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C5 Intermitted starting issue

 
Old 02-20-2019, 02:18 AM
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Mashore111
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Default C5 Intermitted starting issue

Howdy, ive got a 1999 base model, with an LS2 in it. I bought the car this way so im not 100% sure about the details of the swap, but ive read a bit about people that have done it before and they say it runs fine with the original ECU. Anyway, the car starts randomly, sometimes it fires right up first crank other times it will just crank and crank until you stop and try again. sometimes it back fires. Mostly i have to try to start it 3-6 times, turning the key all the way off and trying again. Once running it runs great, does not stall or anything.

Hoping someone has run into this problem before and it isnt related to the swap

Any input would be appreciated, Thanks
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Old 02-20-2019, 02:56 AM
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Moved to C5 Tech.
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Old 02-20-2019, 07:34 AM
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Look up 2 threads from this one !!...sounds similar to what you're experiencing...no ??
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Old 02-20-2019, 08:59 AM
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Just because the engine cranks does NOT mean that the voltage is adequate to keep the C5 electronics happy. Measure the voltage across the battery posts. Needs to be greater than 12.5V. At 12.5 or less the engine may start but thr electronics are NOT going to be happy. Charge the battery fully. A good battery at full charge will measure 12.7 to 12.9 volts or more measured at the battery posts using a digital multimeter.
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Old 02-20-2019, 09:07 AM
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Originally Posted by 73Corvette View Post
Just because the engine cranks does NOT mean that the voltage is adequate to keep the C5 electronics happy. Measure the voltage across the battery posts. Needs to be greater than 12.5V. At 12.5 or less the engine may start but thr electronics are NOT going to be happy. Charge the battery fully. A good battery at full charge will measure 12.7 to 12.9 volts or more measured at the battery posts using a digital multimeter.
How about voltage drop? How much drop during cranking is acceptable? Say down from full voltage to 9v or so. He may want to check that.
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Old 02-20-2019, 09:26 AM
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Yes !!....the cars electronics are very fussy about voltage....ignition relay 42 takes a beating during a "cold start" of the engine...the C5 coil packs draw up to 10 amps each so we're talking almost 80 amps or so plus maybe 8 amps for the injectors !! ...during cranking you can do a voltage drop not just across the battery terminals but across the relay...you can hook the multi meters red lead to battery positive and the negative test lead to one of 2 test points on top of INJ 1 or INJ 2 fuses....crank the engine and read the voltage drop...should not be more than .2-.3 volts...you will also need to determine if you are getting spark AND injector pulse !!
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Old 02-20-2019, 11:25 AM
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Default Ignition Switch

Mine would crank but not start. After wasting 2 months diagnosing I replaced the Ignition Switch, Car started right up. Its a PITA to rip dash apart but it was necessary to fix my car.
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Old 02-20-2019, 11:27 AM
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Since it was a swap, you might want to check ground connections to the engine, the one ground connection to the cylinder head might have not been made or maybe was left loose after the swap.
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Old 02-20-2019, 01:38 PM
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Originally Posted by 73Corvette View Post
Just because the engine cranks does NOT mean that the voltage is adequate to keep the C5 electronics happy. Measure the voltage across the battery posts. Needs to be greater than 12.5V. At 12.5 or less the engine may start but thr electronics are NOT going to be happy. Charge the battery fully. A good battery at full charge will measure 12.7 to 12.9 volts or more measured at the battery posts using a digital multimeter.
I charged the battery and behold is drops back down to about 12.3.. I'll be replacing this battery today at lunch and hopefully everything is happy.
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Old 02-20-2019, 01:40 PM
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Originally Posted by bighank View Post
Mine would crank but not start. After wasting 2 months diagnosing I replaced the Ignition Switch, Car started right up. Its a PITA to rip dash apart but it was necessary to fix my car.
Would it start sometimes? Or not at all? Was it slowly going bad or just one day it gave up? If the new battery doesn't solve my issue that might be something im looking into.
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Old 02-20-2019, 01:42 PM
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Originally Posted by mmartinez View Post
Since it was a swap, you might want to check ground connections to the engine, the one ground connection to the cylinder head might have not been made or maybe was left loose after the swap.
I will check them all tonight, i just rebuilt the TT and flipped the reverse gear, and while putting the new clutch in, i looked around the block from the bottom (was on a 4 post) and didnt see anything looking loose or missing, but this is my first LS.
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Old 02-20-2019, 02:31 PM
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As far as "people that have done it before with the original ECU"....It will not run fine but probably not at all if that "original" LS1 PCM is installed in an LS2 engine without some mods....the LS1 uses a 24X crankshaft reluctor wheel and an LS2 uses a 58 !!...your LS2 should have a 24X installed with the 99's PCM or some type of "ignition controller" if it has a 58X.

Last edited by C5 Diag; 02-20-2019 at 02:39 PM.
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Old 02-20-2019, 03:01 PM
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Originally Posted by C5 Diag View Post
As far as "people that have done it before with the original ECU"....It will not run fine but probably not at all if that "original" LS1 PCM is installed in an LS2 engine without some mods....the LS1 uses a 24X crankshaft reluctor wheel and an LS2 uses a 58 !!...your LS2 should have a 24X installed with the 99's PCM or some type of "ignition controller" if it has a 58X.
What I was told was in this motor is. ls2 block with Ls1 crank Ls6 heads and manifold Ls9 cams. So I guess itís rockin the 24x then and thatís why sheís running?
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Old 02-20-2019, 03:06 PM
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Originally Posted by Mashore111 View Post
What I was told was in this motor is. ls2 block with Ls1 crank Ls6 heads and manifold Ls9 cams. So I guess itís rockin the 24x then and thatís why sheís running?
Then you're golden !!....see if you have Tach movement (ignition pulse) when it cranks....see whether you are missing spark, fuel pressure/injector pulse or BOTH (crank sensor issue) !!
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Old 02-20-2019, 03:22 PM
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Originally Posted by C5 Diag View Post
Then you're golden !!....see if you have Tach movement (ignition pulse) when it cranks....see whether you are missing spark, fuel pressure/injector pulse or BOTH (crank sensor issue) !!
Awesome thank you thatís good news, I will check that as well. have you heard of this combo before?
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Old 02-20-2019, 05:03 PM
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Update*
The battery is now fresh with sufficient volts and the problem persists.
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Old 02-20-2019, 05:18 PM
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Originally Posted by Mashore111 View Post
Update*
The battery is now fresh with sufficient volts and the problem persists.
You have to check for spark and injector pulse like I had mentioned...to check injector pulse disconnect an injector...take a 12 volt test light (incandescent one) and connect it battery POSITIVE (control circuit is ground side switched)...the control wires are the striped wires on each injector....crank the engine and if the test light flickers you are getting injector pulse. I would want to check the feed wire on the injector too....the feed wires are pink...you would connect the test light to a good ground for this (key ON) !!...I'd probably want to check fuel pump pressure with a gauge also !!
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