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.

 

Yes. The misfire happens when temps outside are higher, possibly also heat soak. And if the wires in the ignition harness were in bad shape. I corrected what I could see and access.

I spent a few hours getting the complete ignition and injection harness, fuel rail and injectors off a 2004 Chevy Colorado. I had a single connector to get off under the truck but They kicked me out at 5:05 so I hid everything.

i also dumped some Techron on the tank. We’ll see I guess. Was hoping to give this H3 to my 5 year old someday. Had it since new.

 

Questions 1 & 2 - Yes to both (key word “could”)

3 - Out of the 3x H3’s I’ve owned, I’ve never heard of this being a major issue when specifically related to Misfire codes (some of the veterans on here can tell me if this is false), but I have experienced similar electrical gremlins on my wife’s H3 Alpha after it sat for a few years before we purchased it. Sometimes the ABS/Traction Control lights would come on, some times they wouldn’t. Sometimes the car would not crank with the turn of the key, sometimes it would. It wasn’t until I cleaned some of the fuse box connectors and some of the fuses themselves and started actually driving the car more and more that all the kinks started to work themselves out. Something like this could absolutely be the cause your issue. With that in mind….

4 - I definitely agree with Hummerz that you should look into the wiring. You might ask “what specifically do I look for?” I’ll give you my input:

below are the wiring diagrams from the Schwartz 2006 H3 electrical diagrams.

There are three wires that lead from the ignition coil for cylinder #5. 1x goes to a ground (black) 1x goes to the ECM (green), and one (Pink) goes to the Ignition Fuse #33 (15A) Connector 12 which leads to the Fuse Block above the left front wheel well under the hood.

It might take some time to track down the ends to each of the three wires, but I would definitely start with:

1) measuring the resistance of each individual wire. Put one lead of the multimeter on the ignition coil connector side of the wire, and the other lead on the ground, ECM, or Ignition Fuse side of the corresponding wire. If you notice the resistance is abnormally high in comparison to the other wires, that is likely your culprit.

If you find a wire that has abnormally high resistance, then you could physically cut out that wire, then re-wire a new wire back to the ignition coil connector and that might fix your problem.

2) if you don’t find the resistance abnormally high of any of the three wires from the ignition coil #5 connector, then I would take a can of CRC Electronics Cleaner (don’t use another brand, use CRC) and spray down the connectors to the corresponding wires (ground, ECM, and Ignition Fuse/Fuse Box). Spray the ground area, the PCM/ECM connectors, and the Ignition Fuse/Box Area connector.

Do all of this, hook everything back up, take it for a few hot drives, and see what happens.

If the code exists, post back here and we can go from there.

 

Came back to get the ignition and injection harness. Pulled the bike thing back to transmission. It’s not 4x4 so will just be for spare parts. All got he fuel rail. Don’t really want to pay for it. Might just take 1 injector to test.

techron in the tank car drive great, smooth. No errors in the car scan.

im going to go though the previous helpful post night

 

Visual inspection for melted insulation or heat damage:
Specifically looking for insulation that is melted, charred, or brittle. Look for spots where the plastic is worn away and exposes bare copper wire.
Physical "wiggle" tests to check for broken wires:
Gently pull (tug test) on wires suspected of damage; a burnt or damaged wire will often feel weak and break or stretch easily. You can also "wiggle" wires while testing to find intermittent issues.
Multimeter to check for continuity or high resistance:
Set a multimeter to ohms (\(\Omega \)) and test for continuity between both ends of a wire to find a break. A wire will show an open loop or infinite resistance.