The carbon deposits on intake valves and ports prevent proper valve seat sealing and therefore compression loss. The result is poor engine performance. The process of “walnut blasting” clears the intake ports and valves of carbon deposits using compressed air and crushed walnut shells.
The procedure was developed to enable the cleaning of the intake ports and valves without disassembling the engine. It works because walnut particles are firm enough to dislodge carbon deposits while being soft enough not to damage engine components.
Despite the availability of numerous chemical solutions, walnut blasting is the most effective way to clean the intake ports of any BMW gasoline and diesel engines.
Why Does The Carbon Build Up?
The carbon in intake ports and valves accumulates from turbochargers’ blow-by, exhaust gases through EGR, oil mist through crankcase ventilation, or valve stem seals.
Prior to BMW’s introduction of direct fuel injection, this was not a concern. The fuel injectors located ahead of the intake valves clean them regularly.
After the injectors were moved directly into the combustion chamber, bypassing intake ports, the intake valves were left without those little cleaning machines.
When the first “carbon build-up” problems arose with BMWs we tried to clean the intake with all kinds of chemical solutions with very limited effect. So, the only method to clean the valves and intake ports properly was to remove and disassemble the cylinder head.
BMW quickly recognized the problem and introduced the “Cleaning cylinder intake ports with walnut shell granulate” repair operation through the dealer network.
The Symptoms of Carbon Build-up
In BMW gasoline engines, like N54, N55, B58, N63, etc., carbon build-up manifests as rough idle, misfires after a cold start or at high loads, as well as power loss and slightly increased fuel consumption.
In Diesel engines, like N57, and B57, the only obvious symptom is sporadic cylinder misfire after a cold start. The other, more dangerous symptom, is a melted intake manifold, which in combination with a leaking exhaust gas cooler can even cause a fire.
Beware that the leaking intake valves have roughly the same symptoms as a failing injector or small vacuum leak, so it’s not that easy to pinpoint the exact cause.
Carbon Blasting Procedure
While the procedure for walnut blasting is similar for all BMW engines, on certain engines you’ll have to close the blow-by (CCV) channels between the cylinder head and cylinder head cover (N55). Otherwise the walnut granulate may enter the CCV system and you’ll have to replace the valve cover.
By “may enter” I mean exactly that. Most often they won’t, and most shops will do the walnut blast without this step. But when you understand how BMW’s crankcase ventilation system works and how fragile it is, you may want to include this step.
Engine removal is required for BMWs with V8 engines, such as the N63 or S63. Although you could technically complete this task by simply lowering the subframe and removing the intake manifolds, it is not worth it. The best way is to remove the front subframe completely and lower the engine and transmission with a table lift.
Now, depending on the engine series, condition, and your skills/preferences, there are three different walnut blasting procedures:
Method 1 – Clean intake ports and valves with valves in the closed position. This is the most common procedure used by most repair shops and DIYers. You are basically cleaning both the intake port and intake valve up to the valve seat part. The rationale behind this method is that when there’s no new greasy carbon coming to intake valves, the seats will clean themselves while the engine is running.
For example, this procedure is recommended for the N54 engine by BMW.
Method 2 – Clean intake ports and valves with valves closed and then clean the valves seats with valves open. The first part is the same as above with an additional step of cleaning/polishing the valve seats with valves slightly opened.
After you clean the intake port, you position the intake valves so the opening gap is about 3-4 mm and blast the seats with walnut shells. After you are done with the cleaning, you need to remove the spark plug and blow compressed air into the cylinder to remove the residue from the combustion chamber.
ISTA recommends this procedure for the N55 engine. If you are afraid of filling the cylinders with blast media, don’t worry, it’s easy to blow them out with an air blower from one side (spark plug) and a vacuum cleaner from the other.
Method 3 – Clean intake ports and valves with valves closed and clean the combustion chamber separately. BMW recommends this for V8 and V12 engines like N63 and N74. I don’t get it why completely, but let’s say while the engine is already out, let’s clean all we can now.
This procedure is recommended by BMW for N63 and N74 engines.
So, which method to use? I’ve tried all of them, but after countless walnut blasts, I’ve come to the conclusion that Method 1 works just fine.
Step 1: Remove the intake manifold
To begin walnut blasting, first remove the intake manifold to have access to the intake valves. With four-cylinder and straight-six engines, you should spend less than half an hour. Hot Vee V8 will take you about half a day or more.
Step 2: Close cylinder head blow-by passages (N55, B58)
ISTA procedure states that you should close blow-by passages between the cylinder head and valve cover to prevent the entering of blasting media into the CCV system. The procedure also suggests making six plates from a feeler gauge tape (0.05 mm). So, cut six pieces to a length of about two inches. I made those little covers from plastic.
I’ve seen this done without this step and you can find a lot of cases online, like this one, for example:
Rotate the crankshaft at the center bolt into the TDC setting – valves at cylinder 1 are completely closed.
To save time and walnut shells, first clean intake ports as much as possible with other suitable tools, like a knife, screwdriver, or a small wire brush. When done blow the intake port using compressed air and an extraction adapter connected to the vacuum cleaner. Then clean this area with scotch Brite pads.
3 Walnut Blasting
Position the extraction adapter over the intake port of the 1st cylinder and insert the blasting probe at the extractor adapter.
81 29 2 208 033 N54
Connect compressed air and turn on the vacuum cleaner.
Move the blasting probe in a circular pattern while moving the probe slowly inside and outside. Depending on the carbon build-up, it takes about a minute or two per intake port. Move up and down very slowly with a bit faster circular movements.
With the extractor at the intake port and vacuum cleaner turned on, blow clean compressed air to remove the remaining walnut granules.
Clean the intake port with brake cleaner and compressed air. Check that no walnut granules are left in the intake port and seal the port with masking tape.
While moving to the next cylinder you need to make sure the intake valves of that cylinder are fully closed before you start the cleaning/blasting procedure.
Clean the remaining intake ports the same way.
Assemble The Engine
After blasting all intake ports, crank the engine at the crank hub for at least two full turns. Do not crank the engine in the reverse direction.
F30 335i N55 Example
After the cleaning procedure, we need to check how the engine runs and delete mixture adaptations. If you’re working on your own car you can just drive it and see how it behaves.
Depending on the case and many other factors, some engines will work smoothly from the first start after the cleaning while others will misfire, sometimes even for a few days. To prevent this from happening, you can follow this procedure.
Start the engine and immediately press the gas pedal fast and briefly (full throttle) several times – do not rev the engine! (1.5-2k RPMs is more than enough) – to blow remaining walnut particles from the combustion chamber.
If the engine runs smoothly, delete fault memory and the mixture adaptations. The cleaning procedure is concluded successfully.
If the engine runs erratically/misfires, switch off the engine and wait for 5 to 10 minutes. This will allow the hydraulic valve clearance elements gradually return to their previous position and the valves closes completely.
If the engine still runs irregularly, warm it up for 15–20 minutes at idle. Once the engine reaches operating temperature, switch it off and wait for at least 15 minutes (no longer than 1 hour). The oil at operating temperature has low viscosity and HVA elements will return more easily.
Start the engine again and immediately press the gas pedal fast and briefly several times.
Reset mixture adaptations and clear fault memory. Diesel: reset injector correction values.
Only after a long drive and adaptation relearning process under all relevant conditions (high speed, heavy load, extended overrun phases) and a subsequent cold start can you be confident that the engine is functioning smoothly.
Note! For V8 engines there’s an ISTA service function “Start-up after walnut-shell blasting.”
How to Prevent Carbon Build-Up?
It’s virtually impossible due to the design of the injection system. It’s just how direct-injection engines operate. Yes, you can remove and disable the EGR valve, and install the oil catch can, but this will only help as much in gas-powered Bimmers. With Diesel it’s another story – here most of the carbon deposits come from recirculated exhaust gases.
And while not much of a help with gas-powered Bimmers, disabling the EGR on a BMW diesel engine will almost fully prevent the carbon build-up in a healthy engine. There’s also a handy ISTA service function that allows you to increase the EGR fresh air rate on M47, M57, N47, and N57 engines. Unfortunately, it only works with older DDE software versions (I-Level) for N47 and N57 engines. I believe up to 2018 or so.
So, what can you do? Using premium fuels like Shell V-Power or fuel additives (never both at the same time!) will help with cleaner combustion and therefore cleaner recirculated exhaust gases. But they are just a small part of the mix.
The blasting media is 20/30 SAE or 0.45-0.80 mm walnut shells that can be sourced locally
N54 Walnut Blasting Kit
How to make a vacuum adapter
When BMW first began walnut blasting at their dealer network, vacuum adapters for the N73 engine were not available, so they wrote a handbook on how to make one from available parts.
So, this is also an option.