Fresh LS2 L92 / L76 build, engine will turn over but not fire...please help! - Pontiac GTO Forum
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post #1 of 8 (permalink) Old 07-18-2014, 02:34 AM Thread Starter
 
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Fresh LS2 L92 / L76 build, engine will turn over but not fire...please help!

So a few months back I ended up spinning a rod bearing, which lead to a complete engine rebuild. Went with a factory style bottom end rebuild, and decided to do an L92 / L76 top end swap with a new cam.

Went to crank the car up earlier tonight, and it would turn over just fine, but would not fire. We found out that there is no spark getting to the plugs (pulled a plug still connected to a pack, turned over for 5 seconds, no spark)

Things I have done so far to troubleshoot:

-checked fuel pressure via valve at end of the fuel rail (sprayed out with moderate force)
-checked to make sure all sensors were connected (could possibly have missed one, but have been looking extensively)
-checking for any missing ground wires (are there any hidden ones that are easily overlooked?)
-Hooked up Diablo Predator to check for codes, none posted
-tried re-loading a bobcat tune
-rechecked all coil pack connections

I ended up having to replace the MAP sensor during the install (broke via hoist chain snapping it off), and the sensor I got was quoted to be an exact replacement from a local GM dealership, but had a different part number than originally on the engine. would this cause it not to fire at all? Is this common possibly due to there being various MAP sensors that could work?

I am at a loss, and extremely frustrated with the situation. I feel like there is something big that I am missing, because it's odd that it seems none of the coils are getting any spark. Does anybody have any thoughts or suggestions on things to try, or procedures to use to check / rule out any common or uncommon faults? Any help would be greatly appreciated!
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post #2 of 8 (permalink) Old 07-18-2014, 09:52 AM
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Just a thought but the crank position sensor is in kind of a hidden place and will make it not fire

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post #3 of 8 (permalink) Old 07-18-2014, 11:00 AM Thread Starter
 
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Originally Posted by svede1212 View Post
Just a thought but the crank position sensor is in kind of a hidden place and will make it not fire
Its located behind the starter, right? I'm pretty sure it's hooked up, but will have to double check that.
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post #4 of 8 (permalink) Old 07-18-2014, 11:34 AM
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Ditto, check the crank and cam sensors.
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post #5 of 8 (permalink) Old 07-18-2014, 12:58 PM Thread Starter
 
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Is there any chance it could be the anti theft system? Maybe the computer needs to be reset?
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post #6 of 8 (permalink) Old 07-18-2014, 02:53 PM
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No, the computer memory is non-volatile. The only way you can affect it so it wouldn't run is not have it hooked up or fried and I don't think it's either

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post #7 of 8 (permalink) Old 07-19-2014, 10:08 PM Thread Starter
 
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*update* my friend who owns a shop came by today with his fancy pants snap on scanner tool, and he found a fault with the camshaft position sensor. He checked continuity in the plug / harness for the sensor and everything read correct values. He is 75% sure it's the sensor, but there could be something not correct internally. Where does the sensor pick up its signal from, the camshaft or the gear? He attempted to run the old camshaft across the sensor plugged into the harness, and got no signal whatsoever from the sensor.

Hoping it's just a bad sensor and not something internally with the timing or camshaft / cam gear. My thinking is if the timing was off, it would still send a signal, just at the wrong time.
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post #8 of 8 (permalink) Old 07-21-2014, 06:04 AM Thread Starter
 
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*Update*

While trying to crank with the scan tool attached to OBD, it was registering around 100rpm's, which seems about right since my car has always been kind of sluggish during crank...but the point being that it was definitely sending a signal, which tells me that it was working.

After doing some research, found that the sensor reads off the gear, not the camshaft, and that people have said that it will run without a cam position sensor, but will have a 'long, hard crank."

By a long crank, how long are they talking? 10 seconds or more? Because I stop at around 3-4 seconds of crank time, the reason being the fresh rebuild and am not certain the oil pump is circulating oil to the new bearings at such a low RPM; worst case scenario, I would hate to burn through the assembly lube and roast a bearing before I even got it running, so I won't crank it for more than about 4 seconds.

I know for a fact, that the diagnostic tool was picking up a crank position signal during the cranking procedures; and nothing - NOTHING - was coming from the cam position sensor.

Checked all my fuses at the block in the passenger side engine bay - is that all the fuses present? are there more in a kick panel inside the car or under the hood elsewhere?

Is it possible to install the camshaft gear on backwards, or is it dished and can only be installed one way? Maybe I have it put on backwards and the timing bumps aren't present to send a signal to the sensor. Are the magnets located in the gear itself, or within the sensor? or both? and random thought...assuming the crank position sensor is operating (since it is giving a value on the scan tool), even if I had mechanical timing off internally, it should still send a spark, just at the wrong time...correct?

at any rate, I won't be able to get a new cam position sensor until Wednesday. I suppose until then it will just be a wait and speculate game.

ugh...i hate electrical gremlins

Last edited by blackwidow; 07-21-2014 at 01:36 PM.
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