tanmanski

Opti advice please! This is on my mostly stock 96 Z28 6 speed.
I have SES light on steady, occasionally going to flashing SES light. I can feel it's got a bad ignition miss, especially under load at low RPM (less than 3k) Code P0300 was set, which appears to be a general ignition misfire, not specific to any one cylinder. Im thinking this eliminates a bad plug or plug wire. The condition is present cold or hot, but worse if coolant temp is above 180. The car starts and runs (rough) and hasn't died yet. I just completed a 1500 road trip and it got worse during the trip, some of which was through rain & high humidity. I'm thinking the opti has become contaminated, perhaps from a bad vent system? I'm in a bad spot, away from home, no access to my garage and I only have a basic tool kit on board.
Im considering rolling the dice on an $140 aftermarket opti replacement, just to solve the problem as quickly as possible.
Any advice?

thanks guys

timnfxr

It's more likely to be a plug and/or wire, than the opti. It's a lot easier to change the plugs, than replace the opti. Especially on the road.

Good luck...

BTW: You should change you signature, since it still indicates you have a 95.

tanmanski

Thanks for the input, but I would expect a single plug or wire to generate a specific cylinder misfire code? eg P0303 would be a misfire specific to cylinder 3. I have the "general misfire" code P0300, not specific to any one cylinder.
I do still have both the 95 AND a 96 (glutton for punishment eh?)

Any other thoughts on possible contaminated opti?

Injuneer

Are you able to pull up the misfire count by individual cylinder? Requires a scanner with "GM enhanced parameters".

tanmanski

Hi Fred, thanks for your interest and input. I certainly value your thought and opinions.

I am unable to get misfire count by cylinder right this moment, but can probably get it later today. Anything key I should be looking for?

Also, assuming same condition still present, how many drive cycles should it take to set SES light on for code P0300? I did a reset yesterday and the SES light is out for now, even though I can still feel it missing badly. I think I've put 3 cycles on it so far today.

thanks Fred!

tanmanski

Also, I do have laptop and interface cable with me, but no diagnostic or logging software on this particular laptop. Im not sure my Harbor Freight scanner has enhanced GM parameters, so if it doesn't I'll have to download some data log software.

moparman

I've had the P0300, and it can be a pain to track down. I could feel the misfire(s), but couldn't narrow it down to which cylinder(s). Finally, one night, I took a spray bottle of water and misted each plug wire. It was #2 that started sparking. Next day I took the wire off, and sure enough, it had a crack in it. Replaced it and everything ran OK. I only had a code reader at that time, but after that I got a scan tool.

tanmanski

Thanks for the input Moparman, I did not expect that P0300 could in fact be attributed to a single cylinder misfire. I did visually inspect for plug wires arcing but couldn't detect any. I'll try your water mist idea. The misfiring is so bad, I'm still dubious it could be just one cylinder, but I'll at least remain open to that possibility.

Injuneer

That's the advantage of pulling up a misfire count by cylinder. One misfiring cylinder can affect the entire bank of the engine - unburned fuel and air exit the misfiring cylinder, the O2 sensor sees all that oxygen, tells the PCM that bank is running lean, and the PCM adds fuel to the entire bank. Now there's 1 cylinder misfiring and 3 cylinders running excessively rich, possibly misfiring as well.

The culprit will most likely have the highest misfire count. If two adjacent cylinders have high misfire counts, could be those two wires arcing to each other, for example.

The actual definition of P0300 in the factory manual is "Engine Misfire Detected".... not random misfire. To set the code, the PCM calculates the percentage misfires over 1000 revs of the engine. Don't know what the specific percentage is that trips the code. The first time it happens, it does not turn on the SES light. The second time it happens, under similar operating conditions (load, RPM, temperature, etc.) the SES light turns on.

SES light will turn off after 3 drive trips with the same operating conditions that were present when the code was set. Could take a long time to turn the SES light off if the misfires set under some odd combination of operating conditions that are infrequently encountered.

The code can be caused by road conditions... tire/wheel out of balance, variable thickness brake rotor, drive shaft not balanced, rough road. The PCM actually looks at the ABS system to see if it is showing signs of rough road conditions.

GaryDoug

For future reference, I have written OBD2 scan software that can read the misfires by cylinder, It is called GDScan and is available for free as a download here:

https://www.mediafire.com/?0elo4c935uc5s

Get the GD-scan2a-GMenhanced.zip file.
You will need an ELM327-type OBD2 cable. Avoid the cheaper ones from China. Most of them do not work for our cars.

Fatdog2

I would check the coil wire first as I've seen white corrosion on my coil on my prev. bird,and cleaned it up and put a new coil wire on.The symptom was no power under load.Also check the wire connector that plugs into the opti you can see it from the top(my 93 anyway)as you standing in front of the car it is on the left side of the water pump,just look from the tb and straight down you can see the plug on the left .It goes from there to the ecm and can become melted.Get bandages.I'm only chiming in because you have limited resources at your disposal away from home.My last bit of wisdom would be to gently spray your mass air sensor ever so gently with a cleaner and see if that helps.Good luck.

tanmanski

GaryDoug, Awesome, thanks, that will help me for sure
Fatdog, appreciate the input, it's easy to check, so I will!
Injuneer, still trying to get a live data capture that will show miss count by cylinder

much appreciated guys!

I am able to limp it around, but I'm concerned it's gonna die completely if I don't get this figured out soon.

tanmanski

I have confirmed I don't have enhanced GM parameters on my scan tool, so tonight I'll try GaryDoug's scan software with my DLC cable I got with Ion's Mad tune a few years ago (for the 95). Hopefully be able to post some useful data tomorrow.
SES has returned and I now have P0305, so I'll put #5 plug wire under the microscope. Too bad it's not a little easier to get at the plug, but I might be able to cobble together the right socket, swivels and ratchet to get it out.
Really appreciate the advice and contributions guys, this board is a great band of brothers!

GaryDoug

That cable won't work for any car after 95. 95 was OBD1, 96 and later ones are OBD2, totally different.

tanmanski

Just wanted to update the board. The problem is still present, I have ordered OBDII interface cable and will post misfire count by cylinder as soon as I have it. Code has reverted back to P0300. It sure feels like more than one cylinder, and I'm starting to suspect worn/dirty plugs. At about 3k RPM it actually starts firing on all 8, but will miss if its loaded or throttle above 50%. So, what are the possible results from a misfire count by cylinder?
1. It reveals #5 (P0305 code has appeared in history) and (maybe) #3 or #7 are frequently misfiring. I would then focus on two adjacent cylinder plug wires arcing to each other, correct?
2. It reveals frequent misfiring on two or more cylinders NOT adjacent to each other. I'm guessing this points to Opti (terminals, rotor, etc)
3. It reveals frequent misfire on all cylinders. I'm guessing this would point me to worn, dirty plugs? Maybe Opti?

thanks guys