ISM 320V 2010 No Boost

[Rawze]

(07-18-2023 )Lakepremix Wrote:  ...When accelerating, we get a air fuel control derate. Mechanic got a max of about 4 psi of boost.
...

a 'fuel air control derate' is simply the engine limiting fuel because of too low of boost. It is a reaction to missing positive air pressure, and not a cause.

Even if the exhaust cans were partially clogged up, it would most likely make more than only 4 lbs of boost.

Disconnect the down-pipe before it reaches the first can under the truck and take it on a test drive around the block. See if the boost comes back. If it does then you know its an exhaust blockage (a severe one). If nothing changes then you know it is not the exhaust making low boost. If it does, then take pictures of the races of the DPF, DOC, etc. and post them here for opinions on what is wrong that caused them to clog up abnormally.

Is the engine consuming any coolant or oil>?.

ALSO look at the turbo rpm, commanded positions, and measured positions, when it is struggling like that and in that air derate. If that turbo is spinning really fast + has high measured position and only making 4 lbs of boost, and you have eliminated the exhaust not being the cause, then you need to eliminate the intake as being the cause.

Possibly has a very severe restriction in the CAC unit/intake (or a garbage after-market CAC unit was installed recently)?, or perhaps the CAC is full of dirt and dust, etc. from someone running it with a missing or failed air cleaner? ... or the turbo has a severe issue.

While your at it, check the CAC boots, maybe one of them is blown out on the back or under side?. Check the turbo exhaust housing to ensure it is not cracked on its back side. Do a CAC pressure test to 30 psi and ensure the CAC is not blown apart on one side.

Check the EGR piping.. Yank on it!.. check any boots. .. See if one of the boots or pipes pops loose. Not sure abut an ISM.. but on the ISX's the EGR pipe coming off the top of the egr valve likes to rot out from the inside and then pop loose.. cause severe low boost, because all the boost pressure is leaking out at that joint.

And if that thing had an intake butterfly (intake throttle device), some trucks/engines have them, .. make sure it is not closing or restricting intake air in any way.

You also did not mention any engine faults that might be re-occurring in the ecm after test driving it and seeing low boost. Fault codes can tell someone a lot about what is wrong.

If it comes right down to it NOT being a restriction in the intake or exhaust (or an EGR circuit leaking all the boost pressure out), the turbo IS turning a lot of rpm's and going to high commanded/measured positions, but not making any proper boost. Then that is a sign that the exhaust turbine for the turbo is damaged, or that the control arm in the turbo itself is slipping, etc.