Engine light remaining on and car is drinking a lot of fuel

Tiny
DANNIE86
  • MEMBER
  • 2000 HYUNDAI EXCEL
  • FWD
  • AUTOMATIC
  • 240,000 MILES
The check engine light has remained on in my Hyundai excel sportz 2000 model and it is drinking a lot of fuel (I only get around 250km out of a full tank) I have taken it the mechanics and diagnostics were done. It said that it was the oxygen sensor. We changed it, cleared the codes but on a test drive the engine light came back on and remained on once again. We then thought that it could be the fuel injectors so we changed all of them and also installed a new computer, thinking that it could be something wrong with that. We then took it for another drive and again the check engine signal came back on. We also changed the MAP sensor as I read somewhere that, that might be the cause. We still had no luck and my mechanic has told me that it might be time to say goodbye to the excel, but I don't want to let it go if it is a small problem that can be fixed easily. Any help would be much appreciated as I am at my wits end :(
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Monday, January 13th, 2014 AT 10:00 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
It's time to find a different mechanic. The first mistake was a very basic one. Diagnostic fault codes never ever say to replace parts or that they're defective. They only indicate the circuit or system that needs further diagnosis, or they describe the unacceptable operating condition. The sensor referenced in a fault code is actually the cause of that code about half of the time. You also have to test first for wiring problems.

The next mistake is there are dozens of fault codes related to each oxygen sensor. They mean vastly different things from the sensor not working to it simply reporting the condition it sees, meaning running too rich or too lean too long. It tells that to the Engine Computer which sets the fault code and turns on the Check Engine light to tell you. You don't replace the messenger because you don't like the message. Why on earth replace the Engine Computer? It's doing exactly what it is supposed to do.

When there is a fault code for the oxygen sensor, what would possibly lead your mechanic to injectors or a MAP sensor? A MAP sensor can fail to report the correct manifold vacuum value, and it will not set a fault code as long as that value stays within the acceptable range, but it won't take long for it to fail completely and set its own fault code. You've added a lot of variables to the fuel metering calculations that the Engine Computer has to sort out. It's not going to do a full job of that until the cause of the fault code is repaired and that code is erased.

The first thing you need to do is find out the exact code number, not a generic description. From that I can offer some suggestions of possible causes and possible tests.
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Monday, January 13th, 2014 AT 11:13 PM
Tiny
DANNIE86
  • MEMBER
Hi thank you for your fast reply. The code numbers that I can remember that came up were 81 AIR FUEL CORRECTION and 82 OXYGEN SENSOR. Any more suggestions and or possible causes would be much appreciated.

Thanks again :)
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Tuesday, January 14th, 2014 AT 7:01 PM
Tiny
CARADIODOC
  • EXPERT
I don't have a reference for those codes. '96 and newer models use three-digit fault codes that get a lot more specific. In this case all you know is there's a problem with the fuel / air mixture, but you don't know why. If it's too lean, you'll have hesitations and stumbling on acceleration. If it's too rich, the engine will run fine but the extra fuel will just go out the tail pipe.

With non-descriptive codes like these, your mechanic needs to start with viewing live data on a scanner. That will show if the oxygen sensor is staying too rich or too lean too long. That sensor can only detect unburned oxygen, not fuel. Under normal operation, the exhaust gas switches from too rich to too lean one or two times per second, with the average being correct. Most commonly the exhaust will be too lean, and the computer will command more fuel to correct that condition. There's four places to look for a lean condition. The most common is a spark-related misfire. With old spark plugs or wires, when a cylinder misfires, the unburned fuel and oxygen go into the exhaust where the oxygen is detected as a lean condition. The Computer doesn't know that only one cylinder is responsible for that so it commands more fuel from all of the injectors. You'll smell raw fuel at the tail pipe, but the oxygen sensor will continue to report a lean condition no matter how much fuel is added. Based on oxygen sensor readings, the Engine Computer can modify fuel metering calculations by about plus or minus ten percent.

The next thing to consider is low fuel pressure. That is almost always accompanied by other problems. There's two forces that determine how much fuel sprays from an injector when it is pulsed open. Those are fuel pressure and intake manifold vacuum. When that relationship changes, it shows up in the exhaust and the computer corrects for that by adjusting the amount of time it holds the injectors open.

The last two things involve the air entering the engine. All of that air has to go through the mass air flow sensor. The computer calculates the correct amount of fuel based on the weight of that air. If there is a vacuum leak, the computer won't know about that air. The same happens when there's a leak in the fresh air tube between the mass air flow sensor and the throttle body.

In all of these cases it's a lean condition that results in too much fuel entering the engine. You can also look at the short and long-term fuel trim numbers on the scanner. If they are high positive, the computer is adding fuel to the metering calculations above and beyond what was programmed in at the factory as a starting point. If you find high negative numbers, the computer is trying to reduce the amount of fuel, and in this case, without success. That would indicate the need to look for the cause of running rich. Fuel pressure that's too high is one possible cause, but a leaking injector will allow too much fuel also. This is where a good engine performance specialist will look at the readings from all of the other sensors to figure out why the computer thinks more fuel is needed.
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Tuesday, January 14th, 2014 AT 8:49 PM
Tiny
DRYSKY
  • MEMBER
Good morning, I have the same problem, replaced almost the car. Only error code is code 81 air/fuel correction. Compression @140psi. Car runs great no problem with power or idle, although cleaned the throttle body out last week now my revs are higher 1500rpm. Was getting around 540km per tank. Then suddenly over night (almost) gone down to 360km. Replaced all sensors, (map, o2, tbs, cam, con, purge valve & canister, knock sen) comp, fuel pump relays. Nothing has worked. Had a rom error on first computer check, replaced the computer. Now only have the non descriptive code 81 air/fuel correction. Help need this fixed stuck in a rock and a hard place.
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Wednesday, July 2nd, 2014 AT 6:35 PM
Tiny
CARADIODOC
  • EXPERT
You're doing yourself a disservice by piggybacking on this post. I'm the only one who will see it, and I don't have an answer of value for you. I'd recommend posting a new question so all the others will see it and have a chance to help.
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Thursday, July 3rd, 2014 AT 10:51 PM

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