inactiveuser92616

Hey, in the past 2 months or so my fuel mileage has gone to pot. When I first got the car it was getting 400-500 miles to the tank, verified by my brother (the PO). But since the start of december, I am getting ~200 to the tank. Yes, this is a full tank in the 15-17 gallons range. I have tried switching gas stations and brands, no luck. Fuel grades, no luck. I have replaced the oxygen sensor, nothing. Went from 20-50dino to Valvoline 10-40 syn blend. Helped startup and revs, not mileage. I dont drive it like I stole it.

so, I guess the culprits I have pretty much ruled out are the O2 sensor, thick oil, and I have also tested the barndoor flow meter. It tested fine. Have run a can of valvoline fuel injector cleaner through, made sure to open it up on the throttle.

Where should I go from here? I have a few ideas but would like to know the most likely culprit. The loss was sudden, one tank was fine and the next was crap, and has been since. Power seems unaffected. The consumption gauge is unaffected, still reads as though I am getting appropriate mileage.

Ideas I have:
new air filter (mine isnt that bad)
test throttle position sensor
fuel pressure test
fuel filter
bad jettronic
baddly clogged/sticky/leaky injectors
something in fuel system leaking?

Advice would be great, before I go down the wrong road and blow all my very limited finances on the wrong problem.
TIA

Bhj0887

change the fuel and air filter, they're not that expensive and will make a slight difference. If you had really dirty gas one time, it porbably clogged the filter.

elf89

This is a long shot, but if the car still seems to be running fine, I'm wondering if the Odometer gear could be failing. I've heard if they lose a tooth or two they will still turn the numbers, just more slowly. This would result in fewer "miles" per tank.

RMills944

elf89 - thats a great idea - I could have had 100,00 on my car instead of 130,000!! ;-)

Chris_924s

Maybe a little Techron and some 5w Mobil1.. I concur - fuel filter and air filter will improve mileage, but not that much..

Did you check for a fuel leak??

Driving hillier roads?

Waterguy

The factory manual suggests that you check the following items if fuel consumption is high:

1. Ignition system (distributor cap, rotor, spark plugs, plug wires, coil.)

2. AFM (you have done that.)

3. Fuel pressure.

4. Engine temperature sensor (bad connection or break causes car to run rich.)

5. Throttle switch.

6. Idle CO and speed adjustment.

HTH

Peckster

Any smoke? Have you checked the color of the plugs? Check for signs of rich running.

Manning

Personally, I never get more that 21mpg in mixed driving, mostly stop and go through neighborhoods. Compression is fine, no smoke or gassy smell and plugs look great. I keep up on maintenance like oil, fuel and air filter, etc. The fact that some of you get close to 30mpg is astonishing to me. I must be a total lead shoe or something.

ERAU-944

i get low 20s city/hwy combined, and 27 with long highway jaunts. during any of this i don't baby the car, i'm sure i could get 30 if i really babied it around town and on the highway, but i enjoy the sound of the intake(airbox top cut off) and the flowmaster. maybe i'll baby it and see what happens.... every time i tell myself that i never end up sticking to it.. hee hee.

-Michael-

inactiveuser92616

plugs are all normal, per the color photos in the back of the haynes manual. The tops of my pistons appeared damp though, which concerned me. Not like it was a pool in there or anything, but they were black with a fluid like sheen on them.

Danno

Check the FPR, they really can fail. Also all the vacuum lines going to the FPR and fuel-dampener. Leaks in those vacuum hoses will tend to cause the car to run too rich.

visualpascal

For the record, my '84 gets ~360 miles from fillup to the little red light coming on. Fuel economy over 90 isn't quite as good as it is at 64 1/2 though...

Peckster

I got 400 miles in my turbo once. What's that in miles per U.S. gallon?

Danno

That's 22mph.

OriginalSterm

Or about $3.50 per the current exchange rate...