2018 GMC Yukon Transmission Fluid & Service Guide

Quick answer

• Transmission make/model: GM 8L90 (RPO M5U)
• OEM fluid spec: DEXRON HP (DEXRON ULV is NOT interchangeable)
• Drain & fill capacity: Approximately 6.0 - 7.0 quarts (varies slightly by pan size and drain completeness)
• Full overhaul capacity: Approximately 11.8 quarts
• Filter OEM part number: Verify with dealer or service manual (typically AC Delco 24278401 or equivalent)
• Recommended service interval:
  • Normal duty: 97,500 miles (150,000 km) or 72 months, whichever comes first
  • Severe duty/Towing: 45,000 miles (72,000 km) or 36 months, whichever comes first

Fluid type explained

The 2018 GMC Yukon, when equipped with the 8L90 (RPO M5U) 8-speed automatic transmission, requires DEXRON HP transmission fluid. It is crucial to understand that DEXRON HP is a very specific, low-viscosity fluid designed for the intricate internal components and tight tolerances of this advanced transmission. DEXRON ULV (Ultra Low Viscosity) is NOT a direct substitute and should not be used in the 8L90 transmission. Using the incorrect fluid can lead to premature wear, poor shift quality, overheating, and ultimately, transmission failure. Always consult your owner's manual or a certified GMC service manual to confirm the correct fluid specification for your specific vehicle.

Service procedure

Servicing the 8L90 transmission can be done via two primary methods: a drain and fill, or a full fluid exchange. Both have their advantages and disadvantages.

Drain & Fill

A drain and fill involves removing the pan, allowing the fluid to drain out, replacing the filter, and then refilling the transmission with fresh fluid. This method typically replaces about 50-60% of the total transmission fluid capacity. It's a simpler and less expensive procedure. However, a significant amount of old fluid remains diluted with the new fluid. For this reason, some recommend performing multiple drain and fills over a short period (e.g., every few thousand miles) to gradually replace more of the old fluid.

Full Fluid Exchange (Flush)

A full fluid exchange, often referred to as a "flush," involves connecting a specialized machine to the transmission cooler lines. This machine circulates new fluid through the transmission, forcing out the old fluid until only new fluid remains. This method replaces nearly 100% of the transmission fluid. While more comprehensive, it is also more involved and typically costs more. When performed correctly with the specified fluid, it is the preferred method for ensuring maximum fluid longevity and transmission health. However, ensure the shop uses a reliable machine and the correct DEXRON HP fluid, as an improper flush or incorrect fluid can cause significant issues.

Critical Note: The 8L90 transmission requires specific temperature-based procedures for checking the fluid level. This is not a simple "dipstick" check. A scan tool is often required to monitor transmission fluid temperature and ensure accurate level setting. Improper fluid levels (either too low or too high) can lead to transmission damage and poor performance. It is highly recommended that transmission fluid service be performed by a qualified technician familiar with the 8L90's specific requirements.

Service interval

The service interval for your 2018 GMC Yukon's 8L90 transmission is highly dependent on your driving conditions:

• Normal Duty: For vehicles operating under typical driving conditions (e.g., mostly highway driving, no heavy towing, moderate climate), GMC recommends changing the transmission fluid and filter every 97,500 miles (150,000 km) or 72 months, whichever comes first.

• Severe Duty/Towing: If you frequently tow trailers, carry heavy loads, drive in mountainous terrain, operate in extremely hot climates, or use your vehicle for commercial purposes, your transmission is subjected to "severe duty." Under these conditions, the service interval should be shortened significantly to 45,000 miles (72,000 km) or 36 months, whichever comes first. This shorter interval helps to preserve the integrity of the fluid and internal components under increased stress.

Adhering to these service intervals is crucial for maximizing the lifespan and performance of your 8L90 transmission.

Common transmission problems (8L90 Generation)

The GM 8L90 transmission, while generally robust, has had some known issues that Yukon owners should be aware of, particularly in earlier generations. One of the most frequently reported problems is "transmission shudder" or "torque converter shudder." This can manifest as a vibration or shaking sensation, especially under light load at certain speeds (e.g., 40-70 mph), often feeling like you're driving over rumble strips.

This shudder is frequently attributed to fluid degradation and/or torque converter clutch wear. In many cases, a complete fluid exchange with the correct DEXRON HP fluid, sometimes performed multiple times, has resolved or significantly mitigated the issue. Some owners and dealerships have also reported success with specialized flush procedures or updated torque converter designs.

Other less common but reported issues include delayed or harsh shifts, particularly from a stop or during downshifts. These can sometimes be resolved with transmission control module (TCM) software updates. Always have any transmission concerns diagnosed by a qualified technician.

Find a trusted shop on Repairius.