1994 z28 WILL NOT FIRE

[Lotus49]

I had the whole front of the motor off. timing cover, optispark, radiator, I put it back togeather, im trying to start it but it wont kick over, it sounds like its almost firing but wont catch. i checked the wires they are in the right spot... Could it be my timing? how can i check with the optispak. Also anyone know where i can get photo's of the main vacuum layout .. the AIR tubes, and all thjoes hookups.. thanks

[Chevyracincamaro]

i thought the timing on the opti-spark was computer controlled. i dont know, but make sure all your connections are hooked up, like coil and stuff...

[Lotus49]

yeah they are hooked up... but it backfires and makes a "thud" sound when its being cranked... it def sounds like timing

[Chevyracincamaro]

that what it sounds like to me, but i just thought that was the thing about optispark. i have no clue how to check that, sorry...

[Lotus49]

COuld a bad vacuum leak.. or a air hose be hooked up worng.. cause that?

[twdz28]

OPTI and Spark Test


The opti has two functions in the spark process. The first thing that happens is as the cam turns, the optical section of the optispark picks up the signals by the rotation of the shutter wheel. The pulses are sent to the pcm via the optispark electrical harness. The pcm processes the signals along with other sensor input and determines the proper time for the coil to fire. The pcm sends a signal to the Ignition Coil Module (ICM) and it, in turn, causes the coil to fire. The spark from the coil travels through the coil wire back to the secondary ignition section of the optispark (cap and rotor), to be distributed to the proper cylinder.

If the opti is never sending the signal to the pcm, the pcm will never send a signal to fire the coil.

Here is some testing you can do. Refer to this diagram.
http://shbox.com/1/95_ign_system_schematic.jpg

Disconnect the ICM connector. Leave coil connected.
Turn key to ON.
Check for dc voltage with a digital meter at harness terminal "A" to ground and and also "D" to ground. Note: Use a high impedance meter (at least 10 megohm) when dealing with the pcm.
Result should be 10v dc or more on both terminals. If you get no voltage, use the diagram and chase back toward the coil and the ignition fuse. Power for the ICM comes from the ignition fuse and through the coil, so any of that could be bad.
If you have good voltage, switch the meter to ac scale and connect test leads to terminal "B" and to ground. Observe meter while cranking the engine. You should see between 1 and 4 volts ac (those are the pulses that trigger the coil to fire).

If you don't see the proper ac voltage the problem could be the optispark, the harness to the optispark, the PCM or any of the wiring in between. Visually inspect all the connections you can get to for poor contact or corrosion.

[Lotus49]

ughhh.. i am getting spark.. but it sounds like its at the wrong time :P

[twdz28]

plus there should be an emissions sticker on your radiator cover that shows the vacuum hose routing.

[Lotus49]

yes there is but it only shows pcv and stuff like that.. i need the hook up to the smog pump and things like that..

[twdz28]

not having your smog pump or egr hook up will not cause the problem you are having...

check engine light??? if so whats the code???

[Chevyracincamaro]

lol, i dont think he believes its causing the problem, im pretty sure its a seperate issue that he would like resolved along with the timing...

[Lotus49]

So how do u think ci could resolve this timing issue... its not like i can twist teh cap.. and there is no code

[twdz28]

i had the same problem with my z when i put the motor in, the funny thing is it was the maf sensor...
really there is no way you put the timing on wrong its almost impossible unless you did not have the points right when outting the chain on but I don't think you did that... Check MAF sensor and your Knock sensor... If you need test procedures let me know... I am going to scan the vacuum diagram for you on the A.I.R. and email it to you... Please give me that addy...

[Lotus49]

[email protected] thankyou for going out of you'r way so much to help