Where did my fuel mileage go?

[iRoadster]

1.8 with about 155k on it.
Some time in between tank fill ups not too long ago my gas mileage suddenly dropped from about 29mpg to not much more than 22mpg. And it wasn't a gradual decline in mileage.
It's also having trouble holding an idle sometimes after starting the car after it's been fully warmed up. It will unhappily struggle around 500rpms unless I hold it up by pressing the gas pedal.
Only does it on warm engine start ups and goes away after a couple minutes and doesn't happen while driving.
Also, I filled it up yesterday and had a fuel smell in the cabin for a while afterwards and had to keep the window cracked open. Not sure if this is related or if I accidentally spilt some gas on myself.
Plugs and wires are fine. Fuel filter is less than two years old.
What could be causing this?

[SM16RMSM]

Possible vacuum leak.

[— kung fu jesus —]

What year?

Vacuum leak is a safe bet. I would go to an autoparts store, take off an easy to remove hose, tell them you want to buy 10 feet of hose in that diameter. Start replacing all the vacuum hoses one at a time, just remove one, lay against the new hose, cut to length, replace. You may need more than 10, may need less.

Does the idle stumble get worse when you are sitting in neutral and step on the brakes?

[iRoadster]

Car is 1995. Pretty sure stepping on the brakes does not affect the idle. Turning the heater on makes it drop a tad, and turning the a/c on makes it go up.
I know the vacuum leak is part of the problem, but that also existed before the sudden drop in mpg's and it's the sudden loss of mileage that's surprising me.
I'll usually go maybe two weeks tops in between fill ups. So somewhere in that time period something went awry.
I suppose the cat could be clogged or the o2 sensor might have finally crapped out, but don't know how to confirm it. The fuel smell when I filled up yesterday made me think of these possibilities.

[— kung fu jesus —]

Generally, when I had this sort of dropoff in MPG, I checked for a few things. Besides vacuum leaks, I checked to see if the brakes were binding. Usually this was done by seeing how easy the car free-wheeled at speed by putting it in neutral, then again at a stop on a slight incline. If the car transitioned from forward to backwards smoothly, it was fine. Because you smell gas, it is possible for the O2 sensor to be bad. Are you getting that smell from the exhaust or the engine bay? If the engine bay, there is something potentially more dangerous at play. That could lead to a fire, obviously. So, bad charcoal can, leaking/stuck injectors, bad FPR, etc.

If the O2 is bad, and you smell the fuel in the exhaust, it won't be hard to fix except the actual removal of the sensor. If this is the case, you may be clogging the cat pretty good too.

The idle dip turning on the heater (HVAC fan) is from the current draw. The idle boost from turning on the A/C is the ECUs programming to help run the compressor when it is on. I asked about the brake droop to see if the larger, upper vacuum hose was the cause of the leak.

[WASABI]

Smell gas? I would say Oxygen sensor, or a plain old leak from the gas tank, or fuel filter/fuel line connections.

[iRoadster]

Intriguing. I know there's no sort of leak anywhere. Fuel smell does seem to emanate from the exhaust. O2 sensor sounds like it could be the culprit. That nuts that it would make the car run rich enough to lose 7mpg!
Pretty sure that my car still has the original O2 sensor and cat so it's not a total surprise that they don't want to work after 19 years.
Thanks I'll try these suggestion and see what happens.

[— kung fu jesus —]

Obd1 cars are less sensitive to their sensors. I had wired in a 4 wire on my '92 and one of the wires separated from a bad connection. My MPGs dropped a lot, but no CEL. It also ate up the effectiveness on my cat.

Is there a lot of soot on your rear bumper?

[Raisin]

Easiest way to trace a vacuum leak that I've learned:

0) Buy a small cylinder of propane or any sort of gas typically used for butane torches like a Bernz-o-matic that you can buy at your local hardware store or autoparts store
1) Start the car normally, make sure it's up to temp so that when it the car stumbles it doesn't die completely (you'll see why in later steps)
2) OPTIONAL: attach a small hose to the torch so that you have some control over where the gas is going
3) With the valve cracked open on the gas cylinder, move the torch around your engine bay, notice where the engine will hiccup when it sucks in the gas from the cylinder. This is where one of your vacuum leaks is.
4) Replace hose with proper sized replacement hose
5) Repeat the process until all your hoses are in good shape

Other than fixing your vacuum leak, how do your spark plugs look? Take a picture for us!

[Pyr0monk3y]

I have used that method before, it works very well and it is much safer than the starter fluid method.

I'm betting on o2 sensor, it makes sense that it would only act up when it is hot. When the engine is cold the ECU doesn't care what the o2 sensor has to say.

I'm having a similar issue but it happens regardless of temperature and there is a hesitation as well. I suspect plug wires in my case. On a miata, if the wires are of unknown origin or over 30,000 miles old, they are likely the problem.

[DK Wolf]

I had a very "special" DK-like case when my first NA decided to drop off in MPGs.


I was running low on oil (like... almost non-existent levels of oil) and it killed my MPGs. I was averaging about 20 and it seriously sucked. After a short while.. motor popped

[Art Summers]

I had my gas mileage drop . No codes showing , but I was told to change the Oxygen sensor were probably going bad . So I changed and gas mileage went back up . The Oxygen sensors has something to do with the gas your injectors use.,

[iRoadster]

Bringing this back.
Not long after I made this I replaced the O2 sensor and the mileage went up a little bit, but 24-26mpg is still not acceptable to me.
It seems that it was running quite rich. There was a small dark spot above the exhaust, not a lot, but you could see it if you were looking for it.
A diagnostic was done a couple weeks ago and the car threw two codes.
I can't remember what the guy said they were exactly now but quoting the receipt it says, "code stored for a intermittent air temp signal high and air flow meter."
They cleared the codes but I'm still only getting around 24-26mpg and the idle is still kind of odd (on the low side) sometimes. CEL has never come one throughout all of this.
The fuel smell I mentioned was a combination of a bad O2 sensor and exhaust leak, both of which have been attended to.

Bad cat and vacuum leaks are still possibilities to the bad mileage, but I had to put things on hold for a while until now.

Anything else that could be affecting the car's range?

[— kung fu jesus —]

OK, so both codes point to the AFM. Why not replace it? If the AFM isn't registering correctly, the ECU cannot meter the fueling and ignition properly.

[iRoadster]

Hmmm, the guy at the shop suggested replacing the AFM too...

Anyone got a spare one lying around that works properly?