A cracked exhaust manifold or leaking manifold gasket would explain the loud noise under the hood and also provide a path for fresh air to mix with the exhaust stream which will cause the A/F sensor to report a lean condition to the ECU. The ECU will then attempt to richen up the A/F mixture and cause your mileage to go down.

 

When you downshift to slow the vehicle no fuel is expended. We do this with our LS430 and ES 300 and pick up 4-6 mpg hwy. Do it gently. No need to over rev the engine. It's fun to watch the computer spool up the number. Off-ramps do wonders.

Do a cost per mile study for your vehicle using high octane and then a lower octane. We did this on a V8 auto and found the difference was so small it was insignificant. Don't look at the price spread at the pump to decide. High octane takes you further.

 

can a leak in the donut gasket at the Y pipe cause mileage to drop from 18 to 13mpg? What are the symptoms of a leak in donut pipe? my car has a deep drone like sound but no muffler leak.

 

Have tried slower speeds on my 25 mile commute to work, but 70 mph seems to be the best speed for gas mileage on my 2002 RX300 AWD. Cleaning the throttle body also boosted mileage 2 MPG after it was dirty enough to make accelerator stick. Average 22.8 on highway and 20.8 combined.

 

That's about what I get with my '99 - She's completely clean inside the TB, MAF sensor, etc. There are a few guys on here that say they get more into the mid 20's but I've never gotten above 23.5. Car runs just like new too.

 

would there be an CEL for this?

 

There wouldn't be a direct trouble code for an exhaust leak but you should be able to see a problem with your long term fuel trim numbers if the leak is before the A/F (upstream) sensor.

 

I see. Two different mechanics inspected my car and found no leaks. My mechanic stated that the drone could be vibration from Cat since drone can only be heard when driving and not when lifted on 3k RPM.

Moreover, there is no pressure in my gas tank. My mechanic suggest a bad valve or sensor--and thnk the lack of pressure in gas tank is culprit for my bad MPG.

Any thoughts?

 

If you have a leak in your EVAP system, that will definitely turn on the CEL so if you have no CEL I wouldn't think that was a problem.

 

weird. i pulled ECI fuses and now car MPG back to normal 18+. Before that, it slowly climbed from 13 to 16 but stayed there for a long time. 2+ months.

 

So what triggered the engine overhaul? I have experienced this drop off in MPG myself on my 2000 RX300 with 115K miles. It's been a problem for a few years... 230-250 miles/tank. How poor was your MPG prior to this work?

 

eci = ecu ?

 

You need need to measure fuel economy in MPG, not by how many miles per tank. Then we can try and help. Remember the 99' & 00' had small fuel tanks.

 

I have been getting 14-16 mpg mixed city/hwy... poor in my opinion.

 

I have a 2001 Lexus RX 300 that just hit 120K miles, and I've been noticing in getting very low MPG. 13-14 MPG.. And I have a heavy foot.. I was wondering, what MPG do you get? I get 13-14 city, and around 17 highway.
I will drop in a bottle of Gumout or something similar, and update you guys on the changes.