2006 Hummer H3 3.7L inline 5, recurring P0305 misfire on cylinder 5, even after engine replacement. 150k miles on chassis, 63k on 2007 3.7l

coils from 07 used. New plugs new sensors on front and front pass side of engine (cam etc). Most everything else from 2006 reused I think.

I’m looking for advice on where to go next. I had a cylinder 5 misfire issue on my previous engine. That engine also had major oil consumption, about a quart every 1,000 miles, and cylinder 5 plug would sometimes come out wet with oil.

The engine was replaced with a used 3.7L engine. The same P0305 cylinder 5 misfire has now returned on the replacement engine. That makes me think this is likely something vehicle side, not the engine itself.

What has been done so far:

1. Swapped ignition coils around. Code still comes back as P0305.

2. Inspected spark plug on cylinder 5. It looks normal and similar to the other plugs. Not wet with oil on the replacement engine.

3. Found damaged insulation on the orange wire at cylinder 4 ignition coil connector. Cut out the damaged section, soldered it, used adhesive lined heat shrink, wrapped with Tesa tape, and put the harness back in loom.

4. Replaced the cylinder 5 ignition coil connector pigtail with GM part number 15306431. Used the full lead, soldered the connections, used adhesive lined heat shrink, wrapped with Tesa tape, and put it back in the conduit.

5. After the cylinder 5 pigtail repair, I drove about 90 miles round trip with multiple stops and no code. Then after a couple short trips and hotter weather, P0305 returned again.

Pattern:

1. The code usually does not show up immediately after clearing.

2. It may take a few drive cycles.

3. It seems possibly heat related or heat soak related, but I am not 100 percent sure.

4. It came back on an 85 degree day after short trips, not during the longer highway drive.

5. I have also had P0300 random misfire in the past, but currently it is P0305 only.

Questions:

1. Could a cylinder 5 injector or injector connector cause this pattern?

2. If the shop reused my original fuel rail and injectors during the engine swap, could that explain the same P0305 across both engines?

3. Is there a known issue with the 3.7L H3 harness, injector wiring, coil wiring, PCM driver, or heat soak causing a cylinder 5 misfire?

4. What would you test next before throwing more parts at it?

I’m thinking my next step may be inspecting the cylinder 5 injector connector and possibly swapping injector 5 with another cylinder to see if the code follows.

i might go to the junk yard and get the ignition and injector harness and an injector

 

Cyl 5 coil connector discolored (now replaced)

Other connectors

Cyl 5 plug. Looks like the rest I think.

Cyl 5 connector now replaced

Ignition harness. The larger grey connector towards the firewall on left looked a bit rough, locking clamps broke off. The small black connector also looked a bit rough

 

LOOK CLOSER AT THE WIRING! Heat increases electrical resistance causing components to fail which leads to CEL.