Back firing through intake
Thread Starter
Burning Brakes
Joined: Jan 2017
Posts: 951
Likes: 42
From: Lincoln Ne
Back firing through intake
Need some advice, I recently cam swapped my twin turbo ls3. I can not get it to start, it will just crank and backfire through the intake. I put a BTR stage II twin turbo cam, 7.4” pushrods, ls9 head gaskets, ls7 lifters. I double checked the crank and cam gear were dot to dot, I replaced the cam sensor thinking that was the problem but didn’t help. Reading 55psi fuel pressure at the rail, could it be the crank sensor? The car is pulling no codes, the only other thing I can think of is incorrect push rod length. Is there anyway for me to check that with out a checker? When purchasing this I told btr my set up so I figured 7.4” was right?
Le Mans Master
Joined: Apr 2011
Posts: 7,820
Likes: 411
From: Montreal
St. Jude Donor '13
You should always measure for correct pushrod length. That's first place I would look.
Also checks that coils and plugs are all wired correctly, but that's not that easy to mess up.
Also checks that coils and plugs are all wired correctly, but that's not that easy to mess up.
Last edited by realcanuk; Jan 21, 2018 at 04:56 PM.
Team Owner
Joined: Nov 2007
Posts: 24,035
Likes: 2,342
From: Gilbert AZ
Always measure, ALWAYS.
Crank sensor would cause issues, but after cranking 1-2 times it should throw a code for it.
Plug in your HP tuners and log the various sensors. Easy enough to watch the cam signal, crank signal, etc when cranking.
If you don't have HP tuners and are building a TT modded car, that is also a mistake, anyone with a modded car, especially a boost once should own a logging setup, and tuning setup, even just for stuff like this, or mild changes.
Crank sensor would cause issues, but after cranking 1-2 times it should throw a code for it.
Plug in your HP tuners and log the various sensors. Easy enough to watch the cam signal, crank signal, etc when cranking.
If you don't have HP tuners and are building a TT modded car, that is also a mistake, anyone with a modded car, especially a boost once should own a logging setup, and tuning setup, even just for stuff like this, or mild changes.
Thread Starter
Burning Brakes
Joined: Jan 2017
Posts: 951
Likes: 42
From: Lincoln Ne
Melting Slicks
Joined: Jun 2006
Posts: 2,491
Likes: 261
From: Miami Florida
Is easy to check if push rods are too long.
Bring a piston to top dead center on the compression stroke both valves are close (on their seats) loosen up the rockers some and tight slow to observe if the valve starts to open as you tighten up the rocker bolt it only needs just a few thous to back fire if any doubt loose slow see if it returns to the seat.
Another condition that can easily be overlooked that can bring a quick back fire into the intake manifold is a flipped coil wire bar.
Here's what to look for if that's the problem, likely is only on one cylinder bank and not the other if this is the case disconnect the (coil wires only) top side from all coils and flip the wire bar so the front wire goes to the back connect all coil wires and start it up.
I mark my coil wire bars pf for passenger front and df for drivers front this way I can always be sure they will be installed rite every time.
Often the bars are flipped looking for a better possition to hide the bars is a no go.
Let us know your findings.
PS if you have your timing marks dot to dot after timing chain was installed there's no need to doubt is installed correctly.
Bring a piston to top dead center on the compression stroke both valves are close (on their seats) loosen up the rockers some and tight slow to observe if the valve starts to open as you tighten up the rocker bolt it only needs just a few thous to back fire if any doubt loose slow see if it returns to the seat.
Another condition that can easily be overlooked that can bring a quick back fire into the intake manifold is a flipped coil wire bar.
Here's what to look for if that's the problem, likely is only on one cylinder bank and not the other if this is the case disconnect the (coil wires only) top side from all coils and flip the wire bar so the front wire goes to the back connect all coil wires and start it up.
I mark my coil wire bars pf for passenger front and df for drivers front this way I can always be sure they will be installed rite every time.
Often the bars are flipped looking for a better possition to hide the bars is a no go.
Let us know your findings.
PS if you have your timing marks dot to dot after timing chain was installed there's no need to doubt is installed correctly.
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Thread Starter
Burning Brakes
Joined: Jan 2017
Posts: 951
Likes: 42
From: Lincoln Ne
Got my timing cover off tonight to see the cam gear was off, and off by a lot. I’m not sure what I was thinking when I put it on. Got them lined up for good this time. Checked the first two cylinders and showed 120-130 psi after about 3 cranks before my battery died. Going to check all of them to make sure I don’t have bent valves. I know stock compression is around 150 on a ls3 so with my thicker head gaskets I’m not sure how much lower it would be. Hoping for the best 
Burning Brakes
Joined: Jul 2008
Posts: 923
Likes: 37
From: Green Bay Wisconsin
Got my timing cover off tonight to see the cam gear was off, and off by a lot. I’m not sure what I was thinking when I put it on. Got them lined up for good this time. Checked the first two cylinders and showed 120-130 psi after about 3 cranks before my battery died. Going to check all of them to make sure I don’t have bent valves. I know stock compression is around 150 on a ls3 so with my thicker head gaskets I’m not sure how much lower it would be. Hoping for the best
I think you nailed it. With 130 psi your around 9:1 which is a good number for your turbo.
Looking forward to good news!
Douglas in Green Bay
