missing under load * update found problem *

[scrappyk5]

I have a 305 that has been running fine for many miles. Today though it started missing under load . It has a slight miss at idle but gets alto worse under load. If I rev it up and hold it at about 2 to 3 thousand rpms its fine and revs up fine. but once I take off from a stop it starts missing, stumbling. I got home and change the fuel filter and the plugs, and it still doin it. I looked at the wires and they are fine. the cap looks good and so does the rotor. when I rev it up, looks like alot of black smoke comes out.
I am thinking possibly a coil or ignition module going bad.

any other thoughts ?

 

[4xcrazy]

Still willing to bet you have a wire arc'ing out somewhere, i had a similar issue with my 4.3 a few years back, couldn't get that miss to go away, finally one morning as it was leaving from the house, relative driving away, i heard the familiar tic of a shorting plug wire. I did the same thing of changing out other parts, even went so far as to replace the fuel pump, all but the wires.
It was grounding out behind the engine up against the firewall i believe, i know it was in the back somewhere...

Swapped them been fine ever since.

these of course were the original wires too, they still had the factory numbers on them

 

[scrappyk5]

I will put the car up on ramps and take a look underneath at the wires, maybe I missed it. Someone also mention to check for a vacuum leak.

 

[mudbuggy]

Start the truck at night and pop the hood, see if there is any bright lights, if there are then your plug wires are arcing out.

If you don't want to do that then pop the hood, with the motor off, and inspect each plug wire individually if you find any light gray marks on the wire, or it is really hard, then chances are its arcing out

I've see guys using a spray bottle of water checking their plugs, it is funny, and it looks like it hurts, I wouldn't advise checking it this way!

 

[British Slave]

Have you checked your timing?

 

[diesel4me]

Usuallly weak spark cause a misfire under a load,the compression increases when you open the throttle and can snuff out a weak spark --the cap and rotor can cause this even if they look perfect,but spark plug wires that are cracked,burnt from lying on an exhaust manifold or header will do the same,so can plugs that are fouled,or a weak ignition coil or pickup coil,sometimes the ignition module will lose its dwell advance circuit and still function and cause a misfire under load..

Vacuum leaks usually show up as a poor idle ,stalling,or a bog upon acceleration,they dont usually cause poor running once your past idle...you may want to be sure your EGR valve isn't opening too soon,if you still have one,that might cause a misfire under a load too..

 

[scrappyk5]

Usuallly weak spark cause a misfire under a load,the compression increases when you open the throttle and can snuff out a weak spark --the cap and rotor can cause this even if they look perfect,but spark plug wires that are cracked,burnt from lying on an exhaust manifold or header will do the same,so can plugs that are fouled,or a weak ignition coil or pickup coil,sometimes the ignition module will lose its dwell advance circuit and still function and cause a misfire under load..

Vacuum leaks usually show up as a poor idle ,stalling,or a bog upon acceleration,they dont usually cause poor running once your past idle...you may want to be sure your EGR valve isn't opening too soon,if you still have one,that might cause a misfire under a load too..

I do not have an egr valve on the intake.
I have put about 25,000 miles on it since I last tuned it up, so I will replace the cap and rotor, and plug wires, but check and see if maybe one of the wires are arcing against anything. then if still a no go I will look at replacing the coil then module. the car does idle steady with a slight miss every couple seconds or so, but gets worse as I take off from a stop. I do notice that the exhaust has had a rich smell lately and when I rev it up, it puffs out black smoke. It does rev up pretty strong, just cannot put a load on it without the missing.
I will check things out to see how it goes. including spraying around the carb base plate ad intake manifold just in case for a vacuum leak.

 

[diesel4me]

Black smoke indicates a rich mixture,or it might be unburned fuel passing into the exhaust..be sure the choke opens fully and the air cleaner is not dirty ,bad fuel or water in the gas can cause misfire and stumbling upon acceleration too..

I have seen many ignition rotors have a teeny black pihole punched in them right under that spring tab,and the spark grounds to the distributor shaft instead of firing the plugs under a load..distributor caps can get carbon tracks or crack,but they dont seem to do it as often on HEI than the old points style ones did it...

 

[scrappyk5]

Well I put it up on ramps today to get a betta look at the plug wires and found the culprit. one of the plug wires was cracked and burnt right near the exhaust manifold. where it somehow was touching the manifold. it was right next to a wire holder so it was hard to see, especially when you have the rams horn type manifolds. I replaced it with an old wire from an old set I had laying around and walaa good to go. tomorrow I will pick up a new set of wires,cap and rotor to finish the tune-up.

Thanks all to your suggestions.

 

[magik235]

Excellent that you found the problem.

 

[jeff in co]

Start the truck at night and pop the hood, see if there is any bright lights, if there are then your plug wires are arcing out.

If you don't want to do that then pop the hood, with the motor off, and inspect each plug wire individually if you find any light gray marks on the wire, or it is really hard, then chances are its arcing out

I've see guys using a spray bottle of water checking their plugs, it is funny, and it looks like it hurts, I wouldn't advise checking it this way!

I will echo, thanks! I didn't know about this so yesterday morning I looked at the engine in the dark. Took awhile for my eyes to adjust but eventually, I started seeing the lights! I could see a few of the wires lighting up and also a constant light where the wires were zip tied together. When I touched the wire, the "light" focused on my fingers. As I moved my fingers up and down the plug wire, the light followed.

So thanks! I ordered a new set of AC Delco wires, cap, rotor and fuel filter yesterday. Should help out a lot!

 

[K5dreamer]

its always satisfying to fix something without throwing parts at it.

 

[diesel4me]

What you saw is known as "Corona" effect,its not always an indication of bad wires...its not a good idea to zip tie plug wires into neat little bundles,as the high voltage can cause "crossfiring"--thats when one wire induces a voltage in another wire next to it,which can actually fire that cylinder out of sequence...its best to route them as factory in the holders or at least keep them at least an inch away from each other and away from any 12V wires too--the #5 and #7 cylinders on chevy V8's are the ones most suseptible to cross firing as they are one after the other in the firing order,and right next to each other--..

 

[jeff in co]

What you saw is known as "Corona" effect,its not always an indication of bad wires...its not a good idea to zip tie plug wires into neat little bundles,as the high voltage can cause "crossfiring"--thats when one wire induces a voltage in another wire next to it,which can actually fire that cylinder out of sequence...its best to route them as factory in the holders or at least keep them at least an inch away from each other and away from any 12V wires too--the #5 and #7 cylinders on chevy V8's are the ones most suseptible to cross firing as they are one after the other in the firing order,and right next to each other--..

The zip ties were from the PO and I do plan to get some separators to help the problem. Who knows, the wires are probably original....

315,000 miles.....probably due.