No start, then start

[5_Liter_Eater]

After putting my 408 in I threw a cam sensor code. I went through exhaustive troubleshooting with it and it turned out to be a bad connection which has been fixed. Still, every ~3rd or 4th start will just crank and crank with no start and sometimes a small backfire like it's not firing on the right cylinder as if the cam sensor was not working. If I turn it off and re-start it it always starts right up. Other times it will start on the first try. The tune is not quite lined out for the extra cubes and new cam but since it starts sometimes and not others I don't think that is where the problem lies. Anyone seen something like this?

[DSteck]

Try priming it every time before you start. See if that makes a difference.

[5_Liter_Eater]

Sorry, forgot to mention that fuel pressure is not the problem. Its holding at ~50# even overnight and instantly goes to ~60# when the cranking is happening.

[IDRIVEAG8GT]

I had this same problem on my G8, only when it was warm though. I corrected most of it by LEANING out the Open Loop Gas sections versus ECT. Too much fuel and a big cam sometimes won't start due to being slightly flooded. Make appropriate changes by ~5% to those and then re-write with a Write Entire, not a Write Cal only.

[carlrx7]

Relink VATS? or CASE LEARN?

[MikeOD]

I was having hard starting issues with the new top end on my LS3. After I tuned my Airflow Final Minimum table, I looked back at the stock file and looked at that stock table as well as the startup airflow table. You'll quickly see how they relate to each other if you just glance at them...once I did that, I just did some math really quickly and made a change to the startup airflow table, now the car starts just like it did when it was stock.

[Mike@G-Force]

I had a 07 c6 that was doing this. Classic signs of something funky in the cam sensor/ pick up.

Pulled the timing cover and bolts were backing out of the cam.

[cpolaris502]

This is probably way off from your problem but mine did this recently. Keep in mind though that I have a Chevy Blazer which swapped to an 0411 PCM when I vortec swapped the truck.

When i rewired the computer, I missed an ignition wire which supplied power during cranking. Using NOID lights, I found that it was only firing injectors when I turned the key off. It was getting no power to supply fuel under cranking. Once running, it was fine.

[camaro1]

still sounds like a cam sensor issue,, i have seen several swaps that people went from gen 3 cam sensor on the block to gen 4 cam sensor on the timing cover and they didnt swap terminals a and c on the connector and they would act just like that.

[DSteck]

F-body I tuned had the wires swapped on the cam sensor and exhibited the same behavior.

[5_Liter_Eater]

...Pulled the timing cover and bolts were backing out of the cam.

I dont see a way to link YT videos here Mike so this will have to do.

http://www.youtube.com/watch?v=onE8utryxE8

I loctited the cam retainer plate and cam bolts so I'm pretty sure they are not backing out. I do agree that it's cam sensor related though. I looked into case learn after carl mentioned it but that would only seem to be needed if I was getting a P0315 and I'm not getting any codes anymore, cam or crank sensor. If I let it crank enough on starts when it does it, it backfires sometimes. I went throuh exhaustive troubleshooting on the sensor itself, even replaced it and determined the distance to the cam gear bump and no bump and made sure it read correctly on the bump and off (voltage-wise) and it was still doing it until I cleaned and played with the harness where it connects at the bottom of the cover. No more code since then but still not starting intermittently.

[DSteck]

Swap the wires as a check.

[IDRIVEAG8GT]

What's the commanded AFR during cranking?

[5_Liter_Eater]

Swap the wires as a check.

Meh, even thought the block is a 3rd gen I used the front cover from the LS2 I had so there should be no reason to swap them. I do hae a cam sensor in the rear of the block just to fill the hole. If I decied to try using it I'll look at swapping wires then.

[5_Liter_Eater]

What's the commanded AFR during cranking?

14.4:1 all the time. Here's a pretty recent tune.

[DSteck]

Why are you so adamant about not just checking simple stuff? Rule 1 of problem solvin is clue generation.

[5_Liter_Eater]

just because that doesn't make sense because it's not something that was changed.

[IDRIVEAG8GT]

Try bumping up your hot cranking spark to 8 degrees. See if that makes it better.

[MikeOD]

Why are you so adamant about not just checking simple stuff? Rule 1 of problem solvin is clue generation.

If he didn't change the harness in the vehicle when he changed the engine, then how would the pins in the harness possibly be wrong? I don't think he's being adamant about not troubleshooting...but that'd be like telling someone to check the pinout at the PCM even though they didn't change it from what GM put there that ran fine with the old engine.

That said, it's possible that the cam sensor wires are sketchy and intermittent and need to be repaired/replaced...but if nobody has tried to do that yet, there's no way they're pinned wrong.

[IDRIVEAG8GT]

I hardly glanced it over but is the tune scaled .50?