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megawatthour

2001 LS1 backfire problem

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megawatthour

After a long and lengthy shop stay to rebuild the LS1 engine,  it is finally done and the boat is back in the water.  Unfortunately, I'm trying to work-out the last of the bugs and this one is kicking my butt.  I currently have a bad skip/backfire when I try to exceed 3500 - 4000 RPM.  Not sure what it is but it is a wicked backfire and the engine just won't go above it even at WOT.  I've tried several things to no avail.  The ECM checked out good, all wires and connections are good, spark plugs have been replaced several times with various gap settings tested, fuel filters and pump have been replaced.  I'm going to start replacing coil packs next to see if that helps.  Any help or advice would be greatly appreciated.....

 

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justgary

I'm guessing valve timing.  You will want to get the rebuild shop involved in this before you throw any parts at it. 

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megawatthour

Thanks for the reply. 

The boat was able to run well for a short time, 8 - 10 hours then slowly degraded to the point were we are now.  This degradation is what lead me to thinking it was a fuel or electrical spark issue.  Would a valve timing issue slowly progress like this or would it be more like a step change?  If it is a valve timing issue, what's the solution?  Is this something internal to the engine of something controlled by the ECM? Thanks.....

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96Response

I would get in touch with whom ever did the engine for you. But sounds like ECM. I went old school and ditched all the fuel injection and went to carb and old school ignition. Now its reliable and makes more power than the TBI ever did. 

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justgary

If it ran ok it is probably not valve timing.  You didn't mention a new cap and rotor for the distributor, but I'm not even sure if an ls1 has a distributor. 

I think I would start with fuel pressure to make sure you are in spec at wide open throttle.  If that checks out, then move to spark testing.  Rotten fuel hoses from ethanol gas are a distinct possibility. 

It is possible, by the way, that (if you have a distributor) your guy set the base timing properly but left the screw loose, and the timing has drifted off.  

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Creekrat

If this is a ls based gym motor as stated above then there is no distributor. I would make sure that the coil coils are plugged into the correct connectors as well as the injectors. Those injectors and coils work on a switched ground so they have power to them at all times. Your ecm will pull the ground and complete the circuit. This makes these motors succeptible to grounding issues. I have had several that ran like crap for it to end up being a faulty ground. 

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