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Popping and backfire on decell and Intial Startup

 
Old 08-01-2018, 04:48 PM
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Default Popping and backfire on decell and Intial Startup

Swap from 2004 GTO to 56 Chevy. I'm not the tuner, I'm the owner/builder so I hope to not sound TOO stupid. I used the same returnless fuel system/tank/pump originally in the GTO. Headers with mufflers, no cats. Tuner eliminated all but the sensor on the header collector.

If you put the key in the ignition and immediately turn the key, It wont start. It pops with small backfires, and a weird bellowing pop like a small combustion with a twist. It sounds like a implosion or a weapon on a star wars flick. I know, hard to explain. If I turn the key and wait a few seconds to start it, it usually fires right up. I know just enough to be dangerous and I know the fuel pump shut off after a couple seconds when 12 volts are applied. This seems like a fuel pressure issue but we put a gauge on it and it never reads higher than 45 pounds at any point. The other thing it does is on decell or down shifting, it gets the same weird pops and backfire. No, no exhaust leaks. I'm not sure if it is a tuning thing or if my fuel system is out of wack. No one I know locally knows much about the returnless fuel system.

I know this is strange and thanks for not thinking I am dumb as hell. You aren't thinking that...right?
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Old 08-01-2018, 04:58 PM
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Make sure the cam sensor plug is wired right. Sometimes the OUTER TWO wires have to be switched to make it right.
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Old 08-01-2018, 04:59 PM
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popping out intake and not starting is likely a cam wheel, sensor or wiring issue. if it doesn't start and you leave the ignition on it will start on the 3rd attempt. that gives you an area to look at for that issue. you didn't by chance change the cam or cam wheel during the swap?

the pops on gear change and Decal are likely the engine running lean as a result of deceleration fuel cut off and its probably sounding different because of the different exhaust system. if its only on decal during coast down or a gear change and there isn't any miss or hesitating when you apply the throttle again I wouldn't worry. your tuner can disable DFCO if its what you prefer and that will stop the exhaust pops.
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Old 08-01-2018, 05:02 PM
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Originally Posted by G Atsma View Post
Make sure the cam sensor plug is wired right. Sometimes the OUTER TWO wires have to be switched to make it right.
I did get a cam sensor something code. It was intermittent tho. Ill check with my harness/tuner guy.

Thank you!!

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Old 08-01-2018, 05:05 PM
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Originally Posted by Luke19901 View Post
popping out intake and not starting is likely a cam wheel, sensor or wiring issue. if it doesn't start and you leave the ignition on it will start on the 3rd attempt. that gives you an area to look at for that issue. you didn't by chance change the cam or cam wheel during the swap?

the pops on gear change and Decal are likely the engine running lean as a result of deceleration fuel cut off and its probably sounding different because of the different exhaust system. if its only on decal during coast down or a gear change and there isn't any miss or hesitating when you apply the throttle again I wouldn't worry. your tuner can disable DFCO if its what you prefer and that will stop the exhaust pops.
No, I didn't change cam or cam wheel. So starting on the 3rd attempt...does it disable or bypass something after the 3rd attempt? Do you mean to cycle the key back to off or just re-engage the starter without turning the key to "off"?

DFCO. Roger that. Ill have him turn whatever that is off. It is just annoying at best.

Thank you!!
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Old 08-01-2018, 05:09 PM
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I think after 3 attempts is will revert to the crankshaft for timing reference as default and start off that. but don't quote me here as I'm not certain. just from experience with that fault on gen 3 if you crank and it pops and doesn't start and you leave the key in the on position instead of cycling it off, one more crank it wont start then the next one it'll fire up.

running off the reluctor wheel for fuel and spark isn't as accurate as the cam so definitely something to fix sooner rather than later.

some people like the noise from the exhaust during DFCO. personally I find its dependant on the exhaust. if its deep and a quiet system I like it. if its tinny, no mufflers and loud as hell I hate it. its easy to disable
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Old 08-01-2018, 05:27 PM
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I will replace the cam sensor. Thank you very much for the info!
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