Hi Barry: you are asking some pretty tough questions to answer.. I do not beleive you can interchage the  computers unless,  they have the same part number and that's not likely.. First what are the codes,  need to have the codes.. I don't think having the dealer reprogram the 98 to work in the 99 will help.. I almost doubt the dealer has the software to do that anymore.. One thing you might check if it runs ok cold , Check the grounds at the thermostat housing.. I think they have a ground wire right at the stat housing bolt.. Also make sure the temp sensor is giving the correct signal to the computer.. On a fuel injected engine the temp sensor is like a choke, it tells the computer how much fuel to suppy to a cold ro warm engine .. that is not the temperature sending unit for the guage.. that is still in the block below or between the plugs in the head.. The PC temp sensor is usually right in or beside the Thermostat..most likely a two pin connecter.. You could swap it out with the one from the known good running engine to see if that helps... What missfire codes are you getting, I can't stress how inportant it is have good grounds..especially a vechicle that has been oiled to hell.. Is your code reader just that or can you get live data,, I would like to know the fuel trims when it starts to act up.. They were know to have distributor bushing issues as well, and depenting on the injection system in that year .. if it has the spider injectors with the plastic intake those injecters were really bad for leaking alot I think they were CPI's central port injection and lots of problems.. I have just forgotten the exact years they used them But I am pretty sure it was in those years.. Been a few years since I turned some wrenches in the shop.. Codes would be important.. take them now, then clear them I think pulling the PCM or ECM  fuse  will clear them. or disconnecting the battery cables and holding them together for a few min will work and run it again and check them again.. Pretty hard to sit here on a desk top and do diagnostics with so little information but I understand it's hard to give info if you only have a code reader.. If you were having trouble with the VAT chip system it would not start period so I don't think that is the problem as you say it does crank and start.. You should also do a fuel pressure test, they need at least 55psi to run I beleive.. They were a hard to diagnose system in those years.. also had some round lobes on the cam on some of them.. Need more information to help any more I think.. I think it may have been easier and cheaper to swap out the rad cradle if the 98 was a better running vehicle.. Maybe not much help but it's all I could offer at this point.. codes , codes, hard to work on those without the codes.. they may be the exact answer but should get us into the area of the issue or problem .. sorry I just don't have anything more.. I do know that swapping the PCm will not fix it unless they are the same part number and then only if it is the pcm that is bad and really from my past it was very seldom the PCm that failed..It can happen but very seldom in my experience..Does it smooth out if you raise the RPM.. could be a throttle postion sensor, you should also check and clean the throttle plates they get pretty gummed up and cause idle problems .. Just so many options and maybe this ot that .. need codes, codes,codes,  did I mention we need CODES..     

Hi boss late reacting spent the day pulling the 98 engine tranny and pto I am getting to frikin old for that ****now to answer your question if I can.the first thing I did was change the drivers side o2 as that was first code and I had one then it said maf sensor swapped from my 98 seeing as how I had a parts source starts fairly easy seems to run smooth until a time has run then the **** hits the fan.the next code it threw was 300 no 301 302 etc just 300.son took it to work on friday said showed hot puked a small amount  of coolant (I had earlier changed thermostat I drill 3 1/16 in edge have for years as a relief in case it sticks.he said it acted like thermostat stuck but it only lasted a minute then temp went to what we think is normal on my old girl (11 years ran perfect)each time it (99) threw a code it was different.i told him to take the code reader to and if light comes on clear it then do that each time until he gets home,today he finished swapping the(98) cap rotor and wires he had put new plugs in friday  after work he also swapped the coil and module today.as to the temp gauge I know what you mean that fycker on the 98 drove me crazy a couple of years ago almost exactly same type of codes that is why I had the o2 I put in the 99 then iit said maf also put a new one on they took it back luckily.then I thought what the hell I had a temp sensor in tool box from what I do not know other than probably a chev it never threw a code for that just said to hell with codes as they were different each time I scanned.that old temp sensor  was still in and working perfect.I will try to get you some codes as they happen if I can.I do not think it has vat on either one.that was another learning expience on my other sons 02 chev when geronimos descendants decided to steal his truck took out door locks then destroyed the ignition then left it somebody probably scared them off.I got the pieces from rock auto pretty straight forward until the relearn.pugs I think your question was answered in my ramblings here nice to see you here also.my reason for wanting to change the Pcm was the erratic codes it was throwing.I will do my best to keep you guys updated.

-- Edited by 54vicky on Monday 26th of August 2019 03:49:29 PM

ok boss here is the 2 codes from yesterday 300 and 0174 found a utube video on the spider setup and the fuel pressure regulator exellent video the sound was nonexistent like watching a silent movie from the pre talky era.other than no torque specs and the audio I may attempt changing the regulator if pressure shows a problem.my son bought a tester last night.I will order gaskets from rock auto then tackle net weekend.unless the codes tell you different.