86 C10 Misfire Help

[birdyhues]

I've got an '86 C10 Custom with a 305 V8. Had it for about three years. One owner before me – an elderly man who had it since new, and it came with an envelope of paperwork for every maintenance right down to oil changes, which is to say it had been well maintained. It had about 110K miles on it when I got it, and I've put about 3K on since then. Here's what I've done so far:

Oil change
Plugs and wires
Oil filter
Air filter
Power steering pump (it was leaking; installed a new one and it's working fine)
New transmission pan gasket (it was leaking; had no choice; no leaks now and all is well) and filter
New battery

Since I bought it (October 2022), it would occasionally stutter when I shut it off. Since I didn't drive it much, and only locally, I didn't look into it right away. It has been a secondary vehicle for me until this year, but now I need it as a daily driver, so I've been trying to tune it up myself. I can't afford a mechanic right now, so I'm trying to learn as much as I can, plus I'm interested in learning more. I changed the spark plugs and wires a few weeks ago, and didn't get everything tight enough. It started misfiring on the road, shaking and pulling really hard, and was arcing in rainy weather. When I got home, one of the boots had fallen off, so I double checked all the plug gaps (.045) and made sure the plugs were all screwed in well and the boots are snapped on to the plugs, and I made sure there was a good amount of dielectric grease on each plug in the boot. It's running better, but still sputters (misfires) a bit sometimes after I turn it off. Since that was an issue before I changed out the plugs and wires, I'm thinking it's likely something else that still needs attention.

Took it out last week and it ran fine for about 20 minutes on the road before it started shaking and pulling again like it was when I'd messed up the plugs and wires, but not as hard. I got back home and now I'm trying to figure out what I need to do next. The only plugs that I'm not 100% sure are installed correctly are the back two on the passenger side (6 and 8) just because they're so hard to get to. I was under the vehicle when I installed them, so I can check those again from above and make sure they're all the way in.

I've got a new distributor cap and rotor to install, and I also noticed oil running down the back of the engine on the driver's side only. There's no puddle under the vehicle, but it's down about a quart and I did change the oil recently, so I've also got some new gaskets for the valve covers that I'm going to put on it. I can see oil along the seam of the driver's side valve cover, so I'm hoping that addresses the oil issue. I've got some pics I can share of that, too.

I'm very new to all of this, but I want to learn. Since it's running now, I just want to make sure I don't get myself in a position where I can't drive it to the auto parts store since I don't have another vehicle right now. I'm thinking once I change the cap and rotor I'm going to need to adjust the timing, so I'm thinking I should rent a timing light before I install them, right? I really need to get this truck running right, and I think it's just in need of some basic maintenance, although I'm starting to get discouraged. Any advice from more experienced C10 owners is appreciated.

Dropped some pics below with file names that describe what I was photographing. Couldn't upload video, but I took some and put them in a Google Drive folder so you can see and hear what it sounds like right now. It didn't keep firing when I cut it off this time, but the third video does show how much the engine is shaking while idling right now. Seems like more than it should to me. I'll try to get out an drive it in the next day or two and get video of the shaking to add to the resources.

 

[83Stepper]

I'd double check the firing order on the cap and plugs to start. 1 8 4 3 6 5 7 2 is the sbc firing order. Some times this gets overlooked or a wire gets swapped. If it's "dieseling" after turning off, it could be a timing issue. There are a lot of variables that can contribute to this. Check your distributor cap for any cracks, or visible signs of damage, and/or corroded contacts in the cap and rotor can cause a weak signal and cause it to run poorly as well. I'm sure others will chime in here soon. These old trucks are pretty simple to work on and sounds like you've gotten a good one to start with and learn on.

 

[Sad Sack]

Some dieseling after turning vehicle off can be caused by the "Idle Stop Solenoid" used on 70 and 80s vehicles, it doesn't release and makes vehicle idle higher and run-on happens - sometimes.

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May not apply in your situation.
Hot carbon deposits in combustion chamber can cause dieseling too, making unfired fuel gases ignite.
Running a little rough is another issue or in layman's terms - a rabbit hole. DOUBLE CHECK all work!
A compression check is kinda needed too - to know just what your facing cylinder-wise.

 

[Ricko1966]

Visual Check for vacuum leaks,hoses off,cracked etc.. And look for lightning under the hood. To check for lightning put a Tbs of salt in some warm water and dissolve it in a spray bottle. With engine running after dark start misting the plug wires,plugs and distributor cap with salt water,watch for lightning. If you know somebody you trust to lightly power brake the truck while you spray,that is a help.Note any lightning. No lightning found spray the base of the carburator,the vacuum hoses and their connections and the manifold where it bolts to the heads with carbureator cleaner(NOT BRAKE CLEANER) listen for the idle to change. Report back. The lightning sometimes lights off the carb cleaner,which can get exciting so look fir lightning,and repair that first.