Popular Hyundai Fuel System and Emissions Diagnostics Issues

Hyundai Fuel System And Emissions Diagnostics

Bob Dowie recently had a 2002 Elantra GLS in the shop. This vehicle is a good example of some common fuel trim and emissions problems you'll see.

We recently had a 2002 Elantra GLS in the shop with assorted issues, so let’s take a look at this job since it’s a good example of some common problems we see. The car wasn’t in great shape, and a persistent check engine light was preventing it from passing the state ­inspection.

As is often the case with this kind of job, the customer was reluctant to make a big investment but also wasn’t in the position to buy a new car. The obvious problem was a code for a solid misfire in the No. 3 cylinder (P0303), as well as codes for a slow response from the front O2 sensor (P0133), a system lean code, plus there was a slight, but audible, exhaust leak at the flex pipe before the cat. The customer had already ­installed a new O2 sensor, but the code persisted.

Even though we explained to the customer that there was no way we could give a firm price on the job, we ­received authorization to get started with the diagnosis and to get further authorizations as the job progressed.

The first step was to deal with the misfire since we also had the system lean code. We knew the problem was with the No. 3 cylinder, so we went right to the manifold in that area. A quick shot of intake cleaner at the manifold flange smoothed the ­engine out, letting us know we were on the right track.

We used a stethoscope where the sound of the leak was obvious in order to confirm and pinpoint the problem. Knowing that there was little cushion in this job, we checked compression as well as the ignition components when the gasket was replaced. When we were sure the misfire would be ­repaired and the car would certainly run better with the manifold gasket, we received ­authorization to replace the gasket and install a much-needed set of spark plugs. With the gasket replaced and the engine running smoothly, we checked the output of the rear O2 sensor and it looked to be operating as ­expected in spite of the exhaust leak.

While we were aware that fresh air entering the exhaust system could lead to an O2 code, with our budget constraints in mind, we set out to run the monitors in an attempt to get the car through state inspection, giving the customer some time to budget for the necessary additional repairs.

Our experience with Hyundais taught us that they can be a bit of a challenge when it comes to setting monitors, and it really pays off to be sure the setting criteria is met. We’ve been in the habit of having the scanner hooked up whenever we’re road testing a driveability problem or setting monitors to confirm setting criteria.

In the case of the Elantra, we’re looking for a starting coolant temp below 104º F with a gain to 120º F before the test will run. There are additional parameters that have to be met depending on the monitor, and it’s well worth the time to look them up on a service information system. The ECUs know nothing about being “close enough.” If the system is looking for 104º F, then 105º F won’t cut it and the test will not be run (and some tests will look for an even higher temp). If the thermostat doesn’t let it get there, you’re wasting your time trying to run the test. I don’t know about you, but I can’t tell a 10-degree difference on the average temp gauge.

Photo 1: The Hyundai gasoline direct injector looks like port injection, but this injector does find its way into the cylinder.
Photo 1: The Hyundai gasoline direct injector looks like port injection, but this injector does find its way into the cylinder.

While the Elantra ran well, it took only a couple of drives before we had the O2 code back. When we monitored the front O2 signal, it appeared to be switching well, but it was hard to tell if they were all within a second. Knowing that the exhaust leak could be an issue, we replaced the flex section of the pipe. With the now-quiet exhaust system, the signal from the O2 sensor looked better. It took awhile, but the code eventually still set. At this point, we’re back to looking into our service info and checking for issues on iATN, something that should have been among our first steps.

Photo 2: High-pressure pump, driven off of the camshaft.
Photo 2: High-pressure pump, driven off of the camshaft.

A quick search on iATN covered the steps we ­already took, but also mentioned problems with the grounding of the alternator bracket to the block, as well as the engine ground and body grounds near the battery. I’d like to say that I know exactly how the bad grounds affected the O2 readings, but I’m afraid I can’t. I can say that after we cleaned the grounds, the car set the monitors in two trips, and we had a successful repair and happy customer who no longer talks about needing a new car.


A lot has changed since 2002, to the point that on the more current Hyundais, you’ll find direct injection (GDI); drive-by-wire throttles; timing chains rather than belts on some engines; and idle stop-and-go (ISG) technology.

By now, we should all be aware of direct injection and how it works. Simply speaking, this ­system ­injects the fuel directly into the cylinder rather than before the intake valves in the port. Injecting the fuel into the cylinder takes advantage of the cooling effect of the fuel, resulting in a more dense mixture that permits the use of a higher compression ratio, providing increased horsepower and torque.

More torque results in the same acceleration with less throttle opening, which increases the miles per gallon. Efficiency is also improved with more precise timing of the injector pulse and the elimination of the fuel dropout that occurs with port injection.

Photo 3: Air intake without the mass air sensor. The engine load signal is handled by the MAP sensor.
Photo 3: Air intake without the mass air sensor. The engine load signal is handled by the MAP sensor.

You need enough fuel pressure to overcome the ­increasing cylinder pressure to deliver the fuel into the cylinder. Hyundai uses an in-tank, low-pressure pump to deliver 65 psi to a camshaft-driven, mechanical high-pressure pump. This pump will boost the pressure from 580 psi at idle to a maximum of 1,958 psi at full throttle to ensure fuel delivery as the cylinder pressure increases with rpm. This pressure is controlled by an internal pressure regulator in the pump and is monitored by a fuel rail pressure sensor.

In the event of a failure, a limp-home mode is provided where 65 psi will be delivered. This system also uses a new design and operating strategy for the injectors that can handle the increased pressure.

Speaking of fuel pressure, good work habits are more important than ever when dealing with this type of pressure. Always release the pressure before working on the system. It’s easy enough, especially if you check your service information for the model you’re working on. You will have to remove the fuel pump relay and start the car to run down the pressure.

The only downside to GDI seems to be that with no fuel flowing over the intake valves, there have been some reports of carbon buildup on the valves and ports. There are many theories on what causes this carbon buildup, but all seem to substantiate that the oil that passes through the intake manifold from the crankcase ventilation system is a major cause. Some engineers feel the problem starts when fuel contacts the back of the intake valve, resulting in a soot-like substance that encourages the carbon buildup.

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