okay basics to start off, I have a 1985 z28 with a swap to a 350TPI and its got a 700r4.

Pretty much everything is brand new, I had the engine rebuilt about 3kmiles ago, me and my brother pulled it and put it back in. I never noticed anything wrong till someone told me I had a misfire cause my cat is going bad(Ie running to rich). Now I had had some issues with it for a little so I put a new 02 sensor, new cap and rotor, new ignition, new wires, new plugs.

now here is whats happening, From a cold start, it starts great, idles at about 1300rpm in park. after about 45sec it idles down to about 1000rpm. now this happens I believe because the #2 cylinder stops firing once the engine warms up. and if I shut it off and restart it it drops quickly to 1000rpm but if I let it cool it will idle at the 1300rpm for the 45sec. I know I am getting spark to the plug, I've checked that. Someone said it could be a bad valve or a leaky lifter? I am hoping to do a compression test on #2.

So now Im left with if its something else. I think it might be an injector issue but im not sure how to tell. So if someone could explain that it would be nice (im gonna go search to hehe) but also if anyone had anything that could help me figure this out I would appreciate it thanks ahead

 

So, the idle is high when the car first starts, but as it warms up it drops down? What's your problem? Seriously, that's normal. When the car first starts it is running under warm up enrichment until the temp sender tells the ECM the engine has reached a predetermined temp at which the regular fuel map takes over. It's like the fast idle cam on a carb.

 

I don't think I have ever hear of a 3rd GEN not running like that. They are all supposed to idle higher for a few seconds, (This is contolled by the IAC)Idle Air Control Valve.... Then roughly 30 sec to 1 minute later it will drop the idle down to a set idle in the car. If you engine is already warm, the IAC will already be close to 0-20 count and your car will start at a lower idle already..

 

I am aware that it will idle down lol, but when it does it seems to stop firing on the #2 cylinder

 

Just out of curiousity, How do you know it stops firing on CYL #2? I would think if it is an LSx it would be the reluctor ring or a coil pack or Plug, But on a L98 style block, it could only be either a plug, wire or Distrib Cap. There is nothing from my knowledge that is different from the startup sequence until the vehicle goes into closed loop on the firing system that would allow it drop one cylinder at idle.

 

Tight valve Adjustment on #2

Bad injector check ohms when hot

 

Verifying a misfire is easy. Just pull plug wires one at a time and see how the engine responds. If it stumbles and runs rough then that cylinder was working right. If nothign changes you've found the culprit. If you have an MSD or other similar ignition be careful doing this. They can bite you with quite a bit of voltage.

If all the cylinders have the same effect when disconnected you do not have a misfire.

If you do have a misfire replace the plugs, wires, cap, and rotor. If one is doing it the others are not far behind. It's usually a sign of worn out ignition components.

If that doesn't cure the problem test the injectors as noted.

It could also be caused by a bad lifter that pumps down when it gets warm. Typically you would hear a tick from the lifter though if this were the case.

 

So the car itself will go into closed loop and shut a cylinder off?


That is the thing that gets me, I dont hear a tick from the lifter or the valve, and I just replaced the whole ignition system,(cap, rotor,coil,wires,plugs) Maybe im just dumb and its fine but someone said that its not running right(an exhaust guy) I had had bosch plugs in there and the mechanic said american cars dont like those plus they foul out easily if they get messed up from a loose wire. So I put ACDelco's in there. it seems to run okay to me, but idk

 

After it has been running "missing" on #2, shut down the engine and pull some of the plugs and see if #2 looks any different then any of the others.

 

TKO has it right... Been there done that, it works. Only way to know which cylinder is broke on a TPI without test equipment.

 

havent dont that. good Idea ha

Ill try the method then