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?:whistle:

[Martin]

I have one off a 1996.

http://i1249.photobucket.com/albums/hh514/MartVerd/for%20sale/08945124-1F2C-4A54-91EF-82F948D860F6-4897-0000056B16F3A6E1_zps08f02d5e.jpg (http://s1249.photobucket.com/user/MartVerd/media/for%20sale/08945124-1F2C-4A54-91EF-82F948D860F6-4897-0000056B16F3A6E1_zps08f02d5e.jpg.html)

Been sitting on a shelf since 2000 or 2001. Worked when I removed it from the car.

[iRoadster]

I'm going to try some MAF spray cleaner first, but I'll keep that in mind.
The thing is pretty dirty from having the Racing Beat intake on there for so long, maybe it's just gummed up?

How much do used MAF's go for these days anyways? Hopefully not much, as I am a poor, unhealthy bastard.

[kung fu jesus]

Not sure but the one I bought for my former NB1 was ~$100, remanufactured.

[kung fu jesus]

You want to replace that oiled cone filter with a dry one. AEM and a few others should have one in the size you need.

[iRoadster]

Yeah, I figured the oil from the K&N filter was probably part of the problem.

So then will any AEM or other filter work so long as it fits?

One more thing to add to the list of stuff I can't afford. Lol.

[kung fu jesus]

Yup. If it is roughly the same size, and the coupler is the correct diameter, use it.

[iRoadster]

*time for an update*

So a couple of weeks ago I noticed that under acceleration like if trying to pull away from a stop or speeding up at low speed in 4th or 5th gear, the car would start bucking and shaking and backfiring (some times quite amusingly loud even though I know it's not good). The fuel mileage seems to have gone down to around 23mpg as well. 26mpg wasn't great but it's just dumb when it's this poor.

I've gotten a number of things to try and remedy this problem and the mileage/running rich problem and just to take care of some other maintenance.

So over the last couple of days I've; changed the pcv valve, it's gasket on the valve cover and return lines, the valve cover itself as it's been leaking, and I put on a new air filter along with the MAF from Martin. I couldn't find a filter that would fit with the RB intake other than another replacement K&N after a lot of searching and a couple returns of filters that didn't fit. And just for the hell of it, I also wrapped the Racing Beat elbow and the entire crossover tube in heat reflective tape, because my inner ricer told me to. It took me the four hour special edition of Return Of The King to put tape on to something. Smh.

Annnnnnnd the end result?!

It's still exactly the same.

[kung fu jesus]

Does the bucking start after the car is warmed? I would bet your cat is clogged.

[iRoadster]

Doesn't seem to matter. I've been talking about this in the Random Thoughts thread on CR too and one person suggested a clogged cat, but others said that a clogged cat wouldn't be making the car backfire and that the symptoms always pointed to a bad MAF, which is what I've been thinking as well. I didn't completely rule it out though. When my o2 sensor went bad a while ago I suspected that the cat on the car (which is the original 20 year old bit as far as I can tell) might have gotten messed up from driving around for so long with the car running rich.

I did try to take it off to look at it and put on some new exhaust gaskets I have, but couldn't as I think the shop that did my clutch probably used an impact gun on the cat's bolts, 'cuz they wouldn't budge.
Besides the cat, the only other relatively plausible thing we could come up with was that maybe the timing belt had jumped a tooth and is now off. I don't see how that would've happened though.

[Willywongka]

You know, I've been experiencing a bit of a drop in MPG as well recently. I think I was getting 28-29 mpg and now i'm getting around 25. I also experience that strange stumbling/struggling idle after I start my after my car is warmed up also... but it goes away after I start driving. I've been tinkering with increasing and decreasing the airflow with the screw on the side of the manifold. I could have a possible vacuum leak as well, I'll see if I can find anything and report back on my end.

[kung fu jesus]

Chucking out a few other ideas...

Failing fuel pump, failing FPR, clogged fuel filter, detached wiring connection (has the the harness ever been cut? If so look at the connections...had this happen before), dead/dying coils.

[etikoner]

Is smoke coming from your exhaust under acceleration? Pull your spark plugs and tell us how they look..

[iRoadster]

I can't find any vacuum leaks. The fuel filter isn't that old, I changed that when I changed the o2 sensor after I ran out of gas a couple years ago. The wiring harness has never been cut in to. Plug and wires seem fine, but I have some new ones on the way regardless. The exhaust doesn't smoke. The bucking and hesitating doesn't go away, warm or cold engine.

Just running rich, bad mileage and the stumbling, backfiring hesitation on acceleration.

[kung fu jesus]

When you replaced the MAF and other stuff you cleared the codes?

[atank]

http://indulgy.ccio.co/0D/W2/f4/2703567838506690330tBtFaLAc.jpg

[iRoadster]

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.

Codes were cleared, but that was back in November. Could those have come back as I didn't change the MAF until just recently?

Apparently I had two bad plug wires. Which I had looked at like three times and noticed nothing. Kind of annoying considering I have new plugs and wires that are supposed to show up from R-Speed today.

But, now is that going to be the only problem?

Going to change oil, tranny and diff oils today.

[etikoner]

R-speed plug wires are good.

What color did you get? [/rice]

[Martin]

I had an R-speed plug wire fail on me 2 weeks ago. #1 wire would not stay on the plug.

The car would spit & sputter until I popped the hood and pushed it back down onto the spark plug. A few miles later it would pop off and the car would run like crap again.

[atank]

http://i.imgur.com/4QoyYXB.jpg

[iRoadster]

New plugs and wires showed up today after gettig them once again, from R-Speed.

I get the purple ones since I also have purple Samco rad hoses so it was only natural to get the same color again. Plus at the moment they come with free spark plugs.

...and that seems to have been the only problem.
#2 and #4 plug wires were bad. Bloody hell. The most obvious and easiest choice is always going to be the last thing done.

Now I see if the mileage goes back up. I'll be satisfied if I can get 28mpg now.

So new plugs and wires are now on. I finished wrapping the entire crossover tube in heat DEI tape. Changed the oil in the engine, trans and shift turret, and diff. Fixed my short shifter as well. The lower ring delrin ring has never fit right, but I have a new Dremmel and shaved about a third of the bottom off of the ring. Now it all fits correctly...finally. I've had that thing for like five years and only now gotten around to it. The turret should finally not leak onto the cat. And I through in a bottle of Seafoam just to smoke out the neighborhood.

Car seems to be running quite good now.
Thanks for all the help everyone.

Well, except for O'Reilly's Auto. They can choke on a burrito from the Taco Bell next door. They gave me the wrong battery a month ago. Then today, after I had drained the oil of course, noticed that the also gave me the wrong freaking oil filter. That store employees people that can't do a job that someone with a history of strokes and no arms could do.

[iRoadster]

The old wires lasted probably 30k miles. Which isn't too bad. Not quite as long lived as I was expecting, but they're alright.

Did I mention they were purple and came with free plugs?

[kung fu jesus]

Wires are generally decent for about 15-20k.

[MoFackyRed]

I'd start looking for vacuum leaks. I had a vacuum plug come off of a port on the back of my cylinder head pop off and it caused similar results. I have since zip-tied all of the vac plugs on to avoid future shooting them off under boost. :-)

[iRoadster]

Seems the fuel mileage is back up to around 27-28mpg.
This is just driving around town, no highway miles, which would probably bump it up a little.

Still wish it was getting 32mpg like when I first got the thing, but at least it's not totally crap again.

[iRoadster]

Mileage is back down to 24mpg. Still running rich.

Arrrgh. Should have seen this coming. I can never seem to just fix something and have it be fixed.

Bloody old cars.

[iRoadster]

Filled up today. Came out to 22.34mpg

...?

Still can't figure out why it's doing this. Don't know that I ever will.

250 miles before it becomes a good idea to head towards the nearest watering hole would be really annoying if I had a commute to deal with.
Not a deal breaker, just a pocket book maimer.

Crazy loud valve tick now too when the car isn't fully warmed up. Don't know if it's just from trying Amsoil or if it's something more.

...old cars.