2002 Z24 Cavalier - won't start...sometimes...

[excel74]

My car is totally unpredictable these days - when turning the key in the ignition, it sounds like it's going to turn over but just doesn't. This only happens sometimes. I never know from one try to the next whether I'll be stranded again, or able to be on my way.
The battery is strong, and I've had the fuel pump module replaced as that was thought to be original problem. I've taken it to 2 different mechanics. The car wouldn't start once for each mechanic, but after that it started without problem for them so they were unable to diagnose the issue. Both places hooked the car up to their computers (which computed "no issues" and checked for loose wiring etc. and found nothing.
I've had to call a tow truck 3 times in 3 weeks...and will be calling again tomorrow AM as soon as the neighbourhood mechanic opens up.
Any ideas?

[J-Ri]

Welcome to AF! This thread belongs in the "problem diagnosis" forum, in the future please post in the appropriate forum.

This is a hard one, the only time you can test anything is while it's acting up. That leaves you with two choices, either buy/rent the test equipment so you can test it when it acts up or leave it at a shop for possibly weeks until it acts up enough to be able to test it.
If it's got a tachometer (I think all the Z24s do), make sure the needle lifts off "0" while you're cranking. If not, then there's a problem with the crank sensor or it's wiring. Check that the fuel injectors are being commanded on, they should have constant power and a computer controlled ground. Also check fuel pressure, even though it's got a new pump it's possible that it's bad. A quick test for both is to have someone spray some intake cleaner in the air intake while you're cranking it. It should fire up for a second if you have spark. If it fires up, you have spark. If it doesn't fire up, it doesn't necessarily rule anything out. Just get the stuff that warns it's "extremely flammable", the stuff that's just "flammable" isn't always enough to get it going. If you don't have spark, then check that the ignition module is getting the signal from the crank sensor. It's also possible that the fuel pump relay is bad.

[JStumpalump150]

i dont think my z24's tach lifts off zero until it stops cranking and is running

[J-Ri]

Could you confirm that for him? My '04 shows it while cranking and I've noticed various other years that also do it. You may have to hold the gas pedal to the floor to get it to crank long enough to be able to tell. On mine I only noticed it when my ignition module started to fail and it had a long crank, normally it appears to do what you described.

[JStumpalump150]

mine is a 98 z24 and i just checked, the tach does not move until it is started, in fact it is below zero, lol. but mine also fires up almost instantly after starting to crank it, so maybe if you crank it longer it will jump around.