My ignition timing suddenly changed ??????

Wow this is a first for me, never had to ask for help before but im to the point of throwing parts at this thing and it just isnt worth it, however i need it to go, and by go i mean anywhere in the continental U.S. so here i am hoping someone has and idea. I m no pro but i used to consider myself a reasonably competent shadetree mechanic. I have built a couple of motors, still serve as a free mechanic for my friends and have always been able to keep my vehicles rolling down the road. without rambling on any more suffice to say i know a little bit about cars, but by no means claim to be a mechanic.
so heres my problem. i have a 91 chevy g-20 van w/305 it has a throttle body. Never had any major problems with it, basic stuff alternators starters ect has about 130 k miles on it mostly highway. I was warming it up one day when it started loping/surging at idle. when I tried to drive it down the road it backfired (which blew the muffler off) hesitated and just ran terrible. changed out the plugs, wires, rotor, and cap. Looked at the injectors saw one didnt have a nice cone pattern so i drained the fuel that was in it refilled the tank and added injector cleaner which fixed that. However the backfire/hesitation still was present, refusing to beleive the timing chain skipped i checked fuel pressure, it checked out. So I broke down and put a timing light on it, it didnt jump around but it was way out past the timing marks. at this point i couldnt see any other explanation other than the chain. i went ahead and ran the codes, got an electronic spark timing, electronic spark control and a lean exhaust, which i assume would all make sense if it was driven with the timing off, so i cleared them. I went ahead checked compression and it was fine, which surprised me, considering that i thought the chain skipped. I retimed it, actually by ear because my manual doesnt list timing after 87 or so it says to refer to an emissions sticker that apparently doesnt exist anymore. i was able to get it driveable, however the idle now was higher than before, the idle still surged when it was cold, as well as sometimes while driving (surge), and a vicous spark knock when it was under load but not in park/neutral, of course hesitation. I changed the chain, to my surprise, it was fine, timing marks were still lined up, barely any more slack than the new one, so of course it had no effect. had already checked the egr for operation, but i pulled it to see if it was plugged and it was pretty clean, also snapped the plastic on the solenoid so it was replaced as well. had the ignition module checked. of course vacuum lines, they were fine. wires/harness ok. pulled the distributor to check the gears. hasnt thrown a code since resetting the timing. put premium gas in it which took care of most of the spark knock but not all, still hesitates, seems like its missing sometimes and idles high. i am about out of options here, i bought a rebuild kit for the tbi, but im not really convinced that is the problem and realizing im starting to throw parts at it, Im here, hoping someone has an idea because i could easily spend more than the value of the vehicle this way and still not fix the problem.

Hmm, did you check the base timing procedure for this engine, you cannot just put a lite on it.

yes, i cant remember the name of it, but theres a wire harness you unclip, i think it cuts out the cpu in regard to altering the timing. as well as engine being at opperating temperature if thats what you mean. beyond that i turned it over w/ a coat hanger in the number one spark plug hole to make sure that the timing marks were still accurate,they were. then pulled the dist cap and put the contact right on the rotor and then set it with the light to 4 degrees b4 tdc.

tore down the tbi and cleaned it last night but the kit had one gasket that was the wrong size so i guess ill be going back to the parts store when they open

ok ill try another post here i ve had a couple that didnt go through but yeah disconnected the ...... terminal and had vehicle att opperating temp. did discover something today. if i disconnect the map sensor and plugl its vac line there is a marked improvement, however replacing the sensor didnt help. i know this makes the ecu take data from the tps for fuel data, i also replaced the ox sensor wout improvement and finished the tbi rebuild

Even though the ign module passed a test it could still be bad when hot especially, the pcm could also be an issue here, guesswork can be expensive and annoying but this is about all that is left, short of low fuel pressure which doesn't match your symptoms.

I was under the impression that an ignition module either worked or didnt, or worked when it was cold then cut out when it warmed up. When i had it tested i drove the vehicle there and pulled it out so it would at least be at operating temp, or close when it was tested. but ill throw another one in it if u think its worth a shot. What about adjusting the valves. I ve never pulled the covers on this thing. thanks for the responses btw.

To get the correct base timing for your vehicle call a local dealer with your VIN. You can also order the proper placard replacement from them.

The pinging and back firing to me indicates a big vacuum leak. I know you've checked for that, but that's my opinion.

As for the valves, since the compression check was good, why mess with them??

thanks for the input i had no idea you could get that from the dealer, probably because ive ever owned a new car. ill check the vac lines again, why not.

ok threw a few more parts at it with no change. ignition module, egr valve, throttle position sensor. have a coolant temp sensor yet to be installed

new iac valve huge improvement, still have some spark knock and hesitation, less of both. checked over all the vac lines again theyre good. gave the old carb cleaner test, no change in rpm. still runs a lot better w maf vac line plugged. no backfiring. any ideas on what i havent replaced yet b/c i think im getting to the end of the list