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I finally got my 6.7 cleaned (engine shampoo) at the dealership this morning. It cost $49.95 and the engine looks like it is back to new. I was worried about the diesel and didn't want to try it myself. Service manager said they do a few and never a problem as they are careful. So now the truck is looking good outside and looking good under the hood. Next step is it to clean the interior, especially the back seat. I tend to pile things in my truck and it looks like I live in it at times.
I didn't think others were washing their engines as I thought everyone was reluctant to do that. And when I asked on this forum the response was be careful with the electronics and power washers. I thought the diesel was more of a risk than the gas engine. It took a while but glad I had it done.
The owner's manual shows what to cover, such as the fuse box, etc. before washing. I too, use Simple Green and hose sprayer and it does a good job. I blow off the excess water with a leaf blower so it doesn't leave spots on everything.
I'd be less worried about a diesel than a gas engine. I personally wouldn't do it while running however, I would have it covered over, and would rather leave it overnight (to dry any water that may have gotten where it shouldn't have... i.e. electronics) before starting it.
I clean mine with Simple Green and spray off with garden hose. Wipe off residual water including those pesky little cups on the plastic manifold. Then I drive the truck to ensure everything dries fairly quick after the cleaning process.
I've been doing this process for years and never a problem, and the aluminum components don't oxidize either which keeps the appearance looking good.
I did mine also following the instructions in the manual which shows what to cover up. It's not bad at all. Don't sweat the petty stuff, pet the sweaty stuff. Snug up the sac and go for it.