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

[NoSeeUm]

I recieved a cooldown timer from UPS yesterday. It is the type (BD) that shuts off on EGT or a 5 minute timer. Reading the instruction manual it said to set the post EGT cooldown temperature 400F as a maximum. The reason being that it was important to let the turbo stop rotating before lubrication was shut off.



Up until then I had read, mostly here on TDR, that 350F worked, but that 300F was the maximum routine shut-off temperature to prolong the turbo and engine component life. I had thought the main reason for this was to prevent coking of dino oil in the turbo bearing.



Which method is the best?



Generally I let my turbo cool to 290F before shutdown, will this work well for the turbo timer? Typically, my cool down takes less than 5 minutes no matter what, so does that let the engine idle for too long?



Thanks;

Jim

 

[EricBu12]

I generally let my turbo cool to less than 400 post turbo... I have a hybrid 35/40 and when you shut off the truck, the tubo does still spin down for a few seconds so the only thing is temp...



Before I had the engine rebuilt, I had synthetic oil in and that helps prevent coking...



I used Valvoline prem. blue extreme 5W /40 full synthetic.

 

[Scrappy]

I have a TTM in mine that shuts down based on temp, unless I have been pulling or running @ high speed it rarely takes more than a minute, but if i have been pulling or running high speed it can take 5 min or longer. I don't like the time based ones because they can either run the engine longer than necessary or worse not long enough. As far as I have ever heard the only reason for the timer is to cool down the turbo to keep from burning the oil after shutdown.

 

[kelly4463]

I let my turbo cool down to 400 before shut down

 

[OL RATLR]

I set mine on 2 minutes works for me.

 

[Mundgyver]

The real enemy is the "COKING" of the oil. I think as a general rule, all "full Synthetics" are designed to withstand temps approaching 400* while the regular Hydro-Carbon is something less by about 50 to 100* (estimate on my part here).



Also you have to remember that you have oil flow when the engine is running and the oil temp will be around 200* (my experience with my oil temp gage). So the bearing or bushings in the turbo are cooler than what your EGT gage shows.



The damage comes when the oil flow stops with the engine. Since the shaft is in the 200 range and the exhaust side impeller can be anywhere from 280 to 400* depending on wha you were just doing and the inlet side of the turbo will be 100 or less, the heat is going to migrate to the cooler side.



The oil in the bushing or bearings is either going to fry and turn to coak and become hard and brittle causing abraision or it is going to withstand the heat.



Cool down helps prevent this. I run nothing but synthetic because of this very reason. It will withstand allot more heat before becomming a problem. I know when towing with 23,000 lbs I will see my engine oil temp go to 265* on a long steep uphill grade and it takes it awhile before it will come back down.



Around town driving I just shut down because my EGT temp will be at 300 or less. On the road with a load, I will wait till my EGT drop to 300. I have seen that take as long as 8 minutes in a rest area at the top of the grade



Just my experience and I offer it as food for thought.

 

[litespeed]

I have read elsewhere that it is more about TIME than EGT temperature. Especailly if you are pulling a load.



Just because your EGT's read an indicated value (IE: 300) does not mean the rest of the engine has cooled. The engine and drive train can become "heat soaked" when run for long periods and under a heavy load. The EGT's may only read 325 or 350 (usually less) after 5 minutes of idling. But everything else can still be VERY hot. In some cases more cool down time is required.



I was reading an interesting thread about this recently. I cannot find it now. If I find it... ... I will post.



AJ

 

[Scrappy]

somewhat correct BUT while the engine is running it is actually pushing alot of the heat out the radiator and the tailpipe. Isspro recommends that their probe be put POST turbo since if it is cool, then the rest of the engine should be somewhat cool. I do know that for the TTM, the probe needs to be post turbo. Mine is post and the gauge is pre and I can see the gauge between 250-300 degrees and the TTM is still active. If I don't let the TTM shut the truck down and manually shut it down when the gauge reads 300 degrees, after a few minutes the gauge has gone UP to 400 degrees+. It does not go up much if at all if I let the TTM work.

 

[Mundgyver]

The subject of Heat Soak is long and it is viable. There is a thread on the VP-44 woes and it is devoted to cooling and in particular how to eliminate the Heat Soak affect which can drive the internal temps of the VP up alot.



The same would be true of the turbo also, guess we need to come up with some active cooling method after shut down.



Any ideas.

 

[MRose]

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.

 

[HBarlow]

In my opinion the cool down temperature requirement is probably based in part on good science and in part on "internet myth" as you suggested. It is passed down as gospel on this site.



I follow the 300 degree shut down requirement with religious fervor because that was what I learned on the TDR when my '01 was new and I was learning everything turbodiesel related. But I see drivers shutting down Dodges as well and inferior brands C and F without sufficient cool down time all the time. I suspect I am wasting time staring at my EGT gauge unnecessarily. An unidentified Cummins employee once told me and a couple other TDR members that 350 degrees was satisfactory.



The 350 degree comment was made by a Cummins employee at the end of a session held at the Cummins Mid Range Engine Plant (MREP) in Columbus, IN during the 2002 TDR Rally introducing the new HPCR Cummins ISB and the new Gen III Dodge. I have often wished I had asked him his name and position with Cummins but at the time I didn't think of doing so. TDR members who attended the TDR rally were treated to a VIP tour with several days of briefings by the MREP manager, Cummins, Jacobs, Fleetguard, Valvoline, and Dodge platform engineers and technical people as well as plant tours, videos, movies, etc. I don't remember now what the subject of that particular session was or who delivered it but as the session closed the group was discussing shut down temps. The 300 degree "internet myth" figure was quoted by one or more of us and tossed around. As the attendees rose from our chairs and started leaving the room a guy dressed in slacks, not work clothes, who was a Cummins employee commented to me in an offhand manner that 350 degrees was cool enough for shut down. Guessing from what I knew about the people attending the session, the man's manner of dress and the way he spoke I assumed that he was a Cummins engineer in the MREP that manufactures the ISB engine.



Our Dodge owner's manuals don't use a temperature figure since the trucks don't have OEM EGT gauges but advise approximately one minute wait before shut down after unloaded easy driving around town and up to five minutes delay before shutdown after full load, full power operation. I suppose we'll never go wrong by waiting until the pre-turbo EGT sinks to 300 degrees but I silently grumble to myself every time I sit waiting for the needle on my gauge to slowly descend to 300.



Harvey

 

[Scrappy]

HBarlow wrote;

"I suppose we'll never go wrong by waiting until the pre-turbo EGT sinks to 300 degrees but I silently grumble to myself every time I sit waiting for the needle on my gauge to slowly descend to 300. "



That is also why I got the monitor, so I don't have to wait for it.

 

[NoSeeUm]

Thanks for all the replies, got a little smarter. Armed with more knowledge I think I will stick with 300F post EGT for the shutdown timer.



The manual says that excessive idling is bad, and recommends 1-5 minute cooldown on shutdown. Depending upon engine loading just prior to shutdown. However, as was mentioned above, DC does not supply an EGT indication as OEM. So this is a generic common sense requirement, at least 1 minute should probably be adhered to for maximum engine and component life.



The cool down timer that I have says that it primarily uses EGT, but resorts to a maximum of 5 minutes should EGT not drop below the set point with in 5 minutes. I am OK with that, kind of why I bought that model.



For those of you with out pre and post EGT indication. The post EGT takes much longer to cool off. The pre temperature can be below 250F and the post can still be in the low to mid 300F's. At least on my truck. I kind of see the post EGT as a "soaked in" metal temperature and the pre as more of a "real time" exhaust temperature. My cool down timer recommends using post EGT for this reason. In fact, just below 250F is about as low as my pre EGT will ever get on a warm engine.



Jim

 

[Mundgyver]

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

 

[cojhl2]

Actually in theory the pre and post should be about the same if the turbine is not doing work, ie: at engine idle.



The reason for temp difference between pre and post is not distance from the heat source, but is because the turbine is taking work out of the exhaust gas therefore lowering the heat value(measured by temp in this case) of the exhaust.



We must keep in mind the biggest reason to let the engine cool down (normalize) is becasue the internals are very hot after hard pulls. The turbo is but one element in the whole picture. We are measuring internal engine temps by the exhaust gas it expels.



Ever notice at a truck stop when a big truck is coming in it is going slow and easy. This gives time for cool down in addition to just being safe.

 

[Mundgyver]

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). My concern was the "

Pre turbo 250's and Post reading of 300's

I don't see this happening as it was stated and all I wanted was a clear clarification that everyone was on the same understanding.



Not arguing just trying to clarify.

 

[cojhl2]

Mundgyver,



I did not mean to question your knowledge, but there is a lot of confusion among members regarding this topic.



I was responding to a the general population.



You are correct in asking that question. The numbers seem to be reversed. They cannot be if the system is measuring correctly.

 

[Mundgyver]

Sorry, it was not my intent nor did want or mean to sound testy, I guess it was just the way it was worded (not yours) the original that caught my eye and I just wanted to make sure everyone was on the same page or Pre and Post positions.



I suppose that running two temp guages would be benifical as you would be able to see how much work was being done by the turbo. I suppose there is a mathimatical formula someplace that would give a graph of what was going on and two guages would probably allow for some diagnosis should the turbo start going out.



I monitor my oil temp at the top of the filter housing. Wonder if it would be a good thing to put a cooler inline with the turbo feed line. I just happen to have a very nice aircraft hydraulic cooler that might work well for this. I have had this for about 30 years and never used it. Was built in 1968 by Stuart Warner Corp. for the Marine CH-53 A model heavy lift helicopter.



Hmmmmm ideas, ideas, ideas.

 

[Mundgyver]

So is there a way to prevent heat soak, or is this really a worry with the new grades and capabilities of the oil we now have on the market ???



With the amount of mileage we get out of one of these engines when taken care of, do we really need to worry about this??



Lots of good stuff here to keep the mind sharp and tuned up

 

[cojhl2]

I love the looks of that Hyd oil cooler. You're right ideas, ideas!!



Im always goining to worry about heat soak, (a term I had not previously heard prior to this thread) if it means the engine has hot spots at shutoff.



My opinion is that you want the engine internal parts, Turbo included, to all be at such a temp that minimal heat transfer takes place after shutdown.



Can't imagine a piston for instance being almost red hot in the middle having to move its heat to the cyl walls after shutdown.