engine knock on 1999 GMC Jimmy

i was driving today and the vehicle overheated. i checked the engine oil and it was 3 quarts low! had a problem similiar to this one about a month ago with the same vehicle. i took it to the shop and the mechanic told me that my spark plugs were not proper for this vehicle and of course the engine oil was low at the time too. he replaced the intake manifiold, changed the spark plugs. he also said my engine oil was contaminated with dex-coolant? i am getting very frustrated with this vehicle. it's old but it has only 126,000 miles on it.

Asked by for the 1999 GMC Jimmy
If you have oil in the coolant, or coolant in the oil, then you have internal engine problems which need diagnosis, or if it's really transmission oil in the coolant, then it's due to an internal leak in the radiator, where the trans fluid circulates for cooling.
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