I have a temporary crown and from that I got a "pimple" above the tooth and the ref. endodontist wants root canal retreatment.

It sounds like you have a failed root canal that is allowing recurrent infection. Your choices are:
Retreat the root canal conventionally.
If that fails, then an apico-ectomy. This means doing a surgery procedure that completes the root canal from the root tip.
If your original root canal is problematic then your endodontist may be able to redo the root canal conventionally. However, if the tooth root is cracked then you may lose the tooth. 
Your last option is to remove the tooth and place an implant.
Without xrays I cannot tell you more. I am sure you are in good hands with your endodontist. Good luck to you with your treatment.

I agree with Dr. Cone.  The only other thing to consider here is that depending on the size of the abscess that is growing in your bone and how much bone loss is present, it may be wise to consider having the tooth removed.  I know that sounds drastic, but I would recommend a CT scan be taken of the area prior to treatment.  We have found that teeth that have already developed abscesses and cysts, that are treated with root canal therapy, tend to have a higher rate of failure than teeth that are acutely infected or affected without pathology at the apex.  Teeth that are abscessed are often "wet" and for a root canal to be successful long term, one of the stipulations is that the canal stays dry and sealed.  If not, then the crown and or post placed in the tooth will experience early failure.  I realize that my description and answer might sound confusing to a lay person, but the bottom line is this.  Consider the cost over the long term of having the tooth fail prematurely and then having to replace it with an implant anyway.  I have seen this time and time again.  You may have to do a little searching to find a dentist that sees both sides of this. The fact that the endodontist not sounding convincing would be a red flag for me. You may be able to have your dentist work with you from a treatment/financial aspect if the temporary doesnt work out and you have to switch gears towards something like a dental implant.  I would recommend a dentist that primarily treats adults and does root canals as well as dental implants in their office.  Warm Regards,

The tooth that you were referred for likely has an infection at and around the root tip (which can often be seen on an x-ray).  The infection will sometimes presents as a small abscess or "pimple" on the gums and will seep puss and other nasty exudate while it drains.  The solution is to have a root canal completed to remove the infected nerve.  However, if there is not enough tooth left to place a crown, the recommended treatment may be to extract the tooth and either place an implant, or possible a bridge.  What you are experiencing is not uncommon.  I hope this helps!
All the best, 
Miles