New here, tried searching old threads to no avail. 02 blazer, 4.3 L vortec. Over 200k miles. Had rough idle and hesitation. Did tune up (plugs, wires, cap & rotor, PCV). Ran better but fuel economy dropped. Replaced oil & fuel regulators as got a code for it. Started to diesel at stop lights. Had to keep 1 foot on gas & other on brake. Would suddenly accelerate on its own. Changed TPS. Then wouldn't start. Changed IAC. Ran but badly. Idle was all over, no power while driving. Strong gas fumes in exhaust. Changed MAP & MAF. Ran good for 2 weeks. Started losing power again & mpg dropped. Then hard time shifting. Found cat was clogged. Removed it, replace with straight pipe. Runs great but gas in exhaust worse along with mpg. Started to stumble & hesitate like it was chained down to something, but after over 2k rpm runs fine until on brake, wants to die. Idled great until in gear. Now dies once in gear. If stays running dies once off throttle. No EGR on this model. I've got no clue as to what else to try. Only code is random misfire & exhaust gas something, which I looked up could be gas cap to exhaust leak. No vac leaks, hoses are good. Need help please.

 

I would do a compression test. Could be you have burnt exhaust valve(s) that is/are allowing raw fuel into exhaust. That could have been what cooked your cat. There are several other issues it could be. But that's where I would start. Compression test will also identify any other base engine problems that could be your issue.

I would also do an exhaust back pressure test to see if possibly your exhaust beyond where the cat was clogged. You can get the loaner tool from an auto store. Pressure should not exceed 1 psi at idle, 2 psi at 2500 rpm.

Next I would do the fuel pressure/ leakdown test linked in my signature block below to see if there is indication of a leaking injector.

You might want to consider investing in a scan tool that will read and display live data. You can get a ODBII connector dongle fairly cheaply off ebay that will connect to your phone via downloadable software. Search the forum to learn more on that.

 

 

He did replace his cat with a straight pipe. Which I do not recommend at all, but that would take away the clogged cat possibility. Thought the exhaust could be clogged further downstream, in which case I suggested a back pressure test to determine one way or another so he is not replacing parts needlessly.