1994 chevy 2500 5.7cid erratic timing

**TRBBenicia** · 06-04-2013

So being new here I hope this is where I post this, if not tell me where. thanks
so this truck has had a few mechanics trying to fix this thing. when driving at a steady speed and the lighter the throttle the more noticable, it has slight surge and can hear popping thru the exhaust.I can also get it to miss/pop in neutral, accel the throttle quickly and hold it in the 2500 to 3000 RPM range. it has had two distributors replaced,cap and rotor, spark plug wires and plugs. the throttle body, map sensor, pcm...but the original eprom was used. checked the timing chain for slop, non found. cam was checked for being flat. was ok. valve springs are new. leak down chk was 10-12% all even. so when I chk the ignition timing with the connector connected,at idle timing appears to be adjusting properly.(with the connector disconnected the base timing is correct). with it connected, when I get it to missfire the timing is very erratic and goes very advanced and sometimes the T-light will not flash for a second or two. so since everything that controls timing has been replaced...except the eprom. I am leaning towards that. Is there anybody near Benicia on here that has an eprom i can use to verify that. or does anybody have a different idea. I am open to what ever will fix this pos! thanks for any help you can give.
p/s I have a movie capture from my modis of the secondary pattern which shows the firing line moving way up on the kv line when it misses. If it comes to that I can try to put that on here, never did it before, I just bought the modis.

**EagleMark** · 06-04-2013

Were the new distributors AC delco units with the EST module? Aftermarket distributors are not that bad but EST modules are.

Watch the MAP KPA when this is happening, should be a smooth line, if it has dips and swings this changes timing on spark table. If there's any spark plug wires near the MAP sensor or wiring? Move them.

**1project2many** · 06-06-2013

Disconnect timing bypass and try again. Computer does not control timing with bypass disconnected. Look for loose grounds, broken wires at coil, loose terminals at distributor, etc. Move wires with engine running see if problem occurs. Watch primary side pattern with modis see if it goes crazy like secondary. Install test coil and try again. Put modis on fat 12V wire to coil and see if steady 12V when problem occurring. Look for wires laying on engine brackets, etc making contact as engine speed increases.

Just something to think about, I've had trouble with weak magnets in the pole piece. Went through three reman distributors on one truck and finally bought a new shaft / pole piece from GM.

hth