I joined this forum several moths ago and posted a question but have been busy on other projects. I have a 91 convertible, manual trans, 92K miles. When I last drive it routinely (abt 10 years ago), it had no issues. Been in the garage since then. Licensed it last fall and took out for drive. Within a couple hundred feet, the Check Engine Light came on and it went into limp mode. I've had this issue before and was diagnosed as OMP - replaced and problem went away.
I don't have access to the SST code reader and haven't found a dealership that has one, so I tried the ground wire/count long and short flashes of the CEL alternate method. Does not flash. When I turn the key to on, the light is illuminated for a couple of seconds, goes out for a couple of seconds, and comes back on steady. Again, no flashes at all. Read a couple of threads that others have had the same issue and replaced the ECM with success so I tried that with used from eBay. No change - same CEL, same limp mode. I don't know that the replacement ECM is good.
Does anyone have any thoughts? Will anything other than OMP put it in limp mode? Might have to go the premix/re-program ECM route. Anyone know if that will pass emissions testing?

 

plan B might be to change the capacitors in the ECU, there are a couple of threads on this, maybe try it on the spare ECU?