2002Express 3500 5.7 engine/trans oil coating on inside of radiator filler neck under cap. Loss of oil at 1/2qurt per month. Oil pressure dummy gauge rests at different places randomly. Performed combustion leak test on highest point of cooling system (coolant reservoir) for 2 minutes. No indication of exhaust in coolant. Also no bubbles or circulation visible with radiator cap off. Temperature dummy gauge stays very near bottom until engine is stopped and then the needle shoots for 220°.
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First step is to verify which fluid is leaking into the coolant. Best way to do that is to perform an engine oil change. Does it have coolant in it? If not it’s most likely not oil. Check your transmission fluid dipstick and look for discoloration. Trans fluid will be red or orange tinted when new, light brown to black when old. Shouldn’t be super “runny”.
I am unsure if the 5.7 express has a transmission cooler but my 6.0 express has a transmission cooler that connects to the radiator and has a radiator passage that acts as a heat exchanger. It also has an oil cooler that does the same thing. Only place that could be causing oil or trans fluid and coolant to mix is at the radiator if it internally ruptured.
These engines do consume oil and to be honest for a 02 5.7L 1/2qt isn’t unexpected. As piston rings wear or trap oil sludge they allow blow by and allow oil past the rings into the cylinder where it is burnt.
Actually there is one more place on these engines where coolant and oil could mix… if you have a head gasket failure between the coolant jacket and oil gallery that would also do it. Drain oil, pressurize cooling system and look for coolant leaking out of the oil pan. If so you found it.
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