TBI 350 jumpy timing

[Colorado2500]

Truck has had a rough idle for a while now, but other than that, it ran well.

Recently, it begin to idle so poorly that it would barely run and would stall. Diagnosed as a bad EGR valve, valve was stuck open with bits of something. Replaced EGR valve & solenoid as well for peace of mind.

After those, it starts (after random starter failure between there), and runs but the idle is erratic, and stills runs rough but could be driven and hasn't stalled since. I've narrowed the issue to jumpy timing, with the tan/black wire disconnected it idles between 0 degrees and about 4 degrees retarded but is all over in there. I noticed it sounds normal when it jumps to 0 degrees (makes sense). Only pulling ~15" of vac at idle.
MAP and coolant temp sensor check out.
Harris at tbichips.com says on his tbi troubleshooting page jumpy timing is likely a bad pickup coil or magnetic reluctor ring. Under my cap, stuff looks a bit corroded, should I just go ahead and replace those two parts?

Unsure how to proceed, I can't find anything to test either component. Any advice is greatly appreciated.

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Edit: other notes: no vac leaks, idle condition much more pronounced when engine is warm/hot.

 

[Colorado2500]

Did some more testing.

This time with the timing wire still connected, idle the timing was all over from 16 degrees to 20 degrees. I heard the ecm controls that at idle and some moving around is normal.

However I then revved it to about 2000 rpm and watched the timing hold steady but then jump and could hear the engine stumble when it jumped, no good.

Don't think the timing chain jumped since it would only affect the timing one way, not erratically so it looks like the pickup or the magnetic reluctor in the distributor, anyone have any input?

Probably going to just buy a new GM unit like Harris at tbichips.com suggests, and to try to avoid any weird other headaches, only it's $460. Ouch. Anyone have a cheaper recommendations or advice?

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[89RCLB]

Buy the pieces parts and rebuild it, there's videos on YouTube for rebuilding a dizzy.

 

[Colorado2500]

Buy the pieces parts and rebuild it, there's videos on YouTube for rebuilding a dizzy.

True, I was just watching one a few mins ago actually. Doesn't look too bad at all.

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[rydeon]

I know this is a shot in the dark. But my girlfriends son and i both have a 1990. He seemed to always have a problem getting the distributor hold down clamp on the shoulder correctly and would always have it just pushed up to the distributor and not clamping downward. You sound very knowledgeable and I am sure you have checked this but i was just throwing it out there. His dizzy would move slightly.

 

[Colorado2500]

I know this is a shot in the dark. But my girlfriends son and i both have a 1990. He seemed to always have a problem getting the distributor hold down clamp on the shoulder correctly and would always have it just pushed up to the distributor and not clamping downward. You sound very knowledgeable and I am sure you have checked this but i was just throwing it out there. His dizzy would move slightly.

I was around it enough, it didn't move, but I should triple check it.

Thanks for the input though, maybe someone will read this in the future and have an Ah-Ha! moment!

 

[rydeon]

Also I have pulled these distributors apart and soaked the shaft and reluctor in Evaporust overnight with good results. Just need snap ring pliers and a punch for the roll pin.

 

[rydeon]

Please post what you find in case the rest of us have the same issue!!

 

[DerekTheGreat]

If the dizzy shaft is sloppy you should replace the whole thing. I went to change plugs, wires, cap and rotor on our '89 and when I did that it wouldn't idle, would just stall out at anything lower than 900 rpm or so. New dizzy, everything magic again. Shaft was so sloppy, I'd say you could move it nearly 1/8".

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[Colorado2500]

Update: Ordered a new Davis Unified Ignition distributor, I'll update when I get it installed.