Lost about 10mpg over the past few weeks...

[themightyquinn]

So all of a sudden my MPG has gone way down. I'm used to about 32-33 all highway or 28-29 city. It's down to about 18-19 combined. I've read some other mpg threads and checked the usual culprits.

About 2 months/1.5k miles ago I replaced the following:

>BOTH O2 sensors (Bosch)
>Spark plugs (NGK) along w/ valve cover gasket
>Fuel filter (Bosch)
>Air filter
>Did oil/manual trans/diff fluids

I don't smell fuel nor have I seen leakage from the filter or residue on the ground. I upgraded to 225/45/17 tires about a month prior to said maintenance. Tire pressures are good. Temperature seems good, she starts right up on first crank, and gets down to idle in a few seconds. Drives smooth.

Seems a little down on power, but that could be my head playing games. It's a '97, manual trans, 147k miles.

Any thoughts? I've been using 91-93 octane. Possibly a bad replacement O2? Keep in mind things were fine for a good 1000-some miles and a month and a half. No CEL's. Help?

[greenmeanie]

Compare the new wheels / tyres with the old ones if you can, if they are larger then you have raised the gear ratio.If so you will then find that the fuel MPG will not read correctly as the speedometer is now incorrect, this will also reduce the acceleration slightly.

[Mallard]

camshaft sensor?

But rolling diameter may also give a difference in Miles to Liters..

[spidertri]

I doubt that you would lose 10mpg just from increasing the tire size. The RKs aren't that heavy, certainly not more than the original steelies. Unless you have like the highest rolling resistance tires known to man.

Do you make a lot of short trips as opposed to a month ago?

[greenmeanie]

READ MY POST AGAIN. It may have changed the indicated MPG not the actual MPG.
When you have changed any thing and then find you have a problem check that first,it is cheap and easy to do that .

[spidertri]

Going from a 205/60-15 to 225/45-17 tire results in the speedo reading 1.2% too slow. That means that the new tires would have less rev/mile than the old ones, thus lowering the gear ratio. However, because there is more tire contacting the road the mpg would drop. Not 10mpg though.

And I wasn't attacking you, I agree that he should put on his old wheels for a while and see if there is a noticible difference. The RKs are pretty light though, ~18lbs, and his stock steelies are pretty heavy so I again doubt there will be much difference.

[dave45056]

Is your neighbor siphoning your gas?

[themightyquinn]

Guys: thanks for the replies, but please read my post again. I had the 17's on a good month or two before any of the above maintenance. I had approx the same mpg after as before. After the maint. the car ran great and got the same/slightly better mpg than before (as would be expected). Everything was completely NORMAL and fine until 2 weeks ago, then a very sudden drop in mpg.

Originally Posted by spidertri

Do you make a lot of short trips as opposed to a month ago?

Nothing more than usual city driving. The very worst I've ever gotten on full city/stop-go was around 26/27. The 18/19 mpg figure of late is including about 1/3 highway driving!

Originally Posted by dave45056

Is your neighbor siphoning your gas?

I'm really starting to think that's it...

[spidertri]

I'm guessing you haven't gotten any CELs?

[b.u.ti-ful]

Did you modify your air intake?

Is your engine getting enough air?

[dave45056]

I would scan the OBD2 port and see what it says.
2 things come to mind:
1. faulty o2 sensor
2. cracked intake boot
if you happen to have the old o2 sensors... try re-installing unless you got a CEL that said they were bad.
I wonder if the system is getting into closed loop mode.
Also you will need to remove the intake boot to see if it has a crack.

[L84THSKY]

Definitely scan for CEL faults. Are the new O2 sensors OEM fitment for the car, or universal? Don't trust universal sensors.

[themightyquinn]

Nope, no CEL...but it can still throw a code and not have the light come on, correct?

I did Fogg the intake. I didn't do some ghetto-hackjob...it was really clean, ran ducting down to the brake duct, screened it off, insulated it, made sure it was sealed from inside-enginebay air.

A cracked intake boot might actually be very plausible. I inspected it a few weeks ago because sometimes I hear a whistle/high pitch noise under accel. I didn't see anything...but I'll have to look more carefully. Could a tiny hole in the boot really drop my mpg this much?

The O2 sensors are OEM fitment...ordered from pelicanparts using the realoem diagrams/part numbers. I'm trying to get a hold of a scanner soon.

What do you guys think about a bad Cat? It seems a little down on power, and I'm not sure if the Cat was changed recently.

Also, I wanted to let you know that I don't have an OBC...so the mileage I'm calculating is based on miles driven/gallons used when I fill up at the station. (just to avoid confusion on not resetting the OBC etc)

[b.u.ti-ful]

Did they happen to recently add 10% moonshine to your gas?

Ethanol reduces mileage.

[spidertri]

MD has had 10% ethanol for a while now.

Quinn, my cat is pretty crappy yet I still manage ~33mpg (mostly highway), as you know, I'm waiting for it to implode one of these days.