DrocheGMC

Hey guys, I need your help.

I apologize in advance for the long post.

I recently had my LQ4 turned into a 408 Stroker, and I have been struggling to get the engine to run for the last month. The engine will turn over, fire, run for a few seconds, back fire a bunch and then shut off, it is also shooting pretty big flames. I have made progress, when I first tried starting it the coil pack wire ground was in the wrong location, it now runs better but is still backfiring. Here is what I have tried so far:

• Cleaned all grounds/checked all grounds
• Checked Coil Pack Wiring/orientation
• Checked main coil pack plugs
• Replaced the Crank and Cam Sensor even though they were brand new
• Checked Injector Wiring/firing order
• Checked fuel rail pressure
• Put another set of new plugs in, and checked the gap (The motor did foul cylinder 2 & 4, every other cylinder definitely had a wet layer of gas on them)
• Checked plug wires
• Checked to make sure new throttle blade is working right with the x link
• Cleaned MAF
• Data logged tune and sent to tuner (tuner stated nothing jumped out in the tune)
• Used GM wiring diagram to compare sensor plug, to verify sensors are plugged into proper location.

Here is a video of the truck starting and the build sheet for the motor:


Motor – Iron Block 408 Stroker – CR: 11.1 (I did not build the motor but very reputable company did)

• 4” Stroke Forged Callies Crank – ARP Main Bolts and a Billet 24x Reluctor Wheel
• 6.125” Forged Callies H-Beam Rods – 2000 ARP Rod Bolts
• Mahle Forged Pistons - Coated Skirts and Plasma Moly Rings
• Clevite Performance Bearings
• Camshaft – 239/247 .620/.632 w/ 114 LSA
• Heads – PRC Stage 2.5 Heads – Ported & Polished – Stainless Valves 2.02”/1.575”
• Rocker Arms – Stock Arms w/ CHE Trunion Kit
• Meiling High Volume Oil Pump W/ Billet Timing Set
• Powerbond 20% Underdrive Damper
• EVAP/PCV Deleted just running breathers
• Intake – Holley High Ram Intake Manifold
• 102mm Nick Williams Throttle Body w/ X-Link
• Stock MAF
• Stock Fuel Pump
• 8 AN Supply line from tank
• Holley Fuel Rails
• Deatschwerks Fuel Injectors (35U-01-0060-8) - 60LBS
• 1-7/8” Longtube Headers
• OEM Electric Fans that will be Pinned into harness
• Trucool 40k Trans Cooler

FFDP

iTrader: (2)

I would ask to see the tune if you can do that.

RedXray

Have you done a compression check in all the cylinders looking for something mechanical that may be causing the backfiring? Something like incorrect pushrod length keeping a valve(s) off it's seat. You said you've data logged would this be with HP Tuners? If so you can setup the VCM scanner to count misfires on each of the eight cylinders and even misfire history during the logging. Then you can swap things like coil packs and wires around and see if the VCM scanner shows the misfires following the coil and wire swap.

DrocheGMC

Unfortunately I do not personally have HP Tuners, I data log on an autocal and send my log files off to Blackbear for tuning. But I assume the truck is tuned with HP Tuners.

The compression check is on the list of things to complete, hopefully I will be able to complete the test today or Thursday.

DrocheGMC

I can try but my tuner may keep their tunes private. I will have to ask.

Bgbldodge

Blackbear does EFI Live for the record.

DrocheGMC

ahah thank you for the correction, I should have know that based on the fact that I have to use EFI live to upload/download my tune. 🤦🏻‍♂️

dantheman1540

Is it possibly a crank relearn problem? Any engine codes?

00pooterSS

iTrader: (40)

If it's backfiring or sneezing you have a cam or ignition timing problem, or stuck open valves.

Do a compression check, that will show stuck open valves and if the cam is far enough off the compression will be off across the board.

Check all wiring going to coils and verify each coil is in the right location. I had this whip my *** one time when a buddy pulled his coils, brackets, and wiring harnesses off the original valve covers and swapped them over to another set. He crossed some of the wires to the coils and my firing order was all over the place.

Another way to verify ignition timing is to make a mark on the damper for each cylinder TDC and hook up a timing gun and watch for it to fire on the mark, if it doesn't its firing out of time.

BUT I ALWAYS start with checking ignition and cam timing when it comes to backfiring on cranking.

EDIT: sorry just noticed you said you verified coil wiring and injector wiring for firing order.

Try disabling one cylinder at a time by pulling the injector connector and see if you can at least whittle this down to which exact cylinders are causing the back fire. If it's all of them I'd be looking at cam timing and reluctor ring orientation.

Does it have cam crank correlation codes?

One thing that helped me diag my buddies car was I pulled the connector for one whole bank of coils at one point and it ran perfect on the other 4 cylinders and wouldn't run at all on the 4 I had unplugged. That helped me at least narrow my focus to one side. Sometimes just having a smaller target is helpful.

chris99gmc

I noticed you mentioned wiring as well... if you swapped coils or changed wiring for the coils, make sure you have them wired correctly for your Truck and the PCM for that year vehicle and PCM operating system. Not all LS vehicles have the same coil wiring configurations. I’ve ordered coil pack wiring harnesses before that ended up having the coil wiring backwards on all the plugs. If wires are correct, check to see if any of them are shorted to ground, including plug wires.

if it’s been sitting for awhile, check to make sure your injectors are not clogged up. This happens more than ppl think. Also make sure fuel pressure is good at the rails.

Since it won’t run at all, another suggestion might be the cam is 180* off.

start with the basics first. Mechanical first, electrical second.