Perplexing LS3 issue. Turns over fine but zero spark for 6 seconds, then it fires up

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05-02-2018
mightyquickz28

Perplexing LS3 issue. Turns over fine but zero spark for 6 seconds, then it fires up

But in limp mode.
Car:
_2011 Camaro SS
_416 stroker
_Turbo'd

Story:
My friends 2011 Camaro SS has developed an irritating issue. Status... New Battery, New GM Starter, New plugs, new wires. Behavior : Last year Steves car started having trouble starting when Hot. He drove to/from Sturgis and on the way back (to Upstate NY), everytime he stopped for gas, he had to let it set there for an hour or so till it cooled down before it would start (it spun over lazily). He and I tried all the usual culprits, new batter, new (autozone rebuilt) starter, new plugs, heavy cable from battery to engine, remote solenoid, copious heat shields, etc. No change .I talked him into buying a brand new GM starter and we put that in last weekend. Viola! (temporarily) It now spun over quickly and started after just a second of cranking. But, he drove about 30 miles and when he shut it off, it wouldn't start 'normally'. It still spun over quickly but you have to crank about 6 seconds and it only starts in the 'default' mode (Stabiltrack?) . We pulled plugs and O2 sensors. They were light tan. I scanned his car with my HPTuners. It only shows O2 heater circuit issues. Fast forward to today, I pulled a plug wire off cylinder 1, leaving the coil side connected. Then took a spare spark plug and grounded it against the coil bracket while Steve cranked it. What the ??? There's no spark at all till about the 5 second mark, then it fires up and the engine would start. No spark till then though, and it's always in that limp home mode. Any ideas ?

1. After arriving in Sturgis, it began having the occasional hot start problem.
2. When Steve got back to N.Y., it had the hot start issue a couple of times a week.
3. Then it started having the hot start issue every day.
4. Then, even cold starts were difficult.

Today, we replaced both the crank and cam sensors with brand new ones. No change. Cranks for 6 seconds with zero spark at the plug. Then fires up, but in Stabiltrack /default/limp mode. The connectors looked fine.

Added a short scan of when I was cranking and it firing up in default mode.

What all has to be satisfied before the PCM commands spark? We'd hoped it was just the cam or crank sensors but there must be something else.www.taekwondoplus.org/z28/Turbo/20180502.hpl
05-02-2018
10_SS

sounds like a bad cam or crank sensor. I had a few cars act this way. Sometimes it only happened after sitting idle in traffic for 15mins. The sensor gets real hot when sitting like that compared to moving. Log your crank and cam triggers. See if something doesnt look consistent. Isnt the crank sensor on the side of the block near a header or exhaust manifold?
05-03-2018
mightyquickz28

We replaced both yesterday. It behaved exactly like it did before we replaced them. I guess all that is left is to pull the wiring harness out and check from the PCM connectors to the cam and crank sensors to see if something is amiss there.

There's no Tach feedback when it's running. It just sits at zero while it's idling.

Quote:

Originally Posted by 10_SS

sounds like a bad cam or crank sensor. I had a few cars act this way. Sometimes it only happened after sitting idle in traffic for 15mins. The sensor gets real hot when sitting like that compared to moving. Log your crank and cam triggers. See if something doesnt look consistent. Isnt the crank sensor on the side of the block near a header or exhaust manifold?
05-03-2018
10_SS

disconnect the crank sensor completely, then try it, then plug it back in and do the same with the cam sensor. May this will help narrow down what one it is. Almost sounds like its running without one of them. Similar symptoms. This was a common problem with the early 2000-2002 GM 8.1L, cam sensor failure. When they fail, It takes forever to start, but eventuall would, and run not so good.

If disconnecting the cam sensor changes nothing then most likey that system is the problem. Bad wiring, or even wrong cam gear.
05-03-2018
Tom @ Overdrive

What crank is in it? Possible reluctor wheel issue?
05-03-2018
mightyquickz28

Hi Tom. He got the engine from Venom Racing in Louisiana a few years ago. It supposedly has an aftermarket crank (Lunati?) but the thing is, it has ran fine in the past. Especially when it was naturally aspirated. Upon reflection, I think the tubing from the turbo hotside might have compromised the crank sensor wires. I found an LS3 pinout and will go check the wiring tomorrow.

Quote:

Originally Posted by Tom @ Overdrive

What crank is in it? Possible reluctor wheel issue?
05-03-2018
mightyquickz28

Thanks for the suggestions. I think the first thing I'll do is check the wiring to the crank sensor. Steve said the connectors to both the crank and cam sensor looked fine, but I'm suspecting the wiring harness going from the PCM/ECM to the crank sensor might be compromised.

Quote:

Originally Posted by 10_SS

disconnect the crank sensor completely, then try it, then plug it back in and do the same with the cam sensor. May this will help narrow down what one it is. Almost sounds like its running without one of them. Similar symptoms. This was a common problem with the early 2000-2002 GM 8.1L, cam sensor failure. When they fail, It takes forever to start, but eventuall would, and run not so good.

If disconnecting the cam sensor changes nothing then most likey that system is the problem. Bad wiring, or even wrong cam gear.