I have a 91 z28 5.7 tune port, it has been bored and camed. Built for 1/4 mile, roll cage and all. Built from the ground up. Most parts on it are aftermarket proformance. It will not start under its own power. I can pray either in it and it will start and run, not for very long and does not run well. Does not sound like its missing thou. Just running rough and ideling up and down. It will not start without being eithered. Sounds like a fuel issue right......But its not. I have replaced the fuel pump, even thou it has had constant and correct fuel pressure. I also replaced the ignition control module, cap and rotor, coolant temp sensor, redone the intake gaskets and cleaned the injectors, and replaced the key and switch. I have noid lighted the injectors, I have tested the compression on one clyinder and it was good. It turnes over really good. The problem just don't make sense, seeing as it will start and run on either, but not on its own. I'm a diesel/auto mechanic so I have covered the basics, I.E fuses, battery, vats. Any advice would be great. I'm at a wall here........

 

I see that you changed various ignition parts, but have you checked for a good spark at all plugs? Have you checked the spark timing with a timing light? May also want to check cam timing and valve adjustment.

 

I have checked spark at coil and and seems to be a good spark. I have checked the wires to see if they are getting spark and all are hitting good. Its got all MSD ignition wires and coil and seem fairly new. I have checked timeing with a light. But not checked the valve adjustment. When I do start it, it does not have a nock or valve slap or any type of ticking like you normaly hear with that issue. It would seem to me that if it was a timeing issue it would not start at all. If I remember correctly when I put the new rotor on, I adjusted the timeing to make sure it was correct. Let me ask you something, would a oil pressure sending unit cause one not to start???? Just was wondering. I did get the scan tool to link to the computer, and If I can remember I belive the sending unit had pressure showing on the computer. I've done so many tested shooting in the dark, that its hard to accuratly remember.

 

Now about the cam timeing???? How could it be the cam timing? I don't know how its done on the tune port but if its done with a chain than its either right or wrong or worn? See what I'm saying? I just cant see the cam timeing to be the problem. Unless it was way way worn....To the point its throwing the timing off that bad. But I don't think it would run as good as it does when I do get it running. It don't sound bad, maybe some stumbble and rugh ideling, and the ideling up and down. But the throttle response is great and does not sound to be missing or anything. I'm not the smartest mechanicn, just going off my own knowlage.

 

Also, this may help. Its a aftermarket motor, built at a performance engine shop in springfild. Can't have many miles on it. When I redone the intake. It looked like brand new motor. I mean no deposits nothing. Clean. Didnt even have to scrap off the gasket. Just peeled off. So I imagine the internal parts are all new.

 

Just throwing in some ideas. The oil pressure switch provides an additional path for current to the fuel pump, but the contacts only close after the oil pressure goes over 5 PSI. When the engine first starts, power to the pump is only through the fuel pump relay. The cam timing could be off if the engine was assembled improperly, like the timing marks not aligned. If the rockers are adjusted too tightly the valves would not close all the way, and that would make the engine spin easily but hard to start. VATS prevents the starter from cranking the engine and tells the ECM to disable the injectors, but the starter is working and the NOID lamps are blinking, so VATS is not the problem. Also make sure all sensors are working, especially the MAP and ECT as these two tell the ECM how to adjust the air/fuel ratio at start up.

 

Thanks for the ideas, I've looked up the map sensor specs and will test it. I've already replaced the ect sensor. So i'll test it and get back with ya. It does not have any codes in the ECM but I know how that goes sometimes so I've been testing sensors and circuits like crazy lol

 

Sensors can go bad and give the wrong info to the ECM. As long as the ECM gets some sort of data it thinks it is OK and won't set codes. Test the sensors with a scanner and see if the info they send makes sense for the conditions. Something like a demented ECT showing -20 while the air temp is +75 will make the engine run really bad.

 

z28pete has you on the right track, your going to be hunting sensors, Erratic Idle/shut off is a common issue when the ECM is getting wrong info

let me ask this, seeing as the motor has been bored and has a cam, has the ECM been tuned?

I hunted down an issue where i could get my RS to run but the Idle was extremely erratic and if i didnt keep my foot in the gas when i was stopped the car would shut off, Changed just about every sensor, vacuum hose and EGR and IAC valves, nothing worked until i swapped my ECM with a Re- manufactured one from the part store, everything was golden after that

 

As far as I know the computer has a hyper chip but I have not messed with tryin to get to it. The plug is in the right fenderwell. And has been coated with somthing so I didnt try and unplug it. As far as the easily seen stuff the sensors look good on the scan tool. But when it idels its up and down. Plus when you put a load on it, it drops RPM bad and Idels even worse, up and down. But still thats after haven to start it on either. But you can drive this thing all day long on the highway. I know the coolant fans are not kicking on but as soon as you go to stop after drivin it for a while it will die and you cant start it on either. I see problems all the time come in the shop. But only mechanics get to have problems like this lmao. Still have not got the IAC valve yet, but I don't think thats the no start issue.