Could be caused by many things including low boost, bad turbo vacuum controls, turbo vacuum piping issues, leaking boost hoses, leaking intercooler, leaking EGR valve if EGR system is still in service, FCV stuck partly closed, etc... You need to know what your actual boost is doing to diagnose properly.
Enclosed list can get you started,
Check List for low boost issues on Jeep CRD engine wrote:
- 1. Check both boost hoses very closely for any crack or leaks. Check connections at both ends of hoses and make sure they are secure.
2. Check rubber intake boot between air filter box and turbo and make sure it has not collapsed or come apart and sucked a piece of it into turbo suction.
3. Failed FCV will stick partly closed and can severely limit air flow and boost into the intake manifold. The small gears in the operator for the FCV are notorious for failing and sticking.
You can remove the butterfly plate out of the FCV very easily. There are only two small torx screws holding it to the shaft. Removing it only takes about 15-20 minutes. See picture.
4. If your EGR system is still in service:
A failed EGR valve stuck in a partly OPEN position will allow air flow and boost to bypass the intake manifold and dump back into the exhaust manifold. You can install a block off plate between the exhaust feed tube and the EGR valve to eliminate any leak by. See drawing.
5. Check CAC (Charge Air Cooler/Intercooler) for any signs of leakage. Oily spots on tanks where tubes connect is an indication of leakage. Cracks have been found in the plastic tanks and blown seals where the tanks are crimped to the aluminum core.
6. The Map Sensor controls boost through the ECM, so if it is defective or giving the ECM a bad pressure reading, it will cause low boost issues and poor engine performance. Remove, clean the MAP sensor with brake cleaner or replace with a new MAP Sensor, use a GM sensor, much cheaper than OEM: Part Numbers: GM 55206797 or New GM PN; 55568175. Google part number.
7. Check all vacuum hoses and small plastic vacuum tubes and connections from connection at brake booster to turbo vane controls on right side of engine compartment. They are known for cracking, collapsing, breaking, etc... If this happens, it can greatly affect turbo boost output.
*The small plastic vacuum tube that runs from the turbo vacuum modulator valve over to the turbo vane actuator has been found by some owners to be cracked or partly collapsed limiting turbo vanes operation. They are known for cracking, collapsing, breaking, etc... If this happens, it can greatly affect turbo boost output. Best to replace all vacuum lines and tube with high quality silicone vacuum hose; it is impervious to heat and oil.
8. The turbo vacuum solenoid on the turbo vane control station has been known to go bad and not open fully and allow full stroke of the turbo vanes when the ECM calls for it. You can do a simple vacuum solenoid bypass test to rule it out as the problem. If it found to be bad, simply replace it. OEM PN: Vacuum Solenoid - 4606226AC See picture below.
9. The turbo vacuum modulator valve could be faulty. It controls the amount of vacuum supplied to the turbo vanes actuator from a signal from the ECM. There is no simple test for it other than ruling everything else out and then just replacing it. Its OEM PN is: 52109543AA
10. CAT (catalytic converter) could be stopped up. If it is, it will greatly limit turbo boost performance and will also limit engine power output.
This list is not all inclusive, but it covers most of the main items to check for low boost issues.
