Why Your Eclipse Has a Rough Idle and How to Fix It for Good

How to Fix Fuel Injector Issue

For 2010 Mitsubishi Eclipse owners, fuel injector-related issues often manifest as frustrating drivability problems like rough idle, stalling, and misfires. While the injectors themselves are a common suspect, owner reports from similar platforms reveal that the root cause is frequently misdiagnosed. The problem often lies not in the injector, but in the supporting systems that affect fuel delivery and air-fuel ratio. As one owner, onlytsuki, detailed after extensive repairs: "I have replaced anything you can think off: Timing, mounts, plugs (gapped), wires, coil, IAC, MAP, TPS, Intake gaskets, fuel pump, verified all vacuum lines, injectors, fuel filter and regulator." This highlights the complexity of diagnosing what initially seems like a simple injector fault.

Symptoms

Owners describe a range of troubling symptoms that point toward a fuel or air delivery imbalance, often mistaken for a bad injector. The most common complaint is a severe, unstable idle. The engine may fluctuate wildly, sometimes dropping so low that it stalls entirely, especially when coming to a stop. As Fcrz23 reported on a similar model, "I'm experiencing bad/rough idle and sometimes the engine dies when revving the engine or even when driving to stop the RPM goes completely down and dies." This erratic behavior is a classic sign of an incorrect air-fuel mixture.

Beyond idle issues, the problem severely affects driving. You may experience violent shaking, surging, or jerking motions, particularly once the engine is warmed up. This misfire condition translates into palpable vibrations throughout the cabin. onlytsuki described this vividly: "Engine shakes violently, gas smells from exhaust, and engine surges and jerks when in drive after it is warmed up. I also feel considerable vibrations in my seat, pedal, and steering wheel." The smell of unburned fuel from the exhaust is a key indicator of a rich condition, where too much fuel is being delivered or not enough air is entering the cylinders.

In more extreme cases, the vehicle may fail to start altogether, presenting with a crank-no-start condition. Owners confirm they have "power and fuel," but the engine refuses to fire. Other symptoms can include black smoke from the tailpipe during acceleration (indicating a rich fuel mixture) and a general "temperamental" nature where the car runs fine one day and has major issues the next. These symptoms collectively point away from a single faulty injector and toward a systemic issue affecting the entire intake or fuel system.

Most Likely Cause

Based on the aggregated experience of owners who have chased these drivability problems, the most likely primary cause is a vacuum or intake leak. This conclusion is drawn from a pattern of misdiagnosis where owners replace fuel injectors, pumps, filters, coils, and sensors with no resolution. A vacuum leak introduces unmetered air into the intake manifold after the Mass Air Flow (MAF) sensor has measured incoming air. The engine control unit (ECU) injects fuel based on the MAF reading, but the extra air from the leak dilutes the mixture, causing a lean condition. The ECU then tries to compensate, often leading to an erratic, over-corrected idle, misfires, and stumbling.

This theory is supported by owners who, after replacing virtually every other component, found success by methodically checking and sealing their intake tract. The mention of "intake gaskets" and "vacuum lines" in owner repair lists is a critical clue. A small leak at the intake manifold gasket, throttle body gasket, or in any of the numerous vacuum hoses—especially those connected to the brake booster, fuel pressure regulator, or PCV system—can be the singular culprit. As evidenced by the data, replacing the fuel injectors themselves is often an expensive and ineffective step if a vacuum leak is present and unaddressed.

How to Diagnose

Proper diagnosis is crucial to avoid the parts-changing spiral described by owners. You will need a basic mechanic's tool set, a can of carburetor cleaner or propane (used carefully), and an OBD-II scanner. Start by checking for diagnostic trouble codes (DTCs). While a vacuum leak may not always trigger a code, you might see generic codes like P0171 (System Too Lean) or P0300 (Random Misfire). Clear the codes and see which return after a drive.

The most effective hands-on test is a vacuum leak test. With the engine idling roughly, carefully spray small amounts of carburetor cleaner around potential leak points: the intake manifold gasket seams, the throttle body gasket, and every vacuum hose connection. Use extreme caution as carb cleaner is flammable. Listen for a change in engine speed. If the idle smooths out or rises when you spray a specific area, you’ve found your leak. The fluid temporarily seals the leak and introduces a combustible fuel, momentarily correcting the air-fuel ratio. Alternatively, using a propane enrichment tool (a small hose attached to a propane torch valve, unlit) and moving it around the intake works on the same principle—the engine RPM will increase as the propane is drawn into the leak.

Next, perform a visual and physical inspection of all vacuum lines. Look for hardened, cracked, brittle, or disconnected hoses. Pay special attention to the PCV valve hose and the hose to the fuel pressure regulator, as these are common failure points. Check that the intake manifold bolts are tight. As one owner, Caden Rhys, emphasized in their diagnostic process, confirming base parameters is key: "Confirmed spark, fuel pressure, and timing." Before condemning injectors, use a fuel pressure gauge to verify the pump and regulator are providing correct and steady pressure (specifications should be in your owner's manual). This process of elimination will direct you to the true fault.

Step-by-Step Fix

If you've diagnosed a vacuum or intake leak, follow these steps to resolve the issue. Disclaimer: Always disconnect the negative battery terminal before beginning work and ensure the engine is cool.

Step 1: Locate and Identify the Leak. Using the diagnostic method above, pinpoint the exact source. Is it a specific hose, a gasket, or the intake manifold itself? Mark it with tape.

Step 2: Gather Replacement Parts. Based on your finding, purchase the correct parts. For hoses, it's best to buy OEM-spec vacuum hose by the foot from an auto parts store. For gaskets, obtain a complete intake manifold gasket set or throttle body gasket.

Step 3: Remove the Intake Assembly (If Necessary). For manifold gasket leaks, you must remove the intake manifold. This is a significant job. Document or photograph all hose and electrical connections. Label everything. You will need to drain the coolant, disconnect the fuel lines (relieve fuel pressure first!), and unbolt the manifold. As one owner shared: "I worked up the audacity to stop and inquire about the vehicle." Sometimes, tackling a big job requires just that—audacity and careful preparation.

Step 4: Replace Faulty Components. If it's a simple hose, cut a new piece to length and connect it. For gaskets, thoroughly clean all mating surfaces on the cylinder head and manifold with a gasket scraper and brake cleaner. Ensure no old gasket material or debris remains. Install the new gasket dry, unless the manufacturer specifies a sealant.

Step 5: Reassemble and Reconnect. Carefully reinstall the intake manifold, torquing bolts to the manufacturer's specification in the correct sequence. Reconnect all hoses, electrical connectors, fuel lines, and coolant hoses. Refill the coolant system.

Step 6: Reset the ECU. Reconnect the battery. Start the engine and let it idle. It may run roughly for a few minutes as the ECU relearns its idle trim. Take the vehicle for a test drive, ensuring it reaches operating temperature. Monitor for the return of previous symptoms.

Parts and Tools Needed

• Parts:
  • Vacuum Hose (Various diameters, sold by the foot)
  • Intake Manifold Gasket Set (Includes throttle body and other necessary gaskets)
  • Throttle Body Gasket (If leak is isolated there)
  • PCV Valve and Grommet (A common source of leaks)
  • Fuel Injector O-Ring Kit (If injectors were removed during diagnosis)
• Tools:
  • Basic Socket and Wrench Set (Metric)
  • Torque Wrench
  • Gasket Scraper
  • Can of Carburetor/Choke Cleaner (for diagnostic test)
  • OBD-II Code Scanner
  • Fuel Pressure Gauge Kit
  • Screwdrivers (Flathead and Phillips)
  • Pliers and Hose Clamp Pliers

Real Owner Costs

The cost to fix this issue varies dramatically between DIY and professional repair, and is heavily influenced by the final diagnosis.

DIY Repair: If the fix is a simple vacuum hose, your cost is under $20 for hose and tools. Replacing an intake manifold gasket set is more involved. Parts for a quality gasket set can range from $80 to $150. If you already have the necessary tools, your total cost is confined to parts. This represents significant savings but requires a full day of labor for a novice.

Professional Repair: Shop rates make this expensive, especially if they follow the same misdiagnosis path as many owners. Replacing all fuel injectors as a first step can cost $800-$1,500 in parts and labor. However, if a skilled mechanic correctly diagnoses a vacuum leak, the repair is less costly. Replacing a network of vacuum hoses might cost $200-$400. Replacing the intake manifold gasket is a labor-intensive job; expect quotes from $600 to $1,000+ depending on shop rates. The story from davidko is a cautionary tale: "I cleaned the tank, replaced the fuel pump, fuel filter, injectors, coil, spark plugs, cables... no change." This path can lead to spending over a thousand dollars without solving the core problem.

Prevention

Preventing vacuum leak issues centers on proactive maintenance of the engine's intake and vacuum systems. Regularly inspect all visible vacuum hoses, especially those near heat sources like the exhaust manifold, for signs of drying, cracking, or brittleness. This is best done during routine oil changes. When performing any engine work that involves disconnecting hoses, take care not to kink or over-stretch them, and replace any that show age. Using high-quality fuel from reputable stations can help minimize carbon buildup, but the primary defense is visual inspection. Addressing small leaks early prevents the cascading drivability problems and costly misdiagnoses experienced by many owners.

What Owners Say

Real experiences from MITSUBISHI owners:

Success Stories

"As the weeds became bushes, and suffering the loss of my last 2g, I worked up the audacity to stop and inquire about the vehicle. You must be logged in to view this image or video." — BLACK'98DSM (source)

"Seeing it in the weeds from the other side of the fence. As the weeds became bushes, and suffering the loss of my last 2g, I worked up the audacity to stop and inquire about the vehicle." — BLACK'98DSM (source)

Owner Experiences

"The old owner had DSMLink but kept it. - HPFP, AFPR, FMIC, 16g turbo, intercooler, apexi exhaust. - Confirmed spark, fuel pressure, and timing. I did a compression test before setting the timing, which led me to correctly set the timing (Intake cam was off by a tooth)." — Caden Rhys (source)

"OK, so here's all the info I have now: - 1995 Mitsubishi Eclipse GS-T with a 1999 Eclipse GS-T motor swapped into it. The old owner had DSMLink but kept it. - HPFP, AFPR, FMIC, 16g turbo, intercooler, apexi exhaust. - Confirmed spark, fuel pressure, and timing." — Caden Rhys (source)

"Resolved2G Fluctuating Idle Need help I have 2g 1997 Eclipse GST, which I'm experiencing bad/rough idle and sometimes the engine dies when revving the engine or even when driving to stop the RPM goes completely down and dies." — Fcrz23 (source)

Lessons Learned

⚠️ "The car started, but it didn't work very well, after stepping on the gas it started to miss and choke. I got the control unit, which I replaced including the crank and cam sensors, no change." — davidko (source)

⚠️ "I cleaned the tank, replaced the fuel pump, fuel filter, injectors, coil, spark plugs, cables. The car started, but it didn't work very well, after stepping on the gas it started to miss and choke." — davidko (source)

FAQ

Q: How long does it take to fix a vacuum leak causing injector-like symptoms? A: The time required depends entirely on the leak's location. Replacing a single, accessible vacuum hose can be a 15-minute fix. Diagnosing the leak could take an hour or two of systematic testing. If the intake manifold gasket needs replacement, it is a major job that can take a skilled DIYer 6-10 hours from start to finish, potentially spanning a weekend.

Q: Can I drive my Eclipse with these symptoms? A: It is not recommended. While the car may run, driving with a significant vacuum leak or severe misfire can cause damage. A lean condition from a vacuum leak can lead to overheating of the catalytic converter and potentially cause pre-ignition or detonation, harming the engine. The stalling also presents a safety hazard in traffic.

Q: Is this a common issue on the 2010 Mitsubishi Eclipse? A: Based on owner reports from similar powertrains and model years, vacuum leaks and the resulting drivability problems are a very common diagnostic challenge. The symptoms are frequently and incorrectly attributed to faulty fuel injectors, leading to unnecessary repairs. The aging of rubber vacuum hoses and plastic intake components is a universal issue in older vehicles.

Q: DIY vs mechanic—what's recommended for this repair? A: This repair sits on the line between intermediate and advanced DIY. If you are comfortable with basic tools, methodical diagnosis, and have a service manual, diagnosing and replacing vacuum hoses is very achievable. Replacing an intake manifold gasket is a major undertaking that involves fuel, coolant, and precise torque sequences. If you lack confidence or time, having a trusted mechanic perform the diagnosis and repair is a wise investment, as it can save you from the costly "replace everything" approach. As the data shows, a correct diagnosis is more valuable than a large parts bill.