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Discussion Starter · #1 ·
Well I started the car and shut it off after about 2 seconds. I went inside, came back out and tried starting it again, it took about 4sec to start but when it did I herd a loud pop, like an internal backfire. So I shut the car off immeadiately. Started it again and it was running really rough idle. kind of sounded like an old tracor, really choppy but no check engine lights came on. And I let it idle for about a minute and could start to smell gas. Any help at diagnosing this issue or what it could be? Thanks.
Think it could have just been some unburnt fuel left over from the short 2sec start and shut off I gave it?
 

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Discussion Starter · #3 ·
I wouldnt think so.. it didnt sound like a metal to metal contact bang. And it only happened once. But I dont really know a whole lot about engine mechanics to 2nd that thought. Any other suggestions?
 

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does it sound like it is missing on one cylinder or does it sound like all cylinders are chugging?
 

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Got a code reader? If so plug it in and see what comes up. Also check all plug wires. I popped a plug wire yesterday while driving and it did the same thing.
 

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Sounds like you lost a cylinder... if it smells like gas you may have a bent valve...timing chain sounds possible.

Sent from AutoGuide.com Free App
 

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If you had an intake backfire, you might have blown off a vaccum hose from the intake. I have had the map sensor come out of the back of the intake on an ls1 and they run extremely rich and idle rough. Not sure if the map sensor is located in the same location as an ls1 but it's way in the back up against the fire wall, passenger side of the engine. Just an idea before you go and tear into the engine, a vaccum leak will cause a rich run and rough idle.
 

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If you had an intake backfire, you might have blown off a vaccum hose from the intake. I have had the map sensor come out of the back of the intake on an ls1 and they run extremely rich and idle rough. Not sure if the map sensor is located in the same location as an ls1 but it's way in the back up against the fire wall, passenger side of the engine. Just an idea before you go and tear into the engine, a vaccum leak will cause a rich run and rough idle.
Actually, a vacuum leak will cause it to run lean since more air than would normally be allowed is entering. But a lean condition will cause backfiring out the intake. Very strange that it didn't set the light, though. When you say you smelled fuel, was it from the exhaust or under the hood? If it was from the exhaust, it would seem like more of an ignition problem- perhaps a bad coil?

If it were me, I would hook up a scan tool and watch the misfire counter since it isn't setting the light. Then, if it is uniform, that is all cylinders misfiring, then you know it is an intake problem. But I think you would find that it is a specific cylinder. If that becomes the case, switch coils with the misfiring cylinder and another cylinder and watch the scan tool again to see f the misfire follows the coil you moved. If so, replace the coil. :)
 

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Discussion Starter · #10 ·
Actually, a vacuum leak will cause it to run lean since more air than would normally be allowed is entering. But a lean condition will cause backfiring out the intake. Very strange that it didn't set the light, though. When you say you smelled fuel, was it from the exhaust or under the hood? If it was from the exhaust, it would seem like more of an ignition problem- perhaps a bad coil?

If it were me, I would hook up a scan tool and watch the misfire counter since it isn't setting the light. Then, if it is uniform, that is all cylinders misfiring, then you know it is an intake problem. But I think you would find that it is a specific cylinder. If that becomes the case, switch coils with the misfiring cylinder and another cylinder and watch the scan tool again to see f the misfire follows the coil you moved. If so, replace the coil. :)
I just checked all the lines I could find, all of them are still hooked up. Spark plug wires are all hooked up too. As for the fuel smell, i can kind of smell it up front and i stuck my nose into my exhaust and could smell it in there. The only scanner I have is the Torque Pro app, that says it isnt detecting a missfire, but i dont know how much i trust that app.
 

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Discussion Starter · #11 ·
I would imagine a bent valve would accompany some odd noises comming from under the valve cover wouldn't it? But there is no difference is the valvetrain's normal opperating noise.
 

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Is it still running rough? Pull your spark plugs and check them for fouling. You may have a leaky injector. It could be allowing fuel to fill a cylinder with fuel while shut off then when you start it, it can ignite the fuel built up and cause a backfire
 

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FWIW, should be no question on the Torque Pro. When configured properly, it will show what the ECM reports.
 

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what was going on before all this happened... any reason why you started it and then shut it down so soon? Anything out of the norm happen recently? Ie changing stuff out, new plugs, wires etc?

I'm looking for context to see what's happened with the engine.
 

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Cold start mis-hap... I agree with with a bent valve or even a dropped valve seat which happens more frequently on cold start. Grab a compression gauge at your local auto part store and at least eliminate the possibility.
 

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I would imagine a bent valve would accompany some odd noises comming from under the valve cover wouldn't it? But there is no difference is the valvetrain's normal opperating noise.
Not necessarily, all it takes is a small tweak to the face of a valve so it does not seat and seal.
 

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Discussion Starter · #17 ·
Im going to to check my plugs here in a little bit. So can I start the car and drive it 1 mile to my shop or leave it off. And how does a compression tester work?
 

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Discussion Starter · #18 ·
what was going on before all this happened... any reason why you started it and then shut it down so soon? Anything out of the norm happen recently? Ie changing stuff out, new plugs, wires etc?

I'm looking for context to see what's happened with the engine.
About a month ago i did my cam swap. VMS 2.5 Kit and had my heads P&P. New spark plugs and wires. I was having some tuning issues with Steve. We were trying to get the P0106 code out, which we did about a week ago. And all has been good since the updated tune. I did get a coolant leak 2 days ago, but found that it's just the plug on the rear drivers head. Simple fix and doubt it had anything to do with this issue.

Cold start mis-hap... I agree with with a bent valve or even a dropped valve seat which happens more frequently on cold start. Grab a compression gauge at your local auto part store and at least eliminate the possibility.
How does a compression gague/tool work?

Not necessarily, all it takes is a small tweak to the face of a valve so it does not seat and seal.
So would not putting the valve's back in the same place they came from when i dissassembled the heads probably cause this too happen? and can/should i drive it 1 mile to my shop or just leave it in my garage and work on it there?
 

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If you can tow it I would.

Not putting the valves in the same spot would not bend a valve.

Go to autozone and buy or rent a compression tester.
 

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A compression tester works by removing the spark plugs and threading the tester into the spark plug hole. Then you turn the engine over and the gauge will show how much PSI that cylinder is generating. Should see a minimum of about 120-140 psi in each cylinder. If one cylinder is significantly less than the others then that indicates where your problem is at.
 
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