sdockery327

I have an ongoing issue with my camaro. Its a 94 LT1, 90K miles, basically stock. I have been chasing a problem for about a year and nobody can figure it out. I start my car and it idles fine, but as soon as I increase the rpms above 1500 or so, it backfires once and the ses light comes on, and won't backfire again until I shut the car off and start it again. The code that comes up is for the high resolution pulse from the opti. The problem is... I have replaced the opti TWICE and it still happens. (current opti is from MSD) I checked the wires with an ohmmeter from the opti all the way to the computer and nothing was shorted or open. I tried replacing the computer, plugs, plug wires, ICM, changed the coil twice, checked the wiring for the coil, checked for a voltage leak from the alternator, checked all the reference signals, I couldn't find any bad grounds, no bad wires, nothing. I even connected an oscilloscope to the distributor to graph the signals coming from the opti to the computer, and both the high resolution pulse and the low res looked perfectly fine on the graph. The signal never gets distorted in any way. As far as I can tell, everything underneath the hood is functioning perfectly. Any advice would be appreciated. Thanks

jaycenk

iTrader: (1)

What is done to the car? Exhaust? cats gutted? headers? I would start with checking the Air injection system for leaks and then make sure fuel pressure regulator isn't leaking through the vacume housing and into the intake. Pull the vac line off the regulator and make sure it does not leak any fuel where the vac line hooks up to the regulator. Another thing maybe to check the knock sensor wire to make sure it has a good conection to the knock sensor. If all else falls check compression

jaycenk

iTrader: (1)

Another thought if you get no where is since it's a 94 is to find someone with a cable and data logging software and just data log it. When a value is out of rage on the logging software it will show up in red and may give you what causes the check engine light. It may or may not be related to the back fire.

sdockery327

The car is all stock except for the parts I have thrown at it to try and fix the problem. so far I have done 2 opti's, 2 coils, O2's, plugs, wires, ICM, PCM, and water pump. My guess is that something in the electrical system is messing with the computer. Before I put a new pcm in, the car would actually stall if I had all the lights on... it doesn't stall anymore, but the code for the high resolution pulse keeps coming back. I think its just throwing that code because its the first thing the computer realizes is wrong. The optical disk in the distributor has 360 marks around the outside of it, so thats 360 pulses it sends to the pcm per camshaft revolution, or 180 pulses per crankshaft revolution. So, at idle (say 600rpm to make the math easy) the crankshaft is turning 10 revolutions per second, meaning the opti sends 1800 pulses per second to the pcm. So... if there is a voltage interference in the computer for even 1/100th of a second, the computer could miss 18 pulses and throw the timing off by 36 crankshaft degrees. After the computer thinks the signal is gone (the signal is still there because I can watch it on my oscilloscope) , it batch fires the injectors, which would make the backfire go away. But thats just my guess and I can't find anything wrong with the electrical system.

Big Al

iTrader: (1)

Is the scope a real oscilloscope or something else? The reason I ask my friend has a very expensive Snap-On Vantage Pro meter that he thought was a scope. It misses anything high speed.

Hook a scope across the battery, start the car, let it idle for a few seconds and then bring the RPM up and see what happens. Let me know. I'm assuming you know how to set it, I want it AC coupled about 1 volt per division. Look for some kind of spike up or down.

Have you tried to watch the pulse train at the coil primary under the above conditions? Might be better to look at the ICM on Pin B. Will be lower voltage and safer.

Al 95 Z28