My 91 LX 5 speed seems to be losing mileage on every fillup. Used to be 30-31 mpg consistently, now I've noticed on the last fillups 28, then 27, andtoday26.6

I check mileage with the odometer and always top the tank off. (Fill tank and reset odometer)
I've addressed or checked offthe common issues:

Tire pressure 31psi
New tires and alignment
Recent new fuel tank, fuel pump, fuel filter, air filter
New Bosch oxygen sensor
Valves recently adjusted correctly

Brakes aren't sticking
No "immediate" start, ieno signs of a leaking injector, no gas in oil.
The car seems to run and drive wonderfully. No signs of trouble.

I'm going to replace the plugs tomorrow with a new set of NGKs, and replace the transmission oil (straight 30wt) with 10w30 synthetic

Any suggestions from the experts?

 

You can try to pull the vacuum hose off of the fuel pressure regulator. If you smell gas, then the regulator is ruptured and pulling raw gas into the intake.

You also might want to check that the ignition timing is correct.

 

Ive noticed with my 92 if I go too long without changing the oil my gas mileage will start to drop off rapidly.

 

I would check engine compressions to make sure you don't have a weak cylinder, stuck ring, burnt valve, or excessive wear.

Was timing belt changed at 90K? You'renearthe mileage (180K) for 2nd T'belt. It's possible, but not likely that a timing belt cog has failed allowing timing to slip slightly. Suggest removing upper timing belt cover and rotating engine to inspect entire T'belt to make sure this isn't the problem. If there is a problem, it could get worse and perhaps damage the valve train or pistons.

good luck

 

could also be the fact its getting colder and colder...

 

I never knew that, my old GMC truck would start in a second even when it had a battery that won't hold a charge for more than 72hours

 

At the risk of possibly being considered an 'expert' (an inaccurate assumption!):

You say you have new tyres, could the drop in economy date from the new tyres? A change in brand / model of tyre could mean you now may have new tyres with an inherantly higher rolling resistance than the old ones(?). Even if the new tyres are the same, it could be that a newer tyre with thicker and more 'mobile' tread blocks might have greater rolling resistance than the same tyre when worn down?

It stands to reason that this would be the case, at least in theory to some degree, because the deeper tread blocks would 'squirm' around more, thus unavoidably generating more heat in the tread. This heat energy must come from somwhere, and that somewhere is parasitically from fuel burned in the engine. It would surprise me if this were as significant as you are reporting (as a decrease in economy), but stranger things happen...

Considering that it's getting colder over there, and over there it can get very cold in some parts, what sort of driving are you doing? If it's short trips where the car spends a substantial amount of it's driving time coming up to temp, only to be shut down shortly after, then you'll be spending more time with the ECU telling the injectors to run rich. A very cold engine will take even longer to come up to a temp where the fuel mix leans out at normal operating temp. This could be a bigger issue in winter than summer when the engine will come up yo operating temp faster, and this will affect how much time the engine spends in 'rich mode', and adversely affect the economy in colder conditions.

Cold gearbox oil will also slightlyincrease 'drag' in the box which will have at least some adverse affect on economy when very cold, and could take a lot longer to warm up the lower the ambient temp.

Very cold air may have an adverse affect on fuel vapourisation, causing a less complete fuel burn and poorer economy, especially at lower rpm and throttle openings when induction turbulence is likely to be less due to slower air speed. At higher rpm / wider throttle openings there will be more turbulence in the induction air and this would promote better atomisation, so this would probably not be such an issue at WOT. If so then very cold air may improve max power at / approaching WOT because of the air being denser, but poorer economy because you spend more time at small throtte openings with less efficient atomisation.

One or more than one of these factors above could be responsible. I'm just speculating, but it works for me!

 

use honda manual trans fluid imo.

is your clutch wearing down?

how were the valves adjusted(correctly..?) ?

your bosch o2 sensor is crap imo - did the drop happen soon after replacement?

 

JohnL'spost has some good points. The main reasonis the winter gas mixture reduces fuel economy considerably. And giving a car more time to warm up in the winter puts a dent in fuel economy also.

 

Winter driving will decrease gas mileage. In good weather I get around 32mpg average on my '06 Accord, in the winter it drops to 28-30mpg. Letting the car warm up (which I only do for a minute unless there's a foot of snow on it, thus I start the car, then clean it off while it warms up) can also eat into the gas mileage. There is definitely something about the "winter blend" gasoline that may also cause a change in mileage. What brand of gasoline do you use?