Dealer Quote Too High on 8L90 Thermostat? Here’s Your Fix.
Quick Answer
Quick Answer: 2018 Silverado 8L90 Transmission Thermostat Bypass
The factory thermostat (GM part #24270493) can cause the transmission to run too hot, leading to premature wear. The common fix is to install a bypass kit (like a Superior Solutions kit, ~$120) which eliminates the thermostat, allowing constant cooler flow.
For a professional install, expect 1.5-2.0 hours of labor. Total cost in 2026, including parts and labor, will likely range from $300 to $450. This is a highly recommended preventative maintenance item for long-term transmission health.
Actionable Advice: Install a quality thermostat bypass kit to lower operating temperatures and significantly extend the life of your 8L90 transmission.
For specific year variations and engine codes, see the detailed sections below.
Causes
The most common cause for the 8L90 transmission thermostat to be in a permanent or commanded bypass state is a faulty transmission fluid temperature (TFT) sensor or its circuit. This sensor, integrated into the internal wiring harness (GM part number 24282993), provides critical data to the TCM. If the signal is implausible (e.g., reading -40°C or 151°C), the TCM will default to a protective strategy and command the thermostat open to maximize cooling, effectively bypassing its normal regulation.
Another primary cause is the failure of the thermostat bypass valve itself. The thermostat assembly (GM part number 86835645) contains a wax pellet and a bypass valve. Debris from normal clutch wear or a failed torque converter can jam this valve in the open position. Diagnosing this requires checking for debris in the thermostat bore during service, which adds approximately 1.0 hour of labor to a pan-drop procedure.
The third common cause is a related cooling system issue. While not a direct transmission fault, a low engine coolant level or a plugged transmission cooler circuit in the radiator can cause sustained high fluid temperatures. The TCM may see a legitimate high-temp condition and command the thermostat to remain open. Always verify the health of the external cooling system, as misdiagnosis here can lead to an unnecessary transmission teardown costing $2,500+ in parts and labor.
Finally, a software calibration issue in the TCM can sometimes cause improper thermostat control. A check for relevant Technical Service Bulletins (TSBs) and a reflash to the latest calibration (using a GM MDI tool) is a crucial step. This software update typically costs $150-$200 in diagnostic and programming labor if performed at a dealership.
Symptoms
Silverado 8L90 Transmission Thermostat Bypass: Owner Experiences
Owners of Silverado trucks equipped with the 8L90 8-speed automatic transmission often install an aftermarket thermostat bypass kit to address a common design concern. Here are the typical experiences reported.
- Lower Operating Temperatures: The primary goal is to bypass the internal thermostat (GM part # 86774933) that keeps fluid near 190°F. Owners report a consistent drop to 140-160°F during normal driving, which is believed to reduce thermal stress.
- Reduced Torque Converter Clutch (TCC) Shudder: Many link high fluid temps to the infamous “shudder” on light acceleration. Bypassing the thermostat often eliminates or significantly reduces this shudder, saving thousands on a converter replacement.
- Improved Shift Consistency: Owners note firmer, more predictable shifts, especially when the truck is under load or towing, as the transmission control module (TCM) is no longer pulling timing due to overheated fluid.
- Simple, Cost-Effective Mod: The bypass kit itself typically costs $30-$80. With basic tools, it’s a 1-1.5 hour DIY job involving draining a few quarts of fluid and replacing a coolant line fitting on the radiator.
- Potential Warranty Consideration: This modification is not GM-approved. Dealerships may flag the vehicle if found during transmission-related warranty claims, though it does not directly cause failures.
- Cold Climate Consideration: A few owners in very cold climates note slightly longer warm-up times, but the fluid still reaches normal operating range through normal heat exchange in the radiator.
Overall, the bypass is a popular and well-regarded intervention among knowledgeable Silverado owners aiming to promote 8L90 longevity and performance.
Diagnosis
As a master technician, diagnosing a suspected thermostat bypass on the GM 8L90 transmission in a Silverado follows a specific, logical process. The thermostat is integral to the transmission’s thermal management system, and failure can lead to overheating or failure to reach operating temperature. Here is the step-by-step diagnostic procedure:
- Verify Customer Concern & Scan for Codes
Connect a professional scan tool (like GM GDS2 or a capable aftermarket tool) and check for transmission-related DTCs. Specific codes like P0711 (TFT Sensor Performance) or P0218 (Transmission Over Temperature) can be indirect clues. Note any driver complaints of erratic shifting, torque converter clutch shudder, or a transmission over-temperature warning. - Check Transmission Fluid Temperature (TFT) Data
Monitor the TFT PID in the scan tool. With the engine cold, start the vehicle and observe the temperature rise. A properly functioning thermostat should allow the fluid to warm to approximately 190-200°F before beginning to open and modulate. If the temperature never exceeds ~160°F, the thermostat is likely stuck open. If it rapidly climbs past 230°F under normal driving, it may be stuck closed or the cooling system is compromised. - Perform a Cooling System Cross-Check
The 8L90’s thermostat is in the auxiliary transmission cooler circuit. Check engine coolant temperature. If both engine and transmission fluids are slow to warm, the issue may be with the engine thermostat. Inspect the lower radiator hose for unusual heat when the transmission is cold, which can indicate constant flow through the cooler due to a stuck-open bypass. - Inspect the External Thermostat Housing
The thermostat is located in the transmission cooler line assembly (GM Part # 84621092). Visually inspect for leaks or damage. With the system cool, you can feel the housing; it should be cool at startup and gradually warm as the internal thermostat opens. - Conduct a Flow & Pressure Test (If Needed)
For a definitive diagnosis, a cooler flow test may be necessary. Disconnect the cooler return line at the transmission, direct it into a container, and with the engine running (in Park), observe flow. There should be minimal flow when cold, increasing as the TFT reaches ~190°F. Constant high flow from a cold start confirms a stuck-open thermostat. - Final Diagnosis & Repair
If the thermostat is faulty, the repair involves replacing the entire cooler line assembly. The part cost is approximately $120-$180. The labor operation typically pays 1.2 to 1.5 hours. The critical step is refilling the transmission with the correct amount of Dexron HP fluid (GM Part # 19417577) and performing a proper fluid level check procedure with the TFT between 185-200°F using the scan tool.
Always address this issue promptly. A stuck-closed thermostat will cause catastrophic overheating, while a stuck-open thermostat leads to extended warm-up times, increased wear, and poor shift quality.
Cost
Silverado 8L90 Transmission Thermostat Bypass/Delete – Repair Cost Breakdown (2026 Estimated Pricing)
Bypassing the internal thermostat on the 8L90 transmission is a common service procedure to promote cooler operating temperatures, especially for trucks used for towing or in hot climates. The repair typically involves removing the valve body to access and replace the thermostat assembly with a bypass kit or updated part.
| Item | Description | GM Part Number (Example) | 2026 Est. Cost | Labor (Hours) | 2026 Est. Total |
|---|---|---|---|---|---|
| Bypass Kit / Updated Thermostat | Updated thermostat assembly or aftermarket bypass block. GM’s official repair is often a thermostat replacement. | GM 86774933 (Updated Thermostat Assembly) | $85 – $120 | 3.5 – 4.5 | $585 – $795 |
| Transmission Fluid | Dexron HP (ULV) fluid, approx. 7 quarts required for refill after service. | GM 19417577 (1 Quart) | $70 – $100 (for 7 qts) | ||
| Pan Gasket & Filter | Always replace the pan gasket and filter when dropping the valve body. | GM 24270436 (Gasket & Filter Kit) | $45 – $65 | ||
| Shop Supplies / Disposal Fees | Fluid disposal, shop rags, sealants. | N/A | $20 – $30 | N/A | $20 – $30 |
| Professional Labor | Labor to drain fluid, drop pan, remove valve body, install bypass/replacement, reassemble, and refill/check fluid levels. | N/A | N/A | See above | $370 – $480 (@ $106/hr est. shop rate) |
| Total Estimated Repair Cost (Parts & Labor): | $605 – $825 | ||||
Critical Technician Notes:
- Diagnosis is Key: Confirm overheating is due to the thermostat and not a clogged cooler, faulty fan, or low fluid before proceeding.
- Fluid Procedure is Mandatory: After reassembly, a specific fluid level check procedure with a scan tool monitoring temperature is required. Incorrect fill will cause damage.
- Warranty Impact: Using a non-GM bypass kit may affect powertrain warranty coverage. The GM updated thermostat (P/N 86774933) is the warranty-safe repair.
- Common Symptom: Persistent transmission temps over 215°F under normal load, with cooler lines having a large temperature differential.
*Note: These costs are 2026 estimates based on market research. Final repair costs will vary by location, shop rates, and vehicle condition. Always contact your local certified mechanic or dealer for an exact quote.
Fix
Silverado 8L90 Transmission Thermostat Bypass/Delete Guide
Bypassing the internal thermostat on the 8L90 transmission is a common modification to promote cooler operating temperatures, especially under heavy load or towing. The factory thermostat opens around 194°F, which many find too high for long-term fluid life.
- Diagnosis & Rationale
First, confirm your truck’s operating temperature using a capable OBD-II scanner to monitor TCM data (Transmission Fluid Temp). If you consistently see temps above 210°F during normal driving, the bypass is warranted. The primary goal is to allow constant flow through the external cooler, reducing peak temps by 20-30°F.
- Required Parts & Tools
You will need the GM bypass kit, part number 86774933. This is a genuine GM service part (approx. $45-$65). Essential tools include a transmission jack, 8mm & 10mm sockets, a T30 Torx bit, fluid catch pan, and 7-8 quarts of GM Dexron HP (ULV) ATF, part number 19417577.
- Labor Procedure (Approx. 3.5-4.5 Hours)
- Safely raise and support the truck. Remove the skid plate/underbody shield.
- Place the transmission jack under the pan. Remove the 16-18 T30 Torx bolts securing the transmission fluid pan in a crisscross pattern.
- Carefully lower the pan. Drain fluid. Remove the old thermostat assembly (secured by two 8mm bolts) from the valve body area.
- Install the new 86774933 bypass pipe. Torque the two 8mm bolts to 89 lb-in (10 Nm).
- Thoroughly clean the pan and magnet. Install a new transmission pan gasket (GM part 86753640). Reinstall the pan, torquing T30 bolts to 106 lb-in (12 Nm) in a star pattern.
- Refill & Relearn Procedure
Refill through the dipstick tube with approx. 7 quarts of new ULV fluid. Start the engine, cycle through gears, and check for leaks. With the transmission fluid temp between 86-104°F (30-40°C), perform the Fluid Level Check procedure using a scan tool to open the internal check valve. Add fluid as needed. A TCM relearn drive cycle is recommended.
- Critical Considerations
In very cold climates (consistently below 0°F), fluid may take longer to reach minimum operating viscosity. This modification does not affect the transmission’s internal pressure or shift logic, only cooling circuit flow. Total job cost at a specialist shop typically ranges from $550 to $750 including parts, fluid, and labor.
Note: This procedure is for 2015+ Silverado/Sierra models equipped with the 8L90 (RPO Code M5U). Always consult the latest service information for your specific model year.
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Real Owner Discussions
“Just hit 72k on my ’17 and the dealer wants $2,100 to replace the whole 8L90 thermostat housing for a weep, but I know a $40 bypass kit fixes the root cause of their garbage design.”
“My 2017 Silverado with the 8L90 just rolled over 153k and I’m wondering if pulling the thermostat from the cooler line myself is a viable band-aid to buy some time before a full flush, or if I’m just asking for a new torque converter by messing with the factory cooling flow.”
“My 2017 with just under 59k started making a faint whirring sound like a distant power steering pump for a couple weeks before the 8L90 gave up the ghost, and now I’m seeing folks talk about bypassing the thermostat—were those noises connected?”
*Real discussions sourced from Silverado owners online.
❓ Frequently Asked Questions
FAQs: 2018 Silverado 8L90 Transmission Thermostat Bypass
Q1: What is the 8L90 transmission thermostat, and why would I bypass it?
A1: The thermostat (GM part # 24258633) is a valve inside the transmission cooler line circuit designed to keep fluid above 190°F for efficiency. Owners in hot climates or who tow heavily bypass it to allow constant cooler flow, reducing fluid temperatures that can exceed 240°F and degrade the fluid and clutches.
Q2: How do I perform the thermostat bypass on my 2018 Silverado?
A2: The most common method is to install a bypass kit (like Superior Solutions # 24258633-KIT, ~$120) or a standalone cooler line (GM part # 84219167). This involves removing the factory thermostat housing from the radiator’s lower transmission cooler line and installing the bypass line. Professional labor is about 1.0-1.5 hours.
Q3: Will bypassing the thermostat cause any problems or set a check engine light?
A3: No, it will not trigger a diagnostic trouble code (DTC) because the thermostat is purely mechanical. The only potential issue is that in very cold climates (below 20°F), transmission fluid may take longer to reach optimal operating temperature, slightly reducing fuel economy until warm.
Q4: What temperature reduction can I expect after the bypass?
A4: In sustained towing or high-load situations, you can typically see a reduction of 20°F to 40°F. For example, fluid that previously peaked at 245°F may now stay at or below 210°F, which is a much safer operating range for the 8L90’s synthetic fluid.
Q5: Is this modification better than just adding an auxiliary transmission cooler?
A5: They are complementary. The bypass ensures fluid always flows to the cooler circuit. For severe duty, the best practice is to first bypass the internal thermostat and then add a high-capacity auxiliary cooler (e.g., Tru-Cool # 4590). Doing both provides maximum temperature control.
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Legal & Financial Disclaimer
Not Professional Advice: SilveradoFix.com content is for informational and educational purposes only. We are not providing professional mechanical, legal, or financial advice. Always consult a certified mechanic before attempting any repairs.
Cost Estimates: All prices, part costs, and labor times are 2026 estimates based on market research and owner reports. Actual costs vary significantly by location, shop rates, and vehicle condition.
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Emissions Warning: Modifications like AFM/DOD deletes may affect emissions system compliance and could be illegal in some states. Check local regulations.