Hey guys. I've been searching around this forum and others for a solution to this problem for the past week when this thing popped up, tried several things to no avail. I have an '03 350z with a crate engine LS3 (cammed) in it. The engine was new when I dropped it in some months ago, ran it with just headers for a bit before I got the rest of the exhaust on about a month ago. It was running beautifully aside from throwing an occasional P0300 code (multiple/random cylinder misfire) but I was still using it to commute just fine to work and drove it a city over on the highway (sustained 2800-3200) rpm and it was behaving fine. After the short trip it again threw the P0300 code but also threw a P0174 bank 2 System Too Lean code; mpg was showing between 24-28 which I thought was pretty damn good. One night I did a burnout, keeping it below 5000 rpm, and the engine started running nasty with obvious misfires and just terribly low power. Turned it off and then back on and it persisted with the smell of fuel. The only code cropping up was the P0300 code. I limped it 3 miles back home where it's been sitting since. I measured the temperature of each exhaust tube on the headers with an IR/Laser thermometer and recorded temperatures after about 30-60 seconds of idle.
(passenger rear) 98 104
128 200
184 250
100 250 (driver front)




*Facing front of car as though you're looking under the hood at the engine.



These are the rough temps before and after I threw in a new spark plug (same model) into cylinder 2 (front passenger). The sparkplugs I pulled (2 and 8) are looking a bit rough and possibly fouled but the fact that swapping one of them changed nothing kind of throws me for a loop. No fuses visually blown, I've reset the ECU twice now. Still a rough, gasoline smell idle. I use a cheaper OBD2 adapter and a free bluetooth app that have both worked pretty well, I can see the proper waveform of the O2 sensor voltage readings during normal running. But watching the voltage when starting up the O2 is reading .5 and then quickly and steadily drops to <.1 within a couple seconds, no fluctuation. The engine bounces out of closed loop and jumps into open loop circuit where it stays until I turn it off. New crate engine, new harness, swap canbus translator, reliable ECU (I guess?), thing was running great. I don't hear anything crudely mechanical or super ear-catching when it's running but I won't dismiss a valve spring being broken, though that would be odd considering it's a crate engine with less than 1500 miles on it. Ground wires are fine. Coil packs all plugged in and spark plug wires connected. I have a couple more ideas to try but any "did you check X" you guys could suggest would be appreciated, or even an OBD2 diagnostic tool that would maybe provide better info would be appreciated.

 

What brand plugs are you using?

what ecu (and what harness)

What injectors do you have on there

 

Most of those seem to be low for cylinder temp. I take it this is a MAF tuned engine and running it on open headers isn't doing it any favors when the ECM is relying on sensors for controlling the fueling and timing of the engine. Any idea if the O2's are fouled out and if those should be replaced? Remember this is a computer controlled engine and unless you're a tuner who's already set it up to run optimal and made sure that nothing is turned on that could monitor the engine and make changes to the fueling or timing. I'm no tuner but I believe that's what speed density tuning is.

 

Went ahead and took off the valve covers. Passenger side was fine, pretty, shiny; but the driver side presented something different...






Rocker arm for the intake on cylinder 5 broke but the remaining portion is still holding the spring down and it looks like the spring has the valve in a bind so it isn't dropping. I'll post updates on how it looks once I get the heads off, fingers crossed. Thanks for the help to those that chimed in.

 

Why has it been over a week since my most recent post and it wasn't approved? People come to these threads hoping to see what the problem was and mods not approving posts can end up causing people more headaches. If this one manages to get approved, the problem was a stuck intake valve on cylinder 5. Rocker arm broke against it and I'm currently in the process of figuring out what caused the valve to get stuck.

 

Crate from what maker? Any warranty on it?

 

Glad you found the issue and hope it's warranty covered.

With number 5 being dead that 200* temp reading had to be heat generated by number 3 & 7 cylinders. My process checking for a dead cylinder is old school... run the cold engine about 5 seconds, shutdown, then quickly touch each primary tube with spit wet index and middle fingers. Not very high tech or sanitary but very effective lol

 

I don't want to yet say who the manufacturer is because I don't yet know the exact cause, but the warranty is very fair and I've been working with the customer support and their techs and they've been incredibly accommodating. I'll pull the head this Sunday and see where we are, I'll try to take photos and report back here; it's probably a bent valve but we'll see what damage we are looking at.

 

Couple of photos for you guys, just to grab your input.
https://photos.app.goo.gl/9HnmcU5rBptE3v878
How does the piston look to you guys? That damage is no more than a millimeter deep, is that significant? Also how would you guys go about removing a stuck/bent intake valve from an aluminum head?

 

Take a used valve and insert it backwards to drive the old valve out. Consider sending them into a machine shop to have them gone through to make sure you have adequate clearance in the valve guides. Any idea as to what caused the intake on No. 5 to hang open?

 

Edit: nevermind, sounds like you found the issue.