Oxygen sensors

Hello everyone and thanks in advance for any input. I have a 99 Grand Prix GT with 115000 miles. No mods at all. I have replaced the lower intake gaskets and replaced the upper plenum with the Dorman. Anyway car runs excellent but fuel mileage isnt that great. I cant seem to get better than 19.5 MPG. Mostly drive highway. With the price of gas rising daily right now I was wondering if I would increase my fuel mileage if I replaced the Oxygen Sensors? I figure that with the plenum and intake gaskets previously leaking the sensors may have baked on antifreeze and may be not reading as accurately? I did replace the spark plugs after the repairs and no difference in MPG's. So what do ya all think? Could I get recoup my money in fuel savings if I spend the money on new oxygen sensors?Thanks!

I have a 99 GT with 130k on it, I rarely get on the highway, and get better than 21 mpg. On trips, I can get 32-33 mpg if I keep it around 70. There is a good chance that your converter may be partially blocked due to the coolant intrusion, so I don't think the oxygen sensors are going to net you the results you want, unless you have a CEL illuminated that points to one of them being bad.

Cat converter? I dont really seem to have any power loss and car runs smooth as can be. No check engine light. Hmmm. Is replacing the cat converter something that can be done by a home mechanic without welding? That may be worth replacing since I dont plan on getting rid of the car anytime soon. Is there a certain type(name brand) of cat converter you could recommend?

If you'd like to avoid welding or taking it to a shop to have it replaced, I would say your best bet has to be to purchase a bolt in downpipe with a cat already welded in. My car rusted around the flex section of the downpipe and broke in half. I got the replacement downpipe on my '99 GTP from ZZperformance. They use magnaflow brand cats in their pipes. http://www.zzperformance.com/grand_prix/products1.php?id=551&catid=110

Just be sure to order a pipe WITH a Cat installed as the offer offroad use pipes without them.

-Justin

Yeah but for the cost of the downpipe with cat, he could buy a magnaflow cat and have en exhaust shop weld it in for 40 bucks.

On the O2 sensor, you could remove it and take a look at it. There is a good chance if you were burning coolant that it has a film on it. You could try cleaning it off with some carb cleaner.

Other things to check out when your using too much gas, Tire pressure, trans going into OD, age/milage of plugs wires and air filter, fuel pressure....

Also how are you basing the 19.5 figure? Of the DIC alone is not going to be accurate as I can see up to 38mpg with a strong tail wind even though at the pump I calc it and its considerably lower.

Yes I am basing the MPG off the display screen. I never really did the math to dtermine its accuracy but it always used to be about 24-25 MPG. Plugs and plug wires replaced after LIM gaskets.Tires are in good shape and air pressure is equal all around. I am pretty sure its shifting into OD. What should the RPM's be at when cruising highway speed? I know I am at about 2300-2400 RPM's at 72MPH. If gas wasnt going up so quickly I probably wouldnt be this concerned about MPG's.

What should the RPM's be at when cruising highway speed? I know I am at about 2300-2400 RPM's at 72MPH. If gas wasnt going up so quickly I probably wouldnt be this concerned about MPG's.

That doesn't sound right. It should be lower than that at 70 mph. Around 1900-2100 rpms. Check to see if it it shifting automatically to 4th - OD above 45 mph and then look for torque converter clutch (TCC) lockup where the rpms will dip some.

I dont have any check engine lights. Its hard to tell if its going into OD but I am pretty sure it shifts through all the gears. It may be the torque converter isnt locking up after it goes into OD. If thats the case would it be a bad sensor or the tranny on its way out?

You can check that by manually upshifting. Go from first D1 to 4th - OD. Then look for the TCC to engage. You should see it drop a few hundred rpms when it does. When was the tranny last serviced? Pan drop - ATF and screen filter. On some occasions a basic tranny service may resolve an issue where the TCC is not locking/unlocking. Also, make sure the 20 way electrical connector is tight on the transaxle. Best to reseat it. Located driver's side front.

Hey. Thanks so much for the responses! I have had the car since new and the trans fluid was changed at 30,000 and the filter and fluid again at about 90,000. The car now has 115,000. It shifts very smooth and the fluid looks good and doesnt smell burnt. I will change it again since that is the cheapest attempt at fixing the problem. Also I will check that harness. I do remember sort of feeling it go into lockup after OD and now I dont notice that "5th shift" like I used to so I am leaning towards a problem with the TCC. I just always thought if the TCC wasnt working properly it would set off a code activating the SES light.

Generally if the TCC isn't working, it will trip a type 3 DTC, which wont illuminate the SES light.

I would calc out the Milage by hand the old way. You just unhooked your battery when you changed your LIM gaskets right? Well that reset the milage. You could try resetting it again when you are cruising at 60mph. BTW, cruising at 60mpg, your RPM's should be right around 1600-1800.

Ok. So today I decided to go over everything i can see in the engine compartment with a fine tooth comb. I could hear a slight hissing noise from the alternator. Turns out its not the alternator making the noise but the MAP sensor seal is torn I guess from replacing the plenum when I put that back in it ripped the orange screw looking seal. Does this MAP sensor have anything to do with the TCC? I shifted my gears manually today and it definitely shifts 4 times but doesnt lockup.

I am pretty sure the TCC isnt working properly. I have been really paying attention to the RPM's and at 70 MPH the RPM's are at 2400. I watch the RPM gauge and it drops about 200-300 RPM's then back up to 2400. It seems like the torque converter is about to lockup but it never does. I am gonna try a can of that seafoam then drive it for a day or so then flush the old fluid and replace with new fluid. Hopefully this works. I am thinking maybe the TCC solenoid is gummed up and sticking preventing it from locking up.

You could have a trans shop or a deal scan for DTC's. A stuck TCC should show a code even though the SES light is not on.