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Random Backfire after header swap. (Solved)

shady

shady

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I changed the headers in my big truck the other day.
Pulled the wires, plugs, old headers.
Put in new headers, put same plugs back in, hooked wires all up (correctly, checked 3 times).
Started right up. Idles good, then out of the blue pops hard on the passenger side.... :dunno:
It'll run for a while, pop, run bit pop pop, run a bit, pop. Etc.
Wires are good, and I made sure I felt the click.
Only thing I can think to even do is just change the plugs. Could a cracked one do that? I eyeballed the gap on all of them when they went back in, and all looked the same.
I did drop one though. In the socket still and it appeared to have hit the socket, not the plug when it landed.
I would have thought if one wasn't firing it'd just have a miss or stutter idling.
 
I've seen plenty of misfire from cracked porcelain, can't remember backfire.

When did it last run before this? This is Q-Jet/HEI?
 
Yeah Qjet/hei.
I drove it around that day, and drove it into the shop to do the job. No issues at all.
I keep coming back to that dropped plug, but still think it'd stumble not pop.
 
If one of them isn't firing every time, could the built up gas vapor be igniting no and then causing a pop? It's a weird sound like a gun shot more than a typical backfire.

I tried videoing it but the phone cancelled the loud pops it seems. When I play it back on my phone I don't really hear it.
There's one about 10 seconds in on this one, maybe a few more after.

View attachment VID_20250309_163356918.mp4
 
A fouled plug, or a plug not firing, can let unburned fuel into the exhaust pipe, and then it can ignite in the exhaust pipe and make all sorts of weird noises.

Thats why I freuqently ask, it is true backfiring, in the intake, or is it popping in the exhaust....

It also doesn't sound like its running on all 8, sounds like a 7 cylinder, but its hard to tell from the poor audio quality.
 
It's the cam lope probably. And only being on one side of it. It runs just like always, just does that pop here and there.
I'll get a full day with it tomorrow. I'm hoping plugs do it.
 
cheavyk10 said:
Did you change tube diameter?
Click to expand...
Not positive about primaries. Collector is the same and reused the rest of the exhaust.

I did take the turbo mufflers I had tried off and put my FM 50's back on. But that was the only change in the exhaust piping.
 
shady said:
It's the cam lope probably. And only being on one side of it. It runs just like always, just does that pop here and there.
I'll get a full day with it tomorrow. I'm hoping plugs do it.
Click to expand...
I'm not talking about the cam lope, I'm used to cam lope, it still doesn't sound like it's running on all 8, even with a cam lope, you still get a nice rhythm and it sounds good, this sounds like it's missing one. You have a temp gun to measure exhaust temps on the header primary?

I could be wrong, the audio quality on my end doesn't sound clear at all.
 
I just know it sounds exactly like it always has, (except the pop), and all the plugs looked good and pretty much even color wise while they were out. :dunno:
 
Sooo.... Apparently I'm a f*ckin moron :doah:

So, after this started, I went back and looked over my wires. Then again, and one more....
My #2-#4 wires are wire tied together , and I guess I lost track of them while tracing, because I had them backwards. Lol. Good now.
Still sounds pretty much the same to me. Just doesn't pop anymore lol.
 
What was weird and had me baffled, was if I held it about 2500 rpm or so,it didn't pop and ran great. Sounded good. Let off amd it'd pop.

That's why I was confused and thought the wires had to be right. Lol
 
shady said:
What was weird and had me baffled, was if I held it about 2500 rpm or so,it didn't pop and ran great. Sounded good. Let off amd it'd pop.

That's why I was confused and thought the wires had to be right. Lol
Click to expand...

Lol, you've probably got some old timer neighbor than can diagnose an engine with his ears that was thinking "You crossed the plug wires dummy!" every time you drove past.
 
I did it, backed it out of the shop and left it. Came back a few days later and fixed it in place. Lol
I'm the only guy in the shop neighborhood that knows how to work on cars.
Old dude across the street has a Vette, and old guy up the hill has a mustang.... That's about it. I do the work on the mustang, Vette I've never even heard of going to a shop.

Guy with the mustang did ask what the popping was though lol.
 
When I was 16, my friends neighbor made a huge explosion that ripped his muffler apart. The entire block heard it. My friend and I got back from running an errand and his parents said, the neighbor is working on his old truck and something exploded, you might want to go check out.

So we walked down there and he was looking very puzzled, he muffler was ripped open like a sardine can. This was a 2WD 75 or so C10.

I asked him what happened, he said he put a new magnetic pickup in his distributor. Cranked it over, it wouldn't fire, so he rotated the distributor, cranked it over some more, wouldn't fire, so he rotated the distributor the other way, cranked it, and it went kaboom, scared the sh!t out of him!

I said, walk me through your process.

He said, "I took the old distributor out, changed the pickup, then I rotated the engine over to feel the compression stroke in the spark plug hole, once I found that I put the distributor in and hooked all the plug wires up in order."

I said, which direction?, he said "clockwise".

I said "where was the rotor pointed?"

He said, "what do you mean?"

I think we found your problem....

So I showed him how to do that and how to get the initial timing close, and double checked his plug wires. At this point there was a crowd gathered around.

We were ready to fire it, he was worried, I said, "unless there is something you are not telling me it should fire right up".

The crowd backed up! :haha:

He got in and hit the key and it immediately purred like a kitten with no muffler.

He was grateful, the crowd, may have been looking to see another explosion I'm not sure, it was mostly neighborhood kids. :whistle:
 
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My dad used to down shift going down hill, let it rev up and rumble like hell, then he'd kill the key for a few seconds and turn it back on.

BOOM!
And he'd get back in it. Lol
I was always waiting for pieces to fly through the bed, because usually I was riding in the back of the truck when it happened lol.
 
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