I'm working on a 90 GMC truck 6.0 liter (366) running on propane. It has the dual connector type distributor etc so this should be a standard GM type problem. The truck has lack of power. I've troubleshooted it down to having no ignition advance at all. Base timing stays the same at all rpms. It retards about 1 degree on accelleration but that could be a sloppy chain. When you disconnect the ignition bypass wire to set the timing, the timing doesn't change if the wire is connected or disconnected. I swapped the ECM from another similar truck, no change. I swapped the ESC module, no change. Because the truck has been converted to propane, it has something called a Superfix wired into the system. This is supposed to eliminate fault codes from the EGR, O2 and knock sensor. All 3 of these parts have been removed from the system. What's left? Could it have a bad distrubutor? There are no flash codes. The bulb in the dash could have been removed when the propane conversion was done. Gimmie a good old HEI with weights and springs. This new stuff doesn't work.

Stephen,

When you reconnect the ESC bypass connector, are you certain there is a good connection? Is there a 5 or 12 VDC signal at one of the wires? As long as the ECM was removed, is the PROM from the exchanged ECM the same (do BOTH trucks have the LP conversion?

Are the connections at the HEI amplifer/switch module clean and tight?

I hate to advocate outright parts swapping, but as long as you have a donor vehicle and suspect the distributor, you could exchange them as well. It doesn't sound like the distributor itself is the problem, however. The ECM controls spark advance in concert with the ESC module input. I'd check all the connections first, including the ESC module.

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Later,
Vader
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"No matter how hard you try you can't stop us now"
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I never checked for voltage at the bypass connector. Both trucks are similar with propane conversion and both have very few differences that the ECM would need to worry about. The one I'm working on has a '90 mark IV block and the one I'm using to get parts from has a Gen V block. Other than that they're both GMC Topkicks. The truck I'm getting parts from doesn't have the "Superfix" box installed and has all the sensors.

On Tuesday I'll see if the O2 and knock sensor wires have been bypassed properly before I do a distributor swap. I only had a chance to visually check them before quitting time and it looks like the wires were cut and spliced into the new Superfix harness but someone has removed the knock sensor since then and installed a recurculating heater in the coolent port and one of the o2 sensor wires in the same area has been cut. I'm suspecting the system may be getting a false reading from the missing knock sensor and the ECM or ESC is in constant spark retard mode. It's also possible the Superfix could be defective causing a false reading.

When the knock sensor detects a knock does it open or close the curcuit? I could ground or cut the wire to simulate a good sensor.

Problem fixed.

I checked for voltage at the bypass wire. None. Found a schematic for the wiring and traced all 4 wires from the distrubutor including the bypass wire to the computer (ecm/pcm). Had low voltage on the red/white wire only. Continuity through all the wires.

Changed the idnition module inside the distributor. Everything works fine now.

I think what had me stummped the first time is that I'd never have guessed the module because the engine still ran. I guess only part of it failed since the magnetic pickup and coil connections still worked allowing the engine to run. The 4 wire connector is how the computer controls the spark curve and advance. That part must have screwed up.