V6 LS3 (2 dead holes).

[416velle]

I've built a lot of LS engines, I've diagnosed a lot of em, nothing has thrown me for a loop more than this. Backfiring at idle under 1300 RPM, cylinders 6 and 8 are dead.

416 LS3, ported 823's, ported LS3 intake, 13:1 compression, K1 crank/rods, wiseco pistons, milled heads, [email protected] cam, running off Terminator X Max EFI, sippin on E85 with dual 255lhp pumps and 65lb Deatschworks injectors, built by yours truly.
FRESH engine, just fired it up 5 days ago as of now, knew there were problems almost right away but wanted to get a heat cycle or 2 in it.
Over 1300RPM it sounds like a normal engine would on 6 cylinders; anything less than that is backfires HORRENDOUSLY through the exhaust, sounds almost like theyre firing on intake strokes but when I take out the plugs of 6 and 8 and theyre drenched in fuel, with zero evidence of wanting to fire. The headers are also cold to touch even after running a full heat cycle.

Heres everything I checked:

If I unplug either the injectors or the plug wires of 6 and 8, it doesn't backfire at 1000rpm idle. If I leave injectors plugged in with plug wires off, it doesn't backfire.
TR6 spark plugs gapped at .035. 8:1 AFR according to WB. 60PSI fuel pressure on the dot. Coils have power, coils have ground. Injectors have power/ground and aren't stuck, took the intake out/unplugged injector harness but left fuel lines connected, primed the pump and not a drop of fuel out the injectors. No vacuum leaks. No bent valves, good compression test; 3% deviation between worst and best cylinders. 6-8 have spark (shocking, literally) and spark looks comparable to the good cylinders. The wires are good, swapped em around. Coils are good, swapped around. 6 and 8 are NOT FLIPFLOPPED, plug wires were always snapped in all the way and likewise on coil side with both audible clicks.

Things I haven't checked:
injectors (havent switched em around yet though I don't see why this would prevent a cylinder from firing all together), coil harnesses though I did ohm out bank 2 and it was OK with no shorts/split wires/etc. Haven't looked at the injector harness (not sure why I would but at this point I might be game). Haven't pinned out the entire engine harness from coil connectors to ECU connector.

What else am I missing here? I'm at a loss. Spent my entire 3 day weekend trying to fix this to no avail.

[ryeguy2006a]

Sounds like some sort of electrical interference being too close to another wire. Have you tucked the wiring really close together with other wiring like the coils, or anything? Also, confirm that your power and grounds from the ECU are directly to the battery and that the two ground wires on the harness side are clean solid grounds to the back of the head. If you have painted them, make sure to clean them off and use some dialectical grease or something to enhance a good connection.

[416velle]

Sounds like some sort of electrical interference being too close to another wire. Have you tucked the wiring really close together with other wiring like the coils, or anything? Also, confirm that your power and grounds from the ECU are directly to the battery and that the two ground wires on the harness side are clean solid grounds to the back of the head. If you have painted them, make sure to clean them off and use some dialectical grease or something to enhance a good connection.

Appreciate your response. Grounds are good; sandpapered the bare heads AND the ring terminals to be sure and checked with volt meter to assure continuity.

Today I swapped injectors and that changed nothing; I also swapped coil sub-harnesses and that didn't do it either. So that narrows it down to RMF noise, engine harness (coil circuit and injector circuits, shorts or chaffed wires i guess), or a bricked ECU.

As for RMF/EMF interference...The wires basically branch off normally out of the engine harness and are nowhere near plug wires or the coils (remote mounted coils on the firewall). I did separate bank 2 pigtail and DBW leads from the fusebox/alternator leads with a split rubber hose and a lot of air space, that didn't do anything. Also another thing i noticed is that fuzebox and alternator lead is bundled to the bank 1 pigtails briefly and there are no problems with any of bank 1 not firing/misfiring. I guess more shielding is something I can try to a greater extent than I already have, the wires are bundled in braided loom and it does get kind of tight where it goes through the bulkhead but they're no tighter than I had it with the previous setup...(all of the engine management system is just about 2 years old).

Something I haven't looked at whatsoever is the cam and crank sensors, though from my experience and word from many other people, those failures are typically realized at higher RPM, with which this engine hasn't gotten even close to. I guess I could check crankshaft thrust but that's exceedingly unlikely to affect ONLY 2 cylinders all the time. If the reluctor wheel was spun, the engine wouldn't even start (I know from experience).
Ill be calling the holley techsupport tomorrow to look at some ECU and harness diag procedures. I'm leaning tmore towards bricked ECU drivers somehow, because even looking at the pinout diagram, theres no connections in the coil nor the injector harnesses that are shared by ONLY cylidners 6 and 8.

[slazisme]

Earlier this year I had a problem with #1 cylinder not firing optimumly, changed plug, injector, swapped coil, swapped o2 sensor = no difference. Picked up another coil harness and changed it on the drivers side along with a ground strap to the drivers cylinder head= problem solved. Sounds as though you swapped parts out but maybe check the coil harnesses and to make sure they're not mixed up from bank to bank?
https://www.youtube.com/watch?v=Bu0XWB7_F5g
Or try a new coil harness?

[416velle]

The engine runs, just on 6. It wouldn't run at all if my banks were switched. I did swap coil harnesses side to side and that didn't change anything.
I used the slow motion feature on my phone and confirmed yet again that it has spark, but mainly the fact that they are indeed NOT firing at the same time. Which makes absolutely no sense. I have no way of testing injectors to see if those are going at the same time unless I take the intake off for the 3rd time. I can pull the injector harness and see if theres anything wrong.
I also tried a old completely different pair of plug wires for 6-8 along with my good new test coils, still nothing. I went to the extent of pulling 20% and 50% fuel from cyl 6 and 8 to have some sort of evidence of firing even if it was weak spark; nothing.
I have an extra cam sensor so I might install that for no good reason and try it. May buy a new crank sensor just to 100% rule those out. While I'm there I might inspect the reluctor on the brand new crank for no reason either.
Im at a loss here. I emailed holley tech support, I hope that leaving the ECU out in the non-heated garage for the 3 degree winter last year along with not powering it up for a year and a half killed something vital in the ECU.

[Hotwire]

When swapping coils/injectors are you swapping from a working cyl to 6/8?

Have you tried replacing the spark plugs? I've cracked insulators before and they barely push spark through.

Last resort is to ring the harness out? Did you build it? I would take the time to ohm out or continuity check from ECM pin to coil/inj of 6 & 8, about the only thing left.

I built my harness and swapped 2 cyl in the coil harness. Engine ran but had a weird cycling idle.

https://static1.pt-content.com/images/pt/2022/11/52410688244_8b7cba9225_c-1.jpg

[Chevy Kid]

Sure sounds like somethings wrong in the coil harness. Check the wire pinouts with holleys diagram online. I love my terminator x, but I've seen some things wrong in a friend's harness that scares me a little. I agree the coil sides are correct. It would not run otherwise. Good job verifying the coils are firing, they just might not be firing at the right time. HTH,

Tim

[Lonnies Performance]

If sub harnesses are good & coils are good & compression checks out on those cylinders, it has to be wiring.

Did you check the pin outs at the ECM to make sure the 6 & 8 coils are wired correct there?
An easy check is to cross the 2 plug wires 6 to 8 & see what happens.

Also maybe stating the obvious but verify firing order in the ECM 1-8-7-2-6-5-4-3

[416velle]

When swapping coils/injectors are you swapping from a working cyl to 6/8?

Have you tried replacing the spark plugs? I've cracked insulators before and they barely push spark through.

Last resort is to ring the harness out? Did you build it? I would take the time to ohm out or continuity check from ECM pin to coil/inj of 6 & 8, about the only thing left.

I built my harness and swapped 2 cyl in the coil harness. Engine ran but had a weird cycling idle.

https://static1.pt-content.com/images/pt/2022/11/52410688244_8b7cba9225_c-1.jpg
Swapped parts from cylinder 1-3 to 6-8. I've replaced the spark plugs twice. Using factory unmodified from-holley terminator harness that worked flawlessly in the prior setup I had.

- - - Updated - - -


Sure sounds like somethings wrong in the coil harness. Check the wire pinouts with holleys diagram online. I love my terminator x, but I've seen some things wrong in a friend's harness that scares me a little. I agree the coil sides are correct. It would not run otherwise. Good job verifying the coils are firing, they just might not be firing at the right time. HTH,

Tim

Even looking at the coil harnesses I have rn scares me because theyre GM 2003, will agree there. But I did ohm them out and they checked out fine, I swapped them side to side and the issue persisted with 6-8.

[416velle]

If sub harnesses are good & coils are good & compression checks out on those cylinders, it has to be wiring.

Did you check the pin outs at the ECM to make sure the 6 & 8 coils are wired correct there?
An easy check is to cross the 2 plug wires 6 to 8 & see what happens.

Also maybe stating the obvious but verify firing order in the ECM 1-8-7-2-6-5-4-3

Compression test had all cylinders within 8PSI, which is the best numbers I've ever seen on any engine (would really hope so since its a brand new build basically)
Firing order is right in the ECU. And I used the harness with the previous setup I ran before blowing the motor up (basically why its a 416 now instead of a 6.0), so unless someone went through my wire harness while the car was apart in boxes that's unlikely.

[slazisme]

Just throwing out other possibilities here to try and find the issue so don't be offended. What crank reluctor do you have 24 or 58?? Was the crank reluctor on the stroker crank when purchased and was it indexed, manufacturer? What gen LS engine is this? Where is the crank sensor? What color is the crank sensor, grey or black?

[416velle]

The very last things I went to check today were camshaft and crankshaft position sensors. Swapped with a GM sensor I had (known good) and needed to buy a crank sensor. Put new sensors in....Thing didn't want to start. No power to coils, no power to injectors, no spark. Then I had the genius idea to use the diagnostic LEDs on the ECU and determined the crank sensor is bad right out of the box!!! My luck has never been better. Even O'Reillys doesn't want my car to run. I assume the ECU doesn't ground the injectors without a reference from the crank sensor, then didn't give any spark to the coils because there was no RPM signal. Learned something i guess but its not something I even remotely cared about lol
Swapped back to the other "old" crank sensor and it fired RIGHT up...on 6 cylinders still. I'd post a video of that final startup but you wouldn't be able to hear anything I'm saying from the amount of bleeps bc its extremely profane.

I have no more energy for troubleshooting. I'll pull and ohm out the injector harness tomorrow maybe even at work because its small, that's the only thing left on the list of things that I can check under my jurisdiction other than ohm the engine harness.....after that I'm calling holley for an ECU "service".
And HEAR ME OUT...
Cost for new ECU is $750. Well, a carburetor is $400 and manifold is $300, and I already have a MSD CarbLS harness and ignition box. That'd be really funny (while still crying inside)

[416velle]

Just throwing out other possibilities here to try and find the issue so don't be offended. What crank reluctor do you have 24 or 58?? Was the crank reluctor on the stroker crank when purchased and was it indexed, manufacturer? What gen LS engine is this? Where is the crank sensor? What color is the crank sensor, grey or black?

I know I know and I appreciate your thought experiments, please excuse me for my my hostility.

Its a Gen 3 6.0 block 24x, K1 crank and it the wheel IS welded. I can tell you for a fact that if the reluctor is spun, it will not start up.
Reluctor wheel disgnosis is something I unfortunately have diagnostic experience with(diagnosing a no-start vehicle where I used to work) and that took 4 days of diag before I ripped the motor out and took it apart....so needless to say its been in the back of my mind.
It just doesn't make much sense that it would affect only 2 cylinders given the crankshaft spins twice per 4 stroke cycle and 6 and 8 are 3/4 crankshaft revolution different in the firing order. A reluctor fault at 6 and 8 would mean theoretically that it would also affect the opposing cylinders in the firing order, or cylinders that are 1 full revolution off in the firing order, cylinders 1 and 5 (1 is a full revolution from 6, 5 is 1 full revolution from 8).

[slazisme]

Hey man I understand your frustrations, you're so close yet .....you know the rest. The part that baffles me is that you have said that everything was working with the previous 6.0L fine and now not. That's why I questioned the reluctor because that is one of the main parts that you changed, pistons too.
I'm surprised that the resident mod that specializes in Holley Terminator stuff hasn't chimed in. Maybe send him a PM/DM for his opinion. I run a Fitech LS stuff in my build that has worked flawlessly, maybe check with Holley for help and possibly warranty replacement if its still under. Just confirming black crank sensor and I only used the GM stuff/ AC Delco not taking any chances.
203241
My 03 6.0L, no visible crank sensor.

[416velle]

Hey man I understand your frustrations, you're so close yet .....you know the rest. The part that baffles me is that you have said that everything was working with the previous 6.0L fine and now not. That's why I questioned the reluctor because that is one of the main parts that you changed, pistons too.
I'm surprised that the resident mod that specializes in Holley Terminator stuff hasn't chimed in. Maybe send him a PM/DM for his opinion. I run a Fitech LS stuff in my build that has worked flawlessly, maybe check with Holley for help and possibly warranty replacement if its still under.

100% its definitely a possibility, that is one of a few things that are different in the totality of the engine from the other setup, the others being E85, injectors, plug wires, and thats REALLY it.
ECU is much easier and would take a lot less time to replace than crankshaft, that's a huge part of why I'm deflecting lol I would MUCH rather that be the issue than crankshaft.

I do have a way of troubleshooting the crank reluctor along with cam reluctor profiles (in case the camshaft is messed up, i DO have an identical camshaft to this current one in my stash right now so that would be somewhat simple), by looking at the wave pattern with my strap-on--i mean snap-on scanner and comparing my engine waves to some random truck or van at a stop light

I don't know enough about computers to answer this but when I first put power to the car after having it all apart...the battery was unbeknownst to me at 8 volts, and the ignition swtich was unbeknownst to me bad as well...and I do remember powering it up a few times and the diag indicator lights turned on and off at times (when they shouldn't be) with the key on engine off position...I suppose that could have messed something up in the ECU internally...i hope.

- - - Updated - - -

Yes it is indeed the black sensor, needless to say black cam sensor too (completely different design from gen 4 not interchangeable)

[rennat_2006]

Injector #6 and injector #8 wiring isnt mis matched is it? Meaning #6 injector pigtail accidentally put on #8 injector.

Not familar with the holley setup but easy way to check/confirm with gm ecm and scanner would be unplug #6 and hook pigtail to noid light or test light then shut it off with scanner and confirm its dead. Could do same thing with coils also.

Ran with valve cover off or atleast cranked over and verified rockers are moving and same as other cylinders?

Probably already have thought of this but god knows the simple easy crap bites us sometimes.

Agree that crank sensor issues typically are absolute dead(crank no start) or break up in higher RPM.

[416velle]

Injector #6 and injector #8 wiring isnt mis matched is it? Meaning #6 injector pigtail accidentally put on #8 injector.

Not familar with the holley setup but easy way to check/confirm with gm ecm and scanner would be unplug #6 and hook pigtail to noid light or test light then shut it off with scanner and confirm its dead. Could do same thing with coils also.

Ran with valve cover off or atleast cranked over and verified rockers are moving and same as other cylinders?

Probably already have thought of this but god knows the simple easy crap bites us sometimes.

Agree that crank sensor issues typically are absolute dead(crank no start) or break up in higher RPM.

Ran as much as i was at liberty to (20 seconds if that) with the valve cover off and valvetrain is indeed working (very quiet valvetrain at that I may add) and compression test can support this. 6 and 8 injectors are not reversed, that's one of the first things I looked at. Also I should clarify that the cylinders are not "dead" necessarily, as the injectors are pulsing and coils are working, they're just getting FAR TOO MUCH fuel.
I confirmed with my slow motion camera that the coils are not firing at the same time, I have an idea of how I can see if the injectors are spraying at the same time.

[Hotwire]

I've pulled a fuel rail before and set it up then cranked the engine over to see the spray pattern of the injectors. No, it's not safe, yes it makes a mess, but it answers questions.

One other thing you can do, put a fuel pressure gauge on the rail, prime it, then pull the pump relay and crank the engine. While cranking, watch the fuel pressure, it should drop like watching a stop watch, tick for every cylinder. If it instantly drops or there's a mismatch in the tick down you know where to start looking.

As with others, hoping to help.

[416velle]

I've pulled a fuel rail before and set it up then cranked the engine over to see the spray pattern of the injectors. No, it's not safe, yes it makes a mess, but it answers questions.

One other thing you can do, put a fuel pressure gauge on the rail, prime it, then pull the pump relay and crank the engine. While cranking, watch the fuel pressure, it should drop like watching a stop watch, tick for every cylinder. If it instantly drops or there's a mismatch in the tick down you know where to start looking.

As with others, hoping to help.

I've swapped injectors side-side, I've also confirmed all injectors do not leak, the issue stays with 6-8 regardless.
I have a mechanical fuel pressure gauge on the regulator, as well as a pressure transistor read by the holley EFI and both are reading 60PSI when running.

Just a few minutes ago I ohm'ed out the engine harness as well as the injector harnesses and confirmed the cyl 6 and 8 injector circuits are completely divorced with no shorts.
I could put my volt meter in hertz and look at the values of those cylinders compared to others when cranking but that's just going to confirm what I already know.

Next on my list is compare my crankshaft reluctor waveforms to a stock engine 97-03. Will be a little bit before I figure out how to do that properly with my equipment.

[416velle]

After some searching through reference manuals I've confirmed that ECU damage does result from using LOW impedance injectors....I have also confirmed that i am using HIGH impedance injectors so I don't have ECU damage as a result of that per se.

[416velle]

So it took me a long time to actually visualize and formulate this hypothesis, but I now have almost 100% certainty the reluctor wheel is not my issue

The 4 stroke cycle requires 2 rotations of the crankshaft to complete its firing order. That means on any 1 given location on the crankshaft during its rotation (of any engine more than 2 cylinders), theres 2 different things happening. In my case, I """MAY""" have a problem on the equivalent of the intake stroke of cylinder 8 AND another at the intake stroke of cylinder 6....Well, give that crankshaft 1 full rotation from either of those locations, and cylinder 8 is on it's compression stroke, while cylinder 5 is on its intake stroke, and likewise when 6 is on its compression stroke, cylinder 1 is on its intake stroke. This suggests any given chipped tooth/damaged gear on that reluctor would yield an IDENTICAL FAULT on the cylinder that's exactly 1 revolution off in the firing order. I DO NOT have problems with cylinders 1 and 5.
This alone strongly suggests I have an internal ECU failure, given I have completely exhausted all my troubleshooting options at this point.
It is late and my brain is fried so I may be way off my rocker.

[Big_KID]

Have you pulled the valve covers and rotated over by hand to confirm the valve event order is correct for 6 and 8? It is a long shot but cam could have lobes for 6 and 8 swapped placing the valve events 135/225 degrees out.

[416velle]

Updated firmware for giggles.....nothing.
I messed around a bit with the datalogging feature to look at the pulse width of the screwed up cylinders when the thing is running.....according to the software ALL injectors are at IDENTICAL PW and lb/hr values.
NOW...when i adjust the individual cylinder fueling parameters of 6 and 8, the frequency of the backfires changes significantly, and the datalog does display the correct % changed values when idling....likewise with adjusting the timing. I contemplated switching 6 and

So at this point, if I trust the ECU, that means I err towards reluctor wheel being the failure mode here (somehow). Which kind of lines up with the symptoms at hand, and the ECU values corroborate that to an extent. If spark isn't at the right time this will happen. So this means I have to pin my engine harness for the coil side?

After some thinking and drawing, when 8 is at TDC of compression stroke, 6 is in the middle of its intake stroke and vise versa. I am about to switch 6 and 8 even though I know they're plugged in right because why not and at this point if I blow it up itd be a god send and I can have an excuse to put a different motor in it that doesn't have an ECU or a reluctor wheel

- - - Updated - - -


Have you pulled the valve covers and rotated over by hand to confirm the valve event order is correct for 6 and 8? It is a long shot but cam could have lobes for 6 and 8 swapped placing the valve events 135/225 degrees out.

That's a good idea and I'm game for it right now.

[416velle]

Valve events checked out fine, and I did swap 6-8 [plug wires] only for backfires to occur through the intake instead of the exhaust

[416velle]

Took a video yesterday of the reluctor straight-on through the crank sensor hole: No runout, no bent teeth, it looks fine.
Today I'm going to use my graphing multimeter and read the waveform of said reluctor and compare it to samples online.
I'm at my wits end here, holley doesn't answer their phones or emails (2 hours on hold) and the one time I did get through, he said "theres no way for you to test the ECU to see if its dead".
If the waveforms are the same then Im ditching the Terminator X and putting a pro-flo 4+ or XFI in the thing. I'm 3 parts away (indlucing engine) from driving this GD car out of my driveway. EXTREMELY frustrating

[416velle]

FOUND MY ISSUE. NOT ECU. ITS RELUCTOR. ANY1 HAVE EXPERIENCE WITH CRANK TRIGGER KITS??

So basically what I did is swap injector plugs AND coil plugs between cylinders 4 and 6, and changed the firing order in the tune to 18724653. This effectively uses cylinder 6's electronics on cylinder 4, and vise versa. This was useful because it tells me that IF the issue FOLLOWS cylinder 6's electronics to cylinder 4, then I have a ECU issue. And contrarily, if the issue stays at ACTUAL cylinder 6 which is using cylinder 4's electronics, then the electronics are OK I have a timing issue SPECIFIC TO ACTUAL 6-8 which indicates a reluctor wheel problem. I'm DEFINITELY not pulling the motor out and apart to service the reluctor wheel.

[pannetron]

Sounds like an ignition harness issue to me. Not sure which ignition harnesses you're using but the GM harnesses are side specific.

[416velle]

I meant to close out this thread with my findings a month ago when I fixed the motor............Found the issue! Engine is in perfect running order now.
Some goofball (me) who built the car installed the bank 2 coil sub harness backwards. THATS IT. Fired RIGHT up after I figured that out.

I did get some good practice with a lab scope in determining my crank and cam signals are completely mint. Lol.

Note to self: when taking stuff apart, know where it came from before putting it back together. I messed around with relocating my ignition coils to the firewall, and flipped one of the harnesses to give them a mirror image look on the firewall. Well, i didn't number which one came from which cylinder before doing that.
I appreciate all the responses fellas, have a great rest of your day

[andrewb70]

I meant to close out this thread with my findings a month ago when I fixed the motor............Found the issue! Engine is in perfect running order now.
Some goofball (me) who built the car installed the bank 2 coil sub harness backwards. THATS IT. Fired RIGHT up after I figured that out.

I did get some good practice with a lab scope in determining my crank and cam signals are completely mint. Lol.

Note to self: when taking stuff apart, know where it came from before putting it back together. I messed around with relocating my ignition coils to the firewall, and flipped one of the harnesses to give them a mirror image look on the firewall. Well, i didn't number which one came from which cylinder before doing that.
I appreciate all the responses fellas, have a great rest of your day

I just had the exact same thing happen with a customer. His driver's side coil sub harness was made wrong by a vendor (not Holley).

If I can be of service with the tuning, please email me: projectgattago at gmail dot com

Andrew

[Jeep9783]

I meant to close out this thread with my findings a month ago when I fixed the motor............Found the issue! Engine is in perfect running order now.
Some goofball (me) who built the car installed the bank 2 coil sub harness backwards. THATS IT. Fired RIGHT up after I figured that out.

I did get some good practice with a lab scope in determining my crank and cam signals are completely mint. Lol.

Note to self: when taking stuff apart, know where it came from before putting it back together. I messed around with relocating my ignition coils to the firewall, and flipped one of the harnesses to give them a mirror image look on the firewall. Well, i didn't number which one came from which cylinder before doing that.
I appreciate all the responses fellas, have a great rest of your day

Digging up bones here but couldn't find a way to message you. I am having same issue except on cylinders 2 and 4. Built my setup (5.3 cammed etc holley term x max) drove for 350ish miles until it started skipping and found cylinders 2 and 4 misfire. Changed all coils, all injectors x2 with Holley, installed new engine harness, spark plug wires and plugs. Same issue, pulled heads and lifters all good, checked heads also no leaks. Forgot to mention comp test prior to all of this and was good. I have fuel and spark but using heat gun 2 and 4 ain't doing nothing. Mine is a rock crawler so not a touring but so far ur post only 1 close to mine. BTW what harness did you have backwards and any cheat codes you care to send my way as May will be 2 years I been working in mine.