Immediately starts on 2nd crank

[wlwarnke]

What brand cam sensors are you using? Genuine GM or parts store?

Assuming parts store.

[2xLS1]

So junk then.

[Alvin]

Most of the cars I run into with a cam sensor issue don't have a code.

When the cam sensor fails it will not know if it is on TDC before the power stroke or TDC intake. So it takes guess. If it isn't the right guess it is out of time and you get the backfires. The next start it chooses the other and cranks no problem.

[wlwarnke]

So, if I unplug the sensor, it bitches and pops instant when cranking.

With the sensor plugged in, it will crank and crank, but only backfire if I crank for more than say 10 seconds and when you stop cranking.

I can crank for a normal 2-4 seconds (like a normal amount of time) and it will not pop.

I can bump the key for like 250ms. Then crank again and it will start immediately.

[blindsquirrel]

Only if you turn it to 'start' and the starter actually engages? What about key turned to 'run' and then off, and then crank? Same effect or none?

[wlwarnke]

Only if I engage the starter does it seem to make a difference. No matter how short of a pulse, hitting the starter seems to be the key.

Last night, I loaded a tune that has all 3 prime pulses configured but didn’t try to start the engine. Just loaded the tune and walked away for the night.

This morning was on my way out of the garage, curiosity got me, so I reached through the window and hit the key. It fired right up. Maybe a fluke, idk. Will see what happen when I get home this evening.

[wlwarnke]

The way I read and understand the priming pulses (minus the delays)

Priming Pulse: grams of fuel delivered at key on after fuel pressure delay.

Pulse 1: grams of fuel delivered after 2 reference periods.

Pulse 2: grams of fuel delivered after Pulse 1 plus another X reference periods.

Guessing that reference periods are 1x cam sensor counts?

If this “fixes” the issue, do I leave them, or keep trying to track down why they’re needed?

[abc]

Let us know when you try the starting fluid test, that's the next easiest thing to do.
This is starting to sound less like a CMP issue than a something else issue. One sure way to check the CMPS is as you mentioned, swap it out with a known good one, of course that doesn't mean there isn't an issue with the circuit. May be time to put a scope on the circuit and compare it to a known good circuit.

You could also let it go through the prime function two times, three times, and even four times as individual tests. I do find it interesting that if you bump the key to start position and engage the starter for less than a second and on the next attempt it starts in an acceptable manner. Turning the ignition switch to run energizes certain circuits and then of course energizing the starter sets others in motion, potentially knowing the difference will help solve the puzzle. Obviously we're in a guessing game, but how old is the ignition switch? Have you done a complete visual inspection of the entire engine harness?

Verlon mentioned a theft code. Setting a code to "No MIL" is not the same as "No Error Reported". I don't know anything about that code or sequence of events, but should probably set that one to no error reported if Verlon mentioned it.

As I was typing, new info. was provided. Let us know if the additional fuel solved the issue.

[abc]

The way I read and understand the priming pulses (minus the delays)

Priming Pulse: grams of fuel delivered at key on after fuel pressure delay.

Pulse 1: grams of fuel delivered after 2 reference periods.

Pulse 2: grams of fuel delivered after Pulse 1 plus another X reference periods.

Guessing that reference periods are 1x cam sensor counts? Yes

If this “fixes” the issue, do I leave them, or keep trying to track down why they’re needed?

Well... I guess we should have looked closer at the .HPT file, of course they are needed. Prime pulse mass is the same as pushing a carb. pedal all the way to the floor to set the choke and cycle the accelerator pump for the first fuel squirt. You also do not have a fuel psi delay. So as it stands, when you turn the key on, the injectors fire with no fuel psi, delay that by at least one second. Notice the prime re-enable time also?

This would explain why it starts after bumping the key to crank, letting off, then going for a full crank attempt. The first bump was delivering the prime pulse that it wasn't getting during the key on cycle.

[wlwarnke]

Well... I guess we should have looked closer at the .HPT file, of course they are needed. Prime pulse mass is the same as pushing a carb. pedal all the way to the floor to set the choke and cycle the accelerator pump for the first fuel squirt. You also do not have a fuel psi delay. So as it stands, when you turn the key on, the injectors fire with no fuel psi, delay that by at least one second. Notice the prime re-enable time also?

This would explain why it starts after bumping the key to crank, letting off, then going for a full crank attempt. The first bump was delivering the prime pulse that it wasn't getting during the key on cycle.

I agree with the prime pulses being important but it’s funny that I had to go back all the way to a 2000 file to find them being used.

New stuff seems to use the middle pulse while the 2000 uses all 3.

[edcmat-l1]

"Cam sensor" backfire sounds different than regular backfire. It's not as much of a "pop" than it is a burble. It actually sounds like a muffled marine motor or something. If that sounds like what you're hearing, narrow your focus to the sensor and circuit. I've seen all kinds of variations of issues here. Wrong cam wheel, 4X vs 1X, wrong wiring, too much air gap, you name it.

[wlwarnke]

Put all the prime tables back to stock as they didn’t seem to make a difference.

Started normal this morning. Only ran for 60 seconds or so.
Started normal after work. Again maybe 60 seconds runtime.
Started normal after gym. Let run for a few minutes to build some temp.
Let sit for 10 minutes. Removed air filter and gave a huff.
No start. Forgot to plug in timing light. Cranked for maybe 5 seconds with no start. As soon as I STOPPED CRANKING, I get a single backfire out the exhaust. Next crank, starts normally.

Sitting for 10 minutes now. Have timing light connected this time to see if sparking.

[wlwarnke]

Sat 5-10 minutes. Only cranked for 3 seconds. No start. Maybe a faint pop in the pipe somewhere.
Timing light was blinking. Obviously doesn’t say anything about timing but plugs are hitting.

Guess next step is to either scope the cam sensor and try to find a good wave form to compare or swap in a known sensor.

PCM powers the sensor. It’s like 11.3v IIRC. Is that sufficient?

[abc]

Cranked for maybe 5 seconds with no start. As soon as I STOPPED CRANKING, I get a single backfire out the exhaust. Next crank, starts normally.

Get a wire diagram for the ignition switch and make sure you have power during crank going where it is supposed to go.

Have you tried starting fluid during the crank, no start condition? Sounds like it's still not getting fuel.

[wlwarnke]

In post 32 and 33. Air filter is removed, and both no starts got a short fog if starting fluid.

I’ve have also tried using all 3 prime/crank pulses.

Any chance it’s getting too much fuel?

Maybe once the cam sensor gets some heat the PcM doesn like it? Even though it’s counting pulses.

When I unplug the sensor and test for power and ground, the power is only 11.3 or 11.5 volts. This is fed from the PCM, so it may be normal?

I’ve left the timing light connected to cylinder 2 to indicate spark. This should confirm everything is powered up and trying to start, right? (Ignition power).

[edcmat-l1]

Best way to check if it's too much fuel is crack the throttle slightly when trying to start. But normally if it's too much fuel it won't fire on the second attempt like nothing's wrong.

[abc]

What ignition switch are you using? The pop you are describing when you let the key off sure sounds like an old school ign. switch failure.

The only sure fire way to solve these types of problems is to verify each piece of the puzzle is functioning as it was designed to. In this case that's gonna take professional level diagnostics. A hobbyist can do it, but your gonna have to do your own on the job training and in this case, sounds like your not gonna get it done without a wire diagram and a o-scope.

[Alvin]

Is this a swap car?

I've had several wired wrong where the ECM has power in RUN but not in START. So while cranking the ECM is off and as you come off the key it gets power and it will start.

[wlwarnke]

UPDATE.

Swapped in a fresh from the junkyard OEM cam sensor.

It’s fired perfectly every beer for 10 beers.

First start was cold but the other 9 were after getting up to full temp and sitting for a beers worth of time.

Thanks for all the insight gents.

[2xLS1]

See post #22.