Hey guys, I'm new to this forum, but it looks like an awesome place to give and get monte carlo info. I'm glad to join your community.

I am going to try to be as specific and detailed as possible with this one, so I appologize if it is lengthy.

My daily driver 1997 Monte Carlo LS 3.1L has been sluggish for awhile now, and I have been slowly trying to pinpoint the problem. It's got 200,xxx miles on it, but I've been tryin to take care of her. I will probably buy a different car in the future, but I don't know as much about auto repair as I would like to. So I'm gonna go "hands on" with this car.

A Coworker let me borrow his simple code reader, and I have been taking note of what has popped up. So Far: PO305 two times (cylinder 5 misfire), PO420 multiple times (Catalyst system efficency below threshold), PO440 multiple times (Evaporative Emission Control System Malfunction), PO340 once (Camshaft Position Sensor Circuit Range/Performance), and I had a multiple cylinder misfire once as well. After getting the codes, I would erase them, and wait to see what would come back.

My first instinct for me was to change the plugs. I bought the Bosch Platnium +2's because they were supposedly pretty nice. As I was replacing them, I noticed the old ACDelco's weren't actually that bad, but I went for the swap anyways. Still sluggish, still had the codes

Second thing I did was change the O2 sensor. Car ran better before the check engine light came on, and then went back to being sluggish.

Third thing (should of been with first, or second) was to change the ignition wires. Yesterday, I took the old ones off the coils, I noticed a white crust in the boot, and on the terminals. I wiped it off before putting on the new wires. One of the the coil terminal/boots looked almost as if it was rusty, which I can imagine isnt good. It wiped clean, and on went the new wires. I did not use any dielectic grease on them, because I didnt realize I should of until it was already done. I'm pretty sure the misfire thing is taken care of, but the dreaded PO420 popped up again, and the car was sluggish once again. Confuses me, because it runs fine when the code is cleared.

During this "experimenting period" I also got a high mileage oil change (I think they check air filter as well), put in some slick50 engine treatment, and some slick50 fuel system treatment.

Other possibly helpful info about my car:
-I think it has a leak towards the top of the gas tank, so I never fill it more than 3/4 a tank.
-I always thought the gas cap was the stock one, but It isnt connected to the mouth of the tank (I have to set it somewhere when filling up).
-Used to smell like gas all the time, but it was probably just from the misfire, as I dont notice it now.
-A while ago it was scanned and some guy told me the shift solenoid was bad (it shifts hard sometimes), but I dont get that code, and it shifts fine when all codes are reset.
-When my wheels are turned and I accelerate it bogs down noticebly (unless codes are cleared)

I have been researching this for awhile now, and narrowed it down to a couple of solutions rated by what order I will probably go.
1.) Get a new gas cap- that may get rid of PO440
2.) New coils - I heard that white crust meant they werent functioning right, plus the rusty one made me nervous
3.) Purge sensor- changed o2 sensor but not this one, may get rid of PO420
4.) ..New Catalytic converter- might have to pay up, but I would think it would be sluggish ALL the time, instead of only when I dont clear the codes.

Other than that, I have no idea what it could be. I suppose I should check fuses on it to make sure everything is good electrically. I know some codes are set off by others, so I'm hoping one of you guys has a good idea of the problem, and ways to check with standard tools. If you need any more info, please let me know. Thank all of you for your time, and any input.

 

P0440 is probably your leaky fuel tank or like you said gas cap. I wouldn't worry much about p0440

The crankshaft code is possibly a combination of plugged cat and misfiring. Or the sensor has gone bad or has a rust build up between it and the block. Take it out clean the engine block off carefully with light sandpaper and put so synthetic grease all around the hole.

P0420 is either a bad cat or a bad post cat 02 sensor likely.

 

I agree that the P0420 code is more than likely a plugged Cat. The sensors may also be dirty, but more than likely it's the Cat itself.

I have had a 2001 Grand Prix and a 2001 Impala... both of them threw the P0420 code, and both of them were a plugged Cat.