Hi i just recently re-did my lt1 in y 1995 z28. i went to start it up for the first time and the starter engages, whines/spins fast, and the engine is turning but it just keeps cranks and after about 20 seconds it backfires like a cannon. i check the wire at the ignition coil and it sparks and at normal speed. i pulled 2 wires and they would spark but only every few cycles much slower then it should. and then i pulled a third wire and it didn't spark at all. i checked the grounds and they all are good. my questions is could it be the opti or the ignition module? i already checked cam and crank gear timing and that is fine so could i also have improperly put the optispark on?

 

Did you replace the Opti? If so, did you index it correctly to the cam? If replaced, what brand, and did you loc-tite the rotor screws? What exactly was included in "re-did my LT1"? Have you scanned it for codes? Both the cam position sensor in the Opti and the ICM can set codes that do hot turn on the SES light.

 

I did not replace the opti. What I did was I bought a new pre assembled bottom end and put on all new gaskets and new heads. The only thing I found when I ran a scan tool was that the coolant temp sensor was reading -40 degrees and the desired idle was slightly over 3100 rpm. Would this temperature reading cause this issue as it is dumping fuel in and may be retarding the timing?

 

I'd repair the CLT sensor and/or the harness to eliminate that problem. The PCM does enrich the A/F ratio when it sees a cold start. Did you get any codes?

Sounds like it may be a loose rotor on the Opti. Did you check the plug wires for the correct order? I'm sure you figured that the spark rate at the plug wire will be 1/8th of the spark rate at the coil wire.

I've seen the 3100RPM number in data logs, usually before the engine actually starts.

 

It didn't have any codes. I'm going to fix the sensor and connector and check plugs wires today. If that Doesn't fix it I will check the rotor on the opti. I hate pulling that thin off.

 

Ignore the high idle speed reading. You'll get that before engine start. It doesn't mean anything.

You might have a cap/rotor issue, carbon tracking, plug wire problems or just a bad opti.

 

Well I hauled it up so a mechanic and they are telling me there is no compression in any of the cylinders. I don't get how it can back fire with no compression.

 

Maybe you set the valves too tight. I kinda missed the "spins fast" comment until I went back and re-read the first post. What method did you use to set the valves?

 

The repair manual I used said set to 0 lash then turn 360 degrees but I feel like that might have been too much.

 

thats for the rocker arms

 

Not many people use a full turn. Back all them off by half a turn and see what happens.

 

I will try and get that done tomorrow. Thanks for all help. Most useful site I have found yet

 

So adjusting all the rockers didn't get it to start. But it doesn't back fire anymore. I'm about ready to give up

 

What about compression?

 

I'm gonna have to get a test kit and check it.