The milky stuff is from vapor in the crankcase. Typical HG blow is from coolant passage to cylinder, however there is an oil passage nearby. Let's see some photos of the spark plugs. If the issue is cylinder #4, then you have a cracked block as there are no HG coolant passages on cylinder #4. At this point I would recommend a system pressure test, although it is possible the crack only manifest at higher temps you might get lucky and find the leak even at ambient. Pressure test it overnight and check each cylinder for coolant the next day. Do before and after pics for comparison, be sure to look at the downhill side of the piston, typically you will see a small puddle of coolant.

Regarding the thermostat test, you cannot water pot test a factory design thermostat conclusively. The problem with factory design thermostat is not whether it will open but whether the bypass valve will close. If the bypass valve does not close properly it will overheat due to the coolant bypassing the radiator. The bypass valve is not temperature controlled, it is combination of pressure and opening of the main valve.

 

Here is a picture of a spark plug from cyl 4 (below). They all looked the same on 2/4/6/8. On 1/3/5/7 they looked OK (didn't take a picture). The exhaust leak into the coolant happens with a cold engine already. If I keep the coolant cap closed and start the engine, the coolant hoses get hard quite quickly long before the coolant reaches 180F. I think that rules out a broken thermostat.

Where do you think the oil comes from in cyl. 2/4/6/8? There is a small puddle of oil in cyl 2 visible (see picture cyl. 2) after 24 hours of resting already.

 

Compression test? That's the first thing my mechanic asks.

 

Well hoses getting hard that fast is generally a sign of pressurization from compression, given all the spark plugs look the same I would lean to head gasket and warped head, rather than a cracked block.

 

Well your spark plug is not steam cleaned, but it sounds like you really aren't driving/running it enough for it to get steamcleaned.

So the combustion gas in to the coolant can happen with a cold engine running because of the higher pressure (150psi), my comment regarding cold vs hot was related to the cooling system pressure test which is only capable of around 20 psi and it is pressure from the coolant side to the cylinder side, running it is the other way around.

You clearly have a combustion gas leak in to the cooling system which is not uncommon, but the question at hand is from where. Blown HG is best case scenario and easily repaired, but your original description of misfire on #4 cylinder (possibly caused by excess coolant in the cylinder) at the same time is not usual for blown HG because there is no coolant passage through the hg around #4, only the end cylinders have coolant passages through the hg. You can disassemble the engine and if the HG is blown on the end cylinders it will likely be obvious. However if they are not, then there is the question of where is the leak. Although rare, there have been cases of cracked cylinders and even more rare cracked heads. The engine block and heads can be pressure tested with the heads off, but not as easy as with the heads on. With the heads on it is an easy test by pressurizing the entire cooling system through the coolant bottle. In a normal condition the system will hold pressure overnight with no pressure loss. In cases of blown hg a pressure test will typically result in the coolant being forced in to the cylinder during the overnight test and you can see it in the cylinder as a small puddle like this (BTW this was cyl #4, cracked block).



All of my comments regarding your thermostat relate to the original cause of the overheat, not the current work of diagnosing the overpressure/overheat situation although that also points to bad thermostat.

Simple physics dictate that a running D2 engine full of coolant cannot overheat if coolant is flowing through the radiator. Full stop. True even if the hg are blown or block is cracked. The engine cannot generate enough heat to overcome the cooling of the radiator. Now if the radiator is plugged, or the thermostat limits flow, it will overheat. You have thermostat problem in addition to your combustion gas problem but you probably will not be convinced until you remove it completely from the system and run the truck with no thermostat installed.

 

Will an engine from a 2000 P38 (it's a 4.6l Bosch) fit into my 2004 D2?

 

yes, it will

 

it's the exact same engine, though probably with higher build quality as they were built earlier before the production tooling began to wear out.

P38 4.6s don't have the reported issues that the D2 4.6s have.

 

Exactly, and the blocks are the same so you can take all of your 4.6 internals and put inside of a 4.0 block. The order of desirability is easy and the exact inverse of year model, so 99 is best and 04 is worst.

 

not to detract but “production tooling began to wear out” is not a legitimate excuse imo. There has to be another reason our D2’s have the issues.

It just sprang make sense that an engine manufacturer would let worn tools build crappy engines for years