p0300 code bad ecu?

[383camaro]

ok so my setup is
98 trans am
408 lq9 swap
ls3 heads
248/257 .615/598 cam
patriot gold .650 springs
42lb injectors
vic jr intake with the low profile elbow
90mm throttle body
11.3 to 1 compression

now what im having problems with is a low rpm miss fire under load its not there at wot and runs strong but if I just baby it up to speed ill get a popping or a drop in power. Also im getting a surge but I think its mostly the misfire while cruising around town. Ive logged my runs and my spark table isn't jumping around. I have changed my plugs and plug wires which made it seem to have more low end torque. but when I pull the plugs I cant seem to find the one that's missing. I Bought a new coil pack and started moving it around to see if it made any difference on different cylinders and that didn't seem to help. So I moved on to see if moving injectors around while monitoring misfires and that didn't seem to help. when logging misfires it seems the most misfires are on cylinder5 and 6 so I did a compression check on the whole motor and everything is dead on give or take 5 numbers I also did a leak down test and found nothing everything looks good. pulled the valve covers and no broken springs everything is moving fine. Now my question is can my pcm be going out and this be causing my problems I will attach my tune file along with a log of my run from last night maybe theres something I missed when tuning for the bigger motor

[383camaro]

i forgot that when logging misfires the most misfires are on cylinders 5 and 6 but all of them will hit then go away then hit then go away so it coming and going i made sure that i was watching the data when i had the motor running its smoothest. I did do a crank relearn.

[Chevy366]

Don't forget injectors can cause a misfire event too.

[383camaro]

yea i did take that into consideration to so i moved those around and also logged the misfires and they didn't seem to move the misfire counts any

[joecar]

Inspect the harness wiring for damage (unravel it from the convolute tubing).

Disconnect the PCM, coils, injectors, and measure the resistance of each wire (end-to-end).

Inspect the pins in the PCM connector, and in the coil/injector connectors (look for damage or corrosion).


Measure voltage drop across each PCM power feed, and each PCM ground (e.g. measure voltage between battery and pin/wire at PCM) with engine running.

Measure voltage drop across each ground strap (e.g. between mounting bolts at each end).


What holds your battery down (is it loose)...?

[383camaro]

My battery is held down by the stock hold down. And today I pulled the harness out to inspect it and didn't see anything out of the ordinary so I said what the hell so I pulled the computer apart and right away I smelled burnt electronics. After tracing down the pin outs on the computer and splitting the two halfs of the boards away from each other I found the fuel sending unit ground pin has a chard mark on the board do you think it fried anything else on the board?

[joecar]

Yes, other stuff could be fried.

Question: what caused the over-current to occur...?

[383camaro]

My thoughts exactly I guess its time to pull the sending unit

[383camaro]

Well I pulled the sending unit and this is kind of scary this is what I found also here is what my ecu looks like1586115862

[joecar]

Those wires were shorting...? Wow...!!!

[383camaro]

Yep that was in the tank :(

[joecar]

Seeing that pic makes my brain skip a cycle :boom:

[383camaro]

Well I got a new pigtail for the sending unit and hooked it back up, installed the computer and wiring harness just got back from a test drive and still have the same problem when it throws the po300 code does it fire the injectors in batch mode? And since its a 98 ecu should I just upgrade to the 00 ecu since my computer looks like it took on a short circuit problem

[joecar]

It does not necessarily fire the injectors in batch mode when misfires are detected...

If the CMP signal is not detected during startup then it may run in batch mode...

but this PCM is able to determine the correct phase during startup from CKP response to ignition events (even if the CMP signal is missing).


The only way to tell is to capture secondary ignition kV waveforms using an oscilloscope... these will also show how the coil dwells/charges and how it discharges, and how the burn progresses.



You might try another PCM (and repin the connectors)... but we still don't know for sure... but there is the possibility of further damage on your current PCM.

[383camaro]

OK today I got off my butt and started to check for shorts in my wiring harness . I need a second opinion but I'm pretty sure I found the short just gotta seperate some wires in the harness if I ohm from the pink wire going to the ign coil 12v source to ground I get 2.1 ohms if I ohm the ign coil ground I get .03 ohms all test were with pcm unplugged and battery disconnected. I think the shorts in the pink 12v power wire any ideas were to start

[joecar]

I take it you had the coils unplugged also.


You have to unravel the split/corrugated loom (a seam ripper helps slicing electrical tape) and trace/inspect the wires by hand... follow the pink wire end-to-end.

[383camaro]

yes coils were unplugged. But after tracing wire after wire and finding nothing pinched or burnt I started unplugging fuses till it went away. I found that if I pull the fuse mark eng sen the 2.3 ohms I was getting finally went away. So I got the car manual out and it told me what sensors it powers which only thing I still had hooked up was the oxygen sensors. so I went under and unplugged them and the ohm reading went from 2.3ohms to 1.7....k ohms so I think this whole time I was just reading through the heater on the o2 sensors :( pretty much I think im back to square one

[joecar]

hmmm... this is where you try two things: different O2 sensors and different PCM.

[Chevy366]

Well I pulled the sending unit and this is kind of scary this is what I found also here is what my ecu looks like1586115862

WOW, not good!

[joecar]

I'm thinking PCM has more damage than is visible.

[383camaro]

ok so lets say I can get my hands on a 00 ecu and harness pretty cheap can I use that harness in my 98 firebird I know the temp sensors are different and I have one sitting around in the garage. is it as simple as ill just have to repin the connectors that go back into the car that hook up under the dash. Just curious sounds like a lot less wires to repin :)

[slows10]

Can you change the misfire counts or sensitivity in the tune? I thought I remember seeing or hearing something about it.

[slows10]

Also agree with joecar and change the ecm. You have tried pretty much everything.

[383camaro]

So I re pinned the harness and got a 411 pcm for the junk yard now how do I change the os to an f- body os instead of a 5.3 Silverado os I tried an 00'' f-body tune file and did a full reflash and it said it completed but it never changed

[ScarabEpic22]

You have to full flash the PCM to change the OS. Did you read the tune back out of the PCM after full flashing?

[383camaro]

since the os are different between the truck to the fbody my personal efi live wont do a full reflash So now what are my options :(

[ScarabEpic22]

since the os are different between the truck to the fbody my personal efi live wont do a full reflash So now what are my options :(

Upgrade to a Commercial license is your only option. I forget that V1s didnt all do full flashes, all V2s do because they come with a Commercial license.

[383camaro]

I have the v2 but its not doing the full flash after the count down it says access denied

[joecar]

I have the v2 but its not doing the full flash after the count down it says access deniedPost a screenshot.


Are you running as admin on Win7/8...?

[383camaro]

Win7 and I will try when I get home. I talked to a rep from efilive and he told me that I should be able to do a full flash from my v2 he told me to make sure my v2 was up to date and check the firmware so I gotta do that too I will keep you posted

[383camaro]

OK did the software upgrade and it did the full reflash so now I just gotta do all the segment swaps and go for a drive it started up but still got more tables to fix