1998 Chevy Blazer Idles smooth no power

With only 12hg of vacuum, either a mechanical engine problem, or more likly a restricted exhuast system, recheck the converter.

Whats the engine size in liters? And at this stage you should be doing a compression test of all cylinders, you may have a worn cam or weak valve springs.

While your at it test the TPS should be at these specs, operation smooth and no spikes as you move it!

Worn valve springs may cause idling problems, but a worn cam can run ok at idle but not under load.

12 hg of vacuum still says a breathing problem. Intake is ok no restricctions? Throttle OK?

Just for the hell of it, try removing the EGR, is the pintle OK, closing all the way? Carbon free? And the PCV?

Still sounds like a restricted exhaust, you said you replaced the cat? Was the bed inside broken up? May have a chunk lodged in the exhuast of muffler.
I'm still stuck on the 12hg vacuum! Throttle is working OK? Maybe a strange problem with a bypass hole being clogged on throttle plate?

I doubt the timing is too far off with compression of 180 per cyl. Air filter is clean? Try cleaning the MAF, they make MAF cleaner spray now, worth a shot. MAF's dont hardly ever set a code even when they go bad, try unlpugging it and going for a quick ride, this forces the PCM to use an average input.

Ok let us know what happens.

Could be this TSB appies to you.
SPECIAL POLICY ADJUSTMENT-SEQUENTIAL CENTRAL PORT FUEL INJECTION (SCPI) FAILURES IN CALIFORNIA ONLY (YF5 EMISSION EQUIPPED) SERVICE CAMPAIGN BULLETIN Reference Number(s): 99066F, Â Date of Issue: Â 04/01/2003 Superceded Bulletin(s): 99066E, Â Date of Issue: Â February, 2003 Related Ref Number(s): 99066A, 99066B, 99066C, 99066D, 99066E, 99066F ARTICLE BEGINNING CONDITION Some owners of 1996, 1997, 1998, 1999, 2000, 2001 and 2002 model year S/T, M/L, C/K, G, P and W4/NPR trucks and 2003 NPR trucks that are registered in California, equipped with 4.3L (RPO L35 and VIN Code W or RPO LF6 and VIN Code X), 5.0L (RPO L30 and VIN Code M) or 5.7L (RPO L31 and VIN Code R) engine, and California emissions (RPO YF5), may experience a Service Engine Soon light, misfire, rough idle or hard start due to a deposit build up on the Sequential Central Port Injector (SCPI) poppet valve(s). The deposit build up may cause injector poppets to stick closed. Fuels used in California have been found to interact with the SCPI system to cause the deposits. SPECIAL POLICY ADJUSTMENT This special policy covers the SCPI failure condition described above for a period of ten (10) years or 200,000 miles, whichever occurs first, from the date the vehicle was originally placed in service, regardless of ownership. The repairs will be made at no charge to the owner. This special policy applies ONLY to repairs requiring SCPI system servicing and/or injector cleaning and/or MFI assembly replacement of the SCPI system. The customer should not be charged for performing a system check when it is determined that the SCPI system is not the cause of a customer complaint (labor operation T5532 is provided to submit claims for such system checks). Any additional necessary diagnosis and repairs CHEVROLET: 1996 - 2002 Certain S/T, M/L, C/K, G, P Trucks Equipped with 4.3L (RPO L35-VIN Code W or RPO LF6-VIN Code X), 5.0L (RPO L30-VIN Code M) or 5.7L (RPO L31-VIN Code R) Engine and California Emission Equipped (RPO YF5) GMC: 1996 - 2002 Certain S/T, M/L, C/K, G, P Trucks Equipped with 4.3L (RPO L35-VIN Code W or RPO LF6-VIN Code X), 5.0L (RPO L30-VIN Code M) or 5.7L (RPO L31-VIN Code R) Engine and California Emission Equipped (RPO YF5) ISUZU: 1996 - 2002 Certain W4/NPR Trucks and 2003 NPR Trucks Equipped with 4.3L (RPO L35-VIN Code W or RPO LF6-VIN Code X), 5.0L (RPO L30-VIN Code M) or 5.7L (RPO L31-VIN Code R) Engine and California Emission Equipped (RPO YF5) SECTION: Special Policy Bulletins NOTE: This bulletin is being revised to add the 2002 and 2003 model years to the SCPI Special Policy on certain S/T, M/L, C/K, G, P and W4/NPR truck models. Please discard Special Policy Bulletin Number 99066E, dated February, 2003. Page 1 of 6 SPECIAL POLICY ADJUSTMENT-SEQUENTIAL CENTRAL PORT FUEL INJECTI. That are not related to the SCPI condition are NOT covered by this special policy. The customer should be informed that any further service that is not covered by new vehicle warranty will not be covered by this policy. VEHICLES INVOLVED Involved are certain 1996, 1997, 1998, 1999, 2000, 2001, 2002 S/T, M/L, C/K, G, P, W4/NPR and 2003 NPR model vehicles registered in California, equipped with 4.3L engine (RPO L35-VIN Code W or RPO LF6-VIN Code X), 5.0L engine (RPO L30-VIN Code M) or 5.7L engine (RPO L31-VIN Code R) and California emissions (RPO YF5). This Special Policy covers all vehicles within these model years, with these engine and emission RPOs. PARTS INFORMATION Parts required to complete this Special Policy are to be obtained from General Motors Service Parts Operations (GMSPO). Please refer to your "involved vehicles listing" prior to ordering parts. Normal orders should be placed on a DRO=Daily Replenishment Order. In an emergency, parts should be ordered CSO=Customer Special Order. PARTS INFORMATION Part Number Description Qty/Vehicle 12345382 Sealant, Thread, Med. Strength (6 ML) As required TBD* Cleaner, Top Engine (24 ml) If required and available TBD* Cleaner, Top Engine (15 oz) 24 ml (12346535 size alternative) 12345104 Cleaner, Port Fuel Injector (20 oz) If required 15050573 Asm -Fuel Tank Fill Pipe (97-99 M/L Van) 1, if required 15747585 Asm-Fuel Tank Fill Pipe (99 C/K E63 w/o AC7) 1, if required 15747588 Asm-Fuel Tank Fill Pipe (99 C/K E62 w/o AC7) 1, if required 12568332 Assy, V6 Fuel Injector 1, if required 88894355 Bracket, V6 1, if required 17113215 Kit, V6 Seal 1, if required 93441235 Assy, V8 Fuel Injector 1, if required 93442096 Bracket, V8 1, if required 17113206 Kit, V8 Seal 1, if required * Top engine cleaner part number 12346535 and 1050002 are no longer California compliant. California dealers will be notified via a DCS message when new alternative cleaner part numbers become available. NOTE: Isuzu Parts Ordering: In order to comply with the 10-digit Isuzu part numbering system, Isuzu dealers must add an "8" to the beginning and a "0" to the end of the listed 8-digit part numbers when ordering parts through AIPDN.

The only thing I think could rob the vacuum, is a bad leak, or the EGR system.I would pinch off the brake booster and see if manifold vac comes up too. Just a thought.