engine computer ?

[superchevy247]

I got a 89 gmc sierra i ran the vin# on it and i guess it originally came with a 305 in it now the guy that owned it before me put a 350 in it and always had a problem with it ideling and he blew that motor up so i put another 350 in it and still same problem shitty idle. Would that have anything to do with the computer do to the fact it was originally programed for the 305 and what do you suggest me doing to get it to run better besides checking for vacum leaks.

[superchevy247]

The check engine light isnt on or anything but do u think it has to do with the computer and the different motors

[superchevy247]

ITs a throttle body it runs good when your on the gas but you let it come down to idle for more then a few seconds starts getting all choppy and wehn its under pressure in gear same way sounds like its ready to cut out

[superchevy247]

ok well i just replaced all the sensors tps idel air contro o2 sensor everything engine light works but the engine is not getting up to 195 u think that might be it

[weylinthedirty]

How did you set the timing? I had the same problem with my trans am. Because the computer adjusts the timing for you. But you have to have the t iming set perfect without the computer conected for it to idle right. Cause when you put it in drive and go the computer will compensate and adjust it.. then at idle itll put it back where you had it originally.
IF thats the problem just disconect the ECU from the distributor, set the timing.. let it idle a minute and reconect the ECU then it should be fine.

[randywrench]

The knock sensors are different for 305 or a 350.

[randywrench]

I just went thru the same problem with a customers truck
someone put a 350 where it was a 305 no codes
changed knock sensor to a 350 one and the truck
runs Great now!!

[randywrench]

Same computer different PROM.