Hi folks, I have an LS3 swapped Firehawk. When attempting to start it first time around, car will typically crank for a while then start backfiring out the tail pipes. Car will always fire right up second time around. I’ve verified that car has fuel pressure. I’m thinking the cam sensor may be the culprit. Any input is appreciated. Thanks folks.

 

Cam sensor.....

 

Just curious, are you running a 58x/4x to 24x/1x converter (Lingenfelter swap box), or did you swap the crank reluctor wheel out for the 24 tooth one and the 1x cam gear? Not trying to insult your intelligence but when going from the Gen IV to Gen III, the signal is not going to match, unless you installed a Gen IV PCM to go with it.

 

I have no idea. I purchased the car already done. I would hope it was done correctly.

 

If you have a Lingenfelter box it would be in an obvious spot on the fenderwell or firewall.

 

I don’t believe I do but I’ll have to take a look again.

 

What color is the crank sensor? Black or grey? Also, yes, see if there is a box in there. This could cause some major problems if the signals are messed up. Part of the reason that I don't like buying other peoples swap projects lol. It's always an adventure trying to figure our what they did or didn't do to the car.

 

Cam sensor? It’s black if I recall correctly. No box in there.

 

I have seen installs where the wires were extended and box was installed in the dash to keep it cooler.

 

He asked about the CRANK sensor, not the cam sensor. The crank sensor is on the right side of the block near the starter.

 

Lol, yes the CRANK sensor is how you can tell if the crank is 24x or 58x.

OP- Grey should be 58, black is 24, iirc. You can also remove it and peek in there and see if it has what looks like a gear (one plate), with lots of teeth which would indicate 58x, or if it has 2 plates, with large castellation style teeth, which is 24x. Not having the proper signal will cause some wild issues and backfires. What's weird is that you said it starts the second time around. Hard to say if this would be the case with the wrong signal since I have never seen it attempted personally. In either case, it really sounds like either your crank sensor or cam sensor could be the culprits here. A bad crank sensor will cause it to not start at all, which is where the PCM gets the signal to fire the coils, not the cam.

 

Here is what 58x looks like, and the box, and sensor as well...

58x LS3 crank...

Signal Converter in question ^^^

Grey 58x sensor ^^^

 

Here is what the 24x looks like...

 

Here's pretty much everything you need to know about signal conversion. LS3's are 58x/4x. Your Firehawk is a 24x/1x stock, with stock PCM. Having the wrong sensor, or the wrong signal to the PCM, or a damaged harness, or a damaged signal converter/damaged converter harness could easily cause your backfires and rough starting. In fact, if any of these things are messed up, then I'm surprised it even starts.

https://www.lingenfelter.com/PDFdown...nformation.pdf

 

You are the man! Thank you very informative! Here is what I have:

 

That's a 1x timing gear, so I would have to assume the crank is 24x. The crank sensor is on the passenger side, on the block, towards the back of the engine. Make sure it's a black sensor. From there, I would either change that sensor out, because when they are bad they cause similar problems as you described. I would also take a look at the harness for the crank sensor as well to make sure the signal is getting to the PCM properly. You'll have to go under the car to see it. It's pretty much right next to the starter on the block. You'll probably have to pull the starter to do all this.

I had an issue on mine that caused spark to only fire on cylinders 1,3,5 and 6, but nothing else. I went nuts chasing that down. I had just put the engine together and got it into the car and all that. Turned out it was a bad crank sensor, that was bad out of the box because it was brand new. I ordered an AC Delco one instead of the cheaper one that I originally bought (ICT Billet I think) and suddenly I had spark on all cylinders. So that sensor can absolutely cause some issues with starting and spark timing, to put it mildly.

Also, in all this, I messed up and had 2 wires crossed on the cam sensor, so it was throwing a code. Didn't affect the engine running at all. Just threw a code, even though 2 of the 3 wires were pinned wrong, meaning the signal was incorrect to the PCM. So the cam sensor won't likely cause a host of issues, and I changed mine to to try to correct it, until I figured out the wiring was messed up.

Nice timing set by the way. Looks like this thing is well put together. I don't think you had to put the cover, but I guess that's one way to check lol. Starter is 2 bolts and you can just hang it out of the way to get a look at your crank sensor and check the wires. My bet is that is where your problem is.

 

Thanks again Doc! You are a wealth of knowledge!! Crazy how a part can be bad right out of the box.

 

Bumping this back up. Conversion was done properly with 24x/1x. Cam and crank sensors both replaced and ignition switch replaced. Tune was unlocked and gone through, data logged, fuel and spark all good. Fresh plugs and wires. Problem still exists. Any other ideas folks?

 

Something just hit me- It STILL ACTS like a bad cam sensor, but that is now new (COULD be defective, but for now assume it's good)
I noticed the cam sprocket is aftermarket, not OEM.
I wonder if the gap between sprocket and sensor is up to spec.
Not sure how to check it nor do I know the proper gap.
Just an idea....