Originally had a 420 cat code the past 7 months,prior owner had after market mufflers that I was told might pull this code. I pulled and replaced all the plugs and 2,4,6 and eight were rich, 1,3,5 and 7 had a nice color, also pulled an O2 failure code just after my missing problem and replaced both O2s on the pass side and the 420 is gone, maybe the rich plugs on that bank as well, but still have a terrable pre ignition knock off idle if you goose the car or lug it. Got a 131 code indicating #1 missfiring and thought the miss was that baby, since I just replaced the plugs I seperated all the wires properly and that is solved. Car still idles ruff at about 600 rpm take her to 1000 she is smooth so their is an off idle issue. Goose quickly in the drive way from off idle and you get a pre iginition knock that is bad on both banks. Lug her a little and you can here the knock with the windows down as the knock bounces off parked cars obviously the knock sensor is retarding at that point so performance is upside down being polite. This is when your lugging her and she bucks like a major miss. You go through the gears with some rpm with no lug when shifting and she will blow the doors off. Lug her in 5th and the knock starts and knock sensors jump in and she goes to "hell in a hand basket" shift down, and watch out, because away you go back in you seat. I drained the gas, filled her with high octane, 91, "Ya right" when I was a kid I could by 102 at the pump, unfortunately without me knowing the wife went to a lower grade fuel the past few months thinking she was saving money. She takes good care of MY car but I told her this was a real no no and not in an LT4. I also put fuel booster in, changed the air filter, but have not changed the fuel filter. Something when you lug her is making her go lean, remember she idles ruff like a miss, but clears up at 1000 rpm, the miss just drops off. I am at my really so if any of you out there can make any sence out my explaination I should would like to here from you. Again when you quickly snap the throttle off idle you get a pre ignition knock that in time would burn a hole in any piston. This problem started 2 months ago and has just gotten worse. Currently she is throwing no codes. I wish she would tell me something but no codes just this sick condition that clears up with good shifting and rpm. Boy, the last thing I want to do is take it to chevy Thanks for any input.

 

My guess is a very lean idle/off idle condition.
(My LT4 idles at about 950rpm, if that helps)

Injectors need cleaning????

Larry
code5coupe

 

Thank you I think I am going to increase the idle up a little.

 

Idle speed can only be raised via programming change to the PCM. The screw in the throttle body is for the minimum idle air, not idle speed. Once you turned the screw, the idle will be adjusted until the PCM has time….about 50 miles worth…to relearn a new IAC position that obtains the programmed idle speed. If you open the throttle blades too far, you'll be out of range of the IAC valve and get a whole host of other symptoms. Factory idle spped is ~700rpm. '90-'96 tachs are notoriously inaccurate and typically read a couple hundred rpm high.

I would check the fuel pressure regulator (pull the vacuum line off it and verify there's no fuel in it) as well as the fuel pressure (~38psi (+/- 5psi) at idle with the engine running and the vacuum line *connected* to the fuel pressure regulator). Next I would verify the inside of the throttle body is clean and not coked with carbon. A bad PCV valve (driver's side of the intake manifold) can do funny things as well.

If those things are all good, then it likely is an ignition issue. As much as I hate to blame things on the Opti, these are the exact same symptoms an Opti with a worn cap & rotor tends to exhibit.

 

Thanks I will leave the idle screw alone as now that you mention it that is an rpm that is fixed in the computer. I will check the idle with my meter that I know is very acurate. I am going to replace the PVC because that is easy, will check the throttle body as you suggested and will measure the fuel pressure on the rack to 38lbs plus or minus 5? with vac line on.

I appreciate your candid remark on the opti, Frankly how can you go 78K without changing a cap and rotor I have seen the guts to one of these and there are rotor contacts like any cap and the rotor as you say like any other distributor. I know nothing about them, but unless they are in a vacuum atmosphere how stupid can on engineer get? There life would be about the same 70K outside and need to be changed. Thanks for your input I will let you know this week end how all this turns out. Very happy for all your input. Regards Bob in Stockton, CA