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12v Cummins new head gasket issue... this sucks.

Old 03-20-2016, 01:33 PM
American Thunder's Avatar
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Default 12v Cummins new head gasket issue... this sucks.

I have a '78 Bronco with a non-i/c 1991 12v Cummins. With 230k miles on it, a few months ago, it began pushing coolant from the overflow tank, which I diagnosed as a failing head gasket leaking compression into the cooling system. I got it torn down, and found the reason for the blown HG.. one of the center head bolts had broken in half. All the surrounding head bolts were black with exhaust soot, as it must've been leaking compression for a awhile, and I could see in the gasket where it had been leaking, yet the motor had still been running good, other than the coolant pouring out of the overflow. Anyway, I had the head hot tanked, ground and resurfaced(He said it was warped slightly, by .009" so he took off enough to correct it), valve job done, 60# springs, new seals, by the machinist who has always done my machining work for the last 25 years.
I bought the arp studs for it, and as dictated by arp, I had the rocker pedestals machined .200" so the head stud nuts will clear the valve covers. I cleaned and inspected the block surface with a scraper and scotchbrite pad, installed a new head gasket, part #HS4068 from Mahle/Victor, and the studs with proper arp lube, bottomed out in the block and backed out 1/2 turn each. then torqued with a calibrated snapon torque wrench in 60, then 90, then 125 ft/lbs, in the proper sequence. I reinstalled everything, the motor fired up and sounds amazing, smoother than it used to, and less smoke. I let it warm up, got all the air out of it, the heat was working good, motor stabilized at 180 degrees, I ran it up and down the street some, parked it, checked for any leaks, it was perfect. The next day I checked it over, looked perfect, took it out for a run up the street and back, inspected it again, now I see a trickle of coolant seeping from the front of the head, AND I checked the sides, and saw at least 2 more small trickles of coolant coming from the passenger side rear, and passenger side center of the engine, running down the block. Leaks get worse when I drive it and boost comes up, so it must be pushing some compression past the gasket and forcing coolant out the sides. This isn't just 1 cylinder, obviously, this is at least 3 cylinders doing it. I checked and rechecked my work as always, and I did everything exactly by the book. I'm pretty pissed off at this point. If the gasket had a defect that I missed on my inspection, it wouldn't be leaking from multiple places like it is, so a defect doesn't seem likely. What is any other possibility besides an improper head surface finish? How critical is it on these engines to have the factory precise microfinish on the head surface?
Old 03-20-2016, 06:51 PM
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the block needs to be within spec also...if it is warped a little then you will have a leak..same as if the head is out of spec...scott
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