There is no factory replacement interval, it should just be cleaned when there is significant buildup. This carbon buildup is caused by a number of factors, the EGR gasses mixing with oil mist from the crankcase ventilation (CCV) and soot. Newer cars used finer EGR metering and all diesel fuel sold in North America (not Mexico) is now ultra low sulfur diesel (USLD) so clogging problems should be much less. (Some gas stations may not have the USLD sticker and some areas may have some non USLD left in the pipes but all refineries now make only USLD for road use). A thin film or a buildup about 2-3 millimeters thick is normal and isn't something that needs immediate cleaning. If it's greater than 5-10 millimeters I would clean it soon. Some pictures are at the bottom of the article showing bad clogging. This is also a good time to clean the EGR cooler, the round cylinder behind the intake manifold.
Some people use a EGR block off plate to greatly reduce any future carbon buildup in the intake. Disabling or reducing the EGR cycling will increase emissions and could cause a check engine light to appear. You could also use an EGR restrictor - a block off plate with a hole drilled in the middle. It could still set a check engine light occasionally but it could be a compromise between intake clogging and emissions. I suggest leaving the EGR system intact for legal reasons and emissions. For more EGR information, see 1000q: TDI EGR system FAQ.
Excess buildup can also cause the anti-shudder valve to stick in the closed position, cutting off air to the engine and resulting in a no-start condition. It is unlikely that it would stick while driving since the anti-shudder valve is only used during engine shut-off with the ignition key. It can also reduce performance and efficiency or send the car into limp mode. If it were stick, it could also prevent the air cut off safety function. See 1000q: engine runaway for more details.
Some people prefer to buy a new intake manifold, some clean their old intake manifold because it's much cheaper. A new intake manifold would be cleaner and faster but intake manifolds are just cast metal so there's nothing that can wear out on a used manifold. The best way to reuse an old manifold is to buy a core (a used manifold from someone else) in advance and have it chemically or ultrasonically cleaned. This will save time and a lot of mess the day you remove the manifold. Or you could buy a new manifold and sell yours as a core. Otherwise you have to use a pressure washer and/or brush to scrub it clean. Wear old clothes because they will be stained after you clean the intake manifold. Always wear gloves and eye protection when cleaning the manifold. The carbon is very sticky and a pressure washer or brush alone can't reach the interior corners. Soaking the manifold in carb cleaner, biodiesel, or wood furniture stripper will clean the carbon effectively.
I do not recommend bead blasting since improper cleaning could leave tiny particles stuck in the carbon and cause engine damage. Make sure it's thoroughly cleaned, including the interior corners, if you use a bead blaster. There are chemical cleaners that work while the manifold is attached to the engine but I strongly do not recommend this. Dealers used to do this until engines were damaged. In a diesel, the valves are almost touching the pistons due to high compression and once you see how much crud is inside, you won't want that going through the engine with the cleaner You also don't want the gunk to go through the turbo. If the cylinder head is really bad you should remove the head for cleaning. You will also spill a little coolant if you remove the EGR cooler, check the color of your coolant, do not mix green/blue coolant with VW red/purple coolant, refer the article: 1000q: coolant flush for more details.
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6mm ball end allen wrench or socket
pliers and hose clamp pliers (optional but suggested)
mirror to see the back of the engine
a brush, degreaser, and a power washer if you want to clean the intake
shallow depth EZ outs (pictured below are normal depth EZ outs, these are too thick to fit in the limited space)
EGR gaskets (egr valve to pipe, at least 2) VW# 069-131-547-D
EGR gasket VW# 038-131-547-A
EGR gasket o-ring N-905-216-04
intake manifold gaskets VW# 028-129-717-D
new intake manifold (optional) VW# 038-129-713-K
new intake manifold bolts (x6 optional) VW# N-904-242-01
new EGR (optional) VW #045-131-501-L
Remove intercooler output hardpipe-intake manifold hose (outlined in yellow) and air intake-turbo hardpipe intake accordion hose (yellow). (2 band clamps each). Tape over the
exposed pipes so nothing falls in.
New Beetle only - remove the windshield wipers (2x 13mm nuts) and the rubber firewall gasket. Then remove the 10mm nuts holding the plastic cowling to get access.
Unplug PCV heating element plug (outlined in green above). Disconnect intake manifold change over valve (behind the intake manifold, 2 allen bolts marked with red circles in pic below). Disconnect anti shudder valve and EGR vacuum lines (purple). The plastic will be brittle with age/heat so be careful.
I suggest that you label the various lines and plugs so
that they can be easily identified during installation. Caution: make sure you have the
vacuum lines going to the anti shudder valve's black vacuum bulb and the EGR
valve routed correctly. Swapping them will cause the EGR solenoid to close
the anti shudder valve and result in a no start/start and then die.
Remove turbo intake hard pipe
(pink, behind engine, held by 1 band clamp at turbo and 1 bolt circled in red).
Unbolt 3 bolts holding the EGR cooler down (red circles in below pic). Use a mirror to
them. Disconnect EGR cooler intake (2 allen bolts, marked with green) and outlet (2 allen
bolts, marked with green). If you choose to clean the EGR, remove coolant hoses (3 hoses, marked
with blue). The EGR cooler can now be removed. If you don't want to
clean it, just place it to the side and work around it. Here are both sides
of the EGR cooler.
To finish disconnecting the intake manifold remove the rest of the allen
bolts (6x 6mm allen bolts, in red). These may get stripped so I suggest
pre-soaking in PB Blaster or another penetrating lubricant. Make sure to
tap in the allen bolts to avoid stripping. If they do get stripped, use a
shallow depth EZ out to grip the outside of the allen bolt head. This
gives a greater diameter surface and greater lever arm to turn the stripped
bolt. The EZ outs should be sharp or else they won't bite. If you find
halfway through that it's dull, use a dremel to sharpen it. Sears
does not warranty the sharpness of their EZ outs.
The EGR naturally puts exhaust soot into the intake which must be cleaned out. It mixes with oily crankcase vapors from the crankcase ventilation (CCV) and can form a buildup. The introduction of ULSD ultra low sulfur diesel has reportedly decreased the amount of soot buildup. A small amount of buildup is normal. Do NOT use a chemical dissolver or a vacuum cleaner to suck out the carbon while the intake manifold is still attached to the head. Hard pieces of carbon could fall into the engine and possibly damage the valves or turbo. The intake, EGR, and EGR cooler must be removed from the head for cleaning.
The cylinder head is best cleaned while off the car. If you want to clean the cylinder head while on the engine, I suggest removing the camshaft (follow timing belt removal as spec. in your factory service manual or see 1000q: timing belt removal ALH engine) so that all the valves are closed. You can then use a brush to clean. I would avoid using liquid cleaner just incase it were to leak into the engine and cause hydrolock (engine damage). Make sure to follow up with compressed air to blow out all loose carbon particles.
Below are some pictures showing a moderately clogged intake. This
restriction in air flow hurts engine performance and efficiency. Note that
it doesn't look too bad upstream of the EGR but looking downstream of the EGR shows how clogged it really is. The EGR and the
anti shudder valve (the throttle-like plate used to help shut off the car by
cutting intake air) collect soot
and built up carbon can also jam the anti shudder valve, resulting in a no start
condition. If this happens, just press the lever on the outside of the
valve, connected to the black plastic
vacuum bulb to open the anti shudder valve internally.
Here is another pic by toolnut showing a clogged intake. You have to
look on the downstream side of the EGR to see the really bad clogging.
Obviously you want to make sure it doesn't get to this point. Up to 2 mm
of buildup is normal and I would leave that alone. 1 cm or greater is
where you should consider cleaning the intake.
At this point, you could just buy a new intake manifold because they are not
too expensive. Buying a new manifold will let you change it immediately
and save car down time. You could sell the old manifold to someone who wants a
core so they can clean it in advance for their next cleaning. I used a pressure
washer, soaked it in degreaser, scrubbed, and then used the pressure washer again. A hose
will NOT clean it, you need a pressure washer! It
will cause a big mess, so wear clothes you don't mind getting stained.
You can also use chemical cleaners, diesel purge, or soak the manifold in biodiesel. Make
sure to get the internal corners and backside as well. Here is the EGR cleaned - much
better but it will still undergo one more pressure washing. Here you can
see the relationship between the anti shudder valve (black plastic bulb) and the
lever and the "throttle-like plate".
I have a junk table and clamps to prevent the manifold from blowing
away. A carwash type electric pressure washer will work but these
aren't as powerful as a gas powered pressure washer because they would otherwise damage cars. Pressure
washers are not toys - take extreme care of the water jet they make.
Another method is to burn the buildup out. It's mostly carbon and
unburned fuel and oil turned into glue. Below is a video from a third
party site showing this. I strongly recommend that you remove the EGR and
all plastic parts off the aluminum manifold before heating it or else the EGR
can be damaged. The person who took the video dipped the valve in water
but in my opinion it should have been removed. I would not dip the hot
manifold in water either because it's possible rapid cooling could damage the
manifold. It's also a potentially serious fire hazard
so make sure there is nothing flammable nearby, take all precautions to avoid
getting burned, let it cool before handling, see the TOS Agreement for the full
legal disclaimer. The fumes are also bad for you to inhale and bad for the
environment to have raw soot and oil burning off so make sure you take all
precautions to prevent inhaling the smoke.
Installation is the reverse of removal. Use all new gaskets on the EGR, EGR cooler, and intake manifold. The metal gaskets can be reused if they are in good shape, the o-ring should be replaced. You may find that your torque wrench will not fit into the limited space behind the cylinder head. The cylinder head is aluminum so don't kill the allen bolts. I torque them to about 18 ft lbs.
Caution: make sure you have the vacuum lines going to the anti shudder valve's black vacuum bulb and the EGR valve routed correctly. Swapping them will cause the EGR solenoid to close the anti shudder valve and result in a no start/start and then die.
Do you have more tips on cleaning a clogged TDI intake manifold of soot and carbon buildup? Feel free to share in the myturbodiesel.com forums