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Misfires on cylinders 1-4 with SD tune... out of ideas.
Have a weird one that I've been trying to diagnose for a couple of days and still can't figure out.
Engine is a stock 2003 5.3 with a 2001 12200411 ECM with an HPtuners speed density tune in an offroad rig that a friend just purchased. Harness is a factory harness that was modified to be a retrofit installation. Transmission is a 4L80E with a standalone controller (why they didn't use the ECM to control the transmission, I don't know...)
Noticed when I test drove the thing it was down on power, sounded a bit funny, accelerated slightly rough, and had a bad raw fuel smell. Free revved it from idle with my hand on the engine, and it's obviously misfiring by both feel and sound.
First thing I noticed is both exhaust manifold flange gaskets were leaking... replaced both of those and no more leaks.
Plugged in my Tech2 and looked for codes... no codes. Nothing counting on the Tech2 misfire counts screen either.
Checked all harness to engine grounds as a first step, all connected and clean.
Thought it might be a bad coil since I just dealt with that issue on a friend's 2002 truck that had 2 dead coils. Went around and disconnected the coil wires one at a time while the engine was running. Idle did not change when pulling 1, 2, 3, and 4, but idle did noticeably drop when pulling 5, 6, 7, or 8. Double checked with a timing light-- it didn't flash for 1, 2, 3, or 4. Cylinders 5, 6, 7, and 8 triggered the timing light just fine.
First step was to swap coils and plug wires... I moved the coils on both banks so the ones that weren't firing were now in the spots of the ones that were firing, and vice versa. Still no spark on 1,2,3,4, but spark on 5,6,7,8 according to the timing light. I've seen a whole bank be dead before for a wiring issue, but only the front 2 being dead on each bank? This is odd...
Checked all 8 plug wires and plugs... all appeared near new with 0.040" gap, no cracked porcelains or carbon tracking. Checked compression while I was there, all 8 were running about 160psi. The engine isn't making any noises that would indicate a lifter going bad although I haven't pulled the valve covers and cranked it to check.
Next up was checking for power and ground at all 8 coil connectors... all were good.
Next I disconnected the connector at the ECM and checked all 8 signal wires and all 8 signal ground reference wires between the coil connector and ECM connectors... all good continuity and pinned correctly with no shorts to ground.
Just to see, I pulled the starter and crank sensor and looked at the reluctor wheel... it looked fine. Note-- I did not try and move the reluctor wheel with a screwdriver to see if it was loose.
Checked cam sensor counts with my Tech2 while idling... it appeared to count normally.
Used my oscilloscope and probed between the signal and signal ground reference wires at all 8 coil connectors while the engine was running. All 8 coil connectors, including cylinders 1-4 that the timing light didn't trigger on, show a nice 5V square wave coming from the ECM... that's odd. I expected to see no ECM signal on 1, 2, 3, 4 since those coils don't appear to be firing according to the timing light. Since they all had power and ground I was leaning towards bad coil drivers in the ECM.
At this point I'm questioning the timing light, so decided to do a "direct" test of the coils instead of using the timing light and dug out one of the spark testers that take the place of the spark plug... turns out cylinders 1, 2, 3, and 4 *are* sparking but not triggering the timing light. However, those cylinders are still misfiring. If you pull the coil or injector wires for cylinders 1, 2, 3, 4 while the engine is at idle the rpm or tone doesn't change. If you pull 5, 6, 7, or 8 the RPM drops substantially.
Did a crank relearn, no changes.
Pulled the vacuum line for the fuel pressure regulator, it's not ruptured and dumping fuel into the intake. Fuel pressure is good.
Did an injector balance test with the Tech2, all 8 injectors flowed nearly identical, so it doesn't appear to be bad injector drivers in the ECM or stuck/clogged injectors... although I suppose the front 4 injectors could be lazy at idle/short pulsewidths and not firing consistently at idle and causing lean misfires. I may try swapping the injectors for 1-4 with 5-8 tomorrow and see if the misfires follow the injectors.
A thermal camera shows the exhaust manifolds near the ports for cylinders 1 thru 4 are much cooler than 5 thru 8, so that backs up the behavior noted above when coil or injector connectors are pulled at idle... 1-4 appear to be either not firing at all or only occasionally firing.
There is quite a bit of a "whistle" noise at idle that got me to think about a cracked intake or leaking intake gaskets causing lean misfires, but it may just be the IAC making noise back through the aluminum intake tube. IAC counts are about 120 at a 650 rpm idle. Decided to spray ether around each intake port where it meets the head hoping to find an intake gasket leak... didn't hear any change in RPM or smoothness.
I found it slightly odd the engine isn't throwing any misfire counts so I downloaded a stock 2001 5.3 tune from the tune repository and did a file comparison, and it appears the SD tune that's on this thing has all stock misfire parameters.
Thoughts? I'm a couple days into messing with this thing now and no closer to a solution, so I figured I would ask for input. If there's any specific logs that would help, let me know and I'll get them for you.
I have attached a copy of the tune that was on the ECM as the vehicle was purchased. I have never messed with an SD tune before, so any insight would be appreciated.
Below are things I did notice in the tune that have me scratching my head, although remember I've never messed with an SD tune before...
P0101, 102, and 103 are set to no mil light. Based on some reading here about SD tunes, shouldn't an SD tune have those 3 codes set to "mil on first error" so the ECM "fails" to SD mode immediately?
Fuel stoich AFR is set to 14.22. This is a stock engine with a stock cam... why not leave it at 14.68? Fuel mileage isn't a goal of this rig, but I don't see a good reason to run it richer than necessary.
Finally... this is a stock engine and chances are it's going to stay that way. Any reason not to add an LS3 card style MAF and revert back to a MAF tune? I understand why the outfit that supplied the engine and harness combo provides SD tunes-- an SD tune doesn't care about the air intake and they don't have to deal with tweaking MAF tables for the end user's specific air intake. But once the misfire issue is sorted out, any compelling reason to either stick with the SD tune or add a MAF and go back to a MAF tune?
Thanks in advance!
