You might use an analogue to Capsaicin, one which stimulates all pain receptors, not just heat receptors.
Capsaicin chemically stimulates the heat sensing nerves so that they uncurl just as they do in response to high temperature. The brain interprets the result as a burning sensation, just as if they were actually at a high temperature. Capsaicin is the active component in chili peppers.
Pain receptors certainly include heat sensors burning hurts, but there are several types. You would need a chemical cocktail to activate all of the pain receptors. For example, many of the bodies pain receptors respond to inflammation in nearby tissue.
Ultimately you could develop technology that simply stimulates the appropriate sensations directly in the brain via electrical signals. This would be the most effective because you could quickly turn the pain on/off as needed. This ability would be more effective in torturing than simply constant high levels of pain, because of the additional control over the psychology of pain. Also, you would have more control over pain levels allowing you to maximize all forms of pain over the entire body.
However, you cannot assume that just because the pain is not due to the body being damaged, that you can do this to a person without physical consequence. The body will response to artificial pain just as it does to real pain (which is only logical if the net result is the same according to the brain).
A second limitation is that people pass out in response to pain. This is the basis for some forms of fainting. The level of pain needed to do this varies widely. Perhaps you need to study this response to determine the threshold of pain to induce this response in any given person
Like many medical conditions, you could also treat the underlying symptoms associated with fainting. In particular, giving the patient oxygen can reduce the likelihood of fainting, as could treatment altering blood pressure and blood sugar. Reduce fainting, increase overall pain.