alright, i just recently swaped out my 94 LS for a 99 LS, the other engine was throwing alot of smoke (rings) well anyways im running a chipped p75 obd2 engine harness everything else is obd1 anywayz when i had the other LS it would do the exact same thing, idle rough like its dying, when i rev it, it revs fine no back fire nothing, but when i let go of gas after reving it, it idles low like its about to turn off, the injectors on my other engine were brand new i didnt put em on, i just put the whole swap how it was, and it does the same thing. i changed spark plugs same thing, cant be fpr its a diffrent one. anyone have any ideas of what might be causing this?

 

My h2b did this shortly after putting it in. I replaced my used O2 sensor with a Bosch one. Same symptoms with the other O2 sensor installed, But got tired of changing things. I bought new DSM 440's, changed my Main relay for the new one I just bought for my other car, changed the FPR, already had a new fuel filter, and a new walbro fuel pump. Mine would idle really rough and then stutter and shut off.Turns out after I went to go figure out what my problem was at the dyno, just backed up to it so my tuner and I could use the wideband and the screens to see what was going on with it. Turned out my **** box 2 month old bosch O2 sensor sucked *****, and wasn't doing its job, since it would idle fine running off the map sensor with the O2 disabled. I replaced the O2 sensor and it worked out fine, just the $200 price tag attatched to the sensor that makes it a little undesirable to have to purchase. At least it fixed mine.

 

im thinking it might be the fuel filter or the tank. does anyone else have any other input on this?

 

try the fuel filter, it's cheap and easy. It probably isn't the sock on the fuel pump, but you could take it out and look at it if you want.

 

headgasket. any loss of coolant? or white smoke?

 

yea a little smoke when i rev it, it doesnt throw smoke when im reving it at high rpms and hold it there just when i let go it throws white smoke.