So ive had this this bad missfire on cold starts, ive check fuel pressure, I've checked for air leaks, new chts, check air regulator (by clamping the hose). Checked timing. Idk what else to do! Goes away after warm up. Feels like after the cold start valve gas burns up it falls on its face. I also checked compression. Should I add a resistor to the chts harness?

 

Your cold start Valve, is an injector, it could be malfunctioning, when activated it is flooding, once warmed up it gets shut off. Unplug it and see if it does the same thing. It is an item that guys do delete, when doing a clean up of the intake.

Here is a link to a good writeup

https://www.zdriver.com/forum/240z-2...anifold-19937/

 

Checked def. Not the cold start valve.

 

Does it just die or back fire? What yr is it?

I know some have a allowance for a timing advance depending on the engine temp. Just not sure what years?

Check the factory service manual. You can download one for free.

 

the parts that relate to cold start are the air bypass valve and cold start injector, also the thermotime switch. if any are not working it will cause hard starts. maybe the cold start works fine, but if its not being triggered and not getting powered (i.e. wiring issues or thermotime switch broken) it won't cold start well. more like a carbed car. personally i deleted all of my components and just use my foot for 30 seconds and then it runs fine.

 

When I did my megasqurit ECU I did not remove the cold start valve. I think it can be useful as a trouble shooting tool if someday the car will not start. Just put power to it and it sprays gas right in the intake.

 

You nailed it Shady280...I wanted to say the Thermotime switch.

 

why would you add a resistor to the chts circuit. that is probably your issue. change to a new one and see if your problems go aaway