Hot but not hot, not overheating but check engine temp
I have a '07 supercrew 350 6.0 with 43,000 miles. I only use it to tow the 5th wheel. Since August I've had my first problems with this truck. At 43,000 miles I don't expect problems but, such is life.
Problem: I have a regular route I tow and am very familiar with how the truck normally handles this workload. I've never had problems towing this route even in 95F degree outside temp nor even when loaded slightly higher (I use weigh stations).
Twice I have seen the analog temp gauge run up to hot after the fan cranks up to highest RPMs. Before the needle hits the top I punch the dash buttons to get the digital system reading. All check are "OK" including engine temp. Once the analog needle get to the hot reading the check engine temp light comes on and I hear the bell. I take my foot off the accelerator and simultaneously I punch the dash buttons again to get the system check and engine temp displays "OK". What also happens immediately after I take my foot off the accelerator is the analog needle *immediately* lowers to the normal range (middle of the gauge). And, again, the system check says engine temp OK.
Of course the check engine light never came on and the truck never overheated. No codes at the dealer. The dealer did a fluid system check (pressumer check I suppose?). They said there is nothing to fix because nothing is broken.
I'm not a diesel mechanic but I didn't just fall of the turnip truck either. I've owned a lot of Dodge and Ford diesels over the years. I also know that a bunch supposeldy near-boiling fluid in a really hot block of steel isn't going to *immediately* drop temperature.
Finally, this tow route has never drivin up the analog gauge...ever. Not even with higher load and 95 degree outside temp. This problem happened again three weeks ago and the outside temperature was 47 degrees.
So, friends, since the dealer thinks nothing is broken, can you experts give me some talking points/ideas for the next time I hand the truck over for a full day and a few hundred bucks worth of scratch-head-and-make-bad-guesswork by the boys at the dealer?