I'm running a stock LS6 intake......FAST's are known to seperate under boost, that could be a major culprit, and stock headbolts definetely don't like serious amounts of boost. I use MLS gaskets, but with ARP studs...I'm pushing 20 regularly....

 

im aware of the fast situation but this motor was never suppose to see large amounts of psi it jsut happend. still trying to figure out why my gate isnt working properly. i know should of used studs but again i wasnt planning on large psi. also figured its cheap insurance as to not wipe out the motor. again motor runs fine for most part. AFR is dead on for cruising just its odd when any boost around 10 is encounterd. after that it idles funny like its being mufflerd and then only seeing 9in of vaccum

 

When I lifted the heads on my 4.8 it would immediately push coolant out of the over flow. There would be coolant all over the side of the truck and the pass side of the engine bay. It only did it when I went above 11-12 PSI of boost the rest of the time it drove and ran fine. It would kinda stumble a little after it did it, but it also got some coolant on the air filter when it did it too. It's normal after driving around for the cooling system to have pressure in it, just when there is exesses pressure from boost leaking into the coolant system that it pushes coolant out the over flow there is problems.

 

so what does this sound like a bad fast? yesterday i did noticed a hickup also while in some boost [like a pop].

 

It's hard to say...

 

well i believe i found my culprit. i did a compression check and motor checked out. lowest was 140 and highest was 147. plugs looked good and werent wet. everything seemed fine. more and more i thought of it i thought it could of been a vac leak. As the motor went to 9in idle jumped up some. so i pulled the fast and flipped it over. I had purchased a fast that had been cracked on the bottom. i had used epoxy to fit the lower part and on inside i used RTV over the seems. So when i flipped intake over i had seen one epoxy not attached to intake anymore. so i removed TB and noticed the RTV had been loose. so i got my air line and applied air to the crack on the bottom and felt air coming in the intake. so makes sense why it was intermitted. so im in the process of swapping on my LS2 intake. Fast is on the shelf for now till i build the camaro's motor and then ill fix it. so ill know for sure once i start the truck up tomorrow

 

Glad it wasn't a serious issue!

 

ya i was worried cause i would be out a motor. I just recently sold my 408's block. but as long as motor is fine ive decided to not race the truck anytime soon. as much as i want to i just cant afford to hurt this truck. i want to enjoy it awhile.

 

alright was out running. seems better but still have a leak or issue somewhere. intake is ruled out. now will say going from a ported fast to the ls2 intake. the tune is way rich now. need to pull fuel out.

 

glad you found the problem.

stock mls gaskets kick ***. i haven't heard anyone having problems with the stock bolts either but most swap them to studs since you are not suppose to reuse the stock bolts.