bearsnob

I've recently experienced a pretty big decrease in gas mileage on my xB and I'm wondering what ideas people here might have for things to check. I have a ScanGuage and it is registering a decrease of 5 to 10 miles per gallon which coincides with calculations at the pump. There also seems to be a small loss of power that is evident when climbing a hill or at highway speeds. It is barely noticeable but I'm pretty sure it is real and not just something I *think* I am noticing. Also, about a month ago, gas mileage was normal, then I noticed about a 2 mpg decrease, and gradually it seems to be getting worse.

So I changed plugs and air filter. Both were due for changing, but doing that didn't improve mileage. I ran some Lucas injector cleaner through the system. That didn't help either. Took it to my mechanic today and everything checks out fine. There have been no check engine lights like you might expect with a partly clogged cat/exhaust system. Was going to have the fuel filter replaced just to make sure, but apparently that is not something that needs replacing.

Just to be clear: I'm not running AC or doing anything else that would cause a normal drop in MPG. So it has to be something mechanical. When I took it to the mechanic today, I followed a route that is very familiar. There was no wind or any other conditions that would affect gas mileage. On that trip I usually get 42 to 46 mpg over 15 miles because there is a 500 foot elevation decrease. Today I got 36.7 which is the lowest I have ever gotten on that route. In my mind, the next thing to check is the exhaust system, but there are people here who know a lot more about these things than I do. Any ideas will be greatly appreciated.

Reactor

And your tire pressures are?

bearsnob

Good question. I run them at 32 all around. That has been consistent before and after the mpg decrease.

AnotherDude

iTrader: (1)

PCV Valve. I changed mine at 108K and immediately saw a 2-5 mpg gain.

bearsnob

I don't know why I didn't think of that one. Recently a friend told me about barely making it into a repair facility (the car was running that bad) and it turned out to be a PCV valve. I had never heard of anything like that, but my friend said he heard of it happening several times after he had it happen to him. Worth a shot, that's for sure. Thanks.

opus139

Watch what type of gas your using, if the gas station is selling 10% ethanol blend the mileage and power will drop. I switched back to Shell and the gas mileage went from 27 to 29-31mpg. Ninety percent is city driving stop light to stop light.

LuisRX7

Hey, how's the mileage now.... any improvement?

bearsnob

Installing the new PCV valve tomorrow to coincide with the next gas fill. I am hoping that is it. Things got complicated a bit because I installed a cruise control last week getting ready for a long drive to Alaska. So I've been using that almost every chance I get to make sure it is working properly (it is). That bumped up my gas mileage a bit, but then I wasn't sure if the increase was because of that or because of something the mechanic did. Now I'm sure it was the cruise control. So if the PCV solves the original problem, I'll be golden. Thanks for asking. I'll let you know as soon as I know.

LuisRX7

Awesome thanks !

bearsnob

Well, shoot ... at least the swap was super easy and the old PCV looked really bad (but sounded fine). Anyway, no improvement in mpg. One thing really good about the cruise control is now I know almost exactly my gas mileage from a given point A to B. Changing the PCV didn't even yield 0.1 mpg.

Only other thing I think I can do myself is the MAF sensor, meaning I can clean it. Not sure if that is even a possibility, but in theory it might be. By process of elimination, I guess it is looking like it might be early stages of the catalytic converter breaking up. I guess I'll have that checked and if that doesn't pan out, then I'll see what Toyota can figure out.

LuisRX7

Thanks for the feedback, I have never ever changed a PCV valve on any of my cars, so I was curious if it would even make a difference.

I used to get roughly around 300 miles per tank ~10 gallons and since the end of last year I've been getting around 220 miles per tank for about roughly the same ~10 gallons.

I cannot explain the reason for this other than it is running rich (which I could smell if I turn on the car after a cold start and step out of the car while idling).

I don't know how to adjust the A/F mixture or why it would be out of spec since everything is OEM, and nobody has touched anything under the hood other than routine maintenance per Scion manual.

Maybe has something to do with the fuel injectors?

bearsnob

You know, mine has been running rich at times as well. I was going to report that in last night's post and then deleted that portion because I went out and started the car and couldn't smell gas like I have in the past. I'm still betting it is the cat because, with my admittedly no-so-great understanding of newer engines, that seems like something that would cause problems sometimes and not others (i.e., the clog could get better at times on its own, especially if you are talking about the early stages of its demise). But, then again, maybe sensors do the same sort of thing. I don't know. What I do know is that, like you, my mpg is way off of normal. I'm leaving on an 8,000 mile trip this week and I would really like to have this fixed before I leave. I'm sure the repair would be easily paid for by the increased mpg on this trip alone.

bearsnob

The problem is pretty much solved. I took it to the local exhaust shop and they checked the back pressure. Told me that the cat was "definitely" starting to clog and that the rear O2 sensor was 90 percent which they said meant it could possibly need replacing but they wouldn't know until they replaced the cat. Red flags started going up in my head. My mechanic had recently checked the sensors and they were all fine. A back pressure problem isn't necessarily the cat. I asked if this would definitely solve the MPG problem and they said they couldn't guarantee it. At some point I asked how bad the clog was and they said something about the car having 100k miles on it. So I asked if that meant that most xB's with 100k miles would have about this amount of clogging and they said yes. But, in the end, they still said they were really sure the problem was the cat.

I made an appointment for Monday and then drove over to the scion service department at the local dealer for a second opinion. They basically said the cat theory was baloney. It would throw a code if it needed to be replaced. They also said that anything that would reduce mpg that much would throw a code. Sounded like they were at least being honest because they didn't want to do any work on the car. So I mentioned that someone had told me the MAF sensor might need to be cleaned. They said that might be worth a shot. So I went to AutoZone and bought a can of the cleaner. I pulled the sensor and was disappointed that it was so clean that it almost looked as if it was new. But I had the can of cleaner, so I sprayed it down really good, let it dry, then put it back in. Got about 4 mpg more on several trips this afternoon. That's most of what I thought I had lost and the rest could easily be explained by a few things I was already aware of. Long way of saying that I think the problem is solved.

Btw, the other thing I noticed was that the car had more power. It wasn't downshifting in places where it used to downshift. In fact, on these runs it hardly downshifted at all including times when I would use the cruise control to accelerate. In the past, that would almost always trigger a downshift at highway speeds. So both performance and mpg improved and I guess this skeptic is now a believer when it comes to the issue of whether or not the MAF sensor should be cleaned on a regular basis.

taman86

Absolutely^^^^. it should be checked faily regularly. shouldnt pull it out every oil change depending on your service routine since could run the risk of damaging it and it is an expensive piece to replace. but ide say maybe every 10k or so depending on your environment and also depending on whether or not you are running a stock filter or aftermarket. ill be honest guys, i am all for performance enhancing and will swear by k&n for that reason, however being a mechanic, they DO NOT clean the air, so to speak, as well as a standard paper filter. BUT if properly maintained such as regular cleaning every 10k, properly oiled and not over or under oiled, your engine and MAF will be happy. When you clean the filter, clean the maf as well.

When the maf gets dirty, the heating elements inside the MAF housing cannot heat up properly and the air passing over the elements cannot cool them properly, which the computer senses the signal and either leans or richens the A/F mixture along with a number of other factors. So more then likely, thats why you were running rich.

For the CAT and O2 sensor, O2 sensors do go bad after a while, especially running rich or lean. Running lean will destroy the internals of your CAT fairly quickly since CAT's are meant to operate within a certain temperature range. If the engine is running lean and the exhuast temp is already super heated then it degrades the CAT. Running rich will clog the CAT because it is igniting the unburnt fuel inside the CAT and building up carbon within the screens and building up carbon on the O2 sensor. Most O2 sensors are 4 wires and operate similarly to MAF's with a heating element. However the computer uses them to watch O2 parts / million. more or less within the specified range is either lean or rich. The computer uses a reference voltage through the sensor and it continually swings between 0v and 1v. 1v back to the computer means no resistence, 0v is the most resistence, and uses that range to calculate the parts per million and try to adjust accordingly. Hope this information might help you or someone else in the future.

LuisRX7

that's great news !
I was putting off this issue since I have other projects I'm working on. This morning I did a few things;
I checked the air filter and dusted off the surface with compressed air,
I cleaned the throttle body (TB) with appropriate cleaner,
cleaned the MAF (looks like a resistor) with some rubbing alcohol very gently,
and finally re-gapped the spark plugs to .030in
I have a full tank, so I'll let you know if any improvements.




cool, thanks for the info. I was wondering if there was a way to manually adjust (or override) the A/F ratio. I was reading through the Scion manual, but didn't find anything on the matter. maybe everything is dependent on these sensors and computer controlled.

jct

i clean my MAF once a year and its at 80k

taman86

As far as I know with these ECU's the computer can only adjust the A/F ratio so far before it peaks positive or negative. So from what I have read from another guy's thread, if you keep adding parts that frees up air flow into the engine, the ECU can only richen the fuel so far. According to this guys thread when he went to have his car dyno tuned, his tuner was saying that it was just within the threashhold of running lean, but he had full intake setup, head port, full exhuast, maybe even a mild cam profile. Possibly going to larger TC injectors could rectify this but not sure if the ECU has to be piggybacked to run them. As far as leaning out manually, deffinately piggyback, plus have to have a tuner tune it. Another good way to get the most out of the A/F ratio would be to advance the timing for N/A cars, which again for this ECU would have to be done with a piggyback. The only one that I know of, and I havnt been in THIS game too long with these cars, is the greddy e manage ultimate.

LuisRX7

hmmm, very valid points. I would have definately gone all out with the mods, but I am already deep into that with my other cars. Anything engine or drivetrain related for that matter is OEM on the XB and not planning to make any changes. Just keeping it stock, putting in trouble free miles, just hoping to get some better mileage like I used to and hopefully continue to. I am monitoring to see if I see any improvements with what I did today and go from there.

bearsnob

Thanks again and good luck to everyone trying to get their mpg back to where it used to be. I'm headed to Alaska tomorrow and probably won't be online much. I still find it hard to believe it was the MAF sensor. The thing just looked so clean. But I'm still getting great mileage even with the car packed full (rtt, arb fridge, the three legged cat, the wife, and everything else). The can of CRC MAF sensor cleaner was $8 at autozone and should last a long time.

jct

i've been getting my normal mpgs around 34-38 mpg i get a couple more mpg's in fall and spring weather