Diesel Jay

Sorry, should have noted that in a properly running direct-injected diesel, the star pattern will always be evident. Quote: The way it has been explained to me, the injection should(in a perfect world) be contained inside the bowl.. Different loads may contribute to a variation of how intence the patern might be, but as a rule if the star pattern reaches the outer edges of the piston, it is due to a programer changing the PW and timing, and will begin to burn the edge of the piston.. In my opinion, its not clear enough to say "pay me, you added a chip".. There was more truth to this in the past. Combustion was never completed contained within the bowl, but it was more so than with current engines. However, with today's retarded timing and the use of EGR, a lot of combustion occurs after TDC. With the piston traveling downward, and the fuel still burning, you're going to see that pattern expand beyond the piston bowl. As an aside, the whole science behind piston bowl shape and design is very indeterminate. There are a lot of theories to describe how fuel gets moved around in the combustion chamber, with spray patterns, swirl, tumble, cone angle, etc. However, it's very rare to find the theories actually supported by the testing data. So, piston bowl design is really more trial & error than anything else. That also means, however, that trying to contain combustion completely with the bowl is akin to capturing lightning in a bottle. If you have no idea as to the optimum shape, size or placement of the bottle, it's just dumb luck should you happen to succeed. /forums/images/%%GRAEMLIN_URL%%/smile.gif - Jay

Every DI diesel will display a star pattern -- stock, or not. However, Nav does have engineers who analyze the patterns as part of their failure analysis. Those guys can tell if timing/pressures were altered, or if too much fuel was injected, based upon the placement and shape of the pattern, as well as any damage/melting of the piston. Thus, you'll always find a star pattern.

Huh! I didn't know the grease/lube also reacted to the UV lamp. Learn something new every day! /forums/images/%%GRAEMLIN_URL%%/smile.gif Later, Jay

Nav did some checking into this last year, and found that the majority of the "bellhousing" leaks were related to assembly lube leaking out of the torque converter, over time. These leaks generally appeared as "stains" on the bottom end of the bellhousing, and some trucks take quite a few miles before they fully "dry up." Their main thing to watch for was for actual drips, and/or the presence of the UV dye in the oil. Either could indicate a rear main leak, but they also found many other sources, as y'all have noted. Regards, Jay

I only replied to the quote to note how the problem could with "a stock truck that works really hard and pukes coolant" as well as "a modified truck that pukes coolant?" /forums/images/%%GRAEMLIN_URL%%/smile.gif Regards, Jay

Quote: Okay, sorry to drag this topic from the technical to the political but I have recently seen a new resurgence of posts on the "other" web sites about warranty. Specifically, What's the difference between a stock truck that works really hard and pukes coolant and a modified truck that pukes coolant? Nav has been doing a lot of testing on this issue, and it boils down to overpressurization of the power cylinder. Obviously, mods can cause this, but it's also possible with a stock motor. The most common cause of the problem is heavy acceleration on a cold motor. On a cold motor, there are a bunch of "correction factors" to deal with cold/sluggish engine oil, and it's effect on the injectors. There are also factors that prevent white smoke, and others that account for the fact that some trucks are sluggish when cold. Under certain circumstances, when all of these factors align just right, it's possible for a stock motor to get too much fuel and timing, thus exceeding max CP. That may stress the head gasket, and lead to failure, though the symptoms may take some time to appear. There is some testing being done to determine if altitude also plays a part in this. So far, that does not look to be the case. Both high altitudes and cold conditions generally warrant more timing, but altitude also works to lower CP since the engine doesn't breathe as well. Al things being equal, the whole in-cylinder pressure trace is reduced because the engine starts with a lower manifold pressure. The tests seem to bear this out. So, the most obvious answer to this problem on a stock motor was that it was floored or heavily loaded when cold. Engine oil needs to reach about ~60C to be safe. It has nothing to do with the various rumors of bean counters reducing the # of head bolts, quality of the gasket, etc. It has everything to do with poor code control in respect to the design limits, and mfg'ing variability tolerances. Apparently, some MY and PCM revisions appear to be worse than others, but I don't have any data as to which. Regards, Jay PS -- The 7.3L also suffered slightly from this same issue. However, the 7.3L injectors were better in the cold, and the motor had a higher "safety limit." PPS -- Every Nav engineer I know notes that they consider the 6.0L as being "chipped from the factory." None recommend any sort of power mods.