Hello all,
I posted an introduction on here about a week ago. I just got a 97' Olds 88 L36 w/ 4T60E, brand new engine (bottom end, reworked heads, all new gaskets, alternator, water pump, and battery) with 110,000 on chassis/trans. I have about 500 miles on everything as of now since I got it with the rebuild. I put 8.1 gallons in it the other day (25 bucks), and got about 110 miles to the 8.1 gallons before the tank was bone dry again. This equates to just about 13.5 mpg. I drove about 50% highway, 50% city with this kind of horrible mileage. There are no check engine lights visible, and the tach is mostly under 2000 rpms at all times. Many of you said that the 02 sensor could have gone bad but not caused a check engine light to come on. I do not smell any gas so I am pretty sure that there are no fuel leaks. What kind of thing should I look for that could kill gas mileage other than tires, fuel filter, etc because I am pretty sure these are good for now? How can I check this oxygen sensor? I do know that the reason the old engine blew was because it threw the two middle piston rods through the block. My uncle, who built the new engine, said that this was due to the leaking intake manifold that is common on L36s, and coolant could have been leaking into the combustion chambers, thus destroying the O2 sensor in the exhaust. Can anybody give me some insight?

You have a rebuilt engine that hasn't broken in yet. It'* too tight. That'* probably part of your problem. Not much to do there but have fun with it.

How old is your O2? Is it the same sensor that was exposed to coolant and water when the UIM failed? If you don't know how old it is, or it'* over 50k, and/or it was the one in the car when the UIM puked, change it.

AC Delco or Denso, never Bosch.

Willwren,
I understand that this engine is brand new, but I didnt think it would cause the fuel economy to be this bad. As for the O2 sensor, I believe it is the factory unit. I am almost positive that my uncle did not replace it. Therefore, it is 110k miles old and was on the engine when the UIM failed.

I doubt a new engine is responsible for such a drastic change in fuel mileage or no one would ever buy the car new. Your Uncle has a good point on the coolant getting onto the O2.

Do you have a scan tool available to see what the O2 is doing?

I have about 1200 miles on my freshly rebuilt L36 and was averaging 19 mpg before i blew my tranny, with a 50/50 mix of city and highway. It was about the same when the motor was first built too. I dont think mine is completely broken in yet either so im hoping for even better mpg.

I rebuilt a 3100 in a '96 Corsica a few years ago, and it had TERRIBLE MPGs for the first 1000-1200 miles, around 16-18 highway. After the first 1200 miles, it was averaging 26-28 highway.

I personally do not have a scan tool. My uncle works at a shop where I would assume they would have one. What voltages and electrical behavior should I look for if I actually have a chance to scan this O2 sensor? Can I pull the sensor out and have a look at it to see any signs of neglect? Its pretty sad that my LS1 Camaro gets about 200 miles on the same route that this gets about 120 miles.

Depending on how the motor was built and by whom, that can have a profound effect on mileage, but probably isn't the sole cause. There are always contributing factors.

In your case, with that many miles and coolant exposure, I'd simply replace the sensor. It'* overdue.

I personally do not have a scan tool. My uncle works at a shop where I would assume they would have one. What voltages and electrical behavior should I look for if I actually have a chance to scan this O2 sensor? Can I pull the sensor out and have a look at it to see any signs of neglect? Its pretty sad that my LS1 Camaro gets about 200 miles on the same route that this gets about 120 miles.

Looking at Bank 1 sensor.. watch for flipping around from say 50 to 900'* back and forth...some of the hits will be 200'*..some 700'*..some between..but overall it should be hopping high and low pretty quickly.