Where to install thermal relief valve

Question edited for clarity, full 'make model (what it is)'.
Question moved to a better category.

This says you don't need it. You just need a 3/8 plug.145'F is the temperature it opens.

https://mastertoolrepair.com/portable-gas-pressure-washer-parts-type1-dp3750-p-1687081.html

There is one here, aftermarket type I guess

https://cvindustrialhardware.com/shop/dewalt/valve-thermal-relief/

More here

https://www.google.com/search?q=dp3750+thermal+relief+valve

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thermal relief valve would be a start. but is the pump actually turning... are you on city water or well water...

If they are the tires with the plastic rims then there was a recall and a local service center can replace them for free, if not I have tires that fit on this unit in stock, price is $13.33 each, send me a email at [email protected]

Does the thermal valve just screw into the pump head, if so what size thread is it? I have 1/4, 3/8, 1/2 in stock.

Many of the excel units don't come with a pressure relief valve but instead a thermal relief valve, it really depends on the model number of the pressure washer. You should only replace the thermal relief valve if it is leaking.

I like http://www.ppe-pressure-washer-parts.com/excell-vr2522-replacement-parts-pump See pump upgrade and compare to rebuilding. Another good site is www.pressure-washer-parts.com. Piston seals usual begin by overheating from excessive idling. You might add a thermostatic relief valve to help this.

check oil filter/sump gasket or sump oil drain plug copper washer

Have a helper hold the trigger of the wand open while you try it again. If it starts that way you have a sticky unloader valve in the pump. DO NOT RELEASE the trigger for more than 10 seconds if the engine continues to strain when released! If the engine doesnt run freely when you let go of the trigger, pull it again and shut down the unit!
Add some power steering fluid to your supply hose (about 3 or 4 ounces at a time) and restart the unit with the trigger pulled. Allow the oil to flow through the pump for 1 - 2 minutes for each treatment. Several treatments should make the pump and valve happy again.

Hope this helps.

Some suggestions:
Is the gas old? If in doubt replace it.
Try removing the air filter and spraying carburetor cleaner into the carb Then try starting it. If it runs for just a minute, then stops, the carburetor needs cleaning.