I have been having problems with my truck getting really bad gas mileage. It has 194K miles on it, and was running pretty well with the stock gears and 31x10.5x15's. In November I got new tires in 31x12.5x15. The truck is sluggish and weaker than normal, seems to run rich,and the exhaust smells strongly of gas. I used to be able to get 300 miles per tank (3.0 V6) and I am down to around 200 miles per tank. I suspect that it is the O2 sensor since it is supposed to be replaced every 100K miles and I know that I never replaced it. Any ideas? I recently replaced the cap/rotor/wires and plugs and it runs the same. I bought the O2 sensor and an Equus code reader but I can't do anything since the rig is at the body shop since I was in a wreck in Oregon in December. The truck can still do 70mph on the freeway but only if it is floored and up to 4th gear. Any ideas?

Run a wet and dry compression test on it, sounds like a valve problem My '90 started doing the same thing back in December, ended up with a bad #4 cyl, and #2 & #6 cyls are low as well. I need a valve job again. <img src="/forums/images/graemlins/angry.gif" alt="" />

300 miles to a tank huh? I don't ever remember getting that many miles, right now I get about 200 or 225 in the summer months. If it's the 02 sensor that is the prob, I am going to replace mine too and hopefully I can get 300 miles too. Let me know what's up.

I raplaced my O2 sensor last month and noticed an immediate improvement in fuel economy..i now get 330 miles to a tank

how much is the o2 sensor? i'm assuming there is only one by the cat...

I would buy a compression tester they are only like $25-30. It can rule out alot of things more quickly than tring to doing ecu code diagnostics.

Also if you are running rich it could be just a AFM adjustment you are running a air intake am I right?
It could be even loose air intake fittings as well a gap will cause weird idling and stuff..
It's not always a faulty O2 sensor.
You can definately change it out and see what happens but you might try leaning out the AFM. Don't do it too much you will get detionation.

If it is headgaskets or valves definately spend the money to have the heads and valves re-done by a machine shop it's well worth it then put a fresh set of head gaskets on.

I would suspect bad compression due to valves or headgaskets.
A good sized impact can cause all kinds of things to unseal break loose break contacts all kinds of stuff.

Hell I am on 32x11.50's I can do 80-85 in 4th and the valve gaps and timing cam is off on 2,4,6 bank and it isn't WOT yet.

Just my 2 cents.

Thanks for the ideas. I actually already have a compression tester but it is at my parent's home in CA. I guess that I will have to buy another one. I just hate having to buy the same tool over and over. I will change out the O2 sensor and check the compression when I get the truck back. I will pull the codes with the tool that I bought and try to isolate any further problems.

If you do replace the O2 sensor, get it from sparkplugs.com

They have OE identical (NGK or Denso) for like 45 bucks.

Better than the couple hundred the dealer wants, huh? <img src="/forums/images/graemlins/cheers.gif" alt="" />

Well, I would start with some basic diagnostics and see what you discover. Your O2 is way overdue for replacement.

1. Pull your codes

2. Replace your O2 sensor

3. Rule out engine problems with a compression test

4. A valve adjustment might be in order, depending on how long ago the last one was


A few other hints: Get a Factory Service Manual (availabe on eBay or brand new at Toyota Motor Pubs). There is a notorious head gasket problem on the V-6 that you ought to rule out.

If I was betting on what the problem was, my guess would the O2 sensor died. Replace it with an OEM, NGK, or Denso sensor. Don't use Bosch or any off-brand. Trust me on this one!

Does the check engine light need to be on to make it worth changing the O2 sensor? I noticed a decrease in fuel economy after moving from 32" to 31" tires. I expected an improvement and was disappointed when it dropped. I get about 210 now, got about 250 before.

What kind of gas mileage did you get before you replaced the O2 sensor?

Thanks