diagnosing ignition system/opti

[Craig 94 TA GT]

I've run out of ideas ...changed the opti, plugs, wires, coil, and ignition module yet it still has a terrible low end stumble (the harder you hit the gas) and once in awhile backfires. Is there a sure way to test the whole system with this problem? I still have to fix my intake leak, but I don't think that's the entire cause of the problem. Thanks in advance.

[Joe 1320]

here is a proceedure to troubleshoot the ignition system........but, this is meant to diagnose a no start condition. Ideally, using a scanner to read what all the sensors are doing would be invaluable. A scanner should be a mandatory tool for those who work on their own cars. Here it is anyway......

Step 1. Connect a spark plug tester to a spark plug wire and
check for spark while an assistant is cranking the engine.
Check each plug independantly. this checks for open
spark plug wires. Spark in one or more than one wire
indicates the problem is in spark plug wires or optispark.
If no spark exists, go to step 2.

Step 2. Check for spark at the coil wire with the spark tester
while cranking the engine. A spark indicates the problem
is in the distributer (optispark). If no spark go to step 3.

Step3. Perform check on ignition coil.
a: Unplug coil electrical connector and check pink wire for
12v. If power go to b.
b. using ohmeter low scale, hook up the leads to the
primary terminals on the ignition coil. Should indicate
very low resistance. If not, replace coil.
c. Using the high scale, hook up one lead to the primary
terminal, the other to the secondary terminal. Should
not read infinite resistance. If it does, replace coil.

Step 4. Disconnect the ignition module connector and with the
key on, check voltage at terminals A and D on the harness
side of the connector. (pink/black and white/black wires)
If under 10 volts check for faulty circuit from the ignition
module to the coil or check for grounded external coil
circuit. If 10 volts or more, change the voltmeter to a/c
scale and measure voltage on terminal B while engine is
cranking. (white wire). If 0 volts, Have ECM checked. If 1-
4 volts, with the key off, disconnect the module
connector. Install a LED type test light on the positive
battery terminal and probe terminal C on the harness side
of the connector (black wire). The light should illuminate.
If not, check ground wire in the harness to the module. If
the test light is on, go to step 5.

Step 5. Remove the coil wire from the coil and ground it to the
frame. Connect LED test light into the optispark harness
connector red wire terminal C. The harness connector that
you will use is located on the passenger side of the
engine up near the fuel rail. You will need to insert a back
probe pin into terminal C. If the test light flashes when an
assistant cranks the engine, the optispark is suspect. If
no flashes, replace the ignition module.

If the above tests results are correct and there is still no
start, it is tough to to tell the difference between a
faulty module or optispark. Module = Cheap$$, optispark
=moderate $$$.

[N20LT4Bird]

I've run out of ideas ...changed the opti, plugs, wires, coil, and ignition module yet it still has a terrible low end stumble (the harder you hit the gas) and once in awhile backfires. Is there a sure way to test the whole system with this problem? I still have to fix my intake leak, but I don't think that's the entire cause of the problem. Thanks in advance.

Are you sure you have an ignition problem? It could be fuel.....have you taken a reading of fuel press. as the stumble occurs? It could also be fuel related but be caused by a sensor such as the MAF or ECT etc.

If your not sure, as Joe said, it is best to put it on the scanner and watch the timing advance, O2 readings and other sensors while the stumble occurs to get an idea as to what is happening.

[fastTa]

I think Craig just had most of his fuel system professionally replaced after the sugar incident. It sounds to me like a bad sensor or maybe just a faulty connection between the sensor and the PCM, or it could even be some knock retard.

Craig like said above you really should try to get a scanner on there and see what the PCM is doing while the "missing" occurs. Do you know what your overall timing is set at? You could be getting some pulled timing due to knock or even possibly some false knock reported. A sensor would really help to diagnose this. If you can, I would go get about 3-4 gallons of something like Motorsport 103 or any similar octane unleaded race gas and pour it in with some premium pump gas. Then go drive it for a bit and see if you still get that low end stumble. If you don't then you are very possibly getting some timing pulled because of the KS sensing knock. But again, a scanner will also tell you if you are getting KR.

Did you end up installing the roller rockers?

[Craig 94 TA GT]

Thanks for the info guys. Here's a link to what the car did during my commute to work.
http://s94938520.onlinehome.us/data/commute1.xls
I used Freescan here. The stumbling seemed to be more present past row 10700. I cannot find anything unusual except for the left BLMs which is probably from my intake manifold leak.

Kevin, thanks for elaborating on my fuel system. From the chart I'm getting 0 knock retard, and I installed the LT4 knock module awhile back. I still haven't installed the rockers yet because gmpartsdirect says the LT4 springs are "special order" and haven't shipped yet! Racenet also had a backorder on the chromemoly pushrods (but they at least have shipped). I ordered everything over 2 weeks ago!

Maybe it's worth to try putting the stock MAF ends back in?

[Craig 94 TA GT]

One other thing I've noticed was the IAC values. It seems to be a bit high according to what Fred said the readings should be. However, it seems to be right on at some points. Is it possible that a dirty IAC would cause a stumble at the low end, or does it only affect the idle period? Thanks in advance.