Gas Mileage Question on 99 LTZ

[88red4cyl]

Lately my wife and I have noticed the mileage on the 99 Lumina LTZ (3800 Series II) has been going down. The car has about 137,000 miles on it, and had been getting high 20's on the highway. Now the car is getting low to mid 20's on the highway, which on a long trip really adds up. I think it kind of sounds like the car is running a little bit louder (which may or may not make a difference), and it has been starting a little harder (not like a low battery, just like the car shakes a bit when starting up or something). Another thing I may add is that about the time this started occuring (lower mileage) the IAC went bad on it. When I took it into the parts store, the guy just cleaned it (carb cleaner I think) and told me that it would be fine, and the car would idle again. Now this is probably unrelated, but I just thought I would throw that in there just in case. I am getting no SES light, so I have no idea where to start other than new plugs and wires.. All replies are appreciated.

Thanks,

Will

[Brian P]

when was the last tuneup?

 

If unknown, I recommend:

 

Plugs, wires, fuel filter, air filter, clean the Mass air flow sensor with the proper spray cleaner, and run some seafoam thru the engine (may want to do seafoam first before the tuneup).

 

Check the exhaust manifolds for any leaks, that could be related to the additional noise. Also possibly the catalytic converter could be on its way out, but that's speculation (long, long speculation)

[19Cutlass94]

Also, injector clearner does wonders

[Andrew]

he may have ruined your IAC if he used carb cleaner. you have to use sensor sensitive cleaner(like MAF cleaner).

 

also you mention it sounds like its louder. i wonder if you have a slight exhaust leak, manifold gasket or donut. either way, that could cause your 02 to read that you are running lean via clean air coming in, therefore adding more fuel.

[88red4cyl]

he may have ruined your IAC if he used carb cleaner. you have to use sensor sensitive cleaner(like MAF cleaner).

 

also you mention it sounds like its louder. i wonder if you have a slight exhaust leak, manifold gasket or donut. either way, that could cause your 02 to read that you are running lean via clean air coming in, therefore adding more fuel.

 

What the guy used to clean it was either carb cleaner or WD40 (it has been a couple month, but it def. wasn't a special spray). I am not also 100% sure about it being loud either, as it could be in my head. My daily driver before my 95 Lumina was a Blazer (which was loud and made the LTZ seem quiet). Now that I have the Lumi for a DD it may just make the LTZ sound louder. Also, would a bad O2 sensor throw the SES light? And it not, is there any way to check for an exhaust leak or something?

[runt]

A bad 02 will throw a code. Even the 02 could be on the way out...

 

I'd start where Brian P said and go from there.

[88red4cyl]

Thanks for the replies guys! I will start replacing that stuff ASAP and let you know how it turns out. Since I don't wanna start another thread for this car right now, I have one more question for you all. When I was out there last night listening for an exhaust leak (couldn't find one so I must be imagining the loudness) I noticed a lot of noise coming from the belt area. This car has always had noise coming from that area, I just always thought that was how the engine sounded. But when you lean your ear down close to the belt area you can tell that the motor isn't making that noise the belt area is. I don't know if this is something common on the 3800's or not, but it is really annoying. It's not really a knocking sound or anything, just a little bit of chatter or something. I am gonna have the alternator checked to see if maybe a bearing is going bad in it or something, but it sounds more like it is coming from lower (like down near the tensioner or something). Does anybody have any ideas of stuff for me to start checking?

Thanks again everybody,

Will

[Euro]

When I was out there last night listening for an exhaust leak (couldn't find one so I must be imagining the loudness) I noticed a lot of noise coming from the belt area. This car has always had noise coming from that area, I just always thought that was how the engine sounded. But when you lean your ear down close to the belt area you can tell that the motor isn't making that noise the belt area is. I don't know if this is something common on the 3800's or not, but it is really annoying. It's not really a knocking sound or anything, just a little bit of chatter or something. I am gonna have the alternator checked to see if maybe a bearing is going bad in it or something, but it sounds more like it is coming from lower (like down near the tensioner or something). Does anybody have any ideas of stuff for me to start checking?

 

Is it a sort of higher pitched whining sound? Or does it have a little substance to it, like a bearing noise? Can you hear it when you accelerate? If I think it's what you're talking about, it's common on all 3800's. Check the alternator. Mine makes that noise and I believe I've heard it the loudest around the alternator area.

 

 

[R Dubya]

I know the tensioners on those cars, especially the L67, are prone to failure. I had to replace both of them on my bro's GTP. They were screaming badly. It wasn't the tensioner, it was the pulley bearings.

[Andrew]

I know the tensioners on those cars, especially the L67, are prone to failure. I had to replace both of them on my bro's GTP. They were screaming badly. It wasn't the tensioner, it was the pulley bearings.

yeah, mine was noisy too right before the pulley broke off!!