01 Sienna CE very poor mileage - Please Help!

Hi have an 01 Sienna CE with about 70k miles. A while ago, the Check Engine light came on with the p1135 code. I left it alone for nothing really changed. About 2 months ago, the gas mileage suddenly dropped from 20-ish mixed driving to 15-ish. I figured that I should replace the dreaded rear 02 sensor so I bought a Denso one that was supposed to be OEM (same exact plug too) and replaced it. Sure enough the Check Engine light went away and hasn't returned since (6 gas tanks worth of driving).



However, I could have sworn that the gas mileage improved at first but the last three tanks have been giving me only 15-ish mpg.



Sienna Experts, any ideas?



Also, I can hear the throttle valve screeching so if anyone can give me some pictorial directions or just plain directions on removing the air ducts and cleaning the throttle valve, I'd very much appreciate.



much thanks in advance.

Reply 1 : 01 Sienna CE very poor mileage - Please Help!

Some quick generic directions



1. On throttle body side, loosen the hose clamp with a ratchet/socket not a screw driver



2. Unbolt / loosen the air filter box, top half. Do not remove



3. Identify things connected to the air intake hose and top half of air filter box



3a. Mass air flow sensor, it has an annoying electrical connector

3b. Probably one or two rubber small hoses (i.e. 1/2" inner diameter hose) connected to the air intake hose

3c. Double check for other items



4. Once all items in step 3 are removed, lift the top half of the air box and pull / twist to take out the assemble. This will give some mechanical advantage and something to hold so that you can remove the air intake hose which can be very stubborn to remove. I like the gently pull and twist method, watching for anything that is ready to give way.



5. If all goes well, you can clean the throttle body. Go buy a throttle body cleaner. Spray it on a rag and wipe. Open the throttle body by turning the thing (not sure of the proper name) so the throttle plate opens up. Clean the surfaces. You are welcome to spray the throttle cleaner directly on, but you better have good aim.



6a. Other things that you can check is your PCV valve.

6b. Don't know how winters are in Seattle but in Detroit, the winter blend gasoline and cold weather really cuts the mpg on my 2000 Sienna LE. I'm too lazy to measure it since it gets driven on short trips (<5 miles) and estimate that I am getting 15 - 17 mpg.



Hope that helps.

Reply 2 : 01 Sienna CE very poor mileage - Please Help!

Thanks Plant



I'm going to try this this week. Also, I read up some more and it seems that the check engine light (code 1135) was due to the AF sensor heater circuit not functioning, not the actual performance of the sensor itself. This could mean that both of my remaining AF sensors (California emission car) could be bad. The two remaining ones are 110 bucks a pop so I'm a little leery about throwing these parts at the mileage problem.



I'm hoping that someone has ran into this before.

Reply 3 : 01 Sienna CE very poor mileage - Please Help!

If you are not getting a code, your oxygen sensors are good.

The computer moniters these very closely, if they switch too slow, there is a code for that, if they give too low of a output, there is a code for that. if the ouput is too high, there is a code for that also.



Last winter, I damaged the heater for my front oxygen sensor.

I got the Check Engine Light (CEL) and used my Scangauge to read the code that told me that it was the heater circuit for the front oxygen sensor.

I measured the resistance of the heater, and it was OPEN.

I monitored the status and found that, even in the winter, I went from Open loop (not using the oxygen sensor input) to Closed loop (using the oxygen sensor input) within about 15 seconds of starting the motor.

What this means is, the computer monitors the input from the oxygen sensor, when it becomes warm enough, it starts providing a good signal, and the computer then starts using the input from the sensor (closed loop).

In short, while my Check Engine Light would have prevented me from passing emissions testing, it did not impact the operation of the vehicle.......since these sensors are located at a location that gets them warm quickly anyhow.

My fuel economy was not impacted, nor would my emissions be impacted.



Winter is a tough time to learn much from fuel economy.

If you idle the vehicle to warm it up, it will greatly lower your fuel economy.

Winter weather driving will lower your fuel economy.

Short trip driving in winter (your engine takes longer to get up to ideal operating temperature) will lower your fuel economy.

You should still get decent fuel economy on a long trip (if the weather permits steady speed driving at 50mph or more), but around town can really be bad this time of year.

Reply 4 : 01 Sienna CE very poor mileage - Please Help!

I find that when the mileage drops for no apparent reason that adding a jug of fuel injector cleaner (Costco's Chevron ProGuard) just before a highway trip brings it right back up.

Reply 5 : 01 Sienna CE very poor mileage - Please Help!

My fuel mileage has stayed between 17 and 19 mpg for well over 100,000 miles. 15ish and 20ish are kinda vague-ish. My mileage is always poor due to heavy feet and pizza delivery. She'll get 23 mpg when we drive to LA and that's loaded with six people and luggage.



Unless you check every tank your mileage is really only as good as your mood.

Reply 6 : 01 Sienna CE very poor mileage - Please Help!

It turns out that the throttle body has been recently cleaned by the previous owner so that wasn't the culprit of my poor mileage. I have been very easy on the throttle and over the past 8 tanks, only averaging 17 mpg for mixed highway/city. I'm really tempted to replace the two remaining O2 sensors but as the previous posts suggest, unless I'm getting a check-engine code, they aren't likely to be bad.



On a side but maybe related point. There's a whirling sound like that of a motor spinning something up for a few seconds when the van accelerates from a stop or when it slows to a stop. The sound is deliberate but not that loud. You'd have to listen for it. Is this normal?

Reply 7 : 01 Sienna CE very poor mileage - Please Help!

I would be waiting to read the answers to your question minhmattmai.

Reply 8 : 01 Sienna CE very poor mileage - Please Help!

Quote:

Originally Posted by minhmattmai On a side but maybe related point. There's a whirling sound like that of a motor spinning something up for a few seconds when the van accelerates from a stop or when it slows to a stop. The sound is deliberate but not that loud. You'd have to listen for it. Is this normal?

Vacuum leak?