Head gasket or something else?

[Rawze]

also the passages for coolant flow into the head at each cylinder in the head gasket for the CM870 are larger as well.

regardless... a lot of thermo testing has been done and indeed the CM871's run hotter on #5 and especially #6 if there is no coolant flow to the back of the block via the egr cooler circuit with it plated off.

Also, the CM871 and newer water pump suffers 70+ PSI of head pressure on the now, dead circuit at the back of the pump for that port, if someone plates it off because they removed the egr cooler. This face-loads the bearings and shortens the life of the water pump as well.


The coolant flow is more balanced in the 871's and newer engines overall too ... and yes, CM870's do run actually run slightly hotter on #5 and #6 as well ... even from factory.. but it is not as bad though with them due to larger passageways in the head gasket and more flow thru the older style of water pump.


also.. on all newer-designed 15-litre QSX and non-egr engines that do not have an egr cooler installed, they have in place, a factory bypass for just this reason.

here is a reference to this...
http://rawze.com/forums/uploads/201908/p...1e0970.pdf