Engine/Transmission (1998.5 - 2002) Cooldown EGT - Truth or Internet Myth

[NoSeeUm]

I am not trying to stir the pot here but I do have a question for clarification on the terms pre and post.



In my mind Pre Turbo is between the cylinder head and the turbo in the exhaust manifold and Post Turbo would be after the turbo on the down pipe side.



Is this everybody's understanding? I only ask this because the following statment make me wonder



The pre temperature can be below 250F and the post can still be in the low to mid 300F's.



I do not have two gages but I do have my probe in the exhaust manifold between the cylinder head and the turbo and understand this to be the PRE position.



I guess I am having a hard time grasping the pre below 250F and the post of 300F's. My understanding also is that you loose about 100* of temp for every couple of inches you move down the exhaust system. So how would the down pipe be hotter than the exhaust manifold



Am I making sense here, or have I missed something

Sorry I should have been more clear. My pre turbo T/C is installed in the exhaust manifold cylinder 6,5,4 side common tube. My post turbo T/C is installed in the port provided in the Banks exhaust brake, which is essentially the exhaust elbow. As I recall it is about 1 inch downstream of the flange.



You are correct. For normal running you do see 100F to 300F higher temperatures pre turbo than post turbo. I was talking only cooldown by the statement:

The pre temperature can be below 250F and the post can still be in the low to mid 300F's.

I should also explain this a bit farther when I talked about exhaust gas temperature and metal heat soak temperature. Again, I am only talking cooldown mode. For example, believe it or not, I have noticed that at infrequent times the post EGT will actually increase in temperature when the engine RPM is dropped to idle. Not by a whole lot, maybe 10F - 30F. Keep in mind the Banks brake has more metal mass than a stock elbow.



This makes me think that the post T/C is reacting primarily to EGT if the flow rate is higher. When the exhaust gas flow rate is lower, like at idle, the metal temperature close to the T/C has a greater effect. At any rate the pre EGT has to be cooler for some time, the length determined by engine load prior to going to idle, before the post EGT approaches the same temperature. Hence 250F pre turbo, which is about my lowest EGT that I see after the engine is warmed up.



If you feel like it, check out this TDR thread and read some more of my confusing blabber and the enlightening comments that follow by other members.



Jim

 

[NoSeeUm]

Having worked on jets in the Marine Corps the theory or the turbin taking work out of the exhaust gas is well versed with me (Helicopters).



{snip}

My experience is with frame or aeroderivative industrial gas turbines. The frame turbine will operate close to 1100F EGT normally. On shutdown, it does a 5 minute fired cooldown where EGT goes to around 700F. This turbine has 16 EGT T/C's spread radially in the exhaust gas flow path. These T/C's are well seperated from the metal exhaust duct. You can clearly see the heat soaking effect after the fires go out. The upper EGT T/C's will rise to 200F - 300F higher the lower T/C's.



Jim

 

[bmoeller]

I was talking with the shop forman of the local Freightliner dealer one night and this topic came up. He said that modern oils do not suffer from oil cokking like older formulations did. And for this reason you do not need to let your turbo cool down. YOU DO need to let it cool down for this reason. Diesel engines created alot of sulfiric acid in the oil as part of the combustion process and that you should let the OIL cool down to release the high acidic content. If you do not the hot oil with the high acid levels will eat your babbit bearings and pit them. Makes sense to me, just passing it on.

I doubt letting the oil cool down is going to make a difference in the oil acidity levels. That is what the additives are for. If you dump milk into acid Or say you are eating spicy food), what does it do? Calicum helps to neutralize the acid. Water just washes it around.



The heat still affects the turbos, if they aren't cooled off a bit before shutdown. My boss' brother does that all the time. Once he quit driving for us, I haven't had to replace a single turbo due to the seals and bearing taking a crap on the exh side. Used to replace turbos every 50k miles on his truck. Probably 150k miles on the current one on that truck. Last one I replaced, was due to the compressor wheel snapping in half.

 

[cojhl2]

I suppose there is a mathimatical formula someplace that would give a graph of what was going on and two gauges would probably allow for some diagnosis should the turbo start going out.

Absolutley quantifiable! I don't know how to do it since it's been over 40 yrs since I've been to college and my career did not require me to keep up with it. Knowing the calculations one could determine the work done on the input gas (induction air) and the cost in fuel to do that work. .



Wish I would have kept up with that stuff but i've forgotten all but the very basic knowledge.

 

[cojhl2]

I have noticed that at infrequent times the post EGT will actually increase in temperature when the engine RPM is dropped to idle

This would definitly be true because the turbo is not absorbing work to compress the input air. So,, the output temp would climb to closer to input temp.



I can't figure out though how output could ever be higher, except from the metal mass of the turbo itself as you've indicated.

 

[DLane]

Internet Myth??????

It was a practice in MY house when the intenet was a myth. 300* on the pyro was "beat" into my head as a young child as I was the cool down timer On a side note, in the PHX metro area in late July till mid Sept outside temps may not get below 100*... ... ever!!! even at night, If I Always waited for these temps my pickups may never turn off which would be just as detrimental. So I have come to the unverified conclusion that this ? comes down to a combination of knowledge and a little common sense..... Only you know how you just drove your rig... . Where you working it's guts out the last few miles or did you Start taking it easy as you got close to your destination? Where you loaded to max GVWR or just have the weekly load from the SAFEWAY in the back seat? Make your shutdown a combo of Temps,(engine,exhaust,ambiant) Load, and common sense.

The next ? is..... What would your insurance co say if your rig was stolen while you were in walmart and your cooldown box had your truck at idle???? Would it be the same as doing the TWO key method, when you run into the store for milk and bread???? Everyone with a standard trans knows these new trucks will idle right thru the gears and your truck is just as gone... ...



As you all ponder this thread REMEMBER THIS... ... Driving Diesels becomes a way of life, AND THE BEST PART IS... ... ..... Keep a new TDR in the console cause when the mall is REAL busy you got the PERFECT excuse to sit in the Truck :-laf

 

[NoSeeUm]

Good common sense DLane, thanks.



I did think about the theft issue some.



From the instructions, you pull the key out and lock the doors when you get out. In the process the security system is armed. Additionally, the cooldown timer immediately cuts the engine if the brake is touched. Plus, the truck can only idle for a maximum of five minutes irregardless of EGT. My guess is that only a very savy thief would know this.



Still the risk exists at any time, no matter if the cool down timer is active or just because someone wants to steal your truck and has the know how.



Jim

 

[NoSeeUm]

I got this hooked up and working yesterday. Works great, but a little distressing the first time I pulled the key and the engine kept running.



The only snag that I had was that with the engine running and the key off the TST box was also shut off. The CEL would light. This was caused by the "Low MAP Voltage" code (0237). I ended up switching the power for the TST over to the ECM power wire, which is kept hot by the cool down box during engine cooldown.



Jim