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Build is LQ4/ 243 heads LS6 cam, BTR valve springs currently on factory LQ4 tune with stock 4L80E
Problem: engine will not go over 2000 RPMs under load (not limp mode) will go over 2000 in park or neutral only code is P0300 whether in park, neutral or test drive when revving up to 2k rpm
This engine has less than 2 miles on it. All new build to include gaskets and intake gaskets.
New Delphi MAF; scanner shows correct Lb at idle
New plugs and wires.
Definitely not cat related 👀
swapped entire intake from running engine, no change.
52psi fuel pressure, disconnecting FPR brings it up to 58psi
Fuel trims at idle are attached
Map reading is good 13-15 inHG
Timing at idle is 17
Upon trying to accelerate fuel trims go positive adding fuel for bank 1 STFT or LTFT up to 25% and bank 2 goes -10 or so until I get off throttle so ECU is trying to add fuel and take away depending on bank, swapped 02 sensors left to right and stays the same
Smoke tested intake no leaks.
checked rockers no issues.
Tested under load fuel pressure is 55-60psi
checked and rechecked all grounds.
cleaned throttle body.
Coils have spark
At loss getting ready to throw a fuel pump at, just in case I could have good pressure but not volume.
I’m usually the guy people call for help but this has me stumped🤦♂️ 🤦♂️
I do understand truck needs to be tuned but it’s a small cam and shouldn’t be this bad.
ideas?
You REALLY need to get a tune. The LS6 cam is far enough away from stock to mess things up.
Where are you? I'm near Hanford. I noticed you're in the San Joking Valley too.
You REALLY need to get a tune. The LS6 cam is far enough away from stock to mess things up.
Where are you? I'm near Hanford. I noticed you're in the San Joking Valley too.
I got and responded to your PM.
You said it won't rev under load, but will free rev.
This says fuel restriction to me, and I've read all your mods and fixes above.
One thing not mentioned- clogged fuel filter or screen/strainer. Have you checked EVERY fuel system component from the tank pickup to the injectors?
I got and responded to your PM.
You said it won't rev under load, but will free rev.
This says fuel restriction to me, and I've read all your mods and fixes above.
One thing not mentioned- clogged fuel filter or screen/strainer. Have you checked EVERY fuel system component from the tank pickup to the injectors?
changed filter this morning, same same, yes it will free rev, still pops the code immediately though, just under load, under load fuel pressure stays at 58 psi and never drops, once I do the crank, cam and TB relearn I’ll report back, hoping it’ll show me exactly which cylinder or cylinders are misfiring
Narrowed it down cylinder 4 with massive misfire and 2 minimal, Swapped plug, same.
moved coil and wire all to a cylinder with zeros. Same same, pulled valve cover and rockers are still torqued, no broken springs, rockers all move perfectly while cranking, took for a drive and doesn’t pop code immediately anymore or at all at this point.
Found slightest exhaust leak on manifold on cylinder 4/6 going to swap in a new gasket just in case then an injector. I’m sure I’ll report back.
I checked and checked and looked at data, so while plugging and unplugging the main coil pack harness on passenger side, cylinders 2,4,6,8 what I found is one time it’ll show only cylinder 4 misfiring other times it’s all cylinders, since i know it’s not part related now, time to chase wires or find the ground for that side.
so after cleaning connections it’s mainly 4 but every once in awhile it’ll be 2/6 never 8. does not pop codes for any cylinder just the p0300 this is what I have done now
Changed out number 4 injector
Verified spark at cylinder while running
Ran injector Balance test again perfect
Performed leak down test, best results I’ve ever seen but makes since of new engine
Compression test is good
Moved plug to cylinder without misfire.
Borescoped piston and cylinder just in case looks as advertised, new.
Have fuel/spark/air, good compression, no leak down no vacuum leaks, good fuel pressure but somehow misfires.
might be worth the $200 for dealer to diagnose but I feel so bad for that poor tech 😂.
Last edited by Lowe6417; Mar 30, 2024 at 09:49 PM.
after much turmoil I got the bottom of it, when the cat had come apart it physically was blocking the exhaust after the cat 🤦♂️ 🤦♂️ I never would have found it without a borescope. Runs good, wraps RPMs fine.
if anyone does everything I have done and still has issues scope that exhaust!
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LQ4 engine issues 
