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97 Dodge Neon. Went from averaging about 32 MPG for well over a year to 22 MPG during the past 1.5 months. Tire pressures are normal and there is no check engine light, because of that I think the O2 sensor can be ruled as a possible reason but I'm not sure. IDK much about cars so I'm thinking it's something I need to take to a shop to diagnose but maybe there's something else I can check myself. So my question is, what can cause such a steep decline in MPGs?
(I highly doubt someone is siphoning my fuel)
A few things to consider:
I buy gas mostly from Shell stations and sometimes Chevrons. Different sides of town.
It first starting happening after I let my brother-in-law borrow it for about 2 weeks.
Well, probably this is related to your knucklehead brother in law abusing the car.
Could be the idiot ran it so hard it burnt a valve or cracked a ring?
If the car has not been tuned recently, at least plugs, and fuel filter can be changed.
it may very well be the O2 sensor. sometimes you dont get a code, when you probably should. that is the first thing i'd check. at least make sure that the wire or wires are intact.
Take it to a shop that has emission testing equipment. Sounds like the 02 is junk or you have a bad injector. Both of these will show up in and exhaust sniff test as the car will be running extremely rich.
A few basic things you can do yourself is..
Clean or replace the air filter
Clean the throttle body
Check the tire pressure
Run a fuel cleaner. Only one I ever recommend is Seafoam. You can use it to clean the T-Body then dump the rest in the tank... DO NOT suck it into the manifold. You said you're not a car guy so don't try this. Also DO NOT dump it in the oil.
Check to see if the brakes are dragging.. Get up to speed and put the car in neutral and let it roll. Does it stop too quickly? Is he parking brake up? I've seen this a thousand times so don't laugh.
Bad gas from one station. Go to a different station and fill up.
Is there a ton of crap in the car??? Clean it out...
I can't remember if the 97's had mass air or not and i'm too lazy to go look. If they do clean the mass air meter as well.
maybe someone can correct or confirm, but I've heard that o2 sensors performance degrades over time. Maybe your o2 sensor just hasn't tripped a code yet.
Another thing the OP could do, if he has access to an inexpensive scan tool, is check whether the engine is running open loop, and also check the long term fuel trim. These are good indicators of O2 sensor problems.
A cheap scanner would also give information on the O2 output voltage waveform such as max, avg, and min voltages, time between threshold crossings, rate of voltage rise/fall, avg freq of voltage waveform, etc; all of which are useful if one has the OEM O2 sensor specs or comparison data from a new or known good sensor. Some inexpensive scanners show the output voltage waveform as well.
My brother in law doesn't abuse cars anymore. I think he may have refueled at a cheap gas station but who knows.
Tire pressures are fine, air filter is fine (it doesn't affect MPGs I'm told), I ran a fuel system cleaner, and the brakes are fine. IDK anything about throttle bodies and I only have basic tools so no scanners.
Looks like I'll have to take it to a shop, I just have to find the time. :/
The shop recommended a major tune up which included the replacement of ignition wires and spark plugs. I'll see what the results are in a few days....
Major tune up? That is all you can change on that car anyway. Thats a minor tune up. Plugs and wires is an hour job tops and minimal in parts.
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Christ, dude. you had a sudden and steep decrease in fuel economy. before you DO or CHECK anything else, check out your O2 sensor(s).
