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Mileage Gone Bad: Mazda Protege '98

I bought a '98 Mazda Protege two years ago to drive across the country and consistently got 45 mpg, even hitting 50 mpg on one tank. It was great, especially considering gas prices at the time. About a year ago things started going bad. The engine started missing and the mileage started to drop. I changed the plugs/wires/rotor/distributor cap and air filter, but that didn’t help. I then took it to our mechanic who suggested flushing the fuel system, which we did. The car doesn’t seem to miss as much anymore, but the mileage keeps getting worse. Now it only gets about 32 mpg. No!

One more thing to consider: The check engine light went on just a couple months after purchasing the car. I took it to a dealer (it was on the trip home) who ran it through the computer and he concluded it was a small hole in the catalytic converter which, if I didn’t have to pass a smog test (which I don’t b/c I live in rural Oregon), didn’t need to be fixed. Now I’m not so sure. However, I was still getting great mileage at the time.

Maybe something with the 02 censors? The check engine light? Thermostat? Any thoughts would be a great help!

How do you calculate gas mileage? Because you can’t get 45-50mpg out of a protege - unless maybe you only drive downhill with the wind at your back.

Anyway…are you saying that you’ve been driving around with the check engine light on for 2 years? That’s a really bad idea. If so, you need to go get the codes read and post them - the actual codes (like P0123) rather than anything anyone tells you about them. Many large auto parts stores will do this for free.

Without that info its hard to know where to start. In the midst of messing with the fuel system, did you end up with a new fuel filter?

I calculate gas mileage by topping off the tank, resetting the odometer, driving until it’s time to refuel, topping off the tank, then dividing the miles accumulated by the number of gallons it took to refuel (the amount of gas used). There is always going to be some error from the different filling levels, but not much. Isn’t this the right method? I’m going to feel like a doofus if not b/c I’ve been doing that for years.

About the light: Yes, I have been driving around with it on for two years. As I mentioned, as soon as the light came on I took it to a Mazda dealership who ran it through their computer and told me it was a small hole in the catalytic converter. He certainly could have been wrong, but my mechanic at home has seen it twice since that time and hasn’t seen anything wrong. They could both be wrong for sure. I will search my records to see if I can find the codes that came up on the original visit.

About the fuel system flushing: No new fuel filter, though while they were doing that was probably the time to make the change. Dumb move.

That is the right method to calculate the gas mileage, but this car should make it only about 35mpg tops - and thats with everything perfect & in order on the highway. Maybe you could check your trip OD - ?

As for the codes - there are hundreds (?) - lets just say A LOT. One of the problems with driving around with the CEL light on is that you won’t know when the computer senses a new problem. Go get the codes read and post them all - you’ll likely have several. But do make sure it is the actual codes and not what anyone says and don’t buy anything anyone tells you will “fix” it.

Next time its in the shop, or you want to fuss with the car put a new fuel filter on it. Even if it doesn’t help it most certainly doesn’t hurt.

Also get a good tire pressure gauge and check the pressure regularly - it is the single best thing to do to keep gas mileage in order - other than fixing obvious problems with how the car runs.

It is highly unlikely you got 45 MPG out of an 98 Protege, let alone 50 MPG. Depending on the engine/transmission combo you have, City MPG is 20-26. Highway MPG is 27-33 MPG. Considering you are getting 32 MPG. I’d say you are doing quite well. The issue with the catalytic converter may have something to with your loss in fuel economy. If outside air is coming into the converter it’s throwing off the rear o2 sensor, probably causing it to send a signal to the ECU that the engine is running lean, so the ECU enriches the fuel mixture which can cause the car to use more fuel.

I completely understand your skepticism about the mileage I am reporting, but unless my process is flawed (as described above), then that is what I was getting for the first year and half of ownership.

The mpg that is reported online seems to vastly undershoot what I would expect this car to get. It’s tiny and has very little power, isn’t that a recipe for great mileage? 27-33 mpg on the highway strikes me as a very low standard. Thoughts?

Thank you for the idea on the outside air!

The check engine light (CEL)is on, now? Disconnect the battery for 5 minuets, to reset the CEL. Drive until the CEL comes on. Take the car to an auto parts store for a scan of the CEL (free). Bring the codes (like P301, P0420, etc.) here for analysis and advice.
If there is a small hole in the catalytic converter, it should be plugged. If it’s a hole, a screw could be used. If a split, try JB Weld sealer, or something similar.

When you say an auto parts store do you mean somewhere like Autozone, Schucks, or Napa? I can just drop in and they will run a diagnostic for free?

Most of them do, yes. You can call and ask. I went to one once where store policy was that they didn’t do them anymore. I saw someone in this board say that no one in their area did them. But most people seem to find a place. The big corporates are your best bet.

Fantastic tip, thank you!

I get 41 mpg highway with a '97 protege 5spd manual 1.5 liter and a little better with a tailwind.

Mine is the same specs except one year newer. Verification that that kind of mileage is possible, thank you!

It took about 200 miles for the check engine light to come back on, hence the delay in this response.

I went to an Autozone and the guy plugged in and said the meter read the following: Warm-up catalyst below threshold, Bank 1.

It’s not a code like you asked for, but it’s what he had. Should I go somewhere else and get a code or will that work?

The description of “Warm up catalyst Efficiency below threshold (bank 1)” goes with DTC P0421. It means that there is a report, correct or not, from the rear oxygen sensor (in the exhaust from #1 cylinder side of the engine) that the catalyst action is poorer than it should be.
There are several causes for P0421: a leaking catalyst, a bad catalyst (pre-cat), or a defective rear oxygen sensor.
Fix the exhaust leak. Remove, and examine, the rear oxygen sensor for spots of stuff which may have come from the pre-cat. Read these experiences:

Wow, this is all very impressive and interesting. Thank you! I will do some work and report back.

Maybe you have a bad fuel pressure regulator. This can cause poor running, dropped fuel mileage, and could trigger converter/O2 codes and set the CEL because the system can only control itself up to a point.

The regulator is easy to check. Run it for a minute, shut it off, and remove the vacuum line that is attached to the regulator. If it has gas in it or a strong gas smell it’s defective. Something to consider anyway.

Yes, concentrate on the causes of the poor gas mileage, after you look at the pre-cat rear oxygen sensor. The code P0421 has the same causes as the code P0420, except that P0420 is for the main cat.
Thermostat. If your temperature gauge quickly (after a mile or two driven) shows about half scale, the T-stat is probably ok.
Engine Coolant Temperature Sensor (ECTS) tells the engine computer (ECM) the engine coolant temperature. The ECM makes the fuel/air mixture richer, the colder that reported temperature is. The ECTS can be checked with an electrical multimeter (ohms, engine OFF; volts, engine ON). Or, you may just change it.
MAF (Mass Air Flow) sensor measures how much air in going into the engine to mix with the injected fuel. If the sensing element is dirty, which is likely after 11 years of operation, it needs to be cleaned.
Throttle Body and IACV (idle air control valve) can be dirty and lowering the amount of air going into the engine for mixture with the injected fuel.
It is fairly easy to clean these items. Get a spray can of MAF Cleaner, and a spray can of Throttle Body Cleaner. Remove the MAF and spray clean (carefully). Remove the large black plastic intake tube and spray clean the throttle plate and throttle bore, and spray a little more to clean the IACV.
Let us know the results.
Right cheer’s a pitcher of the stuff I been talkin’ 'bout: