Any clues?

Has had front of sensor replaced by previous owner wen they swapped in a replacement motor. I do not know what brand. Had an o2 sensor code once in the last month. Cleared it and it didn't come back. That was about 5 days after the motor swap. 

 

I have a 98 OBW with 2.2.   I barely get 20 mpg. Plugs and a fuel filter may help.   If I get around to it I may try new injectors in the future. Car is over 20 years old.

knock sensor could be cracked - robs power, kills mileage and often there's no code.

one of the few sensors where an $8 ebay unit seems to work for people.

What year/engine was installed?

read the check engine codes - what do you get?

any other instrument panel lights on?

1. Driving.  City driving  mountains snow letting car warm up heavy on the gas...

2.  Brakes.  “Previous owner...engine install” suggests it sat for an unknown period of time. Its had inefficient brake maintenance because almost all have - pull slides and regressed with Sil Glyde or better  

Knock sensor - I’ve never seen a cheap $6 eBay special cause issues though I can’t imagine they’re awesome for that price  lol   They usually throw a code but stranger things have happened.  They’ll show cracking and rust/putting at their base.

air filter, plugs, vacuum leak, improper timing belt alignment 

Car was from Colorado, bought in Minnesota. Sat for 7 months. Motor was swapped from another 97 OB a month ago.  It sat because old motor over heated. O2 sensor was from the other OB. Brakes do not appear to be dragging. 

Yes, 15-16 mph is too low.  Obviously you should change the o2 sensor since you got the code.  I call that a no brainer.  The knock sensor is also suspect.  They do not always send a code.  So change it.

Get back to us after you make the changes and run the car for a month.

4 hours ago, greg donovan said:

Car was from Colorado, bought in Minnesota. Sat for 7 months. Motor was swapped from another 97 OB a month ago.  It sat because old motor over heated. O2 sensor was from the other OB. Brakes do not appear to be dragging. 

1. Are there any lights on the dash - namely Check engine light or transmission light? 

2. How many miles have you put on it at this rate?  Might just need run in a little bit, get bad gas out of it. 

3. What type of driving - any highway, or all stop and go city driving pulling a dump trailer full of rock?

4. Brakes can light drag and cause no issues at first except a few mpg drop in gas mileage.  It's just that most people don't notice them until they get really bad, but they're typically starting to get bad before those "catastrophic" failures and symptoms, people just don't know it.