You have to pump in thick solutions with a large irrigating syringe. It is impossible to get something as thick as mayonnaise to flow down a tube of any diameter. It doesn't spill out of the jar when you turn it upside down, does it?
Now, if you think back to your bacteriology course (you did take one in college, didn't you??), there is something that you can use that goes in like maple syrup, thickens as it cools and gels inside you, and comes out thick to firm chunks, but not hard. You could boil up some agar agar, let it cool to about 115 degrees F and inject that, let it cool in your body and it will firm up to become like firm jello in the colon. Unlike metamucil which starts thickening immediately and gets stuck in the hose, agar agar must cool before it gels, and even after reaching the cooler temperature must undergo the gelation to get firm. The firmness increases after about ten to thirty minutes or so, depending on how much agar agar powder you mix into solution and the actual temperature it is when you put it in your colon.
This works on the same principle as jello, except that jello comes from animal protein and agar agar comes from seaweed or algae, depending on exactly what agar you buy. Jello will melt at room temperature, so don't expect that to do anything other than stay liquid inside your colon. Agar agar gels at a higher temperature, so it is ideal for this prupose.
The amount of agar agar powder you add to the water will determine how firm/hard it gets when the solution gels when it gels. When you expel it, the agar agar will be in firm CHUNKS, firmer than feces unless you make a fairly dilute solution. You can actually make casts of the inside of your colon with this. I haven't made this in a couple of years, so I don't recall the exact amount of powder I used. It doesn't take much agar agar powder, about one volumetric ounce, to make a quart, which is about all you will want for this kind of enema. You will have to test it out. The consistency is also a bit technique sensitive, so you will have to record EXACTLY what you did so you can repeat it.
WARNING: Do a test run (do SEVERAL test runs) to see how firm your mixture gets when it gels before you actually use it as an enema on yourself. Just boil it up, let it temper and then pour some into a bowl and let it gel. See how hard it gets. If you don't test it first, you could end up with a real impaction that you won't be able to create a believable lie to tell in the ER when you can't shit it out. At best, it will be difficult to pass.
I HIGHLY recommend you read up on this material before trying it on yourself. Agar agar is completely safe, is used in foods and comes from seaweed. Science is just so peachy keen!!! 😎 😎 😎 😎
Read this: http://www.barryfarm.com/nutri_info/thickeners/agar.htm and http://en.wikipedia.org/wiki/Agar and a lot more before using this material.
This stuff is a lot of fun if you know what you are doing.
Trust me on this. I have experience with this material.