Welcome! Sounds to me like you hve an intake problem but need a lot more info. How many miles are on it now and how many since you bought it? What’s your typical type of driving?
Welcome! I would look into Alfaobd. It’s not nessasarly something that gets installed, it’s something that can get turned on and off. My truck, 2014, didn’t have the option to enable or disable them. With alfaobd I was able to enable the option for them and determine if I wanted my fog lights...
Sounds to me like water pump isn’t doing it’s job full time. You said you replaced the alternator in your other post, did you ensure your belt is routed correctly and your tensioner is keeping tension?
Other than your recent highway usage, are you a stop and go short trip driver or do you have a decent commute to work? I’d get a scan tool reader and check to see what code your getting
I wonder if the line joints are failing, not giving the injector enough pressure to fully do its job... deff interesting it’s leaving spots under the truck. Times like this makes me wish everyone lived closer so we could team up and check out these issues and brainstorm.
If you can get ahold of a code reader, that will better direct you to a solution. Modern vehicles are a lot more sophisticated than before and normally the code will pinpoint you to the correct fix. You can replace the entire emissions system for around 5-8k $ but if it’s just the def injector...