480 horse Ls3 first start problems - The 1947 - Present Chevrolet & GMC Truck Message Board Network

the ideaman · 12-10-2019, 09:39 PM

Finally started my 480/376 LS3 and it will fire, but almost dies immediately. Twice it continued to run when we disconnected the maf sensor, but hunted. It continued to run when we connected the sensor. Using the gm computer and harness. No codes read. Help!

87chevy.com · 12-10-2019, 11:27 PM

Quote:

Originally Posted by the ideaman

Finally started my 480/376 LS3 and it will fire, but almost dies immediately. Twice it continued to run when we disconnected the maf sensor, but hunted. It continued to run when we connected the sensor. Using the gm computer and harness. No codes read. Help!

Maybe sensor and tube diameter not the same as stock. Anything other than stock makes it run poorly without retune.

the ideaman · 12-10-2019, 11:55 PM

Thank you, I'm using the GM tube designated for the LS3. Perhaps it's facing backwards. Will check this weekend.

the ideaman · 12-11-2019, 10:09 PM

The MAF was facing backwards. Now it starts and runs well.

the ideaman · 12-30-2019, 03:02 PM

Having put a couple miles on it I've got a few observations. First, it's really fast, like scare me fast. Anybody who tells you 480 plus horsepower is no big deal has bigger huevos than I do. Secondly, I need to move the throttle pedal up higher on the firewall and away from me. My leg is at a strange angle and it's very uncomfortable. We'll have to bleed the brakes again or get a vacuum reservoir for the power brakes. They are a little spongey and there is too much pedal before the brakes really grab. Finally, a hydraulic clutch is the way to go with a swap like this. No squeaks or groans. A nice linear engagement. Truck fires instantly and settles into a lopey idle almost immediately. It's a lot of fun to cruise around in.

**mongocanfly** · 12-30-2019, 11:24 PM

Good deal....what do ya have it in?

kipps · 12-31-2019, 12:11 AM

I second Mongo's question -- what's the ls3 mounted in?

Is it an engine from a donor car, or a GM performance crate engine?

Do you have a clutch pedal position sensor connected? What sensor? How's it mounted? Any pics?

the ideaman · 12-31-2019, 12:18 AM

GMC crate engine with captainfab's hydraulic clutch setup. McLeod hydraulic throwout bearing on a tremec tko.

the ideaman · 12-31-2019, 12:20 AM

Just after 1st fire.

kipps · 12-31-2019, 09:08 AM

Looks like a nice truck. Too bad it's hanging from the roof...

Do you have a clutch pedal sensor wired into the engine computer? If so, do you have any pics of how you mounted that sensor to the pedal?

the ideaman · 01-02-2020, 10:21 AM

Quote:

Originally Posted by kipps

Looks like a nice truck. Too bad it's hanging from the roof...

Do you have a clutch pedal sensor wired into the engine computer? If so, do you have any pics of how you mounted that sensor to the pedal?

No clutch pedal sensor. Why is one necessary? Engine is wired up using GMPP ecu.

kipps · 01-02-2020, 03:15 PM

Quote:

Originally Posted by the ideaman

No clutch pedal sensor. Why is one necessary? Engine is wired up using GMPP ecu.

The LS3 used a clutch pedal sensor in the Camaro's. I assume the Corvette was similar. This sensor would disable the starter(controlled by the ECM in factory applications) if the clutch is not depressed. It would also disable the cruise if you hit the clutch pedal. The third reason is partially speculation. I'm told the Deceleration Fuel Cutoff tables were modified if you had the clutch pedal depressed. Folks have expressed some problem with clutching to a rolling stop, and having the engine hesitate and die. Supposedly, if they had a clutch sensor installed, it would fix that problem.

Being your ECM is from GMPP, they may have adjusted the tune to eliminate the need for a clutch sensor.

12-10-2019, 09:39 PM	#1
the ideaman Registered User Join Date: May 2009 Location: Central Valley,ca Posts: 174	480 horse Ls3 first start problems Finally started my 480/376 LS3 and it will fire, but almost dies immediately. Twice it continued to run when we disconnected the maf sensor, but hunted. It continued to run when we connected the sensor. Using the gm computer and harness. No codes read. Help! Last edited by the ideaman; 12-10-2019 at 11:11 PM.

12-10-2019, 11:55 PM	#3
the ideaman Registered User Join Date: May 2009 Location: Central Valley,ca Posts: 174	Re: 480 horse Ls3 first start problems Thank you, I'm using the GM tube designated for the LS3. Perhaps it's facing backwards. Will check this weekend. Last edited by the ideaman; 12-11-2019 at 12:13 AM.

12-11-2019, 10:09 PM	#4
the ideaman Registered User Join Date: May 2009 Location: Central Valley,ca Posts: 174	Re: 480 horse Ls3 first start problems The MAF was facing backwards. Now it starts and runs well.

12-30-2019, 03:02 PM	#5
the ideaman Registered User Join Date: May 2009 Location: Central Valley,ca Posts: 174	Re: 480 horse Ls3 first start problems Having put a couple miles on it I've got a few observations. First, it's really fast, like scare me fast. Anybody who tells you 480 plus horsepower is no big deal has bigger huevos than I do. Secondly, I need to move the throttle pedal up higher on the firewall and away from me. My leg is at a strange angle and it's very uncomfortable. We'll have to bleed the brakes again or get a vacuum reservoir for the power brakes. They are a little spongey and there is too much pedal before the brakes really grab. Finally, a hydraulic clutch is the way to go with a swap like this. No squeaks or groans. A nice linear engagement. Truck fires instantly and settles into a lopey idle almost immediately. It's a lot of fun to cruise around in.

12-30-2019, 11:24 PM	#6
mongocanfly Post Whore Join Date: May 2015 Location: Alabama Posts: 14,582	Re: 480 horse Ls3 first start problems Good deal....what do ya have it in? __________________ Mongo...aka Greg RIP Dad RIP Jesse 1981 C30 LQ9 NV4500..http://67-72chevytrucks.com/vboard/s...d.php?t=753598 Mongos AD- LS3 TR6060...http://67-72chevytrucks.com/vboard/s...34#post8522334 Columbus..the 1957 IH 4x4...http://67-72chevytrucks.com/vboard/s...63#post8082563 2023 Chevy Z71..daily driver

12-31-2019, 12:11 AM	#7
kipps Registered User Join Date: Jul 2014 Location: North-central Virginia Posts: 1,099	Re: 480 horse Ls3 first start problems I second Mongo's question -- what's the ls3 mounted in? Is it an engine from a donor car, or a GM performance crate engine? Do you have a clutch pedal position sensor connected? What sensor? How's it mounted? Any pics? __________________ 1987 C6P V20 truck, 2010 LMG 5.3, AFM delete, 2010 Camaro exhaust manifolds, 1997 nv4500, 1991 np241c, hydroboost, 2005 14bff axle & driveshaft, drop-n-lock gooseneck, 4.10 gears, stock suspension, rims, and tires. Still a work in progress. Any questions or suggestions are welcome!

12-31-2019, 12:18 AM	#8
the ideaman Registered User Join Date: May 2009 Location: Central Valley,ca Posts: 174	Re: 480 horse Ls3 first start problems GMC crate engine with captainfab's hydraulic clutch setup. McLeod hydraulic throwout bearing on a tremec tko. Attached Images

12-31-2019, 12:20 AM	#9
the ideaman Registered User Join Date: May 2009 Location: Central Valley,ca Posts: 174	Re: 480 horse Ls3 first start problems Just after 1st fire.

12-31-2019, 09:08 AM	#10
kipps Registered User Join Date: Jul 2014 Location: North-central Virginia Posts: 1,099	Re: 480 horse Ls3 first start problems Looks like a nice truck. Too bad it's hanging from the roof... Do you have a clutch pedal sensor wired into the engine computer? If so, do you have any pics of how you mounted that sensor to the pedal? __________________ 1987 C6P V20 truck, 2010 LMG 5.3, AFM delete, 2010 Camaro exhaust manifolds, 1997 nv4500, 1991 np241c, hydroboost, 2005 14bff axle & driveshaft, drop-n-lock gooseneck, 4.10 gears, stock suspension, rims, and tires. Still a work in progress. Any questions or suggestions are welcome!



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