The EGR (Exhaust Gas Recirculation) valve is intended to reduce overall exhaust emissions by recycling exhaust gases back into the engine through the intake manifold.  While it’s essentially a great idea, the downside is that EGR is generally an afterthought by car manufacturers simply to reduce emissions – there’s often little or nothing by way of protection for the engine against the thick oily residue introduced into the intake manifold by the EGR’s exhaust gases.  The TD4 is a prime example of this.

The EGR valve itself is a T-shaped junction, with the intake air passing through the horizontal and the recycled gases introduced through the vertical.  A vacuum valve controls the rate at which exhaust gases enter the air intake, opening the valve at light throttle and steady speeds.
There are three problems with the EGR on the TD4 engine:

• The vacuum valve obstructs the intake airflow by around 35-40%, reducing the amount of fresh air that can reach the intake manifold
• The vacuum valve can additionally attract a build-up of thick oily residue contained in the recycled exhaust gases, further restricting air flow
• The vacuum valve can easily get clogged and stuck in an open (or closed) position

Although the EGR on my car did not exhibit huge amounts of build-up, there was certainly a lot more than I expected, and there is plenty of anecdotal evidence on forums (including photos of dirty EGRs) to show that the build-up can almost completely choke the air flow.

Where is the EGR located?

The EGR is located in front of the engine block and to the right as you look at the engine. 

Location of EGR valve - EGR bypass pictured

The solution

Actually, there are three possible solutions:

1. Disconnect the EGR valve
This is the simplest solution, and requires the disconnection of the braided hose which is attached to the EGR.  The hose should be blocked (a suitably-sized screw to plug the hose and some gaffer tape to keep it in place should do the trick), and secured so as not to dangle freely in the engine bay.  It seems some people have reported no problems with leaving the hose unblocked, but the general consensus seems to be that an unblocked vacuum hose is undesirable.  This solution prevents further exhaust gases being drawn into the air intake, but does not address the blockage and resulting reduced air flow caused by the EGR itself.

2. Clean the EGR assembly
Quite a simple solution still, but requires a hex driver bit, a flat-head screwdriver, an old toothbrush and your preferred cleaner (Mr Muscle, Swarfega, elbow grease, etc).  Remove the braided vacuum hose, and slacken the two jubilee clips connecting the feed end of the air intake line and the exhast gas supply line to the EGR.  Then remove the four bolts attaching the EGR to the engine, at the air intake supply side.  Depending on the flexibility of the 90 degree L-bend pipe to the right of the EGR, you may need to remove the intake duct at the as well – two cross-head screws and some jubilee clips are all that’s keeping it in place.  Clean the inside of the EGR assembly, noting how much oily build-up has accumulated within and the size of the obstruction caused by the valve itself, and replace.  Refitting is a reversal of removal.

Cleaned EGR valve

3. Bypass the EGR
If you’re going to the trouble of cleaning your EGR valve, you may as well replace it with a bypass.  The bypass is simply an EGR assembly without the vacuum valve, resulting in an obstruction-free flow for air passing toward the intake manifold.  EGR bypasses for the TD4 can be found with some diligent Googling (also try eBay).  The Superflow model is the one I recommend; it can be seen here although the listing has ended – some polite emailing for another one to be listed may do the trick.  The Superflow EGR bypass includes a turbo gauge attachment if you feel inclined to fit one, and a convenient connection for the now-redundant vacuum hose.  Note that the vacuum hose must be blocked using the screw and gaffer method in solution #1 if the bypass does not directly support this (the Superflow does), and not left open to the atmosphere or, even worse, connected directly to the air intake (as with inferior EGR bypasses).

The results

The results are largely subjective.  I didn’t feel much of a difference when disconnecting my EGR, but cleaning it helped a little.  Bypassing the EGR made the engine feel a lot more responsive, quicker to pick up revs, and a reduction in that dastardly foot-down-but-nothing-happening-yet feeling.

There is some speculation that the bypass of the EGR causes exhaust emissions to be too high, resulting in an MOT failure.  This is not necessarily the case.  With a bypassed EGR valve, my car achieved smoke test results of 0.79 and 0.07 (not a typo!) in 2009 and 2010 respectively.  The limit for a ‘fast pass’, i.e. passing on the first press of the throttle, is 1.50 while the overall limit for legal requirements is 3.00.