long story short, again. bought a 96 LT4 car. CK engine light came on after a month or so at three drive cycle intervals P0300, random misfire. checked wires and found two bad ones, they were original to the car. replaced the wires and plugs. still had a light every three trips while cruising on the highway. replaced the opti spark. same thing. replaced the coil and ignition module. now i had a code P0303, misfire cyl three. plug and wires looked fine, replaced the injector for cyl three. now went back to P0300. replaced the O2 sensors, same thing. put another optispark in, same thing. did the intake gaskets, had a P0303 again. checked compression, o.k. had a P0300 again. put a third opti spark in and a set of taylor 10.4mm wires as well as NGK TR55 plugs, my original replacements were GM wires and delco platinum plugs. i did find plug wire number 6 corroded at where it snaps onto the plug. but allas i still have a P0300. i also replaced the computer, cleaned the throttle and took a look for carbon at the back of the intake valves while i had the manifold off to do the intake gasket, i didn't see any carbon there. i also pulled the valve covers off and checked the rollor rockers, i didn't notice anything abnormal, but i'm not entirely sure what i'm looking for there, i just heard they can go bad on some early Lt4's, mines number 19. i also replaced the vacuum check valve found on the passenger side of the intake manifold because it looked rotted. i don't hear a vacuum leak. the car dynoed at 305 RWHP, and turned the quarter at 13.32 at 105 mph on street tires at raceway park NJ. not too bad of numbers for a car that supposedly has misfires!! i also replaced tha crank sensor and tried re-routing the wire harness from it as best i could away from the coil, i will try insulating it as soon as i figure out what i could use. i also replaced the AIR pump check valves on both sides as i heard they too can cause grief. i'm gonna re-check compression but i think if i had a bad head gasket the misfire's wouldn't switch back and fort from random to cyl three at will the way it does. i have no coolant in the oil, the coolant isn't low. tha car gets up and goes fine, occasional rough idle, but i heard they are all like that. there isn't a EGR on the car to blame either. i even replaced the fuel pump and filter. i have a brand new car except for the darn light. can anyone tell me if i left anything out? or if they have a similar problem? i also read that the engines harmonic balancer or flywheel being out of balance could set this off. what are the chances of that? i also read that a tire or driveline inbalance could do it, but i don't feel any vibrations. help, help, help.

 

 

Have you considered replacing the entire engine? I swear dude, you have thrown so many parts at this...

I will have to think about it... it's late, and I am tired.

 

see if you can borrow another PCM for a week.....

 

here is something funny, i replaced the PCM twice, the first replacement PCM caused the car to stall after 20-60 minutes of driving and set about a dozen codes or so including PCM memory codes, ignition high and low resolution faults, random misfire, MAF codes, skip shift solenoid code, etc,etc. so i exchanged that one for another from where i bought it from, and i even had a Chevy dealership flash it for me. the car no longer stalls, runs great infact but still sets a P0300, and that's when it isn't setting a P0303!!!!! the only time the car even remotly feels like it is missing is at an idle where the rpm will change about 50-75 rpm and it will have a slight lope to it. i've heard they all about do this. but on the highway when the light does come on i don't feel a thing, nary a hiccup. and get this sometimes the light just pops on steady, and sometimes it flashes several times before coming on steady. very very weird stuff!!

 

Well I don't have a basis to suggest this, but if it really isn't missing then the question is why is it getting a false signal......

Seems to me the reluctor ring is used to trigger a misfire code...not sure and don't have book with me now to look it up...never heard of anyone having problem with reluctor ring...but what I am suggesting is that rather than trying to solve a misfire problem that doesn't appear to exist, maybe you need to look more closely at how a false signal might come about...just sharing a thought I had....wish I could really help..

 

i took a look at the reluctor ring when i replaced the crank sensor. i rotated the engine by hand and checked all the teeth. they all looked the same, which is good. i even thought of a stretched timing chain, but the car only has 55K on the odometer, and it has a true roller chain with steel gears which is supposed to resist stretching very well. normally these chains go for around 100K before showing any problems. besides the car is running too well to have a chain problem. there is no lag or hesitation on accel or decell. i'm gonna try shielding the crank sensor wire with rubber fuel line where it runs next to the ignition coil. GM in there infinite wisdon put a sensor wire that is verry sensitive to electromagnetic fields right next to the coil and behind the passenger side secondary ignition wires. lets see if that changes anything. thanx again for the input.

 

No idea, just subscribe to the thread as I have the same eng. Good luck

 

A stock LT4 doesn't lope at idle, so to me that indicates there really is a problem of some sort.

Since you've replaced so many parts, the thing I would do is check wiring. Make sure you have the correct connectors going to each injector. If two of them are crossed up, that can give the symptoms you have. It may not necessarily be #3 that's wrong so check them all. Next verify that all the plug wires are connected in the proper order. No doubt you've done this, but I would recheck. Check the continuity of the wires from the PCM connector to the various sensors, and also the grounds.

While I agree that GM ran the CPS wiring awful close to the coil, the fact of the matter is that every '96 Vette made is like that and if it was an issue it would seem reasonable to have seen a recall or repeated reports of trouble.

The problem with the rocker arms was the retaining pin backing out. If a rocker arm fails, you'll definitely know it.

You really need a scan tool or laptop diagnostic software so you can datalog the parameters while it's running.

If all the above check good....Just a guess, but it almost sounds like it may be one or more sticking injectors. At idle is when a sticking injector would be most noticeable.

 

i checked and re-checked the injector plugs to make sure they are in the right spots, they appear to be, i also checked and re-checked the fireing order for the plug wires. i just replaced the wires with taylor 10.4mm wires. i just noticed that two wires burned up on my headders. my own dumb fault not taylors, i used zip cord to hold the wires back and the cord melted allowing the wires of cyl 5&7 to touch the headder. so i can't verify if the new wires helped or not till i replace the two burned ones. if my problem still exists i will then check each wire to and from the PCM to the injectors, etc...

 

My LT4 definitely does not lope at idle...and you'd be able to hear it with my exhuast setup if it were there.

 

Too bad you're not in California where Chevy gives a 200K warranty on the injectors for all the V6's and V8's from '96 to 2000 (there's a Bulletin on this and though it's specific to CA, certainly sounds like your symptoms). Suspect you have an injector that clogged or restricted. If you data log, you might see the PCM taking out or adding too much fuel. I'd try cleaning them or have a dealer give it good flush.

 

i had the injector rail appart already, two times. once to check the injector screens and play musical injectors. another to replace the injector that corresponds with cyl P0303, and play musical injectors again. i also replaced the fuel filter and pump.

 

lets get back to basics here.

What causes a misfire?

1) too much fuel

2) not enough spark

3) not enough fuel

4) combo of the above

Let's take this one at a time:

1) Too much fuel - one cylinder running super rich. This is caused by a stuck injector. Unless there are several questionable injectors, I would think these are not the problem because of the injector swap.

2) not enough spark - this would be caused by a bad or weak coil, but then there would be more than one dead cylinder. Also, a bad wire, or bad cap would cause this. A bad rotor would effect all cylinders.

3) not enough fuel - see #1. same theory applies

4) this is where it gets tricky....

now, back to my thoughts at hand. What creates spark? the ECU instructs the opti on a timing setting. That controls when it sparks. Have you checked the advance? Could there be a spike when it misfires?

Have you checked the harness? The signal from the injector to the ECU? There is a tool, think it's called a fiddle light, that will flash when the injector fires.

Something is directly effecting that one cylinder. Could be as extreme as a burnt valve, too. Have you run a leak down test?

 

Check to make sure your 02 sensor wires are not crossed. The shop crossed mine and it took some time to figure out but that was the problem.

 

AIR check valves? Just guessing

 

i already replaced both AIR check valves when i did the headders. but i will check the o2 wires.

 

You can't tell if an injector is good by looking at it. A balance test is needed to know what they're flowing. To test the fuel pump, disconnect the inlet and see it will squirt about a quart into a coffee can with the key on (and it's generally a good idea to do this as a flush when you swap a filter out).

A 300 is more than one misfire. A steady light means the impact is minor on emissions - a flashing light means the Cat is about to melt.

A Tech 2 will index which cylinders are misfiring and if on one side, the chart will have you inspecting the O2 or if paired, the crank sensor (you can't tell if it's good by looking at the reluctor ring). You don't say if this occurred before or after the header install, but if it was after, the headers may be wrong for your application.

At this point, you might reconsider having someone with the right tools diagnose it for you or at least invest in the tools that can help figure it out. An injector balancer tester is about 80 bucks at NAPA. You'll need a decent fuel pressure gage to measure the drop. A hose and can of 3M port injection cleaner will set you back about $50. A TECH 2 with the right cartridge can generally be obtained used on EBAY.

 

just installed my new taylor wires. is it normal for them to glow like a lightning storm. it's pretty intense in the dark with the hood up, they glow all over and then some. any one else experienced this?

 

Glowing is fine. Sparks/Arcing from the wires to metal components (engine, brackets, etc) is bad.