yact: Gas Mileage witch-hunt. what dun it for teh 10% drop in mpg?

[andylawcc]

i must say, I am not 100percent sure with the exact details, since it was a while ago. so here goes:

I changed the Spark plugs (Denso, regular, OEM's, gapped correctly), Spark plugs wires (autozone's Double Silicon), distri cap (autozone's Wells), and rotor (Wells) about 3 months ago. In the mean time of these 3 months, my car was fitted with new exhaust header (the old one was cracked), O2 sensor, and TPS. The gas mileage didn't improve after these subsequence fixes.
The gas mileage before all the changes was hovering around 31mpg, now it's about 27-28. The car is 1996 Toyota Celica 1.8L 7AFE.

so I wonder, is it because of the quality of the set I replaced with? (in other words: should I've gone with more expensive set of wires, rotors, and distri caps instead?)
Does the quality of wires, rotors, and distri caps affects gas mileage at all?

Okay, I may get flamed for this. but the motor has 210k miles on it.
However, how come the drop came so suddenly and not gradually, especially after those changes?


EDIT:
new candidate for the witch hunt: the K&N filter which is now 100k old. Could that be it?
I washed the filter 4 times.

 

[Zenmervolt]

No, it shouldn't affect mileage. What's the temperature difference? I typically see about a 3 mpg reduction during the winter.

ZV

 

[andylawcc]

Originally posted by: Zenmervolt
No, it shouldn't affect mileage. What's the temperature difference? I typically see about a 3 mpg reduction during the winter.
ZV

well, it is getting a bit colder now in LA.

 

[NutBucket]

Could also be the switch to "winter" gas though I've never really noticed much of a difference with my cars.

 

[lobadobadingdong]

That's wierd, I usually get 4-6mpg better gas mileage in winter.

I'm betting low quality parts (I stopped using autozone 2 years ago, thier parts just continually prove unreliable to me)

 

[NutBucket]

You should have gotten the Bosch plug wires But seriously, considering I can usually get OEM NGK wire sets for like $45 why would you bother with Autozone crap? I decided a few years ago OEM was worth the extra money.

 

[tcsenter]

In the mean time of these 3 months, my car was fitted with new exhaust header (the old one was cracked)

It is possible for an exhaust leak to improve fuel economy at the expense of emissions, depending on the design in question. I would also recheck your TPS. An improperly calibrated/adjusted TPS can harm fuel economy without producing any overt signs while running.

Both vehicles on which I've replaced the TPS required a multimeter to verify the voltage at idle and also the voltage change when opening the throttle to 1/4 or 1/2 position. Check the factory recommended procedure for 'proper' TPS adjustment/replacement. Even a non-adjustable TPS could be defective and still seem to be working properly, until you check it with a multimeter and find the voltage is off. The onboard computer may or may not signal that the TPS has a problem unless it has failed completely.

There are other considerations, such as a failing EGR valve (if your car uses one). Troubleshooting cars is even more fun than computers!

 

[andylawcc]

Originally posted by: NutBucket
You should have gotten the Bosch plug wires But seriously, considering I can usually get OEM NGK wire sets for like $45 why would you bother with Autozone crap? I decided a few years ago OEM was worth the extra money.

i can't find NGK wire in the retail (autozone, kragen, NAPA) none. and the Bosch wires are 20 bucks more than the Autozone's cheapos.

but now, (after 3 months), I found NGK.com
45 bucks shipped.

 

[andylawcc]

Originally posted by: tcsenter
It is possible for an exhaust leak to improve fuel economy at the expense of emissions, depending on the design in question. I would also recheck your TPS. An improperly calibrated/adjusted TPS can harm fuel economy without producing any overt signs while running.

well, (long story follows), the 'old' TPS I replaced did not fail - I only THOUGHT it failed. Then I realize the dealership where I bought the new TPS from do not accept return on Electical components. so I went back to the mechanic and asked him to install the new one anyways.

well, I did pay attention this time before&after the TPS change. Gas mileage did not improve nor deteriate. but, i did feel the car is a bit faster, especially in the mid range.

There are other considerations, such as a failing EGR valve (if your car uses one). Troubleshooting cars is even more fun than computers!

how much is an EGR? roughly?

 

[PandaBear]

Is it a sudden drop in milage or is it a gradual drop? I had that problem with my corolla 4AFE before and it turns out to be the distributor cap/rotoer worn out. Since you just replaced it, I would take a look at whether you install correctly.

Things that you did shouldn't affect fuel economy (should help instead) except the header I am not sure, so check for misinstall.

Oh, you are from LA, so they started using winter gas now, and it is not as "efficient" as the summer gas.

 

[andylawcc]

new candidate for the witch hunt: the K&N filter which is now 100k old. Could that be it?
I washed the filter 4 times.

 

[andylawcc]

Originally posted by: PandaBear
Is it a sudden drop in milage or is it a gradual drop? I had that problem with my corolla 4AFE before and it turns out to be the distributor cap/rotoer worn out. Since you just replaced it, I would take a look at whether you install correctly.

well, I can't be absolutely sure, but the drop was a rather sudden one. I mean, I used to enjoy 32-33 all freeway (a 14.5 gallon tank to San Fran anyone?) and 30mpg combined. That is before I had anything 'new' done to my car.
but now I only get high 27 to mid 28 combined.
first of all, there is no improvement in gas mileage with any new parts installed, and now the overall mpg is decreased? that is what bothered me a bit. maybe the car IS getting old and I must accept that as a fact. but I would expect it run better after all the money I poured in.

 

[PandaBear]

Go check again and see if you tighten your spark plugs, it could be loose and all the combustion is leaking.

No improvement with new parts is normal, meaning that your car was running optimal. Car getting old won't get you sudden drop in milage, unless something is broken (or in your case, most likely something not installed right).

IMO K&N is a waste of money, unless you wash it every 10k miles it will be full of dirt and be more restrictive than a paper filter. But in any case, a foam filter should be washed at least every 15k and I would wash it every oil change to be most effective. If you are too lazy, use a paper filter from Fram or generic brand (they are not that critical).

 

[Eli]

Originally posted by: PandaBear
Go check again and see if you tighten your spark plugs, it could be loose and all the combustion is leaking.

No improvement with new parts is normal, meaning that your car was running optimal. Car getting old won't get you sudden drop in milage, unless something is broken (or in your case, most likely something not installed right).

IMO K&N is a waste of money, unless you wash it every 10k miles it will be full of dirt and be more restrictive than a paper filter. But in any case, a foam filter should be washed at least every 15k and I would wash it every oil change to be most effective. If you are too lazy, use a paper filter from Fram or generic brand (they are not that critical).

Uh, ignore this post.

If you wash your K&N air filter every 10k miles, I feel very sorry for your engine.

If his sparkplugs were not tight, he would know it immediately, as they would be blown into orbit(or into the underside of the hood, as the case may be..).

 

[Gillbot]

I refuse to use K&N filters, they allow in too much dirt.

 

[PandaBear]

Originally posted by: Eli
If his sparkplugs were not tight, he would know it immediately, as they would be blown into orbit(or into the underside of the hood, as the case may be..).

No, I had my first plug change loose and it didn't blow out. It hang in there but rattle.

 

[andylawcc]

Originally posted by: Eli
If you wash your K&N air filter every 10k miles, I feel very sorry for your engine.

so I need to wash it more often?