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Poor Gas Mileage BUT All new oxy and cat con

Hello all!

I could really use some help on this one. I have a 1998 chevy metro 1.3 liter car. It has 116,000 miles on it. I have been having some trouble with the fuel mixture/ idle quality/ gas mileage.

A) Changed plugs back in June, had those checked by mechanic–all OK.

B) Code for rear o2 sensor cat con came up, replaced both of those after gas mileage decreased from 45 to 30 and hesitated on acceleration.

C) Hesitation went away for a day, but then returned. Hesitation on idle, sputter at stop lights, gas mileage low. So I changed the FRONT O2 sensor.

D) Ran better for a day, but still have low gas mileage and sometimes, just once in a while, the idle does its stutter.

1) What else causes such a huge decrease in gas mileage (30% loss)? How can I determine what it is without the car throwing more codes (it is not)?

2) Do you have experience with these cars? If so, do they last a long time if properly cared for? That is to say, should I just plan to upgrade in car and forget about chasing this problem down?

Thanks for any and all help!

They can last longer but some will start to fade. One thing that can always happen is that the tappet clearances can get narrow. This isn’t a big gripe with most cars but if it has never been checked, you never know. This phenomenon is caused because the valve face will wear down and the end of the valve stem won’t ever show much wear. This keeps the valves from opening all the way. My other theory is that the engine computer can quit working right. This (of course) is ignoring all other possibilities like plugs, plug wires, coils and EGR systems. The object of setting tappet clearance is to get the gap to the widest specification. If it is .009 to .012, you want the .012. I hope I’m not causing extra work.

1)Recently moved to Wisconsin. We did not experience any of these problems until after the move from D.C. We were told part of problem could be use of ethanol gasoline. Only two gas stations carry ethanol-free gas and we use the highest octane.

  1. I saw in a different thread that cold weather affects gas mileage. It has recently turned winter here. I have trouble believing that could account for a 33% drop in fuel economy, but I figured it was worth noting.

  2. The fuel filter on this car is in the fuel tank and consequently has never been changed. Could the lack of changing, in conjunction with the ethanol, clog the fuel filter and be the cause of hesitation on idle once in a while?

  3. I have not driven the car at highway speeds since making the o2 sensor repair. Do you think I need to get the car to highway speeds, and hence highway temps, in order to bust out any rich gasoline stuck in the new cat con?

As you can see, I am grasping at straws and when I took it to my mechanic, they were also grasping because no codes were coming up. Perhaps I should change mechanics? You tell me!


Hmmm, Mr. Doyle, that is a new path and once I was pondering. I have never opened up my engine before, so is this something you think a mechanic with a warm garage should do?

And if so, should I ask them to check the valve clearance or the tappet clearance?

(Plugs are new, wires are new, they checked the coils and those were fine. I do not know if they checked the EGR system). I do hope the computer is just being a bugger.

And please, don’t worry about creating more work. I enjoy figuring this stuff out and if it does not fit into our budget then I simply won’t proceed!

I like the engine computer possibility and as a result the fuel supply is messed up.

To find the problem, it helps to know how the system works (at least, know a little bit). The engine is running rich (using a lot of fuel). The engine computer controls the amount of fuel based on information from several sensors. Some of these sensors are: the front oxygen sensors, the MAF (Mass Air Flow) sensor, and the engine coolant temperature sensor, etc.
A couple of things your head scratching mechanics can do are: Clean the MAF with an MAF spray cleaner. Clean the throttle body bore and throttle plate with throttle body cleaner. Replace, if it’s too difficult for them to test, the engine coolant temperature sensor (cts).

Thanks, hellokit, for the quick rundown of crucial components.
Computer broken: I don’t think that is the case because the 1st o2 sensor i put in popped and then the computer threw up the o2 sensor code. I put in a fresh one and the code went away.
MAF sensor: The mechanic tossed this out as a possibility but said he was just scratching his head on that. The part cost a couple of hundred dollars so I will look into getting it cleaned, as you suggested.
Temp sensor: The temp sensor was just replaced by a shop not too long ago, so I hope that is not the problem.

1 more question: When you talk about cleaning the throttle core etc, is all of that inside of the MAF or are you speaking about a different part of the engine? Thanks!

How have you determined there are no codes?

They probably call it valve clearance. If you like working and you have a lot of time, it may be worthwhile on a solid lifter system or a system without lifters. If the manual says that the valve clearance should be adjusted at 30,000 miles and it has never been done, you never know what you will find. A compression test may be a more worthwhile. Cold weather will really cut the gas mileage, ethanol gasoline too.

The car has no check engine light and when it is hooked up to an OBD-II reader there are no codes present.

I just checked my maintenance schedule and the valve clearance is supposed to be checked every 15,000 miles or 12 months. We have had the car for three years and NEVER checked that! I completely misread yhe line about which models this applied to. Thanks for this idea–since it should be done anyway, i think I will have the warm garage mechanic take care of it. We’ll see if it works.-Chris

Thanks for all of your helpful suggestions. Here is how the issue resolved:

We took the car out on the highway for the first time since replacing the cat con and oxy sensors. We were on the highway for four hours. At the end of that trip, going no more than 50 mph in the fog, we earned 50 miles per gallon on that tank of fuel.

The next three tanks ranged between 40 and 45 going in the city and at faster speeds.

The hesitation during idle still popped up once in a while. I was going to go chasing after those valves that needed a routine service.

Once we got the car going and could honestly say we trusted the thing–bam! We sold it and got a chevy prizm. Thanks to everyone for the help!

Thanks for telling us of the resolution. Most of the problems were resolved, but one. The next owner is left to deal with the occasional miss. Enjoy your newer car.

i have the same problem
99 chevy metro i bough the car in may, i was getting 38 mpg and in nov from one day to the other i started to get 28 mpg, i did a tun up, change all sensors and still 28 mpg, and i drive 150 miles a day
anybody know what could it be please

I’m going to guess that the long drive was what cleaned the spark plugs enough to make a difference. Since your plugs are new, maybe your combustion chambers are full of clinker, like a furnace. I used to get pinging while going uphill at 55 MPH after doing city driving for three days. I used to downshift and floor the pedal on my 76 Impala and leave a cloud of black smoke. After that, the pinging stopped. Don’t get a speeding ticket.