Mazda Repair: 86 Mazda 626 LX 2.5L Problem, mazda 626, intake hose

Question
John
I took my Mazda into a repair shop to get a new Rack & Pinion gear set and get a tune up.  (the check engine light was on).  After $1500 in repairs and four weeks in the shop the car runs much worse than when I took it in.  As a matter of fact, the car ran GREAT before I took it in. (just a light)  They replaced the EGR valve, ECU (somehow that went bad), had to change the oil again because somehow gas got in the oil....; new rubber intake hose, new intake gasket (somehow it leaked also)  The problem is now: Runs great when cold but eventually gets bad. Misses at 3k RPM when driving.  Idles at 1100 RPM at stop light then drops (surges) to 3-5k RPM range.  Have to put the car in neutral to keep it from stalling.  The dealership tells me that the code they get is from "backfire" and the only thing I can do to fix the problem is a new engine due to low compression.  How can I have a problem now and not before I took it in to get 'fixed'?  I'm thinking the ECU they put in was used and is probably bad (I know, 1:500,000 chance) but really don't throw all this $ I put into it away.
Thanks
Craig

Answer
wow'  who knows what they left off, havent seen many pcm's go bad on those.. i would deff bring it back and explain what you told me..i would have liked to know what code was originaly pulled. never heard of 'backfire' code   new to me and i thought i've seen it all. lol.  sounds like some vacume line or  hose is not hooked up, when your engine is cold it likes alot of vacume, (acts like a choke system)  when it goes into "closed loop"(hot) it dont want any un wanted vacume. supprized it was a dealer, not joe's garage. take it back! my dealership would never let it leave running like that.   let me know how it turns out