Ok, my mileage dropped somewhat suddenly about 2 years ago and I still dont really know what the problem is. I have a 99gs 130k miles, used to get 27+ MPG now I'm lucky to get 22 MPG.

I have a CAI with K&N Filter
New Plugs (didnt help)
225/50/16 Tires.
And the car is an Auto transmission sadly.

What could be effecting the mileage so much.
1. Would the EGR valve effect mileage?
2. Would O2 sensor effect mileage without tripping CEL?
3. What else could cause it since all the simple things have been taken care of, such as tire pressure fuel filter, etc...?

I would say you should get a new upstream o2 sensor. With 130k miles on it, it would make a huge difference if you got a new one. I am going to get a new one on mine soon too.

O2 sensor can read inaccurately but not throw a CEL. If you have access to a vacuum gauge, get the engine warm and idling and get a vacuum reading and post it here. if the needle jumps of acter erratically post that info as well. Do that and i will explain why later.

no cel?

upstream O2, i'll do that this week. Downstream O2 doesn't effect mileage at all does it?

I dont think i have a vacuum meter but i will try and get one.

No downstream wont affect your mileage. Hahn claimed he was able to prove it affected your timing but no enough to affect mileage.

should I go with a Denso oxygen sensor or a Bosch oxygen sensor?

Originally posted by spoof145
should I go with a Denso oxygen sensor or a Bosch oxygen sensor?

I use the Bosch one from Advance. Works just the like other one did, but I get a bit better MPG.

Anyway, 2 years later, been through lots with the car, still bad milage,I'm doing 90% freeway and 10% city and averaging 24-25 mpg.

list of replacements

o2 sensor, plugs, wires, air filter, coil, pcv valve, cleaned egr valve, fuel filter, upped tire pressure.

I used lucas oil treatment once a couple years ago, and the mileage drop was a little after that, would that affect mileage so much and still affect it?

Originally posted by spoof145
Anyway, 2 years later, been through lots with the car, still bad milage,I'm doing 90% freeway and 10% city and averaging 24-25 mpg. list of replacements o2 sensor, plugs, wires, air filter, coil, pcv valve, cleaned egr valve, fuel filter, upped tire pressure. I used lucas oil treatment once a couple years ago, and the mileage drop was a little after that, would that affect mileage so much and still affect it?

If the oil treatment caused the mileage drop you should have seen your mileage return after a couple of oil changes I would hope.

Plus I don't see why that would cause bad gas mileage anyway.

Originally posted by Fasttopass

Originally posted by spoof145 Anyway, 2 years later, been through lots with the car, still bad milage,I'm doing 90% freeway and 10% city and averaging 24-25 mpg. list of replacements o2 sensor, plugs, wires, air filter, coil, pcv valve, cleaned egr valve, fuel filter, upped tire pressure. I used lucas oil treatment once a couple years ago, and the mileage drop was a little after that, would that affect mileage so much and still affect it?

If the oil treatment caused the mileage drop you should have seen your mileage return after a couple of oil changes I would hope. Plus I don't see why that would cause bad gas mileage anyway.

If he used standard lucas additive the high viscosity can cause more drag in the engine and reduce mileage. It's the same difference between running 10w40 and 5w30. You'll get better mileage with the lighter weight oil. When I assemble engines I use Lucas as an assembly lube, and I use a ton of it (better too much than not enough when assembling). Usually I'll notice a jump in mileage after the first oil change when I switch to straight oil.

You are right that the next oil change should have seen the mileage come back though.

To the OP, did you ever do that vacuum test that Terry recommended?

Thats normal mileage

Do you know if any auto parts stores loan vacuum gauges? I thought I had one but could not find it.