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Coolant pressure under high boost

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What would be consider as “normal” pressure increase in the cooling system under high boost, by high i mean over 300kpa absolute or 30psi relative?

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If the head gasket is sealing properly then you should find that your coolant pressure sits relatively close to the rated pressure of your radiator cap (perhaps between 16-22 psi depending on the cap). If the head gasket is leaking then you'll see the pressure spike significantly above this, typically when the engine first hits peak boost. On the dyno you normally will know the head gasket isn't sealing because the engine will push water into the overflow during and after a run.

During a dyno pull i see a raise of coolant pressure to up to 2.5bar only when I boost over 2.3 bar Relative, and immediately drops back to 1.2bar but there is no coolant being push out the overflow, this motor is O ring as was intended for high boost , could it be that the head studs require re torquing? I have put over 50 pases on this motor at lower boost levels with no significant raise.

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Here is a screen shot of the pull

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I decided to inspect head stud torque and sure what I suspect torque was low , from previously torque of 90ft/lbs it was down to 67 in the center 6 studs and 79 the outer 2 ends, I re-torque back to 102ft/lbs , will test tomorrow and hope gasket still doing good

Hope you dodged a failure there, but a common reason for head bolts/studs to lose clamping force (torque) is the head gasket failing, rather than the other way round.

I'd suggest not loading it up again until you have a chance to replace the gasket(s), as what may be a minor weep could rapidly escalate to a 'torched' head and/or block that is rather more expensive and time consuming. Think of it like an earthen dam - if you have a tiny leak, and take care of it straight away, the dam isn't hurt, but if you leave it that tiny leak can very rapidly become a big one that scours away the dam - same thing with the head gasket, with the combustion gases being the water and the gasket/head/block material being the dam.

As Andre said, there should be negligible increase in coolant pressure, with any indication of it requiring immediate attention!

As with previous 4g63 builds is was common practice to run the engine for a few heat cycles at mild boost then re torque head studs and never had an issue after

I would agree, that's generally good practice even with gaskets that are supposedly just torqued when fitted without requiring re-torquing - but that is BEFORE there is leakage evident, which may already have compromised the seal.

As I said, I hope you dodged a more serious failure - but if you still have ANY signs of pressure buildup I'd strongly recommend pulling the head and taking whatever remedial action is needed. Sure, it's PITA, but so is having a head replaced or welded up.

Just test it again and seems like the problem is fix, I get a slight raise of 20kpa at the peak load area and rapidly lowering to 1.10bar which is my cap press specs.

For now this will do, turbo is maxed out anyway i think it will be safe to back target load to 320 from 360 and call it a day, next time we upgrade the turbo will freshen up gasket and block copper rings


That little bit may even be from the increased pump speed pressurising the cooling system slightly - easy enough to rul it out by free reving the engine to around those rpm.