98 Montana 3.4 P0300

Just because the vaccum reading is low doesn't mean it's leaking! The engine when it's running poorly will not create as much vaccum because of pumping losses! As for the advance reading I'd have to look at what a good running 3.4 has! If the upstream 02 is removed and the engine idled does the vaccum improve?

found a magnaflow direct fit cat for a $100 on ebay and went and ordered, stuck it in and rotten egg smell is gone but P0300 is still there and the rough idle, I need to replace rear 02 sensor is it's casing is loose... live data: ------------------ DTC_CNT 1 FUELSYS1 CL FUELSYS2 N/A LOAD_PCT(%) 3.5 ETC(°F) 212 SHRTFT1(%) -0.8 LONGFT1(%) 8.6 MAP(inHg) 14.2 RPM(/min) 714 VSS(mph) 0 SPARKADV(°) 24 IAT(°F) 77 MAF(lb/min) 0.718 TP(%) 0.0 O2S B1S12--B2S---- O2B1S1(V) 0.165 SHRTFTB1S1(%) -1.6 O2B1S2(V) 0.875 OBD OBD2 ------------------

Okay If u are going to bother replacing the downstream 02 sensor I'd suggest replacing the upstream as well! Use only Denso brand 02 in it and when they are installed recheck what your short term fuel trim is! If it still idles rough and generates that po300 dtc I'd suggest getting it scanned with a scanner that can read the live data from the misfire monitor and narrowing down which cylinders are misfiring so that you can determine which cylinders to address! Otherwise your new converter isn't going to last for long!

yeah, i am not driving the vehicle.. one other difference is now there seems to a little bit of stumble every now and then.. I am actually wondering if the compression tester I used is any good... gonna recheck compression with a different unit

compression tests don't alwasy show valve sealing issues! leak down test is better, I'd still proceed with a better scanner check before yanking plugs back out

my buddy keeps promising to let me borrow his Snap on-scanner but hasnt yet, so still f'n on this thing... I installed a rebuilt set of injectors and it ran 10X worse-company sent bad injectors so they replaced also the car was acting real funky and the ecm smelt burnt and I was getting tps codes so I: Installed calibrated ecu Replaced all injectors with another rebuilt set Replaced all coils and ignition module with known good ones Replaced downstream o2 sensor Car is running mouch smoother but wont idle, keeps dying .. short and long term fuel trims are 0.00% and not changing Fuel pressure 35 koeo, same running, goes to 40 if i disconnect vacuum hose at regulator.. It wont idle or stay running, at idle it dies - if you try to keep it running by giving it gas- voltage will drop-battery light comes on and rpm drops and then it dies.. insult to injury the windshield wipers when turned off now stay in up position the car isn't back firing threw exhaust anymore, I need to check my fuel pressure gauge to verify the readings (its an El cheapo which don't last long I am told) and no codes stored or pending now

does the fuel pressure drop off as the engine is revved in gear?

i have been mail orderin parts for cheap so thats why it takes me a week between posts, anyways I knew I smelt something burnt under the hood after I tested the coils again, the ECM smelt burnt and the CPS was burnt, installed new CPS (already changed ecm) and it fired right up and ran smooth, thought everything was great until it warmed up, as it was warming up I felt a very very slight stumble every now and then, once warm it is still cutting out.. hold RPM at 3K -idles awhile- battery light on - rpm drops to 1k- goes back to 2K drops right down to 1500 and then goes right back up to 3K for a lil while and then repeats, if I don't hold at 3K it will die its no longer running rough at all, nice and smooth just wont idle when warm and cuts out -NO CODES AND NO PENDING CODES its got to be IAC or TPS, I will probably change both as they are cheap live data (rpm held at 3K) ------------------ DTC_CNT 0 FUELSYS1 OL FUELSYS2 N/A LOAD_PCT(%) 13.3 ETC(°F) 198 SHRTFT1(%) 0.0 LONGFT1(%) 0.0 MAP(inHg) 6.5 RPM(/min) 2056 VSS(mph) 0 SPARKADV(°) 41 IAT(°F) 54 MAF(lb/min) 0.869 TP(%) 0.0 O2S B1S12--B2S---- O2B1S1(V) 0.120 SHRTFTB1S1(%) 0.0 O2B1S2(V) 0.090 OBD OBD2 ------------------ DTC_CNT 0 FUELSYS1 OL FUELSYS2 N/A LOAD_PCT(%) 6.7 ETC(°F) 198 SHRTFT1(%) 0.0 LONGFT1(%) 0.0 MAP(inHg) 28.9 RPM(/min) 1226 VSS(mph) 0 SPARKADV(°) 30 IAT(°F) 54 MAF(lb/min) 1.007 TP(%) 0.0 O2S B1S12--B2S---- O2B1S1(V) 0.920 SHRTFTB1S1(%) 0.0 O2B1S2(V) 0.870 OBD OBD2 ------------------ DTC_CNT 0 FUELSYS1 OL FUELSYS2 N/A LOAD_PCT(%) 7.5 ETC(°F) 198 SHRTFT1(%) 0.0 LONGFT1(%) 0.0 MAP(inHg) 24.2 RPM(/min) 1973 VSS(mph) 0 SPARKADV(°) 33 IAT(°F) 54 MAF(lb/min) 0.988 TP(%) 2.7 O2S B1S12--B2S---- O2B1S1(V) 0.110 SHRTFTB1S1(%) 0.0 O2B1S2(V) 0.105 OBD OBD2 ------------------ DTC_CNT 0 FUELSYS1 OL FUELSYS2 N/A LOAD_PCT(%) 5.5 ETC(°F) 198 SHRTFT1(%) 0.0 LONGFT1(%) 0.0 MAP(inHg) 29.8 RPM(/min) 633 VSS(mph) 0 SPARKADV(°) 19 IAT(°F) 54 MAF(lb/min) 2.208 TP(%) 4.3 O2S B1S12--B2S---- O2B1S1(V) 0.070 SHRTFTB1S1(%) 0.0 O2B1S2(V) 0.075 OBD OBD2 ------------------ DTC_CNT 0 FUELSYS1 OL FUELSYS2 N/A LOAD_PCT(%) 6.7 ETC(°F) 199 SHRTFT1(%) 0.0 LONGFT1(%) 0.0 MAP(inHg) 10.0 RPM(/min) 1713 VSS(mph) 0 SPARKADV(°) 19 IAT(°F) 54 MAF(lb/min) 1.300 TP(%) 14.9 O2S B1S12--B2S---- O2B1S1(V) 0.065 SHRTFTB1S1(%) 0.0 O2B1S2(V) 0.045 OBD OBD2 ------------------ DTC_CNT 0 FUELSYS1 OL FUELSYS2 N/A LOAD_PCT(%) 3.1 ETC(°F) 199 SHRTFT1(%) 0.0 LONGFT1(%) 0.0 MAP(inHg) 30.4 RPM(/min) 0 VSS(mph) 0 SPARKADV(°) 15 IAT(°F) 61 MAF(lb/min) 0.656 TP(%) 11.4 O2S B1S12--B2S---- O2B1S1(V) 0.085 SHRTFTB1S1(%) 0.0 O2B1S2(V) 0.830 OBD OBD2 ------------------ DTC_CNT 0 FUELSYS1 OL FUELSYS2 N/A LOAD_PCT(%) 25.5 ETC(°F) 199 SHRTFT1(%) 0.0 LONGFT1(%) 0.0 MAP(inHg) 8.9 RPM(/min) 3400 VSS(mph) 0 SPARKADV(°) 32 IAT(°F) 59 MAF(lb/min) 3.169 TP(%) 14.9 O2S B1S12--B2S---- O2B1S1(V) 0.115 SHRTFTB1S1(%) 0.0 O2B1S2(V) 0.055 OBD OBD2 ------------------ DTC_CNT 0 FUELSYS1 OL FUELSYS2 N/A LOAD_PCT(%) 18.0 ETC(°F) 199 SHRTFT1(%) 0.0 LONGFT1(%) 0.0 MAP(inHg) 8.3 RPM(/min) 3728 VSS(mph) 0 SPARKADV(°) 25 IAT(°F) 57 MAF(lb/min) 3.392 TP(%) 15.7 O2S B1S12--B2S---- O2B1S1(V) 0.095 SHRTFTB1S1(%) 0.0 O2B1S2(V) 0.050 OBD OBD2

If the alternator or batteyr is bad it won't idle correctly and after the battery is unhooked on alot of gms they won't idle correctly until the pcm relearns it!