Topic: Non-Bike question on leak down test on a 4cyl motor

[cafe400f]

Okay, so I'm asking around for a buddy of mine who has a replica Lotus 7, basically a Birkin with a ford ztec engine in it.

He redid the head and in the past month reassembled the motor, new cams, head gasket, adjustable cam gears, etc, etc.  The guy is anal as hell and takes his time in assembly, so I know it's done right.

He's having a problem with excess pressure in the coolant system, pushing out into the dump tank and then overflowing on to the ground.   So, common response is bad headgasket right?  But none of the typical headgasket signs are apparent, white smoke, etc.  He took it to a reputable shop, they work on mostly imports of the British variety and the owner is a racer as well, so they know their #$%*.  They did a leak down test and it passed no problemo.  So the question is, is it possible to have a good leakdown test, but still have a bad headgasket seal, theoretically?  I'm thinking no, but wondering what other motorheads think.

My suggestion to my friend was to pull the water pump, as he has never  had it off the motor.  Check the impeller and the gasket.  It could be sucking air into the system, but not leaking.

Thoughts?  Anyone??

[chickenman_26]

He needs to have each cylinder leak-down tested with the radiator cap removed and the radiator filled to the brim. Any sealing defect into the cooling system will show up as bubbles escaping at the radiator filler neck. There's also a kit available for detecting combustion byproducts in a sample of the coolant, but the leak down test alone should be sufficient. If he can't find any leakage into the cooling system, check for bad radiator cap, clogged coolant passages either in the block, head, or radiator, or trapped air in the block passages. Vacuum filling is necessary on some of these designs to eliminate trapped air.

Stu