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Major improvement in gas mileage accomplished by 5 minute procedure - 2004 sienna

In recent years, I noticed that the highway mileage on my 2004 Sienna dropped significantly from 24 to 25 miles per gallon down to 21 to 22 miles per gallon. To correct this I did a number of maintenance projects: cleaned the throttle body and the mass air flow sensor (MAF), changed spark plugs, cleaned fuel injectors, etc. But nothing I tried gave a noticeable improvement in mileage. Since the malfunction indicator light (MIL) had never come on, I was pretty sure that no malfunction codes were stored, however, I bought a good, but inexpensive, graphing OBD-II code reader to check O2 sensors and fuel trims, etc.

The OBD-II tool indicated that my oxygen sensors were probably fine, but my short term fuel trims (STFT) and long-term fuel trims (LTFT) were too high with my LTFT's under some conditions greater than 20%, which is just short of throwing a code (i.e.: at 25%). Toyota indicates that both trims should generally stay below 5%. (Note: Fuel trims are the adjustments that the Powertrain Control Module (PCM, also sometimes called the Engine Control Unit, ECU) creates on the fly when operating in closed-loop mode, which are added to the table of default fuel injector pulse lengths specified by Toyota in the PCM memory for open-loop operation. Open-loop operation occurs for instance upon first starting the Sienna prior to accepting feedback from the oxygen sensors that are not yet up to operating temperature.)

A Google search eventually netted a Toyota Technical Service Bulletin (attached below) that indicated that several Toyota models from 2004, including the Sienna, have a defect in which they develop a vacuum leak at the Intake Air Control Valve (IACV). This results in messed up fuel trims. Toyota recommends replacing the valve, which does not look that easy. Toyota's recommended test procedure involves spraying carburetor cleaner at the point where a pivoting shaft pierces the wall of the valve while the Sienna is idling with an OBD-II monitoring fuel trims. The solvent temporarily blocks air leakage into the valve and/or enriches the fuel/air mixture and improves the fuel trims showing that a vacuum leak exists at that point. I did indeed have the vacuum leak problem at the IACV. To deal with it temporarily, I applied an all-purpose grease around the shaft in generous amount and found that this did improve my highway gas mileage significantly. Note: the IACV is located under the hood on the passenger side near the firewall as shown in the photo below.

It is now about six weeks later and I just completed a trip of over 1,500 miles. I calculated my mileage to see if the benefits of the grease persist. I calculated overall miles per gallon of 26.03. This was through states which allowed a 70 mph highway speed, so my speed was generally a few mph above 70. Also, this is with the A/C constantly running at a setting colder than midpoint but with the fan on low. Without A/C running, I am sure that I would get at least one additional mpg. This represents better than a 20% improvement in mileage which for someone like me who drives a car about 15 to 16 thousand miles per year is a savings of greater than $500 per year on gasoline -- very substantial!

Now, a couple further details: 1. Do not try this with silicon-based grease which may poison the catalytic convertor and/or the oxygen sensors. A petroleum-based grease will be safest and thick vaseline may be the very most safe, as it contains no other components. 2. Since my present mileage was taken with a heavily-loaded car (more tire resistance) and my mileage is probably slightly better than when my Sienna was new, it is possible that this vacuum leak existed to some extent even when the car was new OR perhaps mileage increases with greater wear (doubtful after the break-in period). 3. My fuel trims are still not consistently less than 5%, but the LTFT's are about half what they were which apparently is great enough improvement that the O2 sensor feedback system now works effectively. It is likely that I have a few other vacuum leaks that would be good to find, but this may not further improve my mileage. 4. The OBD-II scan tool that I purchased through Amazon was Autel (AULAL519) AutoLink OBDII/CAN Scan Tool with Mode 6 and Color Screen and cost $90.
 

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Nice work. While my sienna is a 2005, it would be something to check anyway. One big help would be a picture of where you applied the blob of grease. could a small piece of tape work as well?
Thanks.
 

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I will try to take a picture of that and post it as I will be unblocking the A/C drain after that long trip I took. Tape will not work because it is not a flat surface and it is a pivot point. Vacuum leaks are probably the first thing to look for if you are experiencing poor fuel economy or large fuel trims, no matter what year your Sienna is.
 

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Yep, I posted this before for someone who was dealing with an excessively lean mixture that smacked of a vacuum leak. Here is the full TSB.
 

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Hi Fibber, I had done a search here before but didn't find it. Sometimes it is hard to find what you want when you need it. I should emphasize that, in contrast to what the TSB suggests, you can still have this problem without the MIL light coming on, as in my case, and the fuel cost penalty is quite substantial.
 

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It is always good to see it brought up again, especially as you did a thorough job of documenting cause-&-effect.
 

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Siennadan,
Thank you for this post. I cannot wait to try with my new-to-me 2004. I have only gotten 17 mpg (mixed driving), and am hoping this might be the solution.
 

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Thisyear: It's a good bet that this is at least part of your problem. If you are using an OBD-II scan tool with spray from a can of carburetor cleaner, you will see immediately whether this is part of your problem. If you skip that step and go right to the grease, be sure to grease it a full 360 degrees around the pivot point of the penetrating shaft (see my jpeg file above). It is not so easy to do this with a finger way back there. USE ONLY VASELINE OR 100% PURE LANOLIN, I.E. A GREASE CONTAINING ONLY CARBON, HYDROGEN AND OXYGEN WITHOUT MINERALS, SO THAT YOU DO NOT POISON YOUR CATALYTIC CONVERTOR OR YOUR OXYGEN SENSORS!!! Note also, that this fix is only temporary for a few months as the vacuum leak will most likely recur.
 

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That is some nice post. So just to clarify if I understand right the technical note: if my VIN is higher than the one mentioned I can assume that the issue will not happen in my car? Last digits in my VIN are: 112731, 2WD, "production effective VIN ...112604". If so than I'm lucky by 127 cars ;D
 

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I hadn't read the full TSB by Toyota in detail previously, but like you I would assume that lower VIN numbers are the defective vehicles, i.e. lower than the PRODUCTION CHANGE EFFECTIVE VIN number given.
 

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Thanks, Dan! You are the Man! Our '04 has been in an MPG death spiral for a couple years now... Getting about 18 or less around town. Some times less.. Always thought it was due to running the A/C in Texas and tooling around town hitting the taco stands... No codes! Quite frequently the cat has a sulfur smell... Indicating a rich mixture, IMHE. The description of the problem sounds a lot like the old problem of air bleeding due to throttle shaft wear on a carburetor... You DO remember carbs, right?).

I had had the FI and transmission serviced about a month ago... It runs great... No issues except for the now abysmal MPG around town.

I'm going to check that valve out ASAP.
 

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Hope it helps you Crow. Whether or not your Sienna has this issue (I think most 2004 models do), a vacuum leak is a likely cause of poor mileage. On second look, replacing the "intake air control valve assembly number 2" does not look overly difficult, although it is a reach and not a lot of working room. I am taking that on in the next few days. Yesterday I fixed my passenger side electric sliding door actuator with a $5 motor replacement instead of buying the whole Toyota part (another pattern defect of the 2004 Sienna) and next week will probably be rear shocks that I noticed were leaking oil.

On the IACV, the correct part number is given by the TSB (17320-0A010), about $85. Don't forget to order a "surge tank cover gasket" also (17127-20010). Some Toyota dealerships also refer to it as the "fuel injection plenum gasket".

UPDATE: June 26, 2014 I started the job of replacing the IACV today, then noticed that the metal A/C tubing prevents one from pulling the IACV straight out. (I had not noticed that problem way back against the firewall.) My recommendation is to use vasoline or pure lanolin for a while as you put this job off until you are ready to also change the rear bank of spark plugs as almost identical disassembly is necessary for both jobs including wiper assembly, cowl, and most of the intake air assembly. See this thread concerning changing spark plugs: http://www.siennachat.com/forum/185-problems-maintenance-repair/8594-how-i-changed-my-rear-spark-plugs-2004-sienna.html
 

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Dan, thanks again for your posting. Ours is a late production '04.... I'm about 10K plus above the production run upgrade. I'm gonna leave her alone!!! I'm thinking that the tires are getting down to needing change out and perhaps that is the reason for the decrease in MPG. And, the hot pavement making them sticky on the mcadam.
 

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After observing deterioration MPG for a few years and reading this post, I suspected I had the leak. I don't have a scan tool, but I thought I'd spray the area with carb cleaner as mentioned in the procedure and listened for any change in the idle...nothing, so I left it alone. Finally, the engine light came on. No other symptoms, however the VSC and ABS are disabled by the error. Got codes P0171 and P0174 (lean fuel mix) left and right side, possibly the air valve control, but didn't want to just start randomly replacing parts until I knew for sure. Even the shop had a tough time with this one. While at resting idle, the valve actually was sealed properly with no leak. The shop then suspected the MAF sensor was bad. Not so! The problem was still present on their test run of the vehicle after repair. Turns out, the vacuum leak was on the air control valve, but only when under load and opened up would the leak present itself! Apparently the shaft would seat properly while in one position, but when turning the leak would open up. Anyways, all's well now. I've got my VSC and ABS back and am looking forward to improved MPG.
 

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I am guessing that it would not be wise to use an RTV sealant around the shaft instead of Vasoline? The shaft moves, correct?
 

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Thank you for this post. Noticing a decrease in my miles per gallon this post caught my eye. We drive a 2004 Sienna and it was listed on the TSB as one of the models that would have the valve replaced if it began throwing up codes for gas/fuel mixture.
I put petroleum jelly around the valve shaft and I did experience better mpg. I then ordered the replacement air intake valve 17320-0A010 Valve Assembly, Intake A from Toyota parts online and installed. My initial test drive after installation showed an immediate better throttle response when stepping on the gas and smoother acceleration. My wife mentioned it to me after she drove the van also.
I am also showing a 2 to 3 mpg increase in gas mileage for city driving. With this increase I will get a payback for the $92 dollars I paid for the part in about 15-20 fill ups. This old 2004 Sienna has gotten close to 30 mpg on the expressway driving at a constant 65 to70 miles per hour before. I am curious to see if we can return to that level of efficiency again. Currently in city driving it approaches 22 mpg after the valve replacement. I calculate my mpg by mileage divided by gallons of gas added.
Again thank you for taking the time to post your findings about this valve. I would have never dreamed of replacing this valve to increase performance and mpg.
 
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