Exhaust Brake Surge

[rwitt]

Yesterday, going home from the California coast, I noticed the Exhaust Brake was surging while being used. I was loaded with my Camper and flat pulling my Jeep. The boost would vary from 3 to about 7 psi. It would really start to slow things down at 7psi but would immediately let off to lower psi and then back up again. It obviously wasn't working as well as it has in the past, as I needed to use the truck brakes much more than before. Any thoughts?

Rich

 

[HBarlow]

Boost while exhaust braking? I don't understand. You shouldn't have turbo boost while your foot is lifted and turbo is creating exhaust braking.

 

[rscurtis]

You could. The turbo is running at a higher speed with the housing closed down, and the pressure in the intake system has no where to go because the exhaust is restricted. This wouldn't occur with a conventional after-turbo exhaust brake.

 

[jtwcummins]

I've been reading posts about boost when using exhaust brake and I have never seen anything but 0. Went up Spooner Grade and turned back to Carson City using EB and boost never left 0. Spooner is Hiway 50 from Carson to Tahoe and a pretty step grade. I did this to see if I was missing something, wasn't, 0 boost all the way. Wonder if backpressure is bleeding thru causing the reading? Never seen boost on my 97 with Banks Brake or my 08 towing 11,000 lbs over I80 from Reno to Sacramento and the brake is working hard on several step grades.

I, like Harvey, don't understand how boost develops with EB engaged, must be a reason since you are seeing it on your gage. Just weird!

 

[HBarlow]

You could. The turbo is running at a higher speed with the housing closed down, and the pressure in the intake system has no where to go because the exhaust is restricted. This wouldn't occur with a conventional after-turbo exhaust brake.

Curtis,

You usually know what you're talking about when you post but I'm not understanding. I've never tried to think through the details of VGT operation so maybe I simply don't know.

How about further explanation - teaching if you will?

 

[AH64ID]

Due to the nature of a VGT EB, as the vanes on the VGT close the pressure in the manifold goes up. This is where the back-pressure comes from. Because the vanes of the VGT are closed, well almost closed, all of that pressure is being forced over the turbine blades just like in normal operation. This increases the speed of the turbo shaft, which in turn makes the compressor move more air. The closing of the variable vanes happens just before the turbine blades, so the release of pressure (like the fixed or variable orifice in a 5. 9 brake) blows over the turbine just like normal exhaust flow when driving.

On a pre 6. 7, non-VGT, exhaust brake this doesn't occur. Those block off the exhaust after the turbine wheel, and thus the turbine wheel is in the backpressure and lack of flow.

Harvey, if you have a boost gauge on your C&C you should see boost every time you use the EB.

 

[rwitt]

Yes it does show boost on the Edge CTS, I've seen as high as 8lbs under hard ebraking. The problem mine is having has just started, and has never done it before.

Harvey, try opening your passenger door window and let the ebrake come on, you'll hear the turbo spool up.

Rich

 

[HBarlow]

My truck has had gauges installed since day one. Next time I drive it I'll try to remember to check the boost gauge when the VGT is in braking mode.

I haven't driven the truck in month or more and don't know when I'll drive it again.

 

[AH64ID]

My truck has had gauges installed since day one. Next time I drive it I'll try to remember to check the boost gauge when the VGT is in braking mode.

I haven't driven the truck in month or more and don't know when I'll drive it again.

You should at least take it out every 3-4 weeks to get the oil hot and wet the seals. They are fine sitting for long periods, but do better when used occasionally.

 

[rscurtis]

I trust AH answered your question. Where the energy to run the turbo is usually coming from burning fuel, when the EB is engaged it's coming instead from the kinetic energy stored in the moving truck (and trailer).

 

[JDinNY]

Due to the nature of a VGT EB, as the vanes on the VGT close the pressure in the manifold goes up. This is where the back-pressure comes from. Because the vanes of the VGT are closed, well almost closed, all of that pressure is being forced over the turbine blades just like in normal operation. This increases the speed of the turbo shaft, which in turn makes the compressor move more air. The closing of the variable vanes happens just before the turbine blades, so the release of pressure (like the fixed or variable orifice in a 5. 9 brake) blows over the turbine just like normal exhaust flow when driving.

On a pre 6. 7, non-VGT, exhaust brake this doesn't occur. Those block off the exhaust after the turbine wheel, and thus the turbine wheel is in the backpressure and lack of flow.

Harvey, if you have a boost gauge on your C&C you should see boost every time you use the EB.

Great answer, that's exactly what's happening.

 

[Schumacher]

rwitt,

I had the same problem with my EB but mine was only jumping 1 to 2 pound of boost. Took it to the dealer and they where told to clean the turbo. Lucky me they broke the cleaning port and I got a new turbo. It works great now.

 

[HBarlow]

I forgot to reply to this thread at the time but I did watch the boost gauge on my truck a few days after the VGT was explained and my turbo does produce a modest boost during deceleration as a was explained.

I guess I never noticed boost on my gauge when exhaust braking was occurring or it simply never registered with me what it meant.

Thanks for the lesson.

 

[Cummins12V98]

Worth getting out of bed today, learned something new!