1999 Mitsubishi Diamante Has Rough Idle

Common Causes of Rough Idling in Your 1999 Mitsubishi Diamante and How to Resolve Them

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Reviewed by
Bo Fader
Written by RepairPal Car Genius
Updated January 14, 2025
0%
21% chance it's the
Spark Plugs
0%
15% chance it's the
Ignition Coil
0%
64% chance it's
Something else
The most common reasons a 1999 Mitsubishi Diamante has rough idle are a vacuum leak, an issue with the spark plugs, or a problem with the ignition coil.
  • Spark Plugs: Worn or fouled spark plugs can lead to misfires, poor fuel economy, and difficulty starting the engine.
  • Ignition Coil: A faulty ignition coil can cause misfires, rough idling, and difficulty starting the engine due to inadequate spark delivery to the spark plugs.
Proper diagnosis will determine the exact cause of your symptom, saving you money on unnecessary repairs. The diagnostic experts at RepairPal Certified shops have the right tools and experience to solve any symptom.

How much does it cost to fix a 1999 Mitsubishi Diamante has rough idle?

Common Repairs

The total repair cost will vary based on the condition of your vehicle and the number of repairs needed to fix your symptom. Proper diagnosis will save you money by finding the exact repairs needed to fix your car right the first time.

Here are a few common symptoms of worn-out brake pads:

  1. Low brake pad warning light is on: Not all vehicles are equipped with this system, but if yours is, this dashboard indicator light will likely be the first signal it’s time to replace your brake pads.
  2. Brakes start to squeal regularly: This is often one of the first signs of worn brake pads and is caused by a metallic piece built into brake pads for this exact purpose. This part contacts the brake rotor to make a squealing or whining noise when the pads are low.
  3. Brakes make loud grinding noise: If not addressed early enough, your brake pads may get so worn that the metal backing plate of the pad grinds against the metal rotor.
  4. Brake pads look thin: You may be able to visually check your brake pad thickness to determine if they need replacement.

Most often, a brake system inspection will just be a routine portion of manufacturer specified maintenance, and will be performed at mileage intervals. In those cases, there will likely be no symptoms. However, when a brake system makes a funny noise, causes vibration, is hard to use, will not slow the vehicle, prevents the vehicle from driving, makes the vehicle pull to one side, or leaks brake fluid, the vehicle will need a visit dedicated to the inspection of the brake system.

What are the initial diagnostic steps to take for a rough idle in a 1999 Mitsubishi Diamante?

When addressing a rough idle in your 1999 Mitsubishi Diamante, it's essential to adopt a methodical diagnostic approach. Start by checking the spark plugs, as worn or faulty plugs can significantly impact engine performance. Next, inspect the ignition system, including coils and wires, for any signs of damage that could disrupt the spark delivery. Following this, examine the air intake system; a clogged air filter or throttle body can restrict airflow, leading to an uneven idle. Utilizing an OBD-II scanner to check for error codes can provide insights into underlying issues, guiding your next steps. Additionally, assess the fuel system components, such as injectors and filters, to ensure proper fuel delivery. Don’t overlook the vacuum hoses; any leaks can cause a rough idle by affecting the air-fuel mixture. Finally, test the idle air control valve, as its malfunction can also contribute to idle instability. By systematically working through these steps, you can effectively diagnose and address the rough idle issue in your vehicle.

What are the common problems that could cause a rough idle in a 1999 Mitsubishi Diamante?

When dealing with a rough idle in your 1999 Mitsubishi Diamante, it's essential to consider several common problems that could be affecting your vehicle's performance. One of the first areas to check is the air filter; a dirty or clogged air filter can significantly restrict airflow to the engine, leading to an uneven idle. Additionally, inspect the spark plugs, as worn or faulty plugs can cause misfiring, which is a frequent culprit behind rough idling. Another critical aspect to examine is the vacuum system for any leaks, as these can disrupt the air-fuel mixture and result in instability during idling. Fuel system issues, such as a clogged fuel filter or malfunctioning fuel injectors, can also impact engine performance, contributing to a rough idle. Don't overlook the ignition system; problems with components like the ignition coil or distributor can lead to similar symptoms. Lastly, a dirty or malfunctioning throttle body can hinder airflow into the engine, exacerbating the rough idle issue. By systematically checking these areas, you can effectively troubleshoot and address the rough idle in your Mitsubishi Diamante, potentially saving on costly repairs.

How urgent is it to address the rough idle in a 1999 Mitsubishi Diamante to prevent potential safety risks and further engine damage?

Addressing a rough idle in your 1999 Mitsubishi Diamante is not just a matter of convenience; it is a critical safety concern that demands immediate attention. A rough idle can indicate serious underlying problems such as engine misfiring, clogged fuel injectors, or a malfunctioning oxygen sensor. If left unaddressed, these issues can escalate, leading to decreased fuel efficiency, engine stalling, and even dangerous driving conditions. Moreover, the longer you ignore a rough idle, the more wear and tear you inflict on your engine components, which can result in costly repairs down the line. Therefore, it is essential to diagnose the root cause of the rough idle promptly and perform the necessary repairs or maintenance. By doing so, you not only ensure the smooth operation of your vehicle but also safeguard your safety and the longevity of your engine.

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1999 Mitsubishi Diamante Rough Idle: Causes & Repairs