Injector Pulse

[strkt_&_blwn]

Just curious if anyone can educate me on, what triggers injector pulse on LT-1 cars. 95 to be specific.

[Injuneer]

94-97 LT1's are full sequential fire fuel injection. 1993 is not, using a batch fire injection.

The injector timing is determined by the PCM looking at the low resolution signal from the optical cam position sensor in the Optispark distributor. The timing wheel in the sensor has 8 slots, each corresponding to one of the 8 cylinders. Four slots are variable length, which identifies 4 specific cylinders, and in between those 4 variable length slots are 4 equal length slots that identify the other 4 cylinders. In this way the PCM knows exactly where the cam is, and that tells it which crank revolution that each piston is on (two rotations of the crank are required to complete the 4-stroke cycle - intake/compression/power/exhaust), and where it is relative to top dead center. The PCM works backward from an "end of event" point, and opens the injector at the correct point to maximize the fuel flow while the intake valve is open.

That's how it determines when to fire the injector. The next calculation involves setting the pulse width, which is how long the injector stays open.

The PCM looks at the mass air flow from the MAF sensor. Again, this is only used in the 94-97 LT1's. The 93 uses "speed-density" to calculate the mass air flow, using an equation that determines the volume air flow from RPM, displacement and the programmed table of volumetric efficiency (VE), for variations in engine speed (rpm) and engine load (MAP). It then converts volume air flow to mass air flow by using the inlet air temperature (IAT) and manifold absolute pressure (MAP) in the perfect gas law equation.

From this point forward, doesn't matter whether its an MAF setup, or speed-density. Once the PCM know the mass air flow rate, it divides the mass air flow rate by the target A/F ratio to determine the fuel mass flow rate. It then looks at the programmed flow constant for the injector size, and calculates the pulse width required to provide the calculated mass flow of fuel.

The PCM uses a target A/F ratio of 14.7:1 during "closed loop" operation. This is the way the PCM operates for "normal", part load, part throttle driving. When the particular throttle position sensor reading and engine RPM indicates you want "power enrichment", the PCM switches to power enrichment (PE) mode, and sets the richer target A/F ratio required for peak torque and power. A couple things go into this determination, but stock programming yields a typical PE mode A/F ratio of about 11.7:1. This is the basis for the common belief that "out cars run rich"..... 11.7:1 is a very conservative approach, to minimize the possibility of lean operation causing detonation. Typically, peak power and torque are made in the range of 12.8-13.2:1 A/F ratio, and reprogramming the PE mode "target A/F ratio" tables in the PCM will net you a quick gain of 15HP, on a stock engine.

Its not done yet though...... During closed loop operation, the O2 sensor feeds back the results of the combustion calculation to the PCM. The PCM looks to see whether the engine is running richer or leaner than it is supposed to, and adjusts the short term fuel corrections to modify the calculation of the fuel mass flow rate. The "correction" is a binary number based on 128 as the "0" correction point. If the engine is running richer than its supposed to, the PCM sets the short term correction ("INT") to a value lower than 128, and uses the ratio BLM/128 as a multiplier in the mass fuel flow calculation to lean the engine out. If the PCM sees the engine is running leaner than it intended, it raises the INT above 128. If the condition persists, the PCM sets a "long term" fuel correction ("BLM" for block learn multiplier), which is a number similar to the short term correction. The BLM is stored in the PCM's volatile memory, and this is about the only thing the PCM actually "learns". When you "reset" the PCM by pulling the power, it returns the BLM's to 128.

There are 19 different "cells", each of which contains a BLM for the left bank of the engine, and another for the right bank of the engine. The cells ranging from 01 - 15 represent various combinations of engine load (MAP) and RPM. Then there are three more cells.... one for idle (Cell 16), one for open loop load conditions (Cell 18) and one for decel (Cell 17). Cells 17 and 18 are mathematically derived from the other 17 cells.

For any LS1 owners who have managed to read this far without falling asleep, your PCM works basically the same way. The differences are that the LS1 get cam position from a magnetic cam sensor, and crank position from a magnetic crank sensor. And the LS1 PCM operates at a higher processing speed, and uses a larger number of BLM cells. It has also been suggested that the LS1 PCM stays in closed loop at wOT, but I've never poked around the LS1 PCM to verify this.

Hope that clears it up...... there will be a true/false quiz in the morning

[strkt_&_blwn]

Yeah I think that you definately covered all the bases with that one thanks alot I appreciate it. If I'm right from what I gathered from that, the only way that you can actually lose injector pulse is a faulty optical sensor inside you Opti, a bad PCM, or a problem in the harness somewhere. Am I correct. Some how I lost injector pulse.

[Rob B (shoebox)]

Yeah I think that you definately covered all the bases with that one thanks alot I appreciate it. If I'm right from what I gathered from that, the only way that you can actually lose injector pulse is a faulty optical sensor inside you Opti, a bad PCM, or a problem in the harness somewhere. Am I correct. Some how I lost injector pulse.

Pretty much correct. Have you scanned for any trouble codes?

[strkt_&_blwn]

Pretty much correct. Have you scanned for any trouble codes?

Yeah i don't have any codes. But that would be because I wasn't thinking and unhooked the battery cable. So I erased what was on there. I replaced the opti spark and it still won't start. the optical sensor inside the old one got wet from a coolant leak. So then I went out and bought a NOID light and I don't have injector pulse.

[Injuneer]

Check the harness connector. The PCM shuts down the fuel system when it doesn't see the low resolution pulse from the optical sensor in the Opti. Did you check the harness connecor for damage and/or corrosion before you decided to replace the Opti? Generally, moisture in the Opti does not hurt the optical sensor, it causes the spark to track all over the cap. Check both ends of the Opti harness, including the connector at the Opti, and the gray one on the bracket on the passenger side of the intake manifold (bracket also holds the EVAP purge solenoid).

[strkt_&_blwn]

yeah I did look at the plug, the actual terminals look good but the connecter itself is broke. I had some friends help me pull out the engine and one of them broke the tab on the opti side. It broke off about half of the connecter with it. but it didn't seem to affect the wires any. i checked the dealer for a new one and they wanted $165.00 for it. That's WAY over priced since it's only a foot long and has four wires with two connecters.

I went back and checked the injector pulse, I don't have it all of the time. it's periodical. could a weak battery cause this?

[Rob B (shoebox)]

$35 for a 95-97 opti harness right here.

[Injuneer]

I can't believe anyone even attempts to buy parts from "a dealer". There are well-established sellers of discount factory parts, often at 1/2 of dealer prices. Shoebox has the links to two of these sources on his website. They get posted here several times a month. Don't waste you money at a dealer.

If you don't have the injector pulses, you should be getting codes for faulty injectors circuits, or for the missing low res pulse from the Opti. But you claim you have no codes. What are you checking the PCM with?

[strkt_&_blwn]

A Snap-on Scanner. I check it everytime I work on it. Everytime it says "No Codes Present OK To Start" I don't understand it either. I thought that I could only crank it so much with out it starting before it would throw a code. But for some reason I just can't get any codes. Unless there's something wrong with the NOID light that I bought I don't understand what the deal is. I actualy got it to read injector pulse on #2 injector for a little bit last night. Something must be wrong with the light or I'm just not getting good connection through it or something. But the problem just developed from sitting a couple of days. I'm completely out of ideas.

[strkt_&_blwn]

$35 for a 95-97 opti harness right here.

Thanks alot shoe box it helped a bunch.

[strkt_&_blwn]

Ok everyone thanks for the help on this I finally got this figured out. The Opti harness was old and cracked from heat, and the conector was broke partly so I went ahead and replaced it. When I took it out I noticed the connector like it had a bent prong in it. I used a mirror to look down inside the Opti and saw a bent prong. It was just bent enough that it was in between the plastic of the plug and the metal female part of the plug. so it was still making contact but not solid contact all the time. Which would explain the periodic Injector Pulse. So straightened the prong "which was an absolute pain with everything on the car" and installed the new harness. I swapped out the pluggs...........and WAMMO she fires right up. Smoked ALOT but ran ok. It had alot it needed to clean out. So all in all hopefully in a week or two I can get it on the Dyno and get it tuned, and then off to the track to see where I'm at. Thanks for the help I appreciate it alot.