Constant Miss-Fire

[LTZ C20]

Alright guys, here we go. I've got miss fires with my 79 Chev Cheyenne 3/4 ton, sb350 with HEI. About a week ago I got really bad surging and miss firing out of the blue while driving, after a couple days of screwing with the ignition system I finally found that I had crappy spark plugs. So I got a new set of plugs in, drove the truck for about 2 days, all was well and the miss fires returned. Now I have gone thru and checked everything that I can think of. Just for precaution I got a full tank of gas and cleaned the fuel filter. Now, I have tested my coil, ohm'd my plug wires, cleaned my rotor and checked the cap for any signs of problems. All of these have checked out good with the exception of the plug wires. (Keep in mind this is with the brand new week old spark plugs.) I have headers on my truck so I have heat wrap around my wire boots at the plugs, there's enough heat wrap to cover the boot and wires completely around the header area. However I found the #8 cylinder wire boot had some cracking and was arcing to the head. I pulled that wire off, wrapped the boot neatly and completely in the heat wrap and used a zip tie to hold the heat wrap around the boot, insulating the cracked area. this works pretty good as there is no more arcing on #8. I found a second arcing at #6 wire, the heat wrap was leaning against the header, I repositioned the wire and rotated the heat wrap around so that there was  air space between the header and plug wire. That seamed to solve the seconding arcing problem. The miss fire used to happen constantly, at any speed, being steady cruise or acceleration and at idle in Park or Drive. After all of the above mentioned tinkering, the miss fire has stopped AT IDLE but is still present as soon as throttle is applied. Now on to the coil, the coil I got used but tested working properly a year ago, I tested it again 2 days ago with two different ohm meters, both of which registered as having failing ohm readings. Would this mean that because the coil is used and worked then could be failing now? I have heard it's hard to kill a coil but is possible. I followed the testing instructions in my Chilton's manual, what they called the secondary test is what both my ohm meters provided me with failing readings. So am I just really looking at a bad coil that doesn't operate under load but is still working at idle?? This truck is my only vehicle and chasing miss fires while having to drive it everyday sounding like a straight 6 and lacking power is irritating, I'm trying to fix this properly on my own so I don't have to throw it in a shop. I was thinking about getting a new coil, putting new boots on all plug wires at the spark plugs and getting better quality heat wrap. Is this a good idea? Does it sound like I'm doing a good job of trying to fix it? Thanks for the input.

[Irish_Alley]

replace plug wires, re-check firing order and get a new coil, some fail only when hot some fail cold and hot. but i have headers and dont use heat wrap, havent had a problem since i been running this motor for 10 or so years. granted wires arnt that old upgraded a couple years later but never had a problem just dont them touch hot things or rub on other things

[LTZ C20]

Ok, I was hoping someonee would say that. I'm fairly convinced the coil is crapping out. I will order a new coil and wires today at work and see what happens tonight. Also, I have already rechecked the firing order and timing multiple times, both are correct.

[Irish_Alley]

order? any parts store will have in stock. you dont need performance parts if you do want performance again parts store have it.

[LTZ C20]

Yea I ordered the ac Delco original gm parts, they were here by the end off the day. Tonight I got the wires and coil on, it's running alot better, doesnt miss at idle, steady cruise or normal acceleration. But from a stop up to speed and on hard accel it's still misses, it's not a hard miss enough to be reduced power and slow accel, it powers thru the missing no problem but you can still here and feel it. As soon as you let off enough to cruise at a steady speed it goes away. Sounds like it's got one cylinder not running right or not firing. Its safe to say its definitely drivable but not not running smooth and proper like it should. My buddy has a 79' GMC 3/4 4x4 and that thing runs soooo good. Not a single problem. So it's irritating not getting this running properly.

[rich weyand]

Miss on acceleration is normally ignition, so the things to check first are 1) problem with one contact in the distributor cap 2) problem with one spark plug wire, such as cracked boot etc 3) bad plug (they can have cracked bodies new right out of the box) 4) arcing down the plug body due to water, oil, dirt.

Easiest way to check the distributor cap is visual inspection.  Do any of the eight contacts look different?

Easiest way to check the wires is to use the longest wire from the old set, and sub it one wire at a time until the problem goes away.  No need to loom it, just disconnect each existing wire at the distributor and plug and plug the old (but working) one on, one at a time, and test.

One way to check which cylinder it is is to disconnect one plug wire at a time and test.  You should have TWO misses under acceleration on all but one, which will still have only one miss under acceleration.  Pull that plug and replace it.

[LTZ C20]

Ok so here is the new scoop, I got a new cap and rotor also. So far I have a new cap, rotor, coil, wires, and plugs, all Ac Delco O.E. parts. I had a loose contact in the old cap. The truck is running alot better, only misses and accel from a stop and hard accel from a steady speed. I'm gonna have to check the new plugs for a problem like Rich just said. However, I'm starting to wonder if my problem is not the ignition but my fuel system. Maybe I have a fuel pump not working right under load only or the pick up in the tank is clogged and not enough glow under load also. My other thought was that I have a valve that's burnt or hanging up and I should look at doin a compression check also. Any ideas on if my fuel system is bad or not? The pump is only a year and half old. Any ideas as too compression not being proper? Also I adjusted the idle mixture, it was running a little rich also, running evenly now, just got that miss.

[LTZ C20]

Ok so I just did pulled one spark plug wire at a time from the distributor and start the truck and see if I have 2 miss fires in stead of 1. I couldn't hear any change. For all 8 cylinders, every time I pulled a wire off and started it, then revved it to listen for a miss and hesitation the sound never changed, it always sounded like 1 miss instead of 2. Did I do the test wrong or does that mean it could be my fuel system, my friend mentioned something, that maybe my accelerator pump is not working right??

[74 C-10 Shorty]

Gotta do it with the motor running, pull a wire and listen for a change as you drop a cylinder, keep doing that all the way around if you pull one and there's no change then that's the cylinder that has the issue, if all cylinders cancel when you pull a wire then look elsewhere..

[bd]

Kevin, connect a vacuum gauge to manifold vacuum and post the results (i.e., the vacuum reading and how the needle is acting - steady, bouncy, etc).  How does the oil look?

[rich weyand]

What is your timing set at? Have you checked to see if your vacuum advance is working properly? Which port is your vacuum advance plugged into? Have you checked that your centrifugal advance is working properly?

[LTZ C20]

Well I tried pulling it with the motor running, I had big heavy welding gloves on, insulated pliers, and I got the wire off but when I tried to put it on it shocked the crap out of me. I knew I would bare handed but not with big gloves and pliers. So I also reset the timing again cuz it was off a little. My timing is set at 10 degrees, my vaccum is comming from the vac advance port on my carb, like the carb instructions told me, I'm pretty sure the vac advance is working, is misses on accel, as soon as u let off a little and cruise or accel slowly it's fine, if you get into it then it misses again. I found my distributor hold down was bent up so it wasn't holding fully, I flipped it over so the bend pushed down on the flange and it holds proper now. I've gone thru everything I can think of. I put a stethoscope on my fuel pump and it doesn't make a weird noise, I did notice however that when the truck is shut off, my clear fuel filter drops down, the fuel drops low, then when restarted it rises back up and u can see the flow thru it, is it not supposed to drop down when off? Also, my oil is black but not thin or running, it's pretty fresh oil. And I'm not sure if this is connected but I have a short down at the starter wiring harness, it makes my lights stop working sometimes, is that a source of battery power to the dist? Could that cause insufficient power?

[74 C-10 Shorty]

I use plug wire grips but even with insulated pliers you shouldn't get zapped, might want to check the wires they could be arcing??, easy way to do that is run it in the dark you'll see where it's at..

[rich weyand]

Oh, man.  10* BTDC is not advanced enough, and running the vacuum advance on ported vacuum will make it even more retarded when the throttle plate is closed.  When you get on it from idle, the distributor is in the wrong position and takes time to catch up.  Reset the timing to 16* BTDC (with the vacuum advance disconnected and the port plugged).  As long as you don't hear knocking when you, say, floor the motor from low speed heading up a hill, you're good.  You shouldn't hear any knocking, but if you do, back it off to 14* BTDC.  Then move the vacuum advance line to the lower port ("non-ported" vacuum), I don't care what the instructions say.  The lean mixture at idle burns slower, and so needs more advance, but the pollution control weenies moved the vacuum advance above the throttle plate to pass idle emissions tests.  Which didn't really help real emissions, but there you go, they were playing the game by its own rules.  That's fine, but it makes the engine run crappy.  Prior to the emission controls era, the vacuum advance was ALWAYS taken from below the throttle plate.  On ported vacuum, the idle will be rougher, throttle applications from idle will catch the distributor in the wrong position (fully retarded) causing a transition stumble until the advance catches up, and the advance will be all wrong when decelerating with your foot completely off the throttle, causing incomplete burning and ejection of unburnt mixture.

Black oil almost always means running rich, leaving carbon in the cylinders that gets swept by the rings into the oil.  What kind of carburetor do you have and how many miles are on it?  If it's the stock Quadrajet, it may be leaking gas at high vacuum past the fuel-well plugs, a known failure mode.  These can be sealed up with epoxy if you know what you're doing.  Get the Rochester Carburetors book by Doug Roe and see explanation on page 80, pics on page 81.  It can also have the idle mixture set too rich.  It can also be that the engine was modified (headers, high-performance manifold) without re-tuning the carburetor.

[74 C-10 Shorty]

Oh, man.  10* BTDC is not advanced enough,

Well, maybe maybe not all depends on how the curve is set, total is what's critical need to set that first initial is what it is..