What This Thermostat Does
This is the lube circuit thermostat, not the main engine cooling thermostat. It sits in the oil cooler coolant path and controls how much coolant flows across the cooler. Cold engine, it stays shut and lets the oil come up to temperature. Hot engine, it opens and dumps heat out of the oil into the coolant loop. 215F is the rated opening point, roughly 102C.
When the wax element fails stuck closed, oil temperature climbs and stays climbing under load. Hot oil thins out, oil pressure drops at idle, and the ECM may log high oil temp. Stuck open is quieter and easier to miss. Oil never gets up to temperature, condensation and fuel dilution stay in the sump, and you see the damage later in oil analysis.
The 4952631 unit uses a calibrated wax element in a stainless steel body. Wax elements do not degrade gracefully. They drift, then they stop moving. That is why a thermostat that tested fine last season can be the reason your oil temp gauge is pinned this season.
Why These Numbers Cross
4952631, 3100300 and 181960 all describe the same lube thermostat for the ISX/QSX oil cooler circuit. Cummins carried this thermostat under more than one number over the production run, which is where 3100300 comes from. 181960 is the PAI aftermarket number for the same piece. Different catalogs, same physical part.
What has to match for a cross to be real is the opening temperature, the seat diameter, the body length and the travel of the element. Get the temperature wrong and the oil either never warms or never sheds heat. Get the seat wrong and it will not seal in the housing bore. All three of these numbers share the 215F rating and the same envelope.
Numbers from Cummins and PAI are listed here for identification only. We are not affiliated with, endorsed by, or authorized by any OEM. If your old thermostat carries one of those numbers, this is the part you are looking for, but confirm against the number stamped on the unit you pull out.
Before You Order
Get the engine serial number off the dataplate and use it. ISX, ISX15, X15 and QSX cover a long production span and the oil cooler housing was not identical throughout. A cross-reference is a starting point, not a fitment promise. The only two things that settle it are the ESN and the number cast or stamped on the thermostat you remove.
Pull the old one and look at it before the new one goes in. Check the seat face for pitting and the bore for scale buildup. Replace the seal or gasket, do not reuse it. If the oil cooler core is plugged, a new thermostat will not fix your oil temperature, so check cooler restriction while you are in there.
4952631 Cross-Reference & Interchange Numbers
These manufacturer and aftermarket numbers commonly cross-reference to 4952631. Use them to identify the part — then confirm the exact fit by VIN or the number stamped on your old unit before you order. An interchange is a starting point, not a fitment guarantee.
| Brand / Line | Cross-Reference Numbers |
|---|---|
| Cummins | 3100300 |
| PAI | 181960 |
Frequently asked questions
Is 4952631 the same as the main engine thermostat?
No. This is the oil cooler, or lube, thermostat. It regulates coolant flow through the oil cooler to control oil temperature. The main engine thermostat controls coolant flow to the radiator and is a separate part with a different opening temperature. Replacing one does not address a fault in the other.
How do I know if the lube thermostat is stuck?
Stuck closed shows up as oil temperature climbing under load and low oil pressure at hot idle. Stuck open shows up as oil that never reaches operating temperature, often with fuel dilution or moisture in oil analysis. Bench test the removed unit in heated water and watch whether the element moves near 215F.
Will this fit my ISX if my old part says 3100300?
3100300 crosses to this same 215F lube thermostat, so it is the right starting point. Still verify with your engine serial number and compare the old unit for seat diameter, body length and element travel before installing. Cross-reference identifies the part; the ESN and the old part confirm fit.
Not sure it fits? We check before you pay.
Run your VIN and we’ll match parts to your exact truck, or call the counter — a person who knows trucks verifies fitment by OEM number before the order ships. No wrong-box returns.
More cross-reference guides
Need the part, not just the reading?
Every part below is a Premium Quality aftermarket Direct Replacement with published OEM cross-reference numbers, a 6+ month warranty and same-business-day shipping from Tacoma, WA. Not sure it fits? Run your VIN — or call and a person who knows trucks will verify fitment before you pay.