driver side rear worst, rubber ate and decomposing
had build up of corrosive crud filling 1/3 of hole

coolant flush every 2 years, distilled water and new antifreeze

Repaired and back from a 2k mile 2 week vacation pulling new 19ft travel trailer Pacific Coast Hwy 101

had been losing approx 1 cup of coolant per month, getting worse
showed up in used oil analysis last oil change
tried GM Seal Tabs and Gold Seal powder twice with no success except making a gooey mess of the cooling system

removed intake manifold gaskets and replaced with the upgraded improved 2 piece Felpro kit design
steps
disconnect neg. batt. lead
remove air tube
set ac compressor off to side on top of radiator-13 mm socket
removed alternator
remove bolts from EGR valve and mount 10mm 13mm bolts
removed plenum-13mm socket
bled fuel pressure off and disconnected and removed fuel rail-T8 reverse Torx socket
removed fuel injectors-release electrical plug clip and pull up on inj.
removed lower intake manifold 10mm bolts
filled upper oil gallery with T shirt rags to catch debris
cleaned all gasket surfaces with wire brush and acetone

reassembled with Felpro intake manifold/plenum gasket set
new Motorcraft fuel injector O rings, KEMS would not seal
had light corrosion in fuel rail injector holes on driver side of rail, tried to clean by burnishing with crocus cloth but would not clean up and seal. used Permatex Hi-Temp fuel and O2 sensor rated copper RTV to fill gap at bottom of upper O ring on each injector and let set up solid to force O ring to maximum seating depth in fuel rail and position past the corrosion spots causing leaks

intake valves clean on back side of tulips, no buildup
interior of upper crankcase valley and rocker arms spotlessly clean

ran well on the vacation. pulled lots of hills on Hwy 101 and Sierras, never under 45 mph. averaged 14 mpg. best it's ever ran
used no coolant

GCVW gross combined vehicle weights 8,800 lbs van and trailer

In the Redwoods

 

Hope the holiday went well ! Very good account of the repair.
Did the contaminated oil visibly look different or the oil level change ?
Mine uses about 200cc or 7 fl oz/ month, you've got me thinking maybe I have the same problem?

ATB Aeroman

 

no visual sign in the oil
showed up in UOA high sodium, silicon, potassium and ethelene glycol

far too common problem in the 4L
OEM gasket design problem
i had head gaskets ready to put in but found the I. gasket problem first

 

There are 3 ways to solve your problem.

1. Welding with argone
2. Special molecular glue
3. New ore janked manyfold.


Use no acid in your coolant. :-))