Hey guys,

I’m chasing a weird issue on my rebuilt VQ35DE.

Problem:

I’m getting a misfire/hesitation under ~4,000 RPM, mainly under light to moderate load.
Once it gets past that range, it runs completely smooth and pulls fine.

After 15-20 mins for so it goes away completely.


Details:

• No fault codes (CEL not coming on)
• New coils
• New spark plugs
• Issue only happens under 4k RPM, then disappears
• Engine is rebuilt and has only 62ks on it
• Break-in oil was used and has been running for a short period

What it feels like:

• Slight bucking/jerking under load in low-mid RPM
• Clears up completely once past ~4k RPM
• No issues when just revving in neutral unless I try hold the rpm at 2k or so (noticed it when bleeding the system)

What I’ve considered so far:

• MAF sensor reading incorrectly / dirty MAF
• Possible vacuum leak
• VVT / cam timing issue (oil pressure related?)
• ECU not throwing codes even though trims may be off

What I’m asking:

Has anyone seen similar symptoms on a VQ35DE or similar setup?
What should I check first—fuel pressure, MAF, vacuum leak, or VVT?

Any direction would help, trying to avoid throwing parts at it.

 

Even in the absence of the CEL light, there may still be a fault code. If you have an OBD reader, I would connect that or take it somewhere they can pull the codes for you. It definitely sounds like it's misfiring. A common mistake is swapping the harnesses for coils 4 and 6. For troubleshooting, I would go with what's easiest to rule out first and then go from there before loading up the parts cannon. i.e., make sure the coils are connected properly, none of the connectors are bent, plugs are gapped correctly, check for vacuum leaks, etc.

 

I agree with Heel, this sounds like a misfire issue. Double check all your harness connections to your coil packs, cam sensors, and crank sensor. Look for any signs of damaged wiring. Since the engine was rebuilt, the wire loom could have been compromised during the removal or install process. I would also inspect live data if you have access to a good diag tool, look at cam timing numbers and compare with known good.

You mentioned new coils and plugs, did you use OEM or aftermarket?

Cheers!
-Icer

 

A colleague of mine and I are playing around with AI training. We're using "rag" to ingest data from existing documentation. I trained the AI on the entire "2006 350Z factory service manual" so I can quickly make queries for things like wiring diagrams, torque settings, etc. The LLM portion of the model is connected to the cloud, but everything is running off an Nvidia DGX Spark (blackwell GPU in the home). Here's what it had to say...



I know it's not ground breaking information, but it's still pretty cool how it compiles and presents the data.

 

Here's the wiring colors for each cylinder coil pack harness connector. This is data gathered from a 2006 FSM, so the colors could be different, but I doubt it, still it's possible. It took just a few minutes for the AI LLM (Hermes) to give me this info.

 

Wow this looks insane !! I’ve checked all the wiring and confirmed all coil packs are plugged in correctly. I’ve also inspected for any obvious loose or cracked vacuum hoses but haven’t been able to find anything.



The engine runs slightly better with the MAF sensor unplugged, and I can hold around 2000 rpm for longer before it starts to act up again.



There are no fault codes showing after multiple scans. Unfortunately, my scan tool isn’t advanced enough to read STFT/LTFT data.



The car is running OEM ignition coils and NGK spark plugs.



Next step I’m planning is to go over everything with carb cleaner to check for any possible vacuum leaks. Along with trying to spray the top of the injectors.


Cheers

 

check your grounds too!!