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The cracked joint problems are not rare, but they have taken several years to reach that state, and don't happen with all cars. They are also expensive to keep as a spare, unlike a DME relay. The 'refurbished' DME that was installed may have been on the verge of failure when it was put in your car, and if there was a guarantee, I would have told you to send it back to whoever rebuilt it. If you fixed your unit properly, I really think it's overkill to keep a spare. If you still have your car 15-20 years from now and it happens again, well you know what to do.
There were basically two different 24 pin chip programs used in the U.S. 87 model year, the 1267355236 and 1267355302. The .236 program is the earlier one and idles lower at 800 rpm and rougher like the 84-86 cars. The .302 chip idles a little higher at 880 rpm. Power derived from the .236 chip would be equal to the 207 hp of the 84-86 cars, with the .302 chip at 217 of the 88-89 models. A good chip makes a significant difference to the drivability and everyday usability of your car, along with a smoother idle, more low and mid end torque, more throttle response, better highway fuel efficiency, and a 16-18 hp gain without any detriment to your car.
With regards to AFR adjustment, there's only the idle mixture you can adjust, and if you have a wideband AFR sensor, you want to adjust it to around 14.5-14.7:1 at idle. Since most people don't own one, then a shop can adjust it to 0.6-0.8% CO with their gas analyzer (measured before the cat) or you can do it yourself watching the voltage switchpoint off your O2 sensor. Adjusting the AFRs everywhere else, such as at full throttle and acceleration loads, can only be done by chip reprogramming.
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