Engine oil leak at cold start | OBS 7.3L Powerstroke oil cooler replacement

After a month of winter traveling at high elevation in Utah, truck developed a nasty oil leak at cold starts. Leak goes away after reaching operating temp, yet continues to weep oil after shutting down and cooling off.

Oil cooler exploded assembly

The 7.3L Powerstroke’s oil cooler is a tubular heat exchanger between engine oil and coolant along the driver’s side of the engine block, directly below the exhaust manifold. When it leaks, oil comes out past the O-rings at the tubular section between the front & rear mount (which is also where the oil filter spins onto), then drips on the driver’s side of the engine cradle crossmember.

View of oil cooler rear, and block heater cord.

Cold temps below water freezing point causes O-rings to shrink. These original units with 285k miles will not seal in the cold with Motorcraft 15W40 synthetic oil. (Not able to plug into a block heater while off-grid overlanding.)

California base and 200ft elevation, the motor doesn’t leak like that.

Parts to purchase for complete replacement

Requires draining motor oil and engine coolant.

Issues encountered

Ran into a rounded 10mm hex head on the upper mounting bolt, on the forward mount of the 7.3L’s oil cooler. Without taking any power steering or A/C compressor components off the engine, a 10mm ratcheting box-end wrench (12pt) can reach the bolt head with space to ratchet.

Left it in place and instead disassembled cooler while still on the truck. The assembly has 3 pieces pressed together from the interference fit of the 2x O-rings per side. The 2 ends are elbow bulkheads of motor oil and coolant, bolted to the engine block.

Starting from the rear, there are 3x 10mm bolts through the rear mount, which is shared with the spin-on oil filter mount.


1. Unplug the block heater cord & unbolt the heating element.

2. Unbolt the coolant drain plug. It will unload a bunch of coolant.

3. Remove the 3x 10mm bolts to unbolt the rear cooler mount.

4. Locate the pry tabs on the oil cooler cylinder, and pry against the casting of the rear cooler mount until it comes off.

5. Loop a ratcheting strap around the oil cooler cylinder with a slipknot held in place by the pry tabs; hook other end of the strap to the truck’s frame, and crank it off.

  • Upper left. Cooler’s rear mount and gasket.
  • Lower left. New gasket to the front cooler mount left on truck.

6. Assemble new heat exchanger and O-rings to the rear mount (smaller diameter O-ring outboard, larger diameter O-ring goes inboard), by squeezing them together between a floor jack and rear bumper or frame of the truck.

7. Grease the O-rings and sealing surfaces to prevent tearing or pinching or twisting; anti-seize on the mounting bolt threads.

8. Slip this subassembly under the truck and into the forward mount of the oil cooler. Use the exhaust manifold as a pry surface, and push the oil cooler into place and seat the forward O-rings.

9. Continue prying forward and parallel to the block until the 3x bolts line up.

10. Slip the new gasket between the engine block and rear oil cooler mount,

11. Refitted the coolant drain plug with RTV sealant,

12. Reinstall block heater element and cord.

To further mitigate winter cold starts, switched from Ford Motorcraft 15W-40 synthetic to Rotella Shell T6 synthetic 5W-40 (https://amzn.to/3mjktiq).

Since coolant was already drained, replaced thermostat with a Ford Motorcraft RT1201 190*F / 88*c thermostat (https://amzn.to/2VavAOB).

Reused the relatively fresh diesel engine formulated Zerex coolant (7 gallons, https://amzn.to/2JOlNvq) by pouring it back into the truck’s coolant reservoir through a 100 micron filtration sock.

Fire up motor and check for oil or coolant leaks.

Published by Manolo al Sol

Learning by adventure.. will curiosity kill the cat ?

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