How is it possible to run lean on #7 and #8

#25
I realize my setup is not quite the same as yours but i know your pain. ever since i put my procharger on i have blown head gaskets every time i get on it. i have now blown three sets in around 300 miles and every time it has been on #'s 6 and 7 and at least once i think it was eight. in between the first and second set i went to 24lb injectors from 19lbs. and installed a 255lph fuel pump. didn't make a bit of difference. at WOT my fuel pressure never drops below 90psi (still using FMU, flame me at will). oh, and this was also with two different engines. i put a 96 explorer long block in after the second time. i just got done putting on another set and i have driven it maybe 50 miles but i haven't got on it yet. i know that i'm going to have to go to 42lb injectors and a custom program, and i'm also wondering about going to a larger fuel rail. my theory is that under WOT those two injectors are furthest from the supply and therefore may be getting starved through those tiny stock fuel rails. once again, just my theory. oh, and one more thing to tantalize you-i'm a ford mechanic and almost every time high mileage 5.0's come in the shop (usually grand marquis and town cars) and they are missing, it's because #7 or 8 spark plug is oil fouled. seems they are always the first cylinders to wear out. go figure.
 
#26
I dont care what anyone says, but running too rich will never blow a head gasket, and having too low compression will not create a lean condition. Secondly, like I said before, are you sure your running lean because all you keep saying is that you are blowing head gaskets. Running lean isnt the only thing that blows head gaskets. Lastly, how do you know its the head gasket. I dont see how on Earth you keep blowing gaskets if you keep using new bolts, and everything is decked and true and your torquw specs are right. Obviously, you dont have all the answers needed, and furthermore with gasket blown like symptoms and low compression shared in 2 adjacent cylinders, it sounds more like a cracked head or block. Face the facts. You need to find out if they are really running lean because it sounds like you just guess they are. Have you ever thought, that after assuming something and trying to fix it 3 times, that maybe your original assumptions are wrong. If the engine was just rebuild, and everything is decked and straight and you followed all correct procedures to installing a head,and your electrical is good and your injectors were just rebuilt, your not running lean. I'm going to go with a major crack somewhere.
 
#27
If oil is getting into the cylinder it significantly decrease the octane on the fuel, hence the lean condition and this will cause head gasket to blow.

#7 on most 5.0EFI motors is the lean cylinder due the to way the intake design is, the GT40 isn't the best for fuel distribution, the TFS and Holley are better but you still see most guys checking the EGT on #7, and #3.

Make sure you PCV valve is good and you have proper venting, if not this may cause your oil problem. What does the gasket look like when it blows? Do you have a pic? What gasket are you running? What head? The 3 time sthing blows my mind, something is obviously wrong with something. We found the STOCK GT40 cast iron heads on our 93 Cobra had 1/2" holes (not 7/16" like stock) after blowing the gaskets twice once the blower was on the head would lift.
 
#28
Interesting. Well I know the running lean on the 2 cylinders is a long shot. The heads and block have been checked for cracks and came out fine according to the machine shop. The heads have been milled several times. I don't know if that makes a difference.

I would hate to buy new heads and that not be my problem.
 
#29
did they shve the heads enough that you had to get shorter pushrods, this is surely not the answer to your problem since more than just those cylinders would have no compression too.
 
#32
i forgot to add earlier that i agree, a rich condition will NEVER blow a head gasket. lean conditions cause extremely hot cylinder temps and rich is the opposite. also, the low compression causing a lean condition thing is a long stretch. all i can say is, if i spend all the money to get 42lb injectors and a custom program and it still blows head gaskets, i'm gonna be mega-torqued. :bang:
 
#34
Have seen same problem om 351's between cylinders 5&6. the conclusion was that the two adjacent cylinders fired in consecutive order but both cylinders were firing at the same time due to cross arcing of the plug wires. may also be due to cracked distributor cap.this causes detonation on one cylinder causing blown gasket.gasket failure will usually be between the cylinders. Not saying this is your case but check the cap and the wires. is some suspicion since cylinders 7&8 are in consecutive firing order.
Later,
Dave
 
#36
My wires are bunched together and not in seperators, just zip tied. Makes sense. Also, I will have to rig up a fuel gauge to make sure my psi is actually 40 psi @ WOT, its set to that, but who knows when im driving down the road if my fuel pump is keeping up. 7 and 8 and the farthest away. The shop also said that i was running rich at idle but lean at other rpms.
 
#39
im not the best tech but could it have any thing to do with a piston ring being installed improperly or screwed up smell your oil does it smell like gas that could cause you to run lean.....or get a fel prohead gasket and arp head bolts youll find out what it is when you brake it
 
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