Recent Drop in MPG (1 Viewer)

[Seattle100]

1999 LC, 210,000, running slightly oversized tires on Tacoma Rims.

I don't pay close attention to the mileage, but when I do, it's always around 11mpg. Not great, but it's consistent.

In the past few weeks, it's seemed to be worse, and I just calculated it at 7mpg.

Things that have happened recently:

Had 1 cat stolen, replaced with cheap amazon cat.
Had the fan bracket pulley fail, leading to complete timing belt change.

During that change, I did notice a bit of carbon build up on at the butterfly on the intake manifold. I thought it was a bit blacker than usual, but it didn't alarm me. Now that the mileage is down, I wonder if I'm running rich.

Any ideas? New plugs and coil packs in the past few years. Not sure what else I should be looking at, or the best way to diagnose.

Thanks in advance!

 

[krice118]

Tire PSI?

 

[krice118]

engine air filter

 

[nissanh]

Tacoma or Tundra?

 

[AlpineAccess]

1999 LC, 210,000, running slightly oversized tires on Tacoma Rims.

I don't pay close attention to the mileage, but when I do, it's always around 11mpg. Not great, but it's consistent.

In the past few weeks, it's seemed to be worse, and I just calculated it at 7mpg.

Things that have happened recently:

Had 1 cat stolen, replaced with cheap amazon cat.
Had the fan bracket pulley fail, leading to complete timing belt change.

During that change, I did notice a bit of carbon build up on at the butterfly on the intake manifold. I thought it was a bit blacker than usual, but it didn't alarm me. Now that the mileage is down, I wonder if I'm running rich.

Any ideas? New plugs and coil packs in the past few years. Not sure what else I should be looking at, or the best way to diagnose.

Thanks in advance!

Tacoma wheels are six lug vs land cruiser 5.

Its highly likely to be whatever has changed most recently, so you should start there. I would first suspect the cheap cat you referenced is throwing something or an 02 is damaged (or replacement was done wrong or with wrong part during cat install) Hook up to techstream or at least a scanner and check that bank one 02 sensors are giving similar readings against bank 2.

Check over the t belt install work, looking for disconnected or cracked vacuum hoses and the airbox connections including the air filter seal itself, they can get distorted or twisted during install letting unmetered air past. Ensure the MAF is connected properly. You will likely want to pull and clean it with some MAF spray too.

 

[Mike NXP]

Tacoma wheels are six lug vs land cruiser 5.

Its highly likely to be whatever has changed most recently, so you should start there. I would first suspect the cheap cat you referenced is throwing something or an 02 is damaged (or replacement was done wrong or with wrong part during cat install) Hook up to techstream or at least a scanner and check that bank one 02 sensors are giving similar readings against bank 2.

Check over the t belt install work, looking for disconnected or cracked vacuum hoses and the airbox connections including the air filter seal itself, they can get distorted or twisted during install letting unmetered air past. Ensure the MAF is connected properly. You will likely want to pull and clean it with some MAF spray too.

^^^
Maybe clean that TB butteryfly as well but the reality is if you had better mpg before the recent work it's best to focus on what has changed. I know I drop 1-3 mpgs in winter when constantly driving on snowy icy roads but sub 10 mpg is not normal for most situations.

 

[riffman12]

seeing as the only thing that changed is the cat I would look at that

Is there a chance it's clogged? Or a smaller diameter?

Did any of the 02 sensors get damaged when it was stolen?

 

[Mike NXP]

Does battery / ECU need to be reset when doing this type of work? Maybe disconnect battery while you clean MAF and TB just because it's easy to do before diving deeper.

 

[Seattle100]

Awesome suggestions, thanks! I'll check all of these.

Yes, wheels are Tundra, not Taco. You should have seen the undersized no named chrome wheels that were barely hanging on when I bought the truck.

After I posted this, the alternator gave out. This is has been a chronic problem on this truck, getting about 3 years out of alternators. I'm not sure why, but new alternator has helped. Is it possible the spark was weak leading to incomplete burn?

Wife reports 10MPG now. I'd like a couple more, so I'll clean throttle body and MAF, check for any loose hoses.

For the O2 sensors, wouldn't they throw codes if they were off? I did the upstream ones a couple years ago, and got one new downstream to go with the new cat. I read things about measuring resistance, but those threads all end with "it was in spec"

Is there an accurate way to determine the health of the O2 sensor by measuring the resistance?

 

[nissanh]

Poorly functioning alternator and or battery can account for low MPG: My 92 Geo Prizm which usually do 26 MPG in 100% city started doing 23 MPG. This happened right after installing 205 tires. I thought well it's these heavy tires.. then I noticed a longer crank time so replaced the battery.. guess what I am back at 26 MPG. I later did a big-3 upgrde (just a new wire from alternator B post to battery positive) and saw 27 MPG now. So yes, battery voltage plays a role in MPG too.

How old is the battery?

 

[DBULL]

Strangely enough,changing the fuel filter seems to have picked my fuel mileage up a couple of MPG’s,along with an increase in low end power(less downshifts).

 

[Coin]

Strangely enough,changing the fuel filter seems to have picked my fuel mileage up a couple of MPG’s,along with an increase in low end power(less downshifts).

I just picked up a new fuel filter, as it seems low on power. I have no idea if it's ever been changed. Hopefully it helps.

It mainly feels low on power when compared to my wife's VVT-i GX 470. I don't expect that kind of difference, but the gulf between the two is huge.

Update: no noticable change with new fuel filter in the first 10 miles or so. At least I know that isn’t a problem.

 

[SteveH]

It mainly feels low on power when compared to my wife's VVT-i GX 470. I don't expect that kind of difference, but the gulf between the two is huge.

This is because it has ~500# more bloat of weight. My GX470 feels a lot more powerful, but I just live with it. I didn't buy the 100 series because it was particularly fast.

 

[Coin]

It mainly feels low on power when compared to my wife's VVT-i GX 470. I don't expect that kind of difference, but the gulf between the two is huge.

This is because it has ~500# more bloat of weight. My GX470 feels a lot more powerful, but I just live with it. I didn't buy the 100 series because it was particularly fast.

I understand, It just feels like my engine isn’t breathing fully.

I know intake is all clean and clear. I have no o2 codes in the exhaust. So I’m wondering if evap or vacuum is causing less power. Then again maybe it just is what it is.

 

[abuck99]

Clean MAF.

Disconnect battery neg, remove and clean MAF- wait 30min before reconnecting battery. You’ll have to allow several run cycles for the ECU to learn new MAF values, but give it a couple weeks and see if mileage improves.