Strange 1996 Tahoe ignition sys problem

1996 Chevy Tahoe 5.7 EDI system. Engine stopped after shifting into drive. Tried to restart but engine only cranks – does not start. Noticed it was not the usual constant speed crank of the starter motor, but rather had a strange ‘loping’ sound to it, with a backfire on one occasion even.

Checked all fuses and they were good. Fuel pump relay energized and fuel pump works. Next, started checking ignition and this is where things get strange. Pulled plug wire off number 1 cylinder and checked for spark. NO spark on 1, and engine did not start. Checked next plug (which is number 3 cylinder on this engine). Got a good spark on number 3 and engine still did not start….UNTIL I happened to move plug wire away from ground (which removed the spark) and then the engine STARTED and ran fine (except for the fact it was only on 7 cylinders at this time now)! I wasn’t sure what happened so I repeated this and unplugged and secured the number 3 wire away from everything else and tried to start the car. It starts every time like this. As soon as I reconnect the number 3 wire to the plug, it doesn’t start – just the strange loping crank.

OK next I replaced all the plugs with brand new ones. Exact same symptoms. Next I swapped the entire plug wiresone for one from 1 to 3 to make sure the 3 wire wasn’t the problem. Using the other wire did not help either. Same symptoms. Next I checked all wire connections at both the plug ends and the distributor cap ends for security. All seemed tight and fine, and the wires appear fairly new. Measured the resistance of one random wire and it was fine. Next I removed the cap and checked all the contacts and rotor button. All contacts seemed to have a fine white glaze or ‘coating’ but it was uniform throughout. No one contact seemed different than any other. Took an emery cloth and lightly wiped each contact as well as the tip of the rotor.

Finally I checked the spark at each of the 8 plugs twice – once with the number 3 wire attached to the 3 plug, and once without the number 3 wire attached to the 3 plug. At each plug location, on EVERY start attempt using that procedure, I would see NO spark on the wire for each plug while the number 3 wire was attached to 3 plug, and the engine would just crank but not start. And on EVERY start attempt for each plug, with the number 3 wire DISCONNECTED from 3 plug, I would not only see a good strong spark at that plug, but the engine would start and run fine.

So what I surmise from all this is that somehow, whenever the 3 wire is attached to the 3 plug, it ALONE is firing (hence the ‘loping’ sounding crank and one backfire) and all the others are NOT receiving any spark, and when 3 wire is DISCONNECTED (or not allowed to spark) ALL the other 7 plugs fire just fine and the engine runs.

It’s as if when 3 fires, the discharge seems to ‘steal’ all the voltage from the other 7 plugs. I am at a loss as to what is happening. Hope you can help!:banghead:

The plug wires should be replaced ...also measure the fuel pressure . Not sure what pressures you should have use a pressure guage ....

plug wires over time increase in resistance then the arcing all over ...also the wire covering breaks down, cracks and gets damaged when not properly routed,,,

If the wires are bad there is a good chance that the ignition module is bad...

Hello, tonga51:

Sorry to hear that you wasted $ on plugs, and whatever. Never mind fuel pressure or anything injection-related, unless backfires have caused new problems; fuel pressure, ign. module, plugwire set (you said you changed and/or swapped them, right?) and numerous other things would not limit their effect to only one plug, cylinder or circuit.

Don't get me wrong; I'm no expert on these CMFI/SFI/EDI SOBs....I'm in the middle of my first such project....but fundamentals don't change. Also, if you've already found the problem, then I apologize for wasting your time with this.

Look at the top of your distributor cap, orienting the coil-wire nipple to the lower right and the #1 contact (not the nipple) at about the 10:00 or 10:30 position. Note that the #3 nipple becomes a 'speed bump' that curves under/behind the #7 contact, then up/forward to, and turns right around, the center (coil input) contact and heads for the #3 contact at about 9:00.
If there is a carbon-track in the dielectric between the coil contact and the #3 speed-bump (and there's a 90-degree arc in which one could form), ALL spark pulses would divert to plug circuit #3 and ground after jumping #3's gap. Remove the ground circuit (plug wire) and the remaining pulses follow a path of least resistance and jump to ground at their intended plugs. Leaving #3 plug wire ungrounded/open and running on just seven cyl's. will get you home in an emergency, but will introduce an excess of unburned fuel to your left-bank catalyst and could conceivably damage your ignition module (those tens of kilovolts, having no place to reach ground, are going to turn around and try something else).

The guy at my favorite parts-house says they sold a ridiculous number of these caps during the first few years of production because they were made too thin....you may have one of them.

The Tahoe I'm working on had a leaking Fuel Pressure Regulator which allowed liquid gas to remain in the plenum instead of all going to the intake ports where it belongs, and a backfire did the rest....but that's a knot for another thread.

I hope this has helped. If not, let me know; I'm not above being educated.

GSGregg