// Install guides
3.5" DOWN PIPE — 17-23 DURAMAX 6.6L L5P
The PCV hardline gasket on the back of the compressor cover is not sold separately by GM. Be gentle with it and keep it for reassembly. The turbo oil drain line's gasket is fixed to the upper section and not replaceable.
Removing the turbo and SCR to swap the factory down pipe for the 3 1/2" down pipe on a 2017–2023 Chevy/GMC 2500/3500 6.6L L5P Duramax, then refitting the turbo. The same procedure on both generations; the one difference is noted on the coolant line step. Fitting the down pipe on its own, without the EGR delete? You still need both batteries disconnected, the passenger fender liner off, the coolant drained, and the intake pipe, air box and turbo intake horn removed. Those are steps 2 to 9 of the EGR delete guide. Torque specs: turbo mounting bolts 43 ft-lb; turbo oil drain, oil feed and PCV fitting bolts 89 in-lb.
Steps
1 Optional: lift the cab a few inches instead of removing the turbo
Removing the four cab bolts pictured and lifting the cab a few inches gives enough clearance to get the SCR and down pipe out from behind the turbo without removing the turbo. The manual documents this for 2020+ trucks. If you do it, skip the turbo removal and refit steps.
2 Start the turbo removal: intercooler boot and PCV hardline
Remove the T-bolt clamp holding the hot side intercooler boot to the compressor outlet. On the back of the compressor cover, remove the two 10mm bolts holding the PCV hardline and swing the line out of the way. GM does not sell this gasket on its own, so be gentle with it and keep the hardware and gasket for reassembly.
3 Remove the oil feed line from the centre section
Two bolts on the driver side of the turbo. This gasket can be discarded.
4 Remove the SCR heat shields and the NOx sensor
Six bolts: four on the passenger side, two on the driver side. Then break loose the NOx sensor on the driver side of the SCR and remove it.
5 Loosen the two V-band clamps on the SCR
One joins the down pipe to the SCR, the other joins the SCR to the turbo housing.
6 Break the SCR free from the turbo
It may take some prying between the SCR and the turbo, and between the SCR and the down pipe.
7 Split the turbo oil drain line
Remove the two bolts joining the upper section of the drain line to the lower. This gasket is fixed to the upper section, so it will not fall off, and it is not replaceable.
8 Remove the turbo coolant feed and return lines
Four bolts on the passenger side of the turbo hold them to the centre section. These gaskets can be discarded. The lower feed line stays in place; pull it forward and zip-tie it so it is out of the way for turbo removal and refit. Set the upper line aside for later.
9 Unbolt the turbo from the pedestal
Four 15mm bolts. The turbo is then ready to come off.
10 Remove the turbo, then the SCR
11 Unbolt the down pipe from the front pipe and the engine
From the passenger fender well and from underneath, remove the two nuts and two bolts joining the down pipe to the exhaust front pipe. Then remove the two 13mm bolts holding the down pipe support tab to the side of the engine.
12 Remove the two down pipe brackets
The mount bracket on top of the down pipe, and the bracket that attaches to the cylinder head.
13 Remove the front pipe and the down pipe, and move the sealing ring over
Take the exhaust front pipe out first, then the down pipe. It comes out easiest going down, with some rotating and pulling. Once out, remove the lower metal sealing ring and fit it onto the new down pipe. If it slips off too easily, bend the tabs on the ring's edge out slightly so it holds.
14 Install the new 3 1/2" down pipe from the top
Some rotating and slight pushing, but once it is in the right spot it drops straight into place.
15 Loosely fit the support tab and the stud plate
Reconnect the lower support tab to the engine to hold the down pipe, but do not tighten the bolts yet. There is a lot of slop in these holes to allow alignment once the pipe meets the turbo. The plate with two studs can go onto the down pipe's lower flange now too.
16 Refit the turbo on a new pedestal gasket
Set the new gasket on the pedestal; the locating studs hold it. Refit the turbo and torque the four mounting bolts to 43 ft-lb. Refit and torque the oil drain line bolts to 89 in-lb.
17 Connect the down pipe to the turbo with the OEM V-band, snug only
18 Tighten the down pipe support bracket
Back in the passenger fender well, tighten the two 13mm bolts once the pipe is sitting correctly and the mount plate is flush with the lower flange. This may take some movement of the down pipe.
19 Tighten the V-band at the turbo outlet
20 Refit the oil feed line and the PCV fitting
On the driver side of the turbo: the oil feed line with the new GM gasket, torqued to 89 in-lb, and the PCV line fitting with the gasket you saved, also 89 in-lb.
21 Refit the turbo coolant feed and return lines
On the passenger side, with the new supplied GM gaskets. Use the factory hose to connect the line from the tank to the turbo return.
2017–2019 trucks: refit the upper return hardline upside down from its original position so the pipe faces down. That routes the supplied coolant line, fitted in the reassembly guide, much better.
