Turbo Buick guy trying to fix a '96 Z-28. Help!

[GN1220]

I'm trying to fix a Z for a frind of mine. His A/C compressor took a crap and the C.E. light was on so he took it to the dealer. They told him he needed a new compressor and 2 new cats for a total of.....$3,000! I thought surely we could fix it much cheaper than that. I priced the compressor at $322 plus freon. No big deal. Today I finally drove the car and, other than a little miss, it seems ok. I don't think the convertors are bad but is there a way to tell for sure? I also pulled the codes and it showed a 300 and a 420. I just read a post about all things electrical related that could cause the 300 but with the miss I was thinking a good tune-up would be the place to start. Now, the 400. Would O2's cause this code or could it in fact be the convertors? The few times I've driven a car with bad cats it acted like it was plugged up and it smelled bad. This car shows no signs of either. Since the car has 130,000 miles I figured the O2's are probably the culprit but I wanted to get some expert advice since I'm going to be charging him labor and I don't want to just start throwing parts at it and not getting it fixed.

My plan for now is to replace the A/C compressor, expansion valve, orifice tube, dryer and filter and replace the freon. Also add a new belt. Then the O2's and disconnect the PCM wire to reset the light and see if that solves the problem. I will also tell him a good tune-up (plugs, wires) wouldn't hurt either. Should I replace the Opti as well?

Please give any advice you can because I want to be fair but also make a little side money as well. Thanks, Mike

[Z Ya Later]

I believe the P0420 code is thrown when the PCM detects the cat(s) not working efficiently. In essence, either the cat(s) are bad or the post-cat O2(s) are bad. Check the post-cat O2 sensors and see how they look. Even it the cats need to be replaced, you can probably get it done at a local exhaust shop for cheaper than that.

Welcome to the board, btw. Any pics of your GN? What year do you have? Hope this helps

[GN1220]

My local guy, a friend of mine, told $804 + tax for the two cats! My friend that is going to be doing the work with me found a pair on-line complete with the Y pipe for just under $400 but I would really like to save the owner of the car as much $ as possible. If it needs them thats one thing but I just don't want to start guessing with his checkbook. How many O2's does this thing have? I thought it just had the 2 after the cats closest to the manifolds.

I just took it to the car wash and cleaned it up, looked like it hadn't seen a wash or vacuum since 1996 and boy does it look good now. I thought the wheels were silver, not brake dust brown. LOL Anyway, it didn't seem to miss on the way there but the car had time to cool down and the wash is about a 2 minute drive from my house. Wouldn't it still be operating in closed loop function? Today when I went to the parts store it had plenty of time to warm up and that's when I noticed the miss. That should've been open loop, correct? Does this info make sense or help narrow down the miss problem? Thanks for the input and please keep it coming.

My GN is an '87, pretty much all stock:turbo, I/C, green stripes but it has a Mease downpipe, a Stageright tranny brake and a K&N plus a little chrome here and there. With my ET Streets it was running low 12's and I was hoping to hit the 11's before going with Razor's alky kit, a new turbo and stretch I/C. I love that this car responds so well to just the smallest of bolt-ons. I've only removed the valve covers to replace the springs! Low 12's from a mostly stock V6 is impressive in my book!

[Z Ya Later]

My local guy, a friend of mine, told $804 + tax for the two cats! My friend that is going to be doing the work with me found a pair on-line complete with the Y pipe for just under $400 but I would really like to save the owner of the car as much $ as possible. If it needs them thats one thing but I just don't want to start guessing with his checkbook. How many O2's does this thing have? I thought it just had the 2 after the cats closest to the manifolds.

I just took it to the car wash and cleaned it up, looked like it hadn't seen a wash or vacuum since 1996 and boy does it look good now. I thought the wheels were silver, not brake dust brown. LOL Anyway, it didn't seem to miss on the way there but the car had time to cool down and the wash is about a 2 minute drive from my house. Wouldn't it still be operating in closed loop function? Today when I went to the parts store it had plenty of time to warm up and that's when I noticed the miss. That should've been open loop, correct? Does this info make sense or help narrow down the miss problem? Thanks for the input and please keep it coming.

My GN is an '87, pretty much all stock:turbo, I/C, green stripes but it has a Mease downpipe, a Stageright tranny brake and a K&N plus a little chrome here and there. With my ET Streets it was running low 12's and I was hoping to hit the 11's before going with Razor's alky kit, a new turbo and stretch I/C. I love that this car responds so well to just the smallest of bolt-ons. I've only removed the valve covers to replace the springs! Low 12's from a mostly stock V6 is impressive in my book!

There's 4 O2 sensors, 2 before the cats and 2 after. If it doesn't matter that the cats are in the stock location, go with the aftermarket pipe (unless that also keeps them in the same location...if it does, then do the pipe), but after you're sure the O2s are fine.

Hmmmm, sounds like one might be related to the other though, from what you're saying. Does the SES light flash at all, esp. under hard acceleration? Might wanna see if the plug wires are ok. Also, its open loop when still cold, closed loop when warmed up .

Oh yeah, I love how this little 6 just flat out moves. Mine had the GNX turbo until it came time for a turbo rebuild (at 170,000!!!!). But, it does have a custom cat-back, TTA FPR, valve cover breathers, B&M shift kit and that's it. Surprised a Viper in her too...well, my dad did anyways lol. Welcome to the board man, good to have another GN owner . You belong to any G-body boards?

[GN1220]

Open/closed DUH! I never noticed the light flash at all, it just stays on, even under heavy acceleration. I did see the "Traction Control" light come on, though. As of now, I guess we'll replace all of the A/C parts and the 4 O2's, unplug the PCM and see if the light comes back on. If it does I'll do the plugs and wires because I believe they are the originals. Is there a way to check the Opti or should it be replaced with this many miles? Not sure if it "goes out" over time or if it "either works or it doesn't"
I belong to TurboBuick.com and TurboBuicks.com for the GN and The Impala SS Forum for my Roadmaster Wagon.

[Z Ya Later]

Open/closed DUH! I never noticed the light flash at all, it just stays on, even under heavy acceleration. I did see the "Traction Control" light come on, though. As of now, I guess we'll replace all of the A/C parts and the 4 O2's, unplug the PCM and see if the light comes back on. If it does I'll do the plugs and wires because I believe they are the originals. Is there a way to check the Opti or should it be replaced with this many miles? Not sure if it "goes out" over time or if it "either works or it doesn't"
I belong to TurboBuick.com and TurboBuicks.com for the GN and The Impala SS Forum for my Roadmaster Wagon.

Hmmmm. I'd definitely inspect the plugs and wires also. If the Opti is starting to go, that will definitely cause the sputtering and misfire, but that's usually a constant thing and has its own set of codes. Definitely do the AC and O2s, but be ready for the plugs and wires too. Have they been done before? If not, definitely do the plugs and wires.

Ever checked out www.gnttype.org/forums?

[GN1220]

Thanks for the input so far. As of now he just wants to get the A/C done and in a few weeks do the brakes and O2's. I will post back with info regarding the codes after the work is done.

Btw, is it really that much of a pita to change the plugs and wires? I've heard they can be pretty scary.

[ws6drbob]

Btw, is it really that much of a pita to change the plugs and wires? I've heard they can be pretty scary.

Well.... The dealership charges $400 labor to do em... whats that tell ya?

[Z Ya Later]

Thanks for the input so far. As of now he just wants to get the A/C done and in a few weeks do the brakes and O2's. I will post back with info regarding the codes after the work is done.

Btw, is it really that much of a pita to change the plugs and wires? I've heard they can be pretty scary.

As Bob pointed out, yes, its that bad. You're looking at roughly a full-day's work the first time you do them. Just take your time, keep cool and keep chuggin'. The plug wire to #7 (back cylinder on driver's side) will be kinda tough to route if you use 8mm wires and the passenger side you will be doing based on touch, not vision. It'll take some time, but it can be done. You'll end up pulling the battery, alternator and driver's-side half of the Y-pipe. I did my sister's 96, and believe me, I remember how wrong I was when I thought it would be easy lol. Oh yeah, keep lots of bandages handy too lol. If you need some help on which plugs and wires would be best, just lemme know. Take care and good luck