LQ4 swap from hell, help needed
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LQ4 swap from hell, help needed I have an LQ4 that I am having nothing but trouble with. The combo is a DBC LQ4 with a 4L80E and return style fuel rails. Running a standalone harness from LT1swap.com with a reprogrammed ECM.
Truck always fires up on the second try, even if warm. Runs great until it gets warm then loses power. If you shut the key off and restart it’s fine for a few seconds or a minute. No codes and live data shows a healthy engine.
Here’s what has been done to work out the problem:
knock sensors and harness
exhaust manifold gaskets
Crank and cam sensors
TPS, MAP, MAF and every other sensor on the motor
coils, wires, plugs
fuel rails and injectors
FPR
Fuel pump, filter and lines
52-55PSI at idle. 58 PSI w/o vacuum
Replace ECM
Replace wiring harness and verify good grounds (wiring harness fixed issue for 2 days)
Grounds:
Battery cable directly to the block
block to frame
block to head and head to head
block to body
When the truck acts up you can unplug the passenger side coil harness and there is no difference. Unplug an injector and plug it back in makes the motor run normal, briefly. Unplug the alternator exciting wire and it runs slightly better. The ECM will register everything I unplug but no codes otherwise.
I am completely stumped on this truck. The gentleman that built the wiring harness is completely stumped and he does this for a living. Everybody I’ve had to look at this truck is completely stumped.
Truck always fires up on the second try, even if warm. Runs great until it gets warm then loses power. If you shut the key off and restart it’s fine for a few seconds or a minute. No codes and live data shows a healthy engine.
Here’s what has been done to work out the problem:
knock sensors and harness
exhaust manifold gaskets
Crank and cam sensors
TPS, MAP, MAF and every other sensor on the motor
coils, wires, plugs
fuel rails and injectors
FPR
Fuel pump, filter and lines
52-55PSI at idle. 58 PSI w/o vacuum
Replace ECM
Replace wiring harness and verify good grounds (wiring harness fixed issue for 2 days)
Grounds:
Battery cable directly to the block
block to frame
block to head and head to head
block to body
When the truck acts up you can unplug the passenger side coil harness and there is no difference. Unplug an injector and plug it back in makes the motor run normal, briefly. Unplug the alternator exciting wire and it runs slightly better. The ECM will register everything I unplug but no codes otherwise.
I am completely stumped on this truck. The gentleman that built the wiring harness is completely stumped and he does this for a living. Everybody I’ve had to look at this truck is completely stumped.
Last edited by Gw85; Jun 15, 2026 at 11:14 PM.
