Case: Hyundai Elantra GLS 2001 - Hot run issues, Lean O2 readings.

Hello colleagues, I wanted to share this case to see if anyone has experienced something similar:

Vehicle: Hyundai Elantra GLS 2001, 2.0L engine, returnless fuel system (no external return line, internal regulator in the tank).
Symptom: When the vehicle is quite hot, the engine runs poorly as if it’s starved of fuel, both at idle and under load.
After resting ~5 minutes, the issue does not reappear.

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📊 Observed data
 • O₂ sensors (upstream and downstream): voltage drops almost to 0 V during the failure (very lean).
 • Injector pulse width: rises from ~3 ms to ~18 ms (ECU is trying to enrich).
 • Fuel pump voltage: 14 V with proper ground reaches the connector.
 • MAP sensor: shows increased manifold pressure.
 • Throttle sensor: working correctly (mechanical throttle with sensor).
 • DTCs: during one occurrence, codes appeared, mainly related to the link.

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🧰 Parts history / previous work
 • What I replaced: spark plugs and coil.
 • Other shops: CKP sensor, complete fuel pump module (pump, regulator, float, and reservoir), upstream O₂ sensor, fuses checked.
 • Fuel tank: partially cleaned previously; some dirt was noted in the fuel rail.

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⚙️ Observed behavior
 • Holding the accelerator steady → the engine continues to run poorly.
 • Releasing and quickly pressing the accelerator → responds briefly, then fails again.
 • Cold engine → runs fine.

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🧩 Most likely hypotheses
 1. Internal issue with the returnless fuel pump module (jet pump or internal regulator causing the reservoir to starve).
 2. Partial restriction or sediment in the fuel strainer/tank.
 3. Fuel injectors failing in hot conditions, possibly due to partial dirt in the rail.

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🔧 Planned tests
 • During the failure, apply propane or carb cleaner spray to the intake to see if the engine improves.
 • Observe injector pulse width, O₂ readings, and RPM on the scan tool in real time.
 • Check how the engine responds when releasing and reapplying the accelerator.

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❓ Community question

Has anyone experienced a similar issue in a 2000–2003 Hyundai Elantra 2.0L, with intermittent hot engine lean condition, ECU attempting to enrich at maximum, and O₂ nearly at 0 V, which turned out to be internal fuel pump module issues, partial sediment in the fuel strainer, or hot injector failures?
Any experience or testing advice would be greatly appreciated.
 

To answer your question, I don't think I have seen a Hyundai with a pump that fails only when the engine is hot.
A failing injector will induce a single cylinder misfire, so if there's one cylinder that's not contributing while it's running lean, you may be on to something there. I would compare the upstream and downstream 02s and verify they are both lean. This will prove if there's an issue with the upstream measurement.
I would also check fuel pressure if you have a gauge and kit you can T into the rail with, and if you suspect an injector, I would check the injector cold and hot with an injector timer tool while the fuel gauge is in the system.

Thanks again! Yep, I already monitored both O₂ sensors, and they both drop to almost 0 mV during the failure, so that part is clear.

One thing I want to check next is the current going to the injectors, just to see if it might be affected by the fuse box getting hot, as the customer suspects… though honestly, I don’t put too much faith in that idea 😅.

We tried reproducing the failure yesterday for about 20 minutes, but it didn’t happen, and when I opened the hood, of course, the car behaved perfectly. It’s kind of sad — imagine driving two hours with the failure, then spending time in the shop… and it disappears just because you lifted the hood!