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Badgerx · 06-01-2021, **(Subject: Exhaust brake )**

Old topic im sure but need y'all non the less. I have a 2007 w9 with a 500 hp cm870. Truck had 1.4 million miles and the engine has roughly 900k. Its all intact emissions wise and egr valve is 4 months old. Truck runs fine no major issues but my Jake! I have had a cam replaced (1 not both) a good over head ran, new harness under the valve cover and the Jake harness on the outside. I replaced both switches in the cab. I can hear them come on but they don't do anything at all. My oil pressure is around 35 and oil temp around 200-225 depending on if im in PA or AZ lol. I average 4.8- 5 mpg and do my PMs every 12-15k miles. Not sure what other info y'all would need but ill provide what you ask for. [/size]

Rawze · 06-01-2021, **(Subject: Exhaust brake )**

could be the jake rockers themselves sooted up or hemnoraging oil and not engaging?

just a guess.

also, how many psi boost you get under heavy jake on high?

Badgerx · 06-05-2021, **(Subject: Exhaust brake )**

Sorry took so long to reply, but my boost shows about 18psi

Rawze · 06-05-2021, **(Subject: Exhaust brake )**

(06-05-2021 )Badgerx Wrote: Sorry took so long to reply, but my boost shows about 18psi

Make sure Insite is showing the solenoids being activating when testing the engine... at least verify that the ecm is operating them.

(for CM870)...
18 is a touch low, depends on the situation, rpm, and load on the engine too. Maybe pressurize the whole engine up to 30 psi using some block plates front and back of turbo, and check for leaks.

Could also be a worn out head, allowing for some compression losses.

Ricky · 06-05-2021, **(Subject: Exhaust brake )**

(06-05-2021 )Rawze Wrote: ...
(for CM870)...
18 is a touch low, depends on the situation, rpm, and load on the engine too. Maybe pressurize the whole engine up to 30 psi using some block plates front and back of turbo, and check for leaks.

...

So Rawze, is there a too much on the boost when jakeing hard. I have heard 20 lbs. Before my inframe my boost didn't get high when using jakes while loaded heavy on steep hills. Since the inframe my jakes work like never before.

Chamberpains · 06-05-2021, **(Subject: Exhaust brake )**

On my cm870 it would easily hit 25-28 psi @1550rpm+ on a steep hill. My cm871 is way lower @ 18-22 psi @ 1550 rpm+. Both of them were properly tuned by Mommaburt from the forum. I'd say your low with 18 psi on your cm870.

What psi are you getting under a hard pull? Have you checked to make sure you have no leaks in the air supply to the turbo actuator? Do you have full smooth range of movement from the VG ring in the turbo?

Ricky · 06-05-2021, **(Subject: Exhaust brake )**

(06-05-2021 )Chamberpains Wrote: On my cm870 it would easily hit 25-28 psi @1550rpm+ on a steep hill. My cm871 is way lower @ 18-22 psi @ 1550 rpm+. Both of them were properly tuned by Mommaburt from the forum. I'd say your low with 18 psi on your cm870.

What psi are you getting under a hard pull? Have you checked to make sure you have no leaks in the air supply to the turbo actuator? Do you have full smooth range of movement from the VG ring in the turbo?

I haven't let it get over 20 because I had heard not to that's why I was asking. I just brought a heavy load from Ambridge PA to NC and I was thinking about it coming through the Hillbilly.

Rawze · 06-05-2021, **(Subject: Exhaust brake )**

(06-05-2021 )Ricky Wrote: ...
So Rawze, is there a too much on the boost when jakeing hard?.
...

The ISX uses the turbo to set the strength of the engine brakes. If it is set too strong then it can lead to disaster in a hurry for someone.

If someone's engine is de-mandated and has bad delete programming, it is often the case where some jerk-weed jacked with the engine brake settings. If so then Yes.. it can drop a valve during heavy jake operations + high rpm, due to valve closing delay caused by excessive exhaust back-pressures.

Newer engines with their higher compression ratios cannot handle as much back-pressures as the older engines do. - This however does not mean that their engine brake is weaker. If anything the opposite is true, as a higher compression makes for more resistance during jake operations.

As far as what is "safe" .. different engines have different upper-end ratings on this. Like others have pointed out, CM870's typically cap at about 28 psi, CM871's, a bit less that that, and newer CM2350's etc... its even slightly less. For most people ... if their engine brake programming has not been played with ... They should not normally have to keep an eye on their jake boost pressures while going down a hill unless they suspect something is wrong.

Chamberpains · 06-05-2021, **(Subject: Exhaust brake )**

I let my cm870 fly up to 32 psi on extreme hard down hill braking with no problems ever. Steep goat hills with 10% or greater grades with sharp curves demand this kind of engine brake use as to not smoke the actual brakes. The cm870 is a tough engine if it's well taken care of. As rawze stated the ECM will adjust the back pressure according to demand and settings. It would settle in at ~28ish on most declines.

With gross 80,000 #s it would hold me in 9th gear @ 50-55 mph, and 45mph in 8th on most mountain grades with very light braking. I would maintain the boost @ 25-28 psi using the foot brake as assistance the entire way down the mountain. Don't be afraid to let it do the work. It'll save you a pile on changing brakes in no time. But you have to let it get up in the rpm range. If the decline demands it keep the rpms above 1550. I wouldn't let mine go above 1850-1900 rpm all though on a couple ultra rare occasions I accidently grab the wrong gear early and let it fly up to 2100 rpm and had no ill effect. though I quickly stomped the brakes to get it down in rpm's. Anything below 1500 and they were just noise makers.