Engine Swap Misfire - Please Help :)

[Chewy buildz]

Hi, would appreciate any thoughts you may have. 

I’ve just swapped a known good VQ35DE into my 2003 UK manual 350Z, using the donor engine and harness but keeping the original ECU.

It starts and revs fine, but has a bad misfire at idle. Only code is P0300. I’ve narrowed it down to cylinder 6 because unplugging that coil makes no difference at all.

So far I’ve:

• Swapped the coil with cylinder 2
• Swapped the spark plug
• Swapped the injector
• Confirmed the injector is clicking
• Confirmed injector pulse with a noid light
• Compression tested cylinder 6 at 185 psi
• Smoke tested the intake with no leaks found
• Confirmed 12.5V at the coil connector
• Checked continuity from the coil wiring back to the ECU
• Checked the engine grounds and added a direct ground from battery negative to the engine
• Unplugged the MAF, which made it run worse

The fault has stayed on cylinder 6 through all of this. Bank 2 short term fuel trim was around +25%.

The donor engine ran perfectly before it came out. I did remove and refit the crank sensor during the swap.

I’ve got an oscilloscope arriving tomorrow, so my next step is comparing the ignition trigger on cylinder 6 with a working cylinder.

Has anyone come across something similar after a VQ35DE swap?

[Keith D]

Think the fuel trim is a big clue, 25% is very high.  My 370z is no more than 5%.

 

I'd check with the scope as you plan. 

 

If that's fine I'd do a leak down test (cam lobe, spring, valve bent etc...) compression test doesn't always pick these up.

 

Then I'd look for an air leak (plenum gasket runner 6 maybe).  PCV valve is on bank 2 and connects near runner 6 -can you smoke test the PVC hole?

[Chewy buildz]

Thanks for the input. I thought the PCV valve was at the front of the engine, drivers side. Is that not bank 1? I actually swapped the PCV valve back (had changed the valve covers due to a leak so put back the original one) but still when I remove the coil pack on cylinder 6 (rear most cylinder on passenger side), no change to idle, so dont think its contributing anything. Got the scope coming later today so will check with that, then may have to do a leak down test. 

[Keith D]

Just had to check the manual to remember myself! 1-3-5 on drivers side, 2-4-6 on passenger side (RHD).  Yes the breather is on 2, PCV is on one.

 

I'd still follow the same list:

 

1. Scope the trigger - it could be ECU fault but thinking about it, it might also be old brittle wiring - that's a 23 year old harness that's been disturbed.

2. If all okay, then leak down, looking for valve spring, bent valve etc

3. Then if still all okay, look for air leak around 6 runner, breather etc.

 

Let us know how you get on, too many people drop in for an opinion and are never heard again!  Means we don't all get a chance to learn

Edited 7 hours ago by Keith D

[Chewy buildz]

Thanks for your help bud! So spent a lot of time today checking everything, scope was just about to arrive. Then thought I would compare the wiring routes for the coil packs against the original engine, and low and behold, I had swapped the connectors for cylinder 4 and 6 coil packs. Swapped them back and ran great, but was still getting a misfire. Scanned it and this time said it was on cylinder 3. Was about to list the car for sale because I was so frustrated haha, then swapped the coil packs from cylinder 3 and 1, ran it, scanned again and it had moved to cylinder 1 (jumping up and down in joy), so swapped coil pack 1 with one from the original engine and no misfires, starts and idles like a dream and sounds soo much better than before. Still have a lot to do to it before its roadworthy, but looks to be mechanically sound. Just need to learn how to weld to fix all the holes in the frame next. Thanks for your help mate! 

[Keith D]

Glad you sorted it