O2 Sensor Time? - Fuel mileage has dropped off by 35%

[1999montana]

Hi All,

Van has 227,000 KMs on it now. Was running well, but fuel mileage has dropped off significantly over other winters. Down to 20 mpg (imperial gallon). No codes, and no CEL. Runs a tiny bit rough, but not unusually so.

Plugs and wires replaced at 170,000. PCV is new. Intake is solid, no leaks. EGR is working, but original as well. Fuel Filter at around 110,000 Kms. I run plenty of Fuel Injector Cleaner through it so it's not the injectors as far as I can tell. May change the air filter and clean the throttle body and do a Combustion Chamber Cleaning as well.

I'm thinking it is time to replace the O2 sensor, at least the one on the exhaust manifold, but should I do both?

Any thoughts?

[GTP Dad]

The upstream O2 sensor would be the one of choice as the computer uses it for reference for fuel management. I would expect your fuel economy to improve. As for the rear sensor it wouldn't hurt to replace it as they do wear out. With the mileage on this van it would be a good idea to change it but not absolutely necessary.

[1999montana]

Originally Posted by GTP Dad

The upstream O2 sensor would be the one of choice as the computer uses it for reference for fuel management. I would expect your fuel economy to improve. As for the rear sensor it wouldn't hurt to replace it as they do wear out. With the mileage on this van it would be a good idea to change it but not absolutely necessary.

Thanks for the reply. Waiting for better weather and a new garage (we're moving soon, Again! - Geez I feel like Baby Dinosaur). Then I can work on the van properly using my own tools. Think I'll just do 'Bank 1' first (on the exhaust manifold; - after plenty of PB Blaster and an over night soak) and see what happens after the ECM levels out a re-learns.

[1999montana]

Probably time for a new thermostat too. Don't know if the OEM is 180F or 192F. Would go for the 192, but don't want to overheat in summer with the A/C on.

Looks like I have to take the snorkle off the throttle body to get under the intake and un-bolt the t-stat housing.