My 1991 K5 Blazer has a very bad surging problem. It surges especially bad when the engine is cold. When the engine is warm it doesn't surge as bad but you can still tell it is not running right. The exhaust smells rich. If you try to drive the truck while it's surging, it will buck violently and idle extremely low, it feels like it's going to stall out at any moment. If I unplug the coolant temperature sensor and try to run the truck, it seems to run a little better when the engine is warm. The previous owner tells me he replaced the coolant temperature sensor (doesn't look all that new) and wiring harness for CTS. O2 sensor (doesn't look all that new, plug wires (look new), fuel pump (verified new), throttle body base gasket (can't tell for sure if it's new), I.A.C. valve (doesn't look all that new), T.P.S. (doesn't really look new but I don't think this has anything to do with the problem). The transmission was rebuilt with a new torque converter. Any insight on this problem would be appreciated.

 

UPDATE: Got bored today, decided to further investigate the surging problem. Unplugged MAP sensor while truck was off. Started it up and surging was gone. Idled somewhat decent (but still running rich). Went in gear fine and drove alright. When I really gave it gas, it fell on it's face and wouldn't accelerate, it sounded like it was backfiring or missing, but I can't really tell because the muffler is rusted out. I'm thinking the falling flat on it's face is because the catalytic converter is clogged? Can someone help? Also, with the MAP sensor unplugged, the truck randomly stalled out of nowhere.
I drove the truck a bit with the MAP sensor unplugged today and the Blazer had no surging problem and it drove ok. I am thinking the cat is clogged because the truck has little power when giving it a lot of gas, and the transmission is shifting very oddly even though it has been recently rebuilt with a new torque converter. Any input from anyone?

 

Bump

 

Bump, replaced MAP sensor with GM part, same problem still exists.

 

ive had a truck with a extremely clogged cat, and it didnt run bad at all, it just had no power under acceleration, kinda like pulling a trailer all the time

not sure about your year but it might apply. we have a 94 pick up truck at work and it has two tempature sensors, one for the gauge and one for the computer. it was telling the truck it was like 10 degrees out all the time, causing it to run rich.

the one for the temp gauge should be on the side of the block, the computer controlled one should be at the front top of the engine

 

Replaced CTS with GM part and problem still exists. I came to a conclusion today that in fact, the truck IS NOT running rich, but it is running lean! The injectors look like their barely dribbling fuel while the truck is running. Also, the truck randomly stalled two times today, then took four seconds of cranking for it to start back up. The "rich" smell I thought was coming from the exhaust is the smell of a bad catalytic converter. Rotten eggs/sulfur. Looks like I will be replacing fuel pump, it sounds weak also. Finally, after I shut off the truck, I can still hear the fuel pump running for about 3 seconds and I hear fuel sloshing around in the tank even though the truck didn't move an inch.

 

For those of you who read this thread and have a truck with the same problem, I found out what was wrong with my Blazer. A weak fuel pump and the hose that runs from the fuel pump to the lines in the sending unit was broken and was the cause for low fuel pressure making the truck run poorly. Will post pics of the hose for reference.