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Old 12-05-2009, 02:51 PM   #1 (permalink)
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3.4 V6 - Poor Fuel Economy and High Idle

Howdy folks,

Here's my stats: 97 Tacoma 4wd manual 5-sp with 3.4l V6. Only mod is a K&N Air filter... I'm running 31x10.5r15 BFG ATs (I'm pretty sure that's the stock size).

My Taco has been getting progressively worse fuel economy over the last few months. Over the summer, I was consistently getting 20-21mpg mixed hwy/cty. Now I'm getting around 17... Any ideas? Is this just winter blend fuel that sucks??? Could it be an O2 sensor going bad (even tho' hasn't thrown any codes yet)?

So last weekend, I cleaned up the throttle body thinking it might help a little bit. While I was doing that, I pulled the EFI fuse. Now, my truck is idling up around 1700-1800 rpm! Is this just the ECU still mapping itself out since the 'reset'? How long should this take? I've probably driven 300 miles since then. My TPS checks out just fine. When I tested the IAC Valve, it appears to be working fine although it doesn't quite close 100% (not sure if this a problem?). Also, I just cleaned the MAF sensor with special cleaner when I cleaned the throttle body.

I've searched the forum and tried everything that I've found so far. What am I missing?

Cheers,

Jeremy

Edit: It appears to have recent plugs/wires - the date stamp on the wires is 'Denso 2007' although that was before I bought her...
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Last edited by ubermoto; 12-05-2009 at 02:53 PM. Reason: Forgot to add somethin'
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Old 12-05-2009, 06:12 PM   #2 (permalink)
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Butane in the gas. They put it in during the winter so that cars will start when it is cold. I am getting worse gas mileage now than I ever have. Check your tire pressure. I usually end up putting air in my tires when it starts getting cold.

If it is idling 1700 when it is cold, it is ok when it is warm that is definitely too high. Make sure you didn't over-tighten the throttle cable when you put it back together. The cable should have a little slack. If that is not the issue, while the truck is running, unplug your TPS and plug it back in. This will reset it. Your CEL will come on but then turn off after 5 startups.
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Old 12-05-2009, 07:15 PM   #3 (permalink)
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Quote:
Originally Posted by stir_fry_boy View Post
If it is idling 1700 when it is cold, it is ok when it is warm that is definitely too high.
Yep, it stays up around 1700 even after it is warmed up...

Quote:
Originally Posted by stir_fry_boy View Post
Make sure you didn't over-tighten the throttle cable when you put it back together. The cable should have a little slack.
Check... It is slack.

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Originally Posted by stir_fry_boy View Post
If that is not the issue, while the truck is running, unplug your TPS and plug it back in. This will reset it. Your CEL will come on but then turn off after 5 startups.
Thanks for this tip. I'll give it a try tomorrow!

Cheers!

Jeremy
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Old 12-05-2009, 07:28 PM   #4 (permalink)
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I am curious about this aswell. My runner has been doing the same things. Although my mileage is even worse than yours.

Come on somebody who knows stuff.
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Old 12-05-2009, 08:11 PM   #5 (permalink)
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Quote:
Originally Posted by ubermoto View Post
Also, I just cleaned the MAF sensor with special cleaner when I cleaned the throttle body.
What was the special cleaner and how did you clean the MAF?
What did you use to clean the TB?
(maybe that cleaner was burning off and maybe your ECU hasn't been able to adjust yet.)
Did you use new TB and IAC gaskets and some RTV silicone where the IAC and TB meet? Or did you just clean the TB without removing it?

If you pull the - battery cable off for 45 min it will also reset the ECU.
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Old 12-05-2009, 08:29 PM   #6 (permalink)
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What was the special cleaner and how did you clean the MAF?
I think it was Gunk MAF cleaner I got from Napa. Followed the directions on the can - just remove the sensor, place it on a rag and spray it down then let it evap... Repeat about 10 times...

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What did you use to clean the TB?
(maybe that cleaner was burning off and maybe your ECU hasn't been able to adjust yet.)
Berryman's B-12 Chemtool Carb/Choke/ThrottleBody Cleaner

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Did you use new TB and IAC gaskets and some RTV silicone where the IAC and TB meet? Or did you just clean the TB without removing it?
Last weekend I cleaned it while it was on the truck, so no gaskets were removed... Today, I removed the whole throttlebody and was able to clean it more thoroughly. I was also able to test the IAC valve that way. I re-used the gaskets, but I did put some RTV silicone between the TB and the IAC Valve. No apparent improvement after the second cleaning today, but I only did a quick test drive...

Quote:
Originally Posted by brian2sun View Post

If you pull the - battery cable off for 45 min it will also reset the ECU.
10-4...
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Old 12-05-2009, 08:46 PM   #7 (permalink)
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Ubermoto, it sounds like you did everything right. 17 mpg doesn't concern me, that's pretty average especially for winter gas. I'm more concerned with your 17-1800 rpm idle after it warms up. I still think it's the computer having a hard time adjusting maybe due to burning off excess cleaner.
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Old 12-06-2009, 05:38 AM   #8 (permalink)
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Thanks for the feedback Brian... Curse this winter gas!!!

I'll report back the results of unplugging the TPS while the engine is running'. Hopefully later today!

Jeremy
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Old 12-06-2009, 07:00 PM   #9 (permalink)
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Mixed results today... Most typically, she's idling at about 1500 rpm. A couple times though, when I started her up, she sat at 1200, but after driving a little bit, it would end up back up around 1600...

My feeling is that it's just the ECM remapping after the reset... It was mentioned in the throttle body cleaning thread that this happened, but he said his throttle was only slightly higher...

That's my theory and I'm stickin' to it... For now... Until somebody smarter comes along and proves me wrong!

Cheers,

Jeremy
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Old 12-07-2009, 10:25 AM   #10 (permalink)
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Sounds like a vac leak to me.
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Old 12-07-2009, 01:34 PM   #11 (permalink)
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Quote:
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Sounds like a vac leak to me.
10-4... I'll break out the WD-40 vacuum leak detector kit when I get home!
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Old 12-07-2009, 01:51 PM   #12 (permalink)
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10-4... I'll break out the WD-40 vacuum leak detector kit when I get home!
The time it happened to me, it was just one hose in the back that I forgot to reconnect after doing some work. That one that goes from the PCV valve across (near) the firewall and connects to the intake tube. Its easy to forget and hard to see.
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Old 12-07-2009, 05:53 PM   #13 (permalink)
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Originally Posted by mt_goat View Post
Sounds like a vac leak to me.
Thanks for the ideas so far guys!

Well, it's f-n cold out there, but I want to get to the bottom of this! So I sprayed around with my wd-40 lookin for vacuum leaks, but didn't find any. I did notice that the throttle linkage that connects the throttle cable to the butterfly isn't returning all the way to the stop-nut, even though there is slack in the cable. I can press on the linkage with a finger and it closes the rest of the way and the idle drops down to about 800.

Why wouldn't my butterfly be closing all the way??? Could it be that the gunk that I cleaned out was not allowing the butterfly to close all the way and was blocking the narrow gaps on the top/bottom of the TB, effectively sealing up the main airway?

There are what appears to be two dash pots to help slow the close of the throttle when it gets close to completely closed. I assume to prevent backfires and damage to the valve when it slams shut. The front one is black and is not connected to anything - it doesn't appear to be blocking the linkage. The rear one is silver and near where the TPS plugs in - it has a vacuum hose running to it. Here's a link to a picture (not mine) that shows them. This rear dashpot (or dash pot?) may be the culprit, but I can't really tell because it's behind everything when it's installed on the engine. Is it possible that I just need to clean it up a bit?

Thanks again!

Jeremy
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Old 12-07-2009, 06:42 PM   #14 (permalink)
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This poor guy had very similar issues... Looks like he ended up buying a used throttle body. I don't think mine has the side-to-side play that his did, but I do remember seeing some scoring on the side of the TB bore.

Hmmm... I wonder... Is it possible I could have slightly bent the brass plate portion of the butterfly? More things to check when it warms up outside...
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Old 12-08-2009, 06:22 PM   #15 (permalink)
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More symptoms...

Today, driving home, I stopped for gas. It was idling high on the drive to the gas station. So I turn the engine off, fill up the tank, then start her back up. When she starts back up, she idles at about 1000 rpm! Just slightly above spec. Drive a little bit, then come to a stop light and the idle settles back up around 1700 rpm. I'm so confused... Won't have a chance to dig into it more till Friday...
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Old 12-23-2009, 06:35 PM   #16 (permalink)
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Quote:
Originally Posted by UpInSm0kE
Ubermoto,

Man I am having the exact same issues that you are having. I have replace the IAC and the TPS sensor and i still cant figure it out. Did you find out what your problem was yet?
Got this inquiry via PM so I thought it was about time for an update.

Still idling high - now around 1500 rpm which is better than it was at 1700-2000 rpm. My O2 sensor was going bad (I think it's been going bad for a while, but it took a couple months to throw the CEL), so I think that was complicating the diagnosis. Here's my current hypothesis:

All the gunk that dirties up the throttle body helps to seal the butterfly valve. This forces the IAC to open up wider to maintain the idle speed high enough. When you clean the gunk out of your throttle body, some air or at least more air can 'leak' through the butterfly valve, but the ECU is accustomed to opening the IAC for no air leakage, so it takes a while to adjust. During the period of adjustment, either the ECU learns to close the IAC down more or the butterfly gunks up enough to seal better.

Anyway, that's my theory and I'm stickin to it!

Good luck figuring it out and if you discover anything, please let me know!

Cheers,

Jeremy

P.S. I saw a thread over on ultimateyota that said something to the effect that idle speed typically goes up after throttle body cleaning but usually settles back down over a period of a couple of weeks to a month...

So maybe the correct approach is to worry less and drive more.
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Old 12-23-2009, 07:50 PM   #17 (permalink)
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have the same issue here. practically gave up on it.
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Old 12-24-2009, 09:19 AM   #18 (permalink)
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Something to check that will make your engine idle fast and kill your mileage is the coolant sensor for the ECM.

The ECM and the temp gauge on the dash each have their OWN INDEPENDENT SENSOR. If the one to the ECM is reading a low coolant temp (high resistance) it will make the engine idle up significatly and also keep it in open loop mode which increases fuel usage.

A very easy way to check is just let the engine warm up and plug in a ODB-II scanner that can give you live data and look at the engine temp. If it reads lower than 185-190 after the engine has been running a while or its erratic it means the sensor is probably bad.

Just a random thought.
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Old 12-24-2009, 01:04 PM   #19 (permalink)
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High Idle

What I would suggest is to determine weather it's a mechanical problem or a computer/sensor problem. Remove the TB and check to see if it moves freely and is able to close all the way. Import cars do not typically respond well to TB cleaning. Even approved cleaners can effect TB bushings and cause binding, also the cleaner can kill an O2 sensor regardless of the fact it's O2 sensor safe. Too much can damage it. If the TB is ok, chacking the coolant sensor is a good idea, you need a scanner to check the output.

Air leak is my first guess but some cars just need a proper re-learn. Not sure for a Toyota, you need to find the exact procedure. I'm a 30 year tech but for GM, not Toyota.

Hope this helps.
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Old 12-25-2009, 09:15 PM   #20 (permalink)
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shoot some blaster on the rod that goes through the throttle body, some times when you clean the throttle body you take all of the lubricant out and it gets sticky, then this type of thing happens do not use WD-40 just the blaster. I had this happen at the beginning of the year as a matter of fact Mt. Goat might remember my post of a high idle. I did what I said to do and also I did adjust the limit screw so the butterfly could go farther but ended up screwing it back any how..
No one else mentioned that the TPS is always supposed to read like 5% or in that range
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Old 12-25-2009, 09:15 PM
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