help with 96 Geo prizm
 

hello to everyone, Im brand new to signing up but not to the forum as Ive read tons of articles here.

The reason Im posting is very simple

I have a 96 geo prizm with over 200K miles 5 speed and 1.6L and I cannot get past 34mpg in this car
In the summer with no mods I was averaging 38

I only travel 12 miles one way and leave at 1030pm
The car is fully tuned up, new cap/rotor, plugs, wires, oil and filter, air filter
The header was replaced with a stainless steel one as the original was cracked and the aftermarket was less money

I installed a vacuum gauge and at idle the engine is at a dead steady 22.5psi
I follow toyota/geos recommended shift points from the manual
The tires are all aired up to their max safe psi
I also taped off the grill and the middle part of the bumper with the very bottom still open.

I have a few mods I was waiting to do, which included changing the fuel filter, synthetic trans oil and maybe the moon discs

I do not know what else to do, but I see a few people on here with the same vehicle and either less mods or none and they are averaging 40mpg...


Any suggestions?

thank you for your help

First suggestion: get digital MPG instrumentation. ScanGauge or equivalent.

You need trip-level MPG feedback. You sound like the kind of person who would make good use of it.

As for the shift points recommended in the manual - I'd be willing to bet they're not ideal, in terms of maximum fuel economy.

Your drive time sounds pretty ideal, since you're dealing with minimal traffic. What's your commute like? (Sub/urban? If highway/freeway, what speeds?)

Thank you for responding.

The shift points listed in manual are listed for "best fuel economy"

although its fairly hard to believe because you are short shifting the first few gears.

they say to shift at 15mph, 25mph, 40mph and then 45mph

As for my trip

the first 2 miles are urban at about 45 mph
the next 9 are highway at about 65
and then the last 1 is city/urban approx 25mph

Short shifting is fine. But I'd say those upper shift points are too late depending on what you're doing at the moment. Especially if you're just cruising at very light load. I'll often grab 5th @ 25-30 mph depending on what's going on (maintaining speed, light load). Highest gear you can use without lugging. If you need power to accelerate, downshift. That's what makes driving a manual more interesting.

Are you neutral coasting to as many stops/turns as you can? (Driving Without Brakes)?

I'm sure you've looked through our list of driving techniqes - 100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com

I'd also consider dropping your highway cruising speed, to say, 55 mph. See: http://ecomodder.com/forum/showthrea...you-15182.html

My guess is there is something wrong with the car. Perhaps a failed upstream O2 sensor? I'd put a scan tool on the OBD2 and see if it's going into closed loop mode. Then again, the MIL should come on if one of those components was bad.

Since it's a '96, I would guess the SG or UG (scan gauge/ultra gauge) is compatible with it.

Does the car coast ok, meaning you have ruled out bad bearings, misalignment or a dragging brake?

besides what the other say above; do you have a coolent temperature gage, where does it read? does your exhaust smell rich?

if it smells rich when warmed up, it could be your coolent temperature sender.

Thank you for your help, actually the exhaust does smell a bit rich to be honest, there are no codes on the car (I have a scanner).

The car does coast perfect, no bearings sound, its dead straight.

I do know the car does go into closed loop because when warming up the RPMS will stay well above 1000, and when it is finally warm, the RPMS will drop to around 700, however it is in open loop for a long time (like 2 miles or so) The temp gauge works and I do have heat.

Upstream O2 would be precat? Thats the newest one I have(doesnt mean its working great)

Im going to try shifting 5mph earlier in the later 2 gears (35mph and 40mph) I tried earlier and the car just lugs. I will also try cruising around 60 (its very difficult to even do this speed in NJ)

thank you for all you help

I think I see your problem. Somebody installed one hell of a turbo when you weren't looking (22 psi@ idle). :) that thing must be a rocket!!!

anyhoo, your 4 mpg drop could simply be the cold weather.

Yes, cold weather could easily be the culprit in a 4 mpg drop.

But FYI, I've had a vacuum leak before that significantly dropped the mileage on my Firefly (Metro) without setting a code/check engine light.

Im not trying to sound like a smart ass, but could I have steady vacuum on my gauge if there was a leak?