You know, reading this thread about EGT's, etc. brings this to mind. How many vehicle owners actually read their owners manual where it says to let you engine idle for a given period of time prior to shut down after specific driving conditions? I'm willing to bet at least 50% or more never get the manual out of the glove box. I said that to say this. While idling my truck in the parking lot at work one day, a co-worker pullled up in his Ford PSD and immediately shut it off. He saw me sitting there with my truck running and asked what I was doing. I preceeded to inform him about EGT damage, turbo failure and the like. He said he was unaware of this procedure and always just turned his truck off like any typical gas engine vehicle. I asked if he read his owners manual. You know the answer to that! Anyway, now he idles his truck before shutting it off.

I agree with some of the previous statements about driving these trucks blind...most people do it. I think the manufacturers realize this and have enough margin built into these things to cover excessive EGT's, etc.. As long as the truck is stock, I think they are pretty hard to hurt and they will give years of reliable service. Bombing? Now that's a totally different subject all together!

Those of us on these forums make up a very small percentage of diesel truck owners. Most owners don't even know places like this exist...too bad for them. And truth be known, sometimes too much information is just too dang much. You find youself second guessing things you normally wouldn't just because there's so many different opinions on any given subject. But overall...I dig these sites!

 

Absolutely correct. There are a lot of piston aircraft out there that still run probes in every cylinder because of significant deviations in both cylinder head and exhaust gas temps. I realize that there is a world of difference between an air cooled aircraft engine and what is in our trucks, but I would bet that there are similar cylinder to cylinder differences.

 

I observe this all the time. Gas stations, parking lots, convenience stores - heck, you oughta see all the landscapers around here who drive a PSD or a Cummin, fly into the Wawa parking lot for their coffee and sandwich, and shut her right down on the 90* days. And we won't even mention the UPS drivers.

THat being said, I still wait for cooldown.

 

Spudman,

I don't understand your point? Are you argueing with me that my truck (when it was stock) could not reach 1300 EGT? I have watched my EGT guage hundreds of times hit 1300 pulling big steep passes. Ask anyone else who lives anywhere above sealevel and they have seen the same. I think you are missing the point.

Just because the EGT is 1300 does not mean the piston temp is 1300. The oil jets provide cooling...plus there are a few other really technical thermo reasons why the piston never sees that temp. There are a lot of Cummins users in the mountains...I would doubt if they did not take this into consideration.

 

Have any of you heated up a bar with a torch? The bar does not instantaneously turn red hot and melt away. Even when exposed to temperatures of many thousands of degrees, heat transfer has to occur to allow the material to heat up. Additionally, the use of a heat sink during soldering, brazing and welding can considerably reduce the amount of heat transfer and increase the resistance to heating and subsequent damage to surrounding parts. Therefore, just because a aluminum piston is exposed to temps in excess of the melting temperature for a very brief period during the engine cycle does not necessarily mean you are going to have a melt down as heat transfer has to take place and the oil galleries in the piston, interface with the cylinder, etc. all make very effective heat sinks.

The turbo on the other hand is not only exposed to the full brunt of the engine heat from multiple cyclinders, but has little more than the shaft, bearing and a little bit of oil to convey all of that heat. Under sustained high egt's it is conceivable that the blades on the exhaust side could deform or the leading edges deteriorate--throwing the assembly out of balance. Once unbalanced, the speeds in question that the turbine spin can cause it to come apart and the result is sucking components of the turbo into the intake. I believe that this is a much more plausible failure mode in most circumstances than the piston melting on you....

 

Ok what is the correct cool down time?

I let mine cool down for about 30 secs to 1 min. before i turn her off after a highway run...not towing.


This OK???

 

doitl8r, cooldown times are listed in the owners manual.

 

I can tell you one thing from experience, ($11858.00 worth of experience as a matter of fact) the piston melted and the turbo when they pulled it off the engine was fine and could have been re-used. I have the piston on my desk at work for proof if you would like to see a picture of what melts and scores.

 

I'd love to see a pic of that. I never let my truck cool off before I shut it down. I figure that there are like 100k people out there that don't and they are fine. I've got 67K hard miles on my hy35 turbo (read 35psi and tons of barking) and it's fine. My engine runs like it did from day one. Now, I do have a scotty 2 and 4" exhaust to help keep things a little cooler. There has to be a safety margin built in just for ignorant people.

 

The temperature of the exhaust gasses exiting the cylinder is not the temperature of the piston.

Exhaust gasses can actually rise as they exit the cylinder due to secondary burn and exposure to very hot (sometimes red) exhaust components.

I do not believe you ever could get a piston to melt running sustained 1300 deg as long as all systems are functioning properly. I also would consider the 1500 number that Cummins engineers stated as OK for short spikes. That is heat going out the exhaust, not piston temperature.

The big boys will tell you they hit 1700 all the time... It's true. No pistons are melting because they simply don't get hot enough to.

Most aluminum melts below 1200 deg.

Paper burns at what 500? Fill a paper cup with water and put a direct flame from a torch on it. Guess what. The water will not allow the paper to exceed boiling point of 212F so you can boil the water in a paper cup with and open flame of 2000+ degrees. If the experimemt was onside a box and you had a egt gauge on the flame and did not see the cup filled with water you would assume the paper was history cause paper burns at 500F.

Point being.... many thermodynamic properties of mass, pertaining to aluminum, with respect to heat dissipation, oil jet cooling, liquid cooled cylinder walls, keep the pistons below melting point even at sustained 1500 deg egt. That is the true limit that the Cummins engineers see in the labs.

My Dmax would hit 1500-1600 once in a holeshot. The aluminum heads on it are key to removing heat and transferring it to the block and coolant.

One thing that was not mentioned here is what happens to the lubrication on the cylinder walls and around the piston rings when/if pistons themselves reach 700-800+ degrees. I could imagine a breakdown and coking leading to scoring and friction buildup.

 

Hoot, if you do enough heat cycles, something will give. It may be 10, or it may be 100, maybe more, but something will fail. Thats the point.

Don't forget that the "big boys" are continually tinkering/working on their engines. I've read often enough here that the big boys do hole pistons and blow headgaskets etc..... So not sure that you saying that the big boys hit 1700 degrees means its totally safe across the board. 'Specially hauling the family and RV back and forth across the country.

 

Have to toss in what I got directly from Cummins.

I asked what are safe sustained egt temps in my SO.

They responded that under full load, this engine will see egt's of around 1300 degrees F. This is pre-turbo.

They also said that post turbo temps will be in 700 to 900 degree range.

Also they mentioned that "full load" was while running in the power band of the engine(between peak torque and peak horsepower) and not under lugging conditions(not being able to accelerate).

Maybe this info. will help answer some questions.

 

The piston didn't melt during shutdown, it melted pulling 21000 pounds up a 6% grade at 72 miles per hour with 30 plus pounds boost so the cool down before shut off is not an issue.

I go back to my original reply, I have a melted piston top which then scored the piston causing the engine to develop a serious knock. The turbo was fine when they pulled it off.

I don't care how hot the exhaust gets going through the exhaust components, the piston is inside the cylinder and it is definately melted and it is in the travel path before the turbo. The fellow asked me what would melt first and I gave him my personal experience. I will see if I can remember to get a picture of it and post tomorrow at work.

 

Dieseldude4x4,

Do you think it is fair to compare a 99 to a 3rd Gen? I believe they are not created equal...one has more oil jets/gallery cooling. Maybe that is why some feel 1200-1250 is the max cont for 2nd Gen, and 1300-1350 is the max cont for us new guys....

 

I realize he has a third gen and I have a second, and a first.
His basic question was which melts first. I realize the engines may be different but he still has a diesel engine the same as I do. I suspect that the t-stat stuck in mine which, knock on wood, never happens to his or yours or mine ever again. I didn't have an EGT gauge at the time so I don't know what the temps were running but the bottom line is that I have a melted number six piston and the turbo was fine.