Can't get LS to record engine misfires

[HogSlayerz]

I unplug the harness to the #2 or #4 fuel injector and I can feel the engine missing, but the tech2win misfire data and misfire chart never show a misfire.
I went to table C5622 in TunerPro and set the counts for 0 and 6.25 engine load to the same values at 12.5 so that it should record at idle. Unplug injectors, still nothing.
Changed table C5621 in TunerPro and set the counts for 0 and 6.25 engine load to the same values as 12.5 so that it should record at idle. Still nothing.
Did a CKP relearn. Still nothing.
Drove the CJ7 around town, different speeds, RPMs, etc. Still nothing.
The current counters almost always stay 0 and the historical counters always say 0.
For example, when I unplugged the #2 injector for at least 2 minutes after all the changes above, #5 cylinder threw 4 misfires in the counter.

I have read the parameters for entering a P0300 code, but I'm not sure if those are the same for recording misfires? I would think since it occasionally throws a code like on cylinder 5 above, it's working?

I don't have a fuel sender for the LS. I'm using the CJ7 one to the gauge. In Tech2Win on the Enhanced EVAP Data screen, I have Fuel Level Sensor = 5.0 Volts and * Fuel Level Sensor Rea = 5.00 Volts. My Fuel Tank Remaini is 0%, so it thinks it's empty, so maybe that's why it's not triggering misfires? Should I wire in a 100 ohm resistor to pin 54 on the green ECM connector to fake a half tank of gas? I wouldn't think that was necessary to record a misfire, but I'm out of ideas.

Is it possible to just change the F0501 Fuel Gauge Calibration to something so that the 5.0 Volts tricks it into a half tank or something? I think that's Pulse Width Modulation. but I'm wondering if I change all the cells to "0.00" and put 50% tank to "100.00" maybe the ECM will be confused into a half tank?


The backstory is that my engine runs pretty well, then sometimes, 10 minutes into a drive, it starts missing really bad at idle and under load. Of course I'd like my misfire counts to work on the Tech2Win so I can get some readings when it happens and track down the problem. I found out I had chinese knockoff 41-110 plugs from Amazon and I got authentic plugs and swapped them in this weekend. This hasn't fixed the problem, so back to getting misfires to record in Tech2Win.

Sorry for the long post, but I figure some of you guys have a lot more experience than me and may have better ideas on how to track this down.

[B52Bombardier1]

Check the impedance of your fuel injectors with a multimeter per the shop manual specification. For me, specification is 12 ohms impedance and I have had two go high in the nearly three years I've had this 5.3 liter LM7 engine out of a 150,000 mile 99 GMC Sierra. One injector got replaced at 17 ohms before I installed the motor and another recently had to be replaced at 68 ohms. That 68 ohm injector only threw a DTC P0307 misfire when the engine was cold but I replaced all of them on that side anyway. I'm now getting a very, very infrequent P0308 on the other side so its probably time to replace all of those, too.

Rick

[HogSlayerz]

Check the impedance of your fuel injectors with a multimeter per the shop manual specification. For me, specification is 12 ohms impedance and I have had two go high in the nearly three years I've had this 5.3 liter LM7 engine out of a 150,000 mile 99 GMC Sierra. One injector got replaced at 17 ohms before I installed the motor and another recently had to be replaced at 68 ohms. That 68 ohm injector only threw a DTC P0307 misfire when the engine was cold but I replaced all of them on that side anyway. I'm now getting a very, very infrequent P0308 on the other side so its probably time to replace all of those, too.

Rick

Thank you so much for the idea. I was running crawfish traps all morning and didn't have a chance to reply.

I ran an injector leak down test a few weeks ago and didn't notice much variation on the injectors. I tested the resistance at the actual injector (not through the wires) and I got 14 - 16 ohms on bank 1, and 14 - 19 ohms on bank 2. Bank 2 is the bank that runs rich (high negative trims) when the engine starts missing and running rough. I wouldn't think those injectors are bad, but they are almost all out of spec. Do you think they need replacing?

While testing the injectors I noticed that cylinder 2 injector's plastic "plug" doesn't lock to the injector. I have a junkyard harness in the garage, I'm going to try to swap out the plug so that I can get it locking. I'm wondering if it could possibly be jarring loose when on rough roads.

I also failed to mention that I've flooded my charcoal canister twice (due to how I had it mocked up in the CJ7). I rerouted it to be well above the tank now, but I had to disconnect the hose at the purge valve on the manifold and plug both the solenoid valve and the hose feeding it. I left the solenoid valve plugged in and notice it's usually commanded open around 16%, but I wonder if the computer is trying to compensate for the air that should be coming in through the valve. Today I unplugged the solenoid and disabled the MIL light up for P0443. I haven't had a chance to test drive it to see if that helps, but at grasping at straws at this point.

The part that I just can't understand is that when I unplug the injectors I get a P0200 I think, but I never throw misfires in the Tech2Win misfire counter or graphic. That in and of itself is troubling for me.

Thanks again for the advice and I'm open to any other ideas you guy may have. I checked cylinder compression about 6 months ago and it was OK and fairly even in all cylinders. I also ran a boroscope in each spark plug hole when replacing plugs last week and all cylinders looked OK with minimal carbon on the piston heads.

My rear 02 sensors also fluctuate a lot, much like the front 2. I have no idea of the cats are cooked, or the previous owner who started the swap just hollowed them out. When I tap them I don't hear anything loose inside. When I test the manifold vacuum when accelerating to 2500 RPM, it bounds back to ~ 20 pretty quickly, so I'm guessing it's not a plugged cat.

I noticed my downpipe off the bank 2 exhaust manifold touches the transmission crossmember, so another thought is that maybe when the engine is under load it's causing leaks by the donut gasket or exhaust manifold itself. I'm admittedly just guessing at this point.

[B52Bombardier1]

Hello,

I think there is a way to decrease misfire sensitivity in the tune and maybe the previous owner did that. My problem is that I don't know how that is done. I see it in how my two predominant tunes on my PCM's behave here for misfires. One is quite sensitive, the other is not.

Unless you live in a state with strict emission control laws, I would remove all evaporative hardware or at least cap it off and disable it inside the tune. This will take a variable off the table for you while this gets sorted out.

Boiled crawfish!! Definitely good eating with red potatoes and boiled corn on the cob. And the price is very low here in Bossier City, Louisiana this year.

Rick

[HogSlayerz]

Hello,

I think there is a way to decrease misfire sensitivity in the tune and maybe the previous owner did that. My problem is that I don't know how that is done. I see it in how my two predominant tunes on my PCM's behave here for misfires. One is quite sensitive, the other is not.

Unless you live in a state with strict emission control laws, I would remove all evaporative hardware or at least cap it off and disable it inside the tune. This will take a variable off the table for you while this gets sorted out.

Boiled crawfish!! Definitely good eating with red potatoes and boiled corn on the cob. And the price is very low here in Bossier City, Louisiana this year.

Rick

I'm new to this, learning as I go. I'm a computer programmer by trade, so it's not that hard to pick up. I believe C5621 and C5622 allow you to set the # of milliseconds that the cylinder can be slow/late before it triggers a misfire. I copied the table from a default 2004 Silverado tune I got from a junkyard Silverado. I then activated the 0 and 6.25% engine load lines, as they come turn off from the factory. Only thing I can think now is to turn down the milliseconds on the threshold for 0 and 6.25% and see if that allows it to start triggering properly.

I have capped off the purge solenoid, turned off the MIL light and plugged the vacuum line that goes to the purge solednoid. I'm hoping that removes it from the equation for now.

[HogSlayerz]

I think my ECM fuel level at 0% is the driving problem. It's one of the enabling conditions for P0300 and I'm wondering if it's an enabling condition for a misfire counter.

P0300 code.jpg