8L90 Transmission Thermal Bypass Valve with 3/8 Inch Line

If the transmission temperature gauge on your 8L90 truck or van keeps creeping up under load while the cooler itself is clean, the thermal bypass valve is one of the first parts to suspect. This valve replaces GM 13561467, the version used on applications with 3/8 inch cooler lines: 2015-2018 Chevrolet Silverado 1500 and GMC Sierra 1500, 2015-2017 GMC Yukon and Cadillac Escalade, 2016-2017 Yukon XL, and 2018-2025 Chevrolet Express 2500 and GMC Savana 2500. The van fitment is the one many buyers miss, because most bypass parts are only listed for pickups.

What the thermal bypass valve does in an 8L90

The 8L90 mounts its thermal bypass valve outside the case, in the cooler circuit. Inside the factory valve is a wax thermal element. While the fluid is cold it keeps flow away from the cooler so the transmission reaches operating temperature quickly. GM bulletin 22-NA-182 states that the 8L90 valve opens at 194 F (90 C), much later than the 158 F opening point on the 6L80 and 6L90. That late opening is one reason these trucks run warmer than owners expect, and if the element sticks or the passages restrict, fluid keeps skipping the cooler just when you are towing or hauling. This replacement uses a machined aluminum body with a full-flow path, so fluid goes through the cooler at all times instead of waiting on a thermostat. It also has a removable magnetic plug that collects steel particles from the fluid.

Symptoms of a sticking 8L90 bypass valve

• Transmission fluid temperature that climbs past normal when towing, climbing grades or sitting in traffic.
• A cooler and radiator that both look clean and free of debris, yet temperatures stay high.
• Harsh shifts or shudder that get worse once the truck is fully warm, which are complaints GM groups with overheating in 22-NA-182.
• Darkened or burnt-smelling fluid well before the service interval.
• On 2021 and later ETRS-equipped trucks, GM notes a twisted cooler line can set DTC P27EC; that is a line issue, not the valve, so rule it out first.

How to confirm the valve before you buy

Do the line check first. GM's own path in 22-NA-182 is to inspect the rubber cooler lines for a twist or kink, untwist and reconnect if needed, and only then look at the bypass valve if the line is straight. Our 8L90 cooler line replacement guide walks through that inspection and the fluid reset. Then watch transmission temperature on a scan tool: if it passes 194 F and the cooler outlet line stays noticeably cooler than the inlet, flow is not getting through. Overheating also feeds shudder, so read 8L90 common problems and fixes before blaming the converter alone.

Fitment

This valve is for the 8L90 with 3/8 inch cooler lines on the vehicles listed above. It does not fit the 6L80 or any truck with 1/2 inch cooler lines, and the 8L90 used more than one bypass valve over its run, so hold your old valve next to the photos and compare the line ports and mounting before you order. If you are unsure, send us your year and VIN and we will confirm. You can see every part we carry for this unit on our 8L90 parts page.

Install notes

• Work with the transmission cool and a drain pan under the valve; the cooler circuit will drain back when you open it.
• Release the cooler line fittings at the factory valve properly rather than prying on the plastic retainers.
• Install the new valve in the same position with the supplied sealing rings and keep the original line routing. No cutting or programming is needed.
• Start the engine, check every fitting for leaks, then set the level with the GM check procedure through the pan level plug at operating temperature, not by a fixed quart count. GM lists DEXRON HP for the 8L90.
• If the truck also has a warm shudder, pair this job with the 8L90 valve body shudder fix kit. Owners who want to check level from the engine bay often add the locking dipstick and fill tube.

Keep in mind that full-time cooler flow means the transmission takes a little longer to warm up on cold mornings. On trucks that tow or work hard, cooler fluid is the better trade, since heat is what breaks down ATF and glazes clutches.

Will this fit my 2019 Silverado 1500?

Not as listed. The listed pickup fitment for this 3/8 inch line valve is 2015-2018 Silverado 1500 and Sierra 1500. The redesigned 2019 and later trucks are not on the list, so send us your VIN before ordering.

Does it fit the Express or Savana van?

Yes, the listing covers 2018-2025 Chevrolet Express 2500 and GMC Savana 2500 with the 8L90 and 3/8 inch cooler lines.

How do I know if I have 3/8 or 1/2 inch cooler lines?

Check the line ports on your existing valve against the photos, or measure the steel line. This valve will not connect to 1/2 inch lines, and it does not fit the 6L80.

At what temperature does the factory 8L90 bypass valve open?

GM bulletin 22-NA-182 gives 194 F (90 C) for the 8L90 external valve. This replacement does not wait on a thermostat; it sends fluid through the cooler all the time.

What fluid do I use after installing it?

GM specifies DEXRON HP for the 8L90. Set the level through the pan level plug at operating temperature after the job, because some fluid drains out of the cooler circuit.

Do I need a tune or relearn?

No programming is required for the valve itself. If you also did a fluid exchange or valve body work, follow the GM adapt procedure for those repairs.

• 8L90 Cooler Line Replacement: Diagnosis, Procedure and the Fluid Reset
• 8L90 Common Problems and Fixes
• 8L90 3-4 Clutch Failure: Symptoms and Fix
• 6L80 vs 8L90: Differences, Interchange, and Which Is Stronger