2mpg Mileage drop after timing belt install

Hey all,

So, did my timing belt/water pump a few weeks ago. I was hoping I might see an MPG improvement, but after the first tank, it DROPPED 2mpg.

I got 14.5ish on the tank right before I changed the belt, then filled up again right after I completed the work. I burned that tank in the last 2 weeks and my mileage was 12.5ish.

The truck runs smooth and purrs like a kitten, so I'm wondering if it could just be a fluke with the ECU getting reset after having the battery disconnected. I've heard you shouldn't take your truck to get smogged right after resetting the ECU as it will fail. Is this a thing?

I know of at least a few other reasons for bad MPG: MAF, clogged cat, dirty throttle body, etc... and I plan to inspect those and run a couple tanks of injector cleaner, but just wondering if I should be concerned about the sudden drop in MPG.

The whole reason I did the t-belt when I did is because I was getting tired of mpg's in the 14's. My Crowning achievement several years ago was when I towed a small camper trailer to Washington State and back to Cali at 18.5mpg.

Thoughts?

 

Too many things impact fuel economy to give you a good answer. A new timing belt and water pump really shouldn't make much difference.

 

I'm just trying to rule things out, like the possibility that resetting the ecu could cause a (temporary) drop in mpg.

My driving habits didn't change, and even when I drive around in 4wd or with a heavy right foot, or towing, it's never been that low.

 

Resetting the ECU is likely to cause a temporary drop in fuel economy but I wouldn't expect it to last very long. If you installed the timing belt off by 1 tooth it may still run and would most definitely cause a drop in fuel economy. However I would expect it to not sound right and have a notable loss of power. It is more likely something else caused the your fuel economy to drop off. Maybe you got a tank of fuel with a higher ethenol content or someone stole a couple gallons of fuel from you tank... If you put in a lower temp thermostat, that can impact fuel economy or air pressure in your tires dropped. If I were you I would check the simple things and if all looks good, see how it does one your next tank.

 

I am 100% certain the timing belt is aligned perfectly, I was super paranoid about and cheched it like 30 times before reassembly.

A good theory about the stolen gas, anything is possible I guess. I'll have to wait until I go thru a 2nd tank to repeat the mpg anomoly.

 

Probably just a fluke. Report back after a few more tanks.

 

Well after a couple tanks it's still running around 12 or 13 mpg.

Would a dirty maf cause the low mileage even if it runs smooth?

 

Timing belt change has absolutely nothing to do with mpg. If your belt was off and timing not correct it would not run correctly and most likely throw the CEL light on. How about the rest of the tune up bits? Spark plugs, wires, cleaning the intake stuff. .

 

Dear Jbrandt,

I am sorry to hear about your loss in miles per gallon, but Xbeaus or Tanner is completely correct in his analysis of your timing belt endeavor. The timing belt has absolutely positively nothing to do with how much or how little fuel your engine consumes. You took your old timing belt off, and your motor was, obviously, running smoothly and correctly timed before you went to work installing the new timing belt. You installed the new timing belt, and as you said, your 3.4 is purring like a kitten. If you made even the slightest error in aligning the new belt, your 3.4 would be running like shit if you could get the engine to run at all, and it would be throwing error codes at you. You, obviously, did a fine job doing your homework. With timing belt replacements and timing, your engine is either correctly timed and runs, or is incorrectly timed and does not run. It is that simple.

As far as your 3.4 consuming too much fuel, you are going to have to go back to the drawing board on that one, but don't go redoing your timing belt because that would be a complete waste of time, and totally futile as far as improving fuel consumption. As members here have already told you, resetting the ECU is a good start.

Good Luck,
Paul

 

I've reset my ECU so many times it probably doesn't know WTF is going on. ha only reason I track my MPG is to make sure it's running properly. Too much of a deviance in numbers means something is leaking or going out. I'll have to buy another Honda or small car to actually care about MPG. Driving a Brick down the interstate though I'm pretty happy with what it gets considering it's age and everything else that's been running for 20 years.

 

To Everybody,

The stock Toyota 5VZ-FE 3.4 liter V6 with the manual transmission gets 16-18 miles per gallon. This is what the EPA claims, and they test the hell out of anything they get. The automatic transmission does a little better at 17-19 miles per gallon. This is probably because there is more on board Toyota intelligence on the automatic trucks than the manuals.

Take it easy,
Paul

 

16-18 Is what I get with my Shtickshift. Sometimes higher when it's warmer out.

 

I can't provide any insight on the MPG drop, but in regards to the ECU reset, my understanding is that information is correct for the most part. When you reset the ECU, the readiness monitors will show "incomplete" until you complete a full drive cycle. There are ways to expedite the monitors to "complete" in order to avoid waiting a long time and just driving under your normal driving conditions. And your state may allow TWO "incomplete" on CERTAIN monitors. Not sure how many "fails", if any, you can have.

 

getting back to the reset ECU possibility. did or didn't you disconnect the battery and/or reset the ECU for the repair?

 

That's not good. I get 16-18mpg with 33 inch tires. I fully expect to be back in the 20s when i regear to 4.56s.

 

Yes, I disconnected the battery for a good 2 days (should be enough time to reset the ecu, lol) when I replaced my timing belt.

My 1st tank post-work was 12.5 mpg, the 2nd was 13.5, but I also had about 60 miles of highway on that tank too. The 3rd, which I'm on now, is looking to be again in the 12-13mpg range, but haven't filled up yet.

 

This is not something I have had the time to do as of yet. My next guess would be the MAF, or crud in the throttlebody/intake.

At least it will be sunny this weekend so I can work on it a bit.

My main question has to do with a dirty MAF and other components vs. how smooth the engine seems to run. My understanding is that dirty MAFs would cause a rough idle etc... Mine runs perfectly smooth, and idles around 600rpm like it always has.

 

i get 17-19 on my MT with 33's as well, if i could get a solid 20 i would be totally happy

 

that highway or city?

 

then yes, until your computer fully calibrates your emission monitors, you WILL be seeing a drop in MPG. the monitors need to meet certain requirements before they are fully tested. here is a link to a readiness monitor PDF and how to directly complete them.

and here is an image of what the readiness monitor involves on the torque app for some random vehicle, it doesn't say. its usually the catalyst, heated catalyst, and evap sys that take a long time to complete because the driving requirement is very uncommon. you'll see in the PDF, you have to like, drive for 10 miles at exactly 50 mph, and in those 10 miles you have to come to a complete stop in the middle of the road or something stupid like that.