly, but creating a large sucking noise it had to compensating for something. Checked RPMs and they were swinging +/- 30 from base setting. That's when I knew right off bat what it was compensating for. The one piece that had not been touched since the rebuild.

I told my buddy to watch the tach as I finely played with the throttle linkage. The IAC would slowly start to quiet down. Next I pulled out my drill and opened up the minimum air idle screw seal. I followed the proper procedure including the IAC seating. Cranked the motor back up and rpms was only 320....barely staying running a few seconds before shutting off! Increased the minimum to 550, then buttoned everything back up and disconnected the battery to erase any codes. Restarted the truck and man did she purr like never before.. Smooth idle, no more misses or stumble. Sucking noise was gone. I still have some minor things to touch like more plugs, and fine tuning of the minimum idle air screw, but I will drive it a bit to let the computer relearn itself. Other than that I am very elated!!