dapickupman

Hello Everyone, I have a 1996 LT4 6 speed. I went to start it this morning and it turned over but did not crank up. Fuel pressure is at 45 psi, I can spray gas down the Throttle Body and it will fire right up. I took a noid light and I have no injector pulse. The Injector fuses are good and so are the PCM fuses. What pulses the injectors on these cars? Does anyone have any other ideas? Thanks in advance!

joe paco

there is a thred here by Cliff Harris, "l98 bank fire debunked," which applies to lt1, some of it. the ecm grounds one wire of the injectors to initiate the coil.
If you have a steady light, no blinking light, it is possible that one inj is shorted. you can ohm them to see. also chk all fuses, since ecm circuit goes thru airbag, at least on earlier c4.

if no light, you can check one inj harness for 12v, one terminal to grnd, pink/blak on 91. that's a start. the green or blue is the ecm grnd and should have 12v also -with at least one inj plug connected. should be open if all inj plugs disconnected.

read the thread by cliff

joe

add***forget to mention the obvious -ecm uses pulses from dist to fire injectors, but it runs with start fluid so that probably is not the issue. not acquainted with the opti, but I believe it is the same.

Phat98

What is your Security doing, blinking or solid?

Search for VATS in this forum, it may point you in the right direction for troubleshooting purposes.

dapickupman

The security light stays on solid, until it runs the ignition self check and and goes off. I am going to start some probing tommorrow and get back with ya'll. Thanks!

lt4red

PCM uses the low resolution pulse form the optispark to fire the injectors. If your fuses are good, the harness that runs from the passenger side of the intake down to opti is a place to start. It has been known to lose connection. This is what connects the PCM to the opti. It has a 5 Volt supply to the opti, a ground, and the low and high res opti pulses.

Another thing to check is that the grounds to the PCM are good. As described above, the PCM grounds the injector to make it fire. All injectors should have a full 12V on one side when the key is turned on. I don't remember where the PCM grounds for sure, but I think 1 of them is at the back of engine where the transmission bellhousing bolts on.

Cliff Harris

Check the voltage on the center pin of the TPS. The voltage should be below 1 volt.

The ECM will turn off the injectors if it thinks you're in "clear flood" mode, where the TPS is at 70% or higher. That's 3.5 volts on a 5 volts system. I don't know if the later PCMs use 3.3 volts or 5 volts internally, but it will still be 70% relative to the reference voltage on the TPS.

In 1996 the injectors are fired individually, so blanket tests (one injector means all are OK/bad) doesn't work on these cars. Generally, if one works they all will work, but that's not 100% guaranteed. Check a couple to make sure you're OK. One on each bank is a good test (and also checks the fuses).

John Robinson

I second this information. My TPS went bad and did this. Unfortunately I bought an ECM that I didn't need. My mechanic finally did what you did and sprayed some fluid into the TB and it fired right up. Thats how we new it was the TPS.

Snipescastle2

I found that a blown fuse on either "INJ A" or "INJ B) i the fuse block will cause the same thing, make sure to check both fuses with an ohm meter, I've been fooled before with blown fuses, may look good to the naked eye, but use the ohm meter to very continuity, HTH.
Ben Smith
p.s. a set of "Noid lights" helps you see if the injectors are are pulsing as well

hitmanpty

Firt think is to understand how the fue o.sector work on your car. I recent had problema with my.
Basic think i lean. On site of the injector is all was getting 12 volts the other site nevera Have ground intro the ecm give ground . At this time the injector open a Let the fue past

dapickupman

Ok guys, an update to the fun. I replaced the computer and it will crank up and run, just not great. I can hear popping occasionally through the intake and it idles way too low sometimes. On top of everything else, it looks as if someone dumped water in my tank...........so after cleaning all the water out and putting in some fresh gas, it runs like this. If memory serves me right, when the opti is acting up, they will run like this.........any other ideas are greatly welcomed!