Very simple lost the owners manual. need to know

These units come with a detergent bottle that attaches to the spray wand. The manual describes what detergents to use and the attachment reduces the pressure and siphons the right amount of detergent from the container. It is not a good idea to try to run detergent through the pump.
Here is a link to the operating manual for a similar pressure washer.
http://www.ppe-pressure-washer-parts.com/files/1961581/uploaded/pw0501400.pdf.

Check Coleman's web site Coleman Outdoor Gear for Camping Hiking and Tailgating US

Linky:
Free Coleman Portable Generator User Manuals ManualsOnline com

any wands will work

A model number would have been helpful; however this may be your manual:

http://www.powermate.com/pdfs/2007/11/10/manual_pw0501400.pdf

If the above is not your manual, check your unit for a model number and then review or search the list here:

http://www.powermate.com/productmanuals.php?cat_id=6

Cut and paste into your address bar...>>http://www.ppe-pressure-washer-parts.com/coleman-powermate-pressure-washer-parts

Did you try a new spark plug. Old one may have fouled.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/craig_3fa289bf857b1a3c

The systems are simple but if they aunt stored correctly(allowed to freeze) or the air isn't purged out of it. It can result in damage to the machine.If water is dripping out of the pump it is damaged. if not then it could be air (purge system), or a damaged or stuck check valve.

Here is a link where you can look up your exact model and replacement parts from coleman...
http://www.ppe-pressure-washer-parts.com/coleman-powermate-pressure-washer-parts

Looks like your model may be out of production though so here's a generic kit that will 99% work with your washer...
http://www.silverbin.com/FAIP-PowerWasher-Universal-Pressure-Washer-Gutter/A/B000EEQ9KM.htm

Hope that helps...

your pressure relief valve is stuck open. i know that may sound too simple but it depends on if you use a cleaner with the water when your using it , not flushing with clean water after use can cause the housing of the aluminum housing of the relief valve to corrode. the valve can't move back and forth as it should because the o ring won't slide.