So this is a 1995 LT1.

When I crank it does not start no matter how many times I try, or how long i crank for, however If I use starting fluid, it will fire up just fine.

The fuel pump is brand new, it was replaced as part of the diagnosis. I get 41 PSI at key on and about 43 during cranking, If I start it with the starting fluid it stays at about 39 while running.

I get spark at both the coil and cylinders during cranking, I checked this with a spark plug grounded to the neg battery terminal.

The battery is brand new 800 CCA and the alternator was rebuilt yesterday at a alternator shop.
I suspect that the problem may be weak spark during starting, but am not sure.
When I crank, the voltage (read at the alternator with a DMM drops to about 10 volts) If I use starting fluid and it cranks the voltage immediately jumps to 14 and stays there with the engine running. The battery can be disconnected with no ill effects.
So, this leads me to believe that the alternator is not working while the engine is cranking (should it?) If the key is in the run position but the engine is not running, the alternator emits a buzzing noise from the area where the wire plugs into th eback, not the output , but the wire that trips the check gauges light. Is this normal? Also is the drop in voltage during cranking normal? I even tried hooking jumper cables to another running vehicle while cranking, but the voltage still drops.

So any clues? Suggestions?

How can I test the strength of the spark? GM makes a spark tester that is used for diagnosin this problem in the service manual.

How can I test for injectors firing, can I hook up a test light to the injector harness to test for the pulse from the pcm, or will this be too fast/ too low voltage to work? Even then this still wouldn't garantee that the injector itself is firing.

Thanks!

 

I just had the same problem yesterday.After playing around with changing coil wire,checking plugs wires for being loose,ect.I finally had the beast towed to the dealer.I thought it might be the opti,turned out to be the connector(green thing)and wireing harness that goes to the map sensor on top of the intake manifold(black thing).I guess they get all brittle with time.They also cleaned the throttle body,and replaced the wire to the idle control valve(maybe that was it all the time).The beast screams now big time.Kinda like a new car.Have somebody wiggle the wires there on the pass side of the manifold while you are cranking.Hope this helps.

 

Normally when someone says it starts on ether they follow it with but it shuts right off. This means fuel problem and usually fuel pump. But in your case fuel looks normal. I doubt you have an injector problem since it does start and run ok.

My suggestion is you go back to the basics. Remember it takes fuel, spark and compression to light up. You got spark, and you got fuel (we think). Try checking compression or better yet use a leakdown tester.

A compression problem is the only thing that fits in with what you said. However, I have a problem thinking what would cause enough cylinders to go low so it wouldn't start. Have you done anything recently to say valves or push rods or rocker arms? Even if you did you should hear something. If compression was low it should be running crappy also. Got to tell you this one is a stumper.

I'll be thinking and may get back to you later have to go to work
In the mean time check compression

good luck

 

thanks for the responses.

Turns out that I was gettin only 9 V at the underhood relay box causing the injectors to have problems.

I discovered this after scanning the engine while running and seeing 160 BLM's which pointed to a huge fuel problem.

Thanks.