1997 GMC K1500

The letter in the VIN# is M so if nothing has been done to it it should be a 305. Couldn't find anything on the engine so not sure if it matches up. I was told that the drum brakes on the rear are 11" which was only offered on diesel trucks which means that there is a possibility that the whole rear end was swapped out for a higher geared diesel rear end. I seriously doubt this though because the front would also have to be swapped out .It is possible though if somebody wanted the extra power at the cost of the fuel mileage. They may have done it when it was in the crash.Is there any way I can test the o2 sensor with a multimeter or something? I would really like to improve the mileage a bit.

The engine serial/ID number IS on the block, you just need to know where to look.

"Engine Code Stamping Numbers
All engines are stamped with an engine ID code, consisting of assembly plant code, production date and suffix code. V8 codes are stamped on a pad just forward of the right side (passenger) cylinder head."

Here's a couple of links to identify the numbers.

http://www.nastyz28.com/sbchevy/sblock.php#casting

(Maybe all too old... spend a little time on GOOGLE and look for other sites, as well.)

http://www.mortec.com/

As to the O2 sensor, if it's NOT working it SHOULD set a trouble code... Did I read your Check Engine Light isn't working?

You can read the O2 voltage if you have access to a scan tool, or use a sensitive (digital) voltmeter.

The voltage should cycle above and below about .5 Volts.

Steady reading BELOW .5 Volts indicates a LEAN condition or a failed sensor.

Steady reading ABOVE .5 Volts indicates a RICH fueling condition the 'puter is unable to compensate for (such as a stuck or leaky injector, for example).

Proper base timing and spark advance is important for fuel economy, as well. Have they been checked?

My basic advice to you would be to get the CEL working and learn how to retrieve trouble codes, either with the CEL or a scan tool.

I don't believe the 11" brakes on diesels only. I had a '98 Chevy K1500, 4x4, Z71, that I bought brand new off the lot. It had a 350, AT, a 14 bolt rear end, 3.42 gears, and 11" brakes. If your truck has 2 exhaust pipes side by side BEFORE they get to the muffler, you can gain a little over 2 mpg by doing this.... Take the truck to a muffler shop that does custom work, have the muffler taken off and a "X" pipe put in it's place. Then have dual exhaust pipes going off the "X". It is a little load if you really put your foot in it. If you just remove the muffler and put straight pipes on WITHOUT the "X", it will be even loader! BTDT. I did this with my '98, and gained a little over 2 mpg out on the hwy.

Took the cat off my 1990 with a 305 and picked up 2-3 mpg and it runs a lot better too.