Intermittently running rough, stumbling

[TrybalRage]

Hey everyone. Struggling and getting a little frustrated and hoping for some advice. I think I might be on the right track but not sure.

I have an '85 C10 with the 305 (glovebox code 1LE9)/TH350C and I have had some weird rough running issues that come and go. A week or so ago, I changed the oil and adjusted the timing a little. It had been pinging very lightly under load for a while, and when I checked the timing it was about 15° BTDC. I adjusted it slightly to about 8°, tightened down the distributor, and went about my day.

A few days later I was driving it to/from work, and on the way home it seemed like I had no power, having to go really heavy on the pedal just to get it to move... it seemed to come and go.

When I went to check the timing, it was WAY off... like 6° ATDC-ish... off the marker. I thought maybe the dizzy had rotated or something.. maybe I hadn't tightened it down enough. So I adjusted it again and it immediately started running better. I went around and tinkered with a few other things... and suddenly the RPMs jumped (a lot) and I quickly checked the timing again, it was now up at maybe 20° BTDC. WTF?

I rotated the dizzy back to where it was, and got the timing back to around 8° again. Clamped everything down and wrote it off as a wierd glitch.

Well the engine still had the intermittent rough running. When I got home the one day, fighting in traffic and having to put it in neutral at every stop to keep it from stalling, when I started trying to find the problem I noticed that the exhaust sounded like it was choking or muffled. I checked the valve/flap on the exhaust manifold and it was not being opened by the vacuum. Seemed like either (A) there wasn't enough or (B) the temperature switch on the thermostat housing was malfunctioning (it's less than a year old). I bypassed the switch and directly connected some of the vacuum lines and the valve opened and immediately it started running better.

I started looking around at other stuff seeing if I could just bypass some of the emissions stuff, and while I was under the hood (with it running in park) it would keep switching back and forth... running rough, idle erratic, then smooth out and run fine again. I got my timing light back out and saw the same type of changes as before. Thinking that the vacuum switch perhaps was affecting the vacuum advance in an adverse way, I bypassed it completely (4 lines into the temp switch). Seemed to help, took it for a spin and no issues.

This morning on my way to work, it started up fine, and then acted up again. Sluggish, feeling like the engine was choking when I step on the gas. Even putting it in neutral and goosing the pedal it feels like its missing on a couple cylinders. I limped it to the office and when I was within about a mile all of a sudden it started running OK again.

So here I am. Trying to figure out what in the world would cause these issues to come and go. Been researching online and I'm starting to think that the ESC could be an issue. From what I see my engine code indicates that I would have that, and since it is there to change the timing when knock is sensed, could it be possibly faulting and jacking with the timing at random? Or am I looking in the wrong area? Anyone else experience anything like this?

Sorry for the long post, just wanted to try and include as much information as possible.

[TKCR]

Although I can't offer any help, I am having the same issues. Mines an 86 C10 with the same engine as yours. I'm afraid to drive it half the time, thinking it will stall and won't start again. So I will be watching for answers along with you.

[Dead Parrot]

My 75 did something similar with the timing. Would switch between being so advanced it didn't want to start and normal. Turned out the mechanical advance was sometimes sticking on full advance. Don't know if your computerized trucks have a mechanical advance, but if they do, might be worth checking.

I would also check for shorts/opens in the wiring to the knock sensor.

[TrybalRage]

Originally Posted by TKCR

Although I can't offer any help, I am having the same issues. Mines an 86 C10 with the same engine as yours. I'm afraid to drive it half the time, thinking it will stall and won't start again. So I will be watching for answers along with you.

So far I am reading as possible reasons, in no particular order:

- Bad ESC
- Bad ground or weak battery
- ICM failing (this seems likely from my symptoms)
- Coil failing
- Weights sticking in mech advance
- Vacuum advance gone bad causing vacuum leak (happened on the way home again from work once the truck warmed up, pulled over in a parking lot, disconnected the vacuum and plugged it.. no change)

Looks like saturday is going to be diagnostic day.

[TrybalRage]

Looks like it is the ESC. I performed the bypass that I found online here:

http://www.73-87.com/7387garage/drivetrain/escrepl.htm (Not the replacement, but just splicing the wires as detailed at the bottom)

And it immediately seems to have helped. Also, I had an issue with the engine bogging a little under very light throttle, this seems to be gone as well. I'll drive it around for a few days and see if that was it.

[TKCR]

I printed this off and I'm going to look into it with my truck. Looking at the photo of the coupler, this coupler and others are already just hanging. The previous owner must have disconnected them. I always wondered what they went to. I will pertaining try this when I get time and will update.

[TrybalRage]

Hmm... the truck wont even run with that disconnected, unless the splicing of the wires is also done. So it appears someone already beat you to it.

Next on my list was to replace the Ignition Control Module under the cap. Apparently they can be heat-sensitive towards the end of their life.

[rich weyand]

Some other things.

The distributor clamps can also get bent (or get put on upside down) where they don't clamp anymore. Make sure once it is clamped that you can't move it.

Check vacuum can *and* hose to vacuum can (if you have them). Also, put vacuum advance on manifold vacuum, not ported vacuum.

Check freedom of movement of vacuum advance mech and mechanical advance mech (if you have them).