When EGT's get too low

[Allnew2Me]

I have heard many times that extended idling is not a good thing because the combustion chamber temperature can drop to the point of not being able to combust the fuel, resulting in a washdown of the cylinder walls. This leaves the walls without oil and can damage them.

On long downhill grades of many miles where there is no go pedal, my EGT's will fall to 200 degrees and less and I was wondering if the same thing occurs as extended idling. Or because there is no load on the engine, is all fuel delivery shut off when decelerating? Thanks for the help.

 

[Alan Reagan]

That's a good question. The temperature that's important is the coolant temperature. EGT's will fall but if the coolant temperature is staying in the 190 range then the engine is working hard enough that washdown shouldn't be a problem. If the temp is not staying in the normal operating range, you may have to put a winter front on the radiator.

 

[Allnew2Me]

I have noticed this occuring even in warm weather and although the coolant temp does drop a little, it is still in the normal range. Good point on the coolant temp, if it is normal I suppose it is okay to drive like that. I would still like to hear form others that might have some other ideas. Thanks

 

[Devan Manis]

yeah, when the truck is above idle speed and there is no load on the engine, fuel delivery is pretty much zero. I say pretty much because who the heck knows for certain with an electricaly controlled pump.

 

[Alan Reagan]

I have noticed this occuring even in warm weather and although the coolant temp does drop a little, it is still in the normal range. Good point on the coolant temp, if it is normal I suppose it is okay to drive like that. I would still like to hear form others that might have some other ideas. Thanks

I agree on other ideas. The point I'm making is that fuel washdown occurs when the engine temperature is too low to support proper combustion. If the truck is started on a cold morning or left idling for long periods, the engine temp will drop to a level where fuel washdown will occur. Even though the engine is at "idle fuel", it is still working to develop adequate heat. It's doing this through the increased RPMs. Even though gravity is doing the work instead of the engine to move the truck down the hill, the engine is turning higher RPMs as it goes down the hill and internal friction is creating the heat necessary to keep the temps up. It's when the operating temps start falling is when you'll get washdown.

 

[BarryG]

I know when I drive down some of the passes like Wolf Creek here in Colorado my egts can and do fall below 200 deg. You literally coast down the entire hill and still use brakes. It is about 12 miles down, others are longer. I usually drop out of OD to help keep the brake usage down a bit and a couple times on the way down I slow down enough so that I need to apply some go pedal to get the egts up a bit for a few seconds.



How about offroad? I was out in Moab a while back and going down one of the trails from Island in the Sky down to the river road. Coasted down the hill/trail in 4wd and speeds are not over 3 or 4 mph for 30+ minutes. RPMS are low going that speed. I guess 4 low would be an option to keep the rpms up.

 

[Riverside Joe]

Good subject, oil pressure is based on RPM's, Fuel delivery is a program driven on these ECM models with the peddle position as the reference. Haven't looked that deep into my manual but I would think (never a good thing my wife tells me) that the ecm will try to keep the rpms down to what the throttle position reference is saying the driver wants. Oil pressure will be present and increase with rpms. Higher oil delivery will happen with rpms. Not a pencil pocket person but simple is a program the ecm may not have. ----RJ

 

[Allnew2Me]

Alan I agree with you that washdown occurs when it is too cold to combust. I guess what I would like to know is, does the fuel shut off during decel like it does on fuel injected gas engines? If that is the case then I have nothing to worry about. I understand your point regarding the RPM staying up, but if there is still idle fuel being injected and the temps are less than 200, is there a washdown situation? I would think that the engine, regardless of RPM, would still have a washdown if the temps are low and fuel is still being introduced to the combustion chamber, even though it would be at an idle level of fueling. Maybe that is the answer, the amount of fuel is not enough to actually be a problem whereas at a slower RPM that same amount would be enough to cause trouble. Joe that makes sense to me, oil pressure is safe and the ECM would tell the injector pump to match throttle position. Unless vehicle speed input causes any variation. All interesting thoughts, let's see if we can get a few more ideas from the rest of the experts put there.

 

[Alan Reagan]

What I'm saying is that even though your EGT's are low, if the engine temp is up to normal, you should be OK.



Fuel delivery on the engine will be the same as at idle. The computer is telling the pump to deliver what ever the demand is for the accelerator position sensor position.



If the engine temps drop down below probably 160*, you might start having a washdown problem.

 

[rbattelle]

What I'm saying is that even though your EGT's are low, if the engine temp is up to normal, you should be OK.

I agree. But I am curious what the fuel rate is on an engine in decel mode.



If you're that worried about it, turn the key off once you're headed downhill. That'll guarantee fuel is off.



-Ryan

 

[KDillard]

Fuel injected into the compression chamber is a direct relation to both the TPS sensor and load. If your EGT are dropping its because very little or no fuel is being injected to be burned. Diesel Engines are not stoich like gassers. The air to fuel ratio varies greatly on diesels. Gassers like the ballpark of 12. 5:1 to 14. 5:1 depending on programming. Diesels can go from 100:1 to 25:1 roughly. This explains the efficiency of a diesel to a gasser, at idle the diesel has more air than fuel in chamber. So when coasting the diesel uses little to no fuel at all hence the very low EGT readings. Washdown on the cummins isnt really an issue at idle that much due to preheating with the massive 190amp air heating grids.

 

[I-6DZL]

If the fuel EVER shuts off, the engine will either develop a miss (cylinder specific) or will quit running completely. An internal combustion engine CANNOT turn the fuel/gas off without it stalling, or completely shutting off. EGT's are so low because (as was mentioned earlier) there's very little fuel being injected at idle along with ALOT of air.



As was mentioned earlier, engine temp plays a huge role in wash down. Wash down occur's when engine temp, EGT's and Oil pressure are all low. When EGT's are low coolant temp is going to lower.



For the 20 mins or so that EGT's are *cold*. Just apply the trailer brakes manually with your controller and push on the go pedal slightly. Do this for 10 seconds or just long enough to bring up EGT's and not bake the trailer brakes.



Curtis

 

[Allnew2Me]

Thanks for all the info guys. I don't think I have anything to worry about as it sounds like the fuel being delivered is not enough to cause any trouble unless the engine temps were really low and the downgrade was excessive. I kind of thought that might be the case but it is nice to hear what others have to say. I was aware that the fuel air ratio was dependant upon load demand but still wondered if there was enough of it to cause a washdown effect. These diesels are so different than what I have been used to in a gas engine I'm still very much learning how they work and react to certain situations.

 

[GaryCarter]

You're getting a lot of misinformation.

Engines do not stall in gear downgrade.

Electronic injection systems do not need to inject any fuel while coasting. I know the Navistar diesels shut the injector completely off during coast. I do not know for a fact what the programming is on the CTD. If anyone has a scan tool just hook it up and go for a ride... what's the injector pulse width while coasting? Zero, I expect.



GC

 

[NIsaacs]

Thats one of the advantages of exhaust brakes or on big trucks, the "jake" they both help keep the engine temps up on the down grades. In fact years ago when the first intercoolers become popular, Cummins continued to use the watercooled aftercooler and advertised it to keep the engine temps up better on the down grades over the intercoolers.



I don't think the low EGT's are so bad, however I am not in favor of very fast cool down, as in pulling a steep hill and then bailing off the other side. If I know there is an immediate down grade after a tough up hill pull, I will start to back out of it towards the top and get my EGT's down some before starting down. Here the exhaust brake comes in handy again, slows the cool down.





"NICK"

 

[Alan Reagan]

I agree. But I am curious what the fuel rate is on an engine in decel mode.





-Ryan

The fuel consumption on a engine in decel mode is the same as at idle... ... about . 3 gal per hour for our engines. Heavy duty engines burn about . 8 gal/hour. The fuel does not shut off on deceleration.

 

[rbattelle]

The fuel consumption on a engine in decel mode is the same as at idle... ... about . 3 gal per hour for our engines. Heavy duty engines burn about . 8 gal/hour. The fuel does not shut off on deceleration.

I figured. Thanks!



-Ryan

 

[Buddy's Ram]

Great Thread! I wondered the same thing!

 

[Devan Manis]

now, what about a p-pump?

 

[Alan Reagan]

You're getting a lot of misinformation.

Engines do not stall in gear downgrade.

Electronic injection systems do not need to inject any fuel while coasting. I know the Navistar diesels shut the injector completely off during coast. I do not know for a fact what the programming is on the CTD. If anyone has a scan tool just hook it up and go for a ride... what's the injector pulse width while coasting? Zero, I expect.



GC

If there is no fuel injected, then the engine is off. These engines and the Navistar engines are not "off" when coasting. The pulse width might be narrow but it is not zero. Although minimal, there is still fuel being injected to keep the engine running. I'm not sure how Navistar can shut the fuel completely off. What restarts the engine when coasting at low speeds with the TC unlocked? The engine won't just freewheel until the truck comes to a stop and then start the injectors to make the engine run again.



Sounds like you might be confusing this with the fuel off type of governor system, also know as hit and miss. These trucks and I'm sure Navistar does it too, use changes in timing and fuel injection as the governor. Without a governor, an engine at idle would continue to slowly gain speed. By controlling timing and injection that problem doesn't exist. With the hit and miss type of governor, the fuel is actually shut off for a certain period to control engine speed.