Art-97-Z28

Some history. My 97 Z28 (all stock) was running OK, but with a bit of a miss. If I drove it everyday it would fire right up, however if I let it stand for more than 24hours it would behave like it was flooded and not start. The only way I could get it started was by removing the fuel injector fuse, then replacing it. This would take several attempts but eventually worked out and after blowing some smoke out the tail pipe, things would clear up and the car would go back to running and starting right up. Finally, one morning after dropping my daughter at school I went to start the car, it cranked, then stumbled and didn't fire up. On subsequent attempts it continued to act like it was flooded, but I was not able to get it started.

Changed out the spark plugs, wires, opti, crank sensor and tried a new pcm. Checked connections and grounds. Compression test was OK. Timing is OK. Has fuel. Has Spark. Cranks but does not fire up and run. The mechanics that have checked it out say they are stumped. Could a vacuum leak prevent the car from running completely? My car has been down for a month. I would really appreciate some good advice. Thanks.

Injuneer

Did you ever try starting it with the accel pedal on the floor BEFORE turning the key to start, and holding it on the floor until it started? If it started faster, that would have confirmed a flooded condition, since that procedure puts the PCM in "clear flood" mode.

You indicate you have fuel. Have you checked the fuel pressure under various conditions, including key on/don't start (priming pressure), turn key off, how fast does pressure bleed down? When it was running, did you check the pressure at idle, with and without the fuel pressure regulator vacuum compensation line hooked up?

Since it appears you may have leaking injectors or a leaking fuel pressure regulator, have you checked the fuel pressure regulator vacuum compensation line for wet fuel? Have you pulled the fuel rails up, turned the key to run to prime the pump (do not attempt to start), and then looked for drips from the injectors?

How did you verify you have spark? Have you had the IC Module and the coil tested? Have you scanned it for codes? What does the dash gauge show the system voltage is when cranking the starter?

Might seem like annoying, basic questions, but they have to be asked up front.

[EDIT] In fact I already asked some of these questions before???????

https://www.camaroz28.com/forums/fue...2/#post6911361

Art-97-Z28

I appreciate the advice Injuneer. Here are more details to answer your questions.

The gas to the floor trick never worked out. Only pulling the fuel injector fuse.

Fuel pressure is correct per spec at 45+ PSI.
As for fuel bleed down, that was checked and the pressure held beyond what the spec is, but can't remember for how long now.
Never checked pressure when it was running, as we've only gotten it to run for less than 30 seconds.


Fuel pressure regulator vacuum compensation line has not been checked for wetness.
4 of the 8 fuel injectors have been pulled and checked for leaking, none of those 4 were leaking, the other 4 have not been checked yet.


Spark has been checked with a spark checker, and pulled plugs and have watched them spark while cranking.
Have not had the IC module nor the coil tested.
Originally, when the car was still running, the codes on the computer were PO300 (random missfire) and PO441 (Evap emission control incorrect purge flow). We have not gotten any new codes.
Votage is above 12V when cranking, according to dash gauge. Almost always had the battery on a charger as we were testing.

One additional question. When visually observing the timing of the noid light on the injector it seems fire at the same time as the spark plug is igniting. Is that normal?

Injuneer

The injector should fire when the cylinder is on the intake stroke. At idle, and low loads, that holds true. But as engine load increases, the injector pulse width can extend over multiple phases of the 4-stroke cycle.

Both the injector timing and the spark timing are set by the PCM based on the cam position info from the Opti, so it's hard to see how only one of them could be corrupt.

Art-97-Z28

Thanks for the reply. So as I understand it you're saying that because "both the injector timing and the spark timing are set by the PCM based on the cam position info from the Opti" the timing of one relative to the other should always be correct, as long as the opti and PCM are working. The observations we made about the timing of the noid light relative to the spark appearing to be approximately the same were just done visually so I'm not sure if the naked eye would even be able to tell if the injectors were going slightly earlier or simultaneously with the spark? What is the delay between when the injector fires and the spark ignites?

If the timing was correct and based on the areas that we have tested (mentioned in the previous post), what should we tackle next to get this thing to run? Could the ICM or coils be the cause? What about vaccum leaks? How significant of a leak would you need to prevent the engine from running?

thanks again.

Injuneer

At 800 RPM you have 13.333 Rev/Sec. Two revs of the crank complete a 4-stroke cycle. (2 rev) / (13.333 rev/sec) = .15 seconds for the complete 4-stroke cycle.

Logically, injector timing would work backwards from the intake valve closing (end-of-event reference). At idle, your pulse width is typcally about 2 mSec, so the firing of the injector would be about 0.15/4 + .002 = 0.0395 seconds before TDC. Considering that the ignition occurs at idle about 20* before TDC, that 0.0042 sec would be deducted from the previous result, so the injector would be firing roughly 35/1,000th's of a second before the spark. Doubt you would notice the difference.

As engine load increases, you start firing the injector earlier, and at 6,000 RPM with an injector that tries to fire for 20mSec (100% DC) the injector would be open constantly, because at 6,000RPM the engine requires 20mSec to complete 2 revolutions of the crank.

Art-97-Z28

OK, at 35/1000ths of a second earlier it makes sense that they would look simultaneous. So it seems we have correct timing, good spark, correct fuel pressure and good compression. PCM, Opti and crank sensor are new. What should we look at next to get the beast to run? Does it sound like an Ignition Control module or coil problem? Could a vacuum leak prevent it from running? Where are these most likely to occur? You also mentioned checking the Fuel pressure regulator vacuum compensation line for wetness. What would wetness indicate and what issue could it cause?

Art-97-Z28

Would we still be getting spark if the Ignition control module was bad? What would be diagnostic symptoms of the ICM and /or coils?