kiwi

I am now pushing 17.7 L/100KM / 13.3 MPG. seems to be getting worse. tire pressure is good.

i guess the best plan is to purchase a diagnostic tool instead of just throw parts at it. 400 bucks in O2 sensors may not fix it.

any more suggestions are welcome.

RamAirRckt

I never throw parts at it hoping I fix it. Just wastes a ton of $$. Good diagnostic tools are always a worthwhile investment.

In all honesty, O2 sensors are rarely the problem, but most often first part replaced. It is reporting something isn't right, so most people replace them hoping to get a better result.

So if it is running lean, the O2 reports it, or rich, the O2 reports it. We need to find out WHY it is running rich or lean.

For example, if the fuel pressure regulator is leaking, it will run rich. The ECU will read the O2's and see it is rich and pull fuel out. It gets to a point where it can't manage the fuel accurately enough, and it finally trips a code that shows it running rich. Then the customer comes along and replaces the O2. But the problem is the FPR. So the new O2 might give a short time (day or so) of no codes, but it will come back. The key is to look at the data and see what is going on. If it is running on the edge all the time, it won't be running accurately. It needs to. If everything is centered up in the fuel trims, it takes a lot to get it where the ECU can't manage it.

One thing to remember if you take your vehicle to a shop, parts RARELY fail in pairs. If you go in and they want to replace the O2. Fine, IF it is bad, but don't let them talk you into replacing both. Replace the bad one, not the good one. I have had O2's last 200k miles. O2's if treated well last a very long time.

Ok, this might be waaay over your head, so I appologize, but this can give you some ideas to ponder:

O2's are simple. They report rich or lean. Nothing more. It doesn't know what kind of fuel you have. So if you are running E85, E10, E20, E0, Methanol, NitroMethane, etc, it reads rich or lean only. The ECU knows that for X quantity of air, that Y quantity of fuel is needed. And when that number is exceeded (the O2 reporting rich or lean), it has to start to offset the base fueling it is making. So the ECU sees the fuel coming out the tailpipe as rich, it cuts fuel, then it sees it running lean, and it adds fuel. It does this several times a second. So you get something similar to a sine wave. The proper fuel ratio is a statistical average of the rich and lean fuel commands from the ECU to the injectors.

Things that make it rich, leaking fuel pressure regulator. Note that THIS will make BOTH banks rich. NOT 1 only!!! A bad injector (if leaking) will make 1 bank rich, NOT BOTH, and it will have the other injectors on that bank PULLING fuel out to compensate. A bad injector (not flowing fuel) will make 1 bank LEAN (NOT BOTH), and it will have all the other injectors jacking up the fuel to compensate. A bad MAF or dirty MAF will do all kinds of silly things to screw up fueling. A bad thermostat (not making it 190F or so anymore) will make it richer, and it will spend more time in Open Loop trying to warm up before it can even start closed loop fueling (reading the O2 sensors). A bad coolant sensor, say one that doesn't read properly anymore might be ok to the ECU, but not accurate, so the fueling and timing will be wrong. The intake air sensor if not accurate will cause the TIMING and Fueling to be off depending on which way it is off. If the coolant or intake air sensors read hot, the ECU will remove extra timing thinking it is too warm and will be protective. If too cold it will delay closed loop and it will give more timing when it shouldn't causing the knock sensor to tell the ECU that it is pinging and it will keep yanking timing out to be protective when it shouldn't need to.

Needless to say I can go on and on with this stuff....

Being lean is worse than rich, rich is safe, lean is mean. Lean can put holes in pistons. But it would run very bad if you were that lean.

Things that affect mpg:
Alignment (toe in/out)
Tire Pressure (only if pretty far off, a psi or 2 won't make a diff, 10 psi will)
Wind drag (is the belly pan catching air because some screws are missing?) Just windy out?
Idling? Do NOT let it warm up, get in and drive. It isn't designed to sit and idle, it just wastes gas.
Aggressive driving?
Is torque converter locking up in high gear? You should feel 1-2 shift, 2-3, 3-4, and then 4-5 and then torque converter lock up. You can hold foot on gas (steady speed) and gently press on brake, if you don't feel the driveline change a little and the tach come up in rpms some (50-200 rpm), then this could be hurting things.
Is the fuel good? Buy gas at a name brand station that has Top Tier Fuels. Give it a few tanks there to see if any different.
Are you getting E20 fuel and not knowing it? See what ethanol content really is. It changes every tanker full. http://www.waldeneffect.org/blog/How...t_of_gasoline/ (be careful this is handling fuel, it could be very dangerous!!!)
Tires wrong size or speedo inaccurate. Check it against several mile markers, longer the distance the higher accuracy. GPS isn't too bad to use, I like stopwatch and mile markers. I want the distance over the speed it shows me. I want to know my math is right. Speedo vs Odo can be different in accuracy. Lexus has had issues with this in the past. (2GS's speedo's read high)
Bad or dirty MAF
Bad or dirty injectors
Plugged Cat, this goes back to fueling, high backpressure will skew the O2 sensor readings, the higher the pressure the O2 thinks it is rich and starts leaning out ALL the injectors to compensate. It gets worse and worse.

I am sure I will ponder more about it, but this should be enough to chew on for a while.....

FYI, My best mpg is 28.0 for 70 miles in my wife's RX330. But I was limited in traffic to 55-65 mph. So I had someone breaking wind for me (that just sounds wrong!) and we were pretty capped on speed due to construction. I was in the 29's for a while. Once I got out on open road mpg started dropping. Speed kills mpg in the RX....But on a long trip going through hilly Missouri at 70-75 mph I was getting 24ish, but on open road in Oklahoma at 75 mph I was down to 22, when I got to Texas at 75, I was down to 19-20 (for 300+ miles!). Nothing changed, speed, how I drove, etc. I think I just had enough winds to kill mpg.

kiwi

thanks for the information, all very good stuff.

the main reason i was leaning towards the O2 sensors was the general rule that they can wear but not throw a code after 100k, and since i have 209000km it was just a thought.

the FPR does make alot of sense, but again, why bother until its diagnosed.

as for the list as the end of your post, all is well, i dont drive hard, all regular main. is done, tire pressure is good, tranny is great (dont take that the wrong way) belly pan is properly attached and my driving habits are conducive to good fuel economy, and i dont idle for very long if at all.

i used to be a mechanic (chrysler for 4 years, Acura for 5) so i know the lingo, i just lack the diagnostic tools.

the more i ponder it, the more i am tempted to order the tool you suggested. just too bad the software is so expensive.

Gitchiesus

Keep us updated. I too wondered about my gas mileage. I think last tank I was right around the same. I noticed I got 305kms on one tank. That was on regular fuel from only shell. Yes it's been very cold here in Canada, so it did idle a bit longer than normal.
So next tank, I filled up with premium to see if there is any difference. Off the hop, the computer said driving range was 392kms....that is less than what regular gas said previously(409). Weird.
I've got about 70kms on this motor it's an 04.
Only thing I can think of this car has a really small gas tank. Smaller than my Audi A4 and tiguan.

kiwi

i have ordered a data logger, should be here in 2-4 weeks (china).

i will keep you all updated.

lately i have been between 17-20 L/100KM. it is ridiculous, there is clearly something wrong, i plan on keeping the vehicle, so if it means replacing all O2 sensors, i will.

Bikerdad

my 2006 is showing 16L/100km now and 13 when it was warmer. Regular gas in Toronto.

LazarusLng

I've been averaging 15.5 mpg. Premium gas. City driving. Last trip I took, I got 20mpg highway. 2004 with 95K miles.

Gitchiesus

Some of these numbers look really low. I get its a decent vehicle but I would expect 450kms to a tank? But I think I've come to realize the gas tank is just small, like only 56 litres? Which is more
Was size of a car gas tank

Bikerdad

As far as I know it's a 60L tank. I put in that much the last fill up.

diverchris

I just filled up yesterday. I got 518km out of 53L, mix of highway and city, Not too bad for winter formula. All I use is V-Power Shell

Gitchiesus

I'd be happy with anything over 500kms a tank. I'll have to wait it out and see. I haven't had mine for very long. First tank was like 325kms....on regular gas. The previous owner had said that's all he used, I checked the book and it does only say 87 or higher for better performance.
My mdx gets about 525kms a tank, but that tank is bigger, so I had thought maybe getting 450 would be normal for an rx

Bikerdad

Strange, mine is showing 13+L/100km now.

AussieAuto

All depends on your driving Kiwi, I'm about 14.5 - 15l/100km but i only drive about 6km from home to work and vice versa and in winter it will always run richer and use more fuel, especially if you are driving through snow or slush even rain. When i get out on highway it gets way better down to 10.5L/100k Check you fuel trims long and short in conjunction to a/f and o2 readings.

kiwi

my driving is not the issue, i understand ill but more fuel in winter, but ive had this car for 5 years, and its far worse this winter. i am now at about 16-17 L per 100 km. if i go out on the highway, it might drop to 14, maybe 13.5. this vehicle should be doing better.

i dont let it idle either.

i ordered an ELM 327 online, showed up yesterday. used the Torque App. everything is checking out, i watched the short term and long term fuel trims.

i am at a loss, not sure where else to look\

open to suggestions.

AussieAuto

What are the % numbers on the short and long term fuel trims @ idle
Also the voltages of the a/f sensors and the O2 sensors @ 2000 rpm after about 2 minutes
The MAF sensor reading grams/sec @ idle too