Originally Posted by grtpumpkin
It was the front O2 sensor. Wish I would have replaced that sensor first. At least I got an education.
The last trip I took I got 2 codes, even though it was running better, both for the Bank 1 o2 sensor. Fuel trim numbers are now good except for the rear which is still 99%. Not sure where to go with that but the cat may be worn out. As long as it'* running well and my gas mileage is back I'll be satisfied. It'* running like it was prior to the intake job and the exhaust no longer stinks.
Never realised how important an O2 sensor really is. I will keep an eye out for high percentage fuel trim numbers in the future. Unbelievable how one sensor can make or brake the driveabilty of a vehicle.
this is a great thread and I was compelled to revive it. at 200k miles my diehard reliable '01 Monte developed a similar problem. as it turned out, it had multiple problems and it was a wonder it was even still running. it needed plugs/wires (original wires still on it), and the timing chain was stretched to the point the tensioner could not compensate. the tensioner itself had a 1/8" groove worn in it. while in there I changed the crank sensor, cam sensor, and balancer. checking fuel pressure, it had adequate fuel pressure but would not hold steady with key on/engine off. it still had original fuel pump in it. changed to a new Bosch pump, and changed filter (it was blocked), it got better, but still would not hold. changed fuel injectors, then it would hold pressure. upper intake plenum was changed once before, but had warped again on the ends. lower intake gaskets were changed but the crappy GM originals again. one port was leaking antifreeze into port and could see where the sealant I had poured in had sealed it. changed upper/lower gaskets and upper plenum. throttle body was heavily carboned, cleaned it. also changed MAP, MAF, TPS, IAC solenoid, upstream/downstream O2 sensors, ICM module, and all 3 coils. cleaned all the grounds, and moved the PCM ground to battery side, and shortened battery ground cable too. cleaned air filter (reusable type). replaced small vacuum lines to fuel regulator, charcoal canister purge valve. put an entire NEW Walker exhaust system on it, from manifold to tail pipes, cat conv, resonator, 2 mufflers, all new pipes, gaskets. car ran good for ONE DAY, then developed an even worse miss. like it was running on 5 cylinders. so removed all the plugs, put 6oz of sea foam/diesel fuel/trans fluid/injector cleaner into each cylinder. let it sit a week. cranked it over, pushed that out, reinstalled plugs, fired it up. when it cleared, the car still had the miss. WTF ? checked cranking compression, it came up considerably, so the car had stuck compression rings to some extent, but still a bad miss. finally threw the towel in, had it towed to my friend'* shop. read up on it first, found out it could be the front o2 sensor, and the car needing a CRANK RELEARN, being I changed crank sensor/balancer. my friend has a better scan tool than mine, his can do relearn. he immediately found that the Chinese made MAF, upstream o2 sensor, and one coil were already BAD. even though they were only 1 to 4 months old. I bought new sensors, and gave him one original coil to reinstall, the miss disappeared. it was running on 4 CYLINDERS prior to this. holy crap. I also asked him to do an EXHAUST BACKPRESSURE TEST. he found the new exhaust system already had 3 psi of backpressure. you need to change the cat conv. at 5 psi backpressure. he said the new cats. are known to be a pos and defective, a new spiral design, that doesn't have the valuable rare metals inside that can be cashed in at the scrap yard cheapies made in China. well I got one. so the cat. needs to be pulled off, replaced, or gutted and reinstalled. anyway he did a CRANK RELEARN and the car ran even better. it'* getting 20-21 mpg city at 200k miles now, but occasionally starts and stalls. it has a Taiwan MAF on it that cost me $135, I may return it, and get the good NAPA MAF that costs $180. the cat. conv. cost me $145 on Ebay, I already contacted the vendor for a refund/return. also returned all the sensors that were bad for refunds, and the coil. lesson learned, these cheap Chinese parts are junk. don't put them in your car. get good used GM stuff from the junkyard, or buy new high dollar NAPA or GM Delco replacements. or you will be doing it over again.
if your downstream cat. conv. is reading voltage and % that mirrors the upstream cat. readings, after it has warmed up and is in closed loop, that means your cat. conv. is either blocked, or not working, and the CO and HC gases are passing right through it, without being burned off. the CAT is defective. downstream o2 by design, should read low in voltage in closed loop, if it'* doing its job. in the .100-.400 range, while the upstream o2 reads .100 to .800 range.
do a backpressure test using the upstream o2 sensor fitting threaded hole.. if it'* more than zero, gut or change the CAT to a high efficiency aftermarket unit. my son has the IDENTICAL CAR and engine, but 2 years newer, with 166k miles, with new high quality CAT on it. the downstream o2 readings on his car, are much lower than the upstream readings. his CAT is working properly.
and his car runs smoother and gets 2-3mpg mileage in city.
today it'* all about the QUALITY OF THE PARTS. we are literally being flooded with low-quality car parts, that make diagnosing the problem more difficult. you bolt a new part on, and it'* defective already, and adds to the problem.
keep all your receipts, and don't be afraid to RETURN THIS STUFF if it'* junk. sorry to say to keep these old cars running, you have to ante up for high dollar NAPA or GM parts, or scrounge the junkyards for deals on the oem Delco sensors, etc. that are still good.
one more thing, the dipstick tube seal, if it breaks on the dipstick tube handle, will leak vacuum into the crankcase via the pcv system, and bypass the MAF and also give a problem. change the dipstick to a new one.