bad head gasket symptoms vs bad coolant crossover gasket symptoms on 3.9l gm motors
 

admittedly i didnt search much...
to those who have had bad head gasket symptoms vs bad coolant crossover gasket symptoms on 3.9l gm motors:
how did you distinguish one from the other?
recently noticed oil mixed with coolant in my coolant reservoir... not even sure where to begin diagnosis...

notes:
-there is no coolant in the oil
-there is oil in the coolant... it is floating to the top
-currently the only bad operating symptom is that in ambient temps above 55F the temp gauge will bounce between 220F-225F in city driving. the fans kick on at 225 and the temp goes down to 220 and a few mins later the cycle repeats.
-during highway driving there are no bad symptoms.
- when ambient temps are less than 55F or so and with with the heater on it never/rarely goes above 220F...

i cant find any coolant leaks, the cooling system holds pressure...
i am 100% not sure how to diagnose this problem... i would imagine the dealer will take my ~$100 diagnostic fee and give it a head gasket death sentence... but what are the coolant crossover symptoms? how hard is the coolant crossover fix? how do you verify head gasket fail vs coolant crossover gasket fail?
thanks

leaking coolant
https://static.nhtsa.gov/odi/tsbs/20...25683-2634.pdf

as was said in the other topic you posted in ... compression test.

ah ok, yeah i did some better lookin...coolant crossover literally just gets coolant to the other bank... if those gaskets fail it will be coolant leaking/seeping only, no mixing.
should be fairly straight forward to rule out the crossover: no leak, no problem

i definitely mistitled my thread... coulda swore i read that ppl were confusing this diagnosis... but... ill eat my shoe on this one thank ya both...