Oil in cooling system

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2003 E350 extended cargo service van. 5.4L 338k miles. Never lost a spark plug or timing chain. NAPA Synthetic oil. Trany fine, fluid great condition today. This has been the best work truck I've had, bought in new and took good care of it, maintenance and driving.
There is some black oily substance in cooling system expansion tank. A few days later before appointment it started to blow the relief, so tried a new expansion tank cap 16 psi like original, still blew once.
I've gotten some mixed messages. One said it was probably the intake manifold gasket. Then another guy from the same garage said it was the trany fluid due to radiator.... Does that make sense?
Two other garages noted the pressure blowing when I removed tank cap, one said he could smell combustion products from tank pressure. They said it was a head gasket. That seems to make more sense because cylinder pressure pushed into coolant. What do you think?
.... If that's the case, what should I do? Pay for a head gasket job or put in a rebuilt engine because of the high miles? What approx garage cost quote could I expect to see for both?
I would like to buy a used Econoline just like it, but they are not readily available local-ish. Ford ended that Econoline.
Thanks ahead of time.
 
Originally Posted By: bdcardinal
Oil filter adapter gasket. Both coolant and oil pass through it.


My first guess, as well.

Looks like this:

20680253_fel_70415_pri_larg.jpg
 
Originally Posted By: CaptainFreon

There is some black oily substance in cooling system expansion tank. A few days later before appointment it started to blow the relief, so tried a new expansion tank cap 16 psi like original, still blew once.


Please explain this more....
 
I've had a slow leaking radiator for some time, been adding. After the last oil change, I opened to add, and saw what appears to be motor oil. Stuck my finger in and got black oilish on finger from inside wall of coolant tank. Then the cap on tank started blowing a few times. coolant was not overfilled. I bought a new 16 psi relive tank cap, and it still blew coolant out, most likely because cooling system pressure exceeded 16 psi, I felt the new cap confirmed that.
 
I wonder if that oil changing guy did something to the Oil filter adapter as explained above. I did catch saboteurs in the past doing other things to make a repair needed
 
That sounds like a blown head gasket, does it blow bubbles in the tank with the cap off? With that many miles I would stick a junkyard motor in it.
 
Once I did feel what I thought was air bubbles going through the return hose into the top of the radiator. Good idea about a junk yard motor
 
On my 5.4 it was the oil cooler itself. The gasket was okay, even after swapping it to the new engine, but the oil cooler filter housing failed.
 
Change the oil filter adapter gasket. If that does not resolve it then a head gasket or even crack in block or head.

How is the body and frame? If everything else is ok, I would go for a reman engine if it's not the oil filter adapter gasket.
 
A tranny fluid leak into a radiator trans cooler (side tank of radiator) shouldn't over-pressurize the degas bottle. This type of slow leak would first be found by seeing reddish brown oil droplets in the coolant degas bottle.
 
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