My '89 TBI engine has been running great. It passed smog a month ago with great numbers. The engine has 164K miles on it and only burns 1 quart of oil every 3000 miles. It had been 30K miles since the last tune-up, so I decided it was time to do one. Yesterday, I put in new Bosch platinum plugs, plus a new cap and rotor. With the plugs out I did a compression test and all cylinders were 145 to 155. I didn't replace the wires, but during the cap swap, I decided I would do that soon. I also tightened up the bolts that hold the TBI unit to the manifold, and snugged up the torx screws in the TBI. Most of them were fine and I was careful to not overtighten anything. When I was done with everything, the truck started right up, and when I drove it, I could immediately feel the results of the fresh tune-up. I drove about 2 miles to a freeway on-ramp and then floored it. It accelerated about 3 seconds, stumbled once, with a backfire thru the TBI, and then accelerated like normal, thru all the gears, up to 75 mph, where I backed off due to traffic. It was running strong but I was wondering why it stumbled. I stopped for gas and then drove home. It ran like normal. When cold, the engine will usually start with just a tap of the key, but this morning it fired once and then required cranking for about 15 seconds before it began to start. When it was starting it ran rough, like it was flooded, running on a few cylinders in the beginning, and then adding more as the engine cleared. When it was running on all 8, I started driving and everything seemed normal. I drove about 15 freeway miles at 65 mph with no problems. I then exited one freeway to get on another, and when I accelerated, it started to stumbling again, backfiring thru the TBI. It cleared and I continued on, but after a mile or so, it stumbled again and it was clearly getting worse. I decided to take the next exit to get to a gas station so I could check things out. As I got to the end of the exit and made a turn, it was barely running and I had no choice but to pull to the side of the road. As I came to a stop, it was even sounding kinda like it was dieseling . The engine temp when I exited the freeway was a normal 190F. Now, stopped on the side of the road, I made multiple attempts to re-start it but it would just stumble and backfire thru the TBI. It would kinda start on a few cylinders, but it would never clear the other cylinders enough to run on it's own. Eventually it wouldn't fire on any cylinders. I checked for crossed wires but they were all correct. When a stranger stopped (in a Blaser S-10), I had him crank it while I looked into the TBI. I saw good injector spray patterns and ruled out a clogged fuel filter or fuel line (I may check that again). I then checked for disconnected vacuum lines, plugs on sensors, anything and everything I could find, but everything looked normal. I double checked everything I could have bumped while doing the tune-up. I could find nothing wrong, so at this point I had it towed home. Tonight I bought new plug wires and swapped them in, but they made no difference and it still doesn't start. The starter spins it good but it doesn't even fire. My next move is to replace the coil since it's the original. If a new coil doesn't fix it, I'll check for codes and see if anything is there (even though the check light never came on). If that doesn't accomplish anything, I guess I'll be looking for a bad ignition module, EGR, or maybe a plugged cat converter. What could cause the catalytic converter to suddenly plug or go bad? I'm running out of ideas and may be missing something. Have any of you had this happen, or do you know what is causing this problem? I could use some fresh ideas. Thanks in advance. BTW. I called my friend Rene (tRusty) during this adventure and he helped me think it thru. He also provided the firing order so I could check for crossed plug wires. Thanks Bro, you da man! Rod, (Savage1) also called to offer help. Thanks amigo! /forums/images/icons/cool.gif This problem is gonna teach me a lot. Here's what I have learned so far: 1. Get a fuel pressure guage. 2. Put my repair manuals in the Blazer, permanantly! 3. Carry more tools and spare parts. 4. Add towing to my insurance. (the tow cost me $125 today) 5. Cell phones are great! Today would have been a total nightmare without a cell phone.