will a bad thermostat ruin my engine
If stuck open,no,if stuck closed yes,if truck is heating up,then it may be stuck closed.
Changing a thermostat on a 2004 Yukon is a relatively simple job fro any mechanic or do-it yourself-er. If you suspect a bad thermostat, I have to recommend changing it.
Stuck open will still ruin it just at a much slower rate. Sludge will build up over time and it will never run efficiently and run rich constantly and pollute the oil more quickly. We are talking about years tho. Stuck closed will kill it in minutes
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