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556 Posts
Discussion Starter #1
This is just another tutorial for a repair I’ve recently done on my car, to save others time with the research on how to do this. This repair is related to [ur=]my other topic about the tricky vacuum actuator leak[/url] that causes the engine to run roughly right after start and whenever launching from full stop, but running otherwise fine and smooth. In short: this was caused by the EGR cooler bypass valve vacuum actuator leaking, which in turn would affect also the turbo boost of the car whenever it tried to activate these both at the same time.

In order to understand what and why to do, I will do some explanation first. This might also prove useful in understanding how the EGR system and the EGR cooler in particular work, or what issues it might create when they’re not working as intended, and also when doing other kind of diagnosis or repair on it. That’s why this post is so lengthy. Anyway, let’s start!

What does the EGR cooler do?

The EGR cooler – as the name say – does the cooling of exhaust gases that are recirculated and routed back into the intake. The EGR cooler on most cars attaches directly to (or is part of) the exhaust manifold, and cools the gases passing through it using the coolant that’s also used to keep the engine temperatures at bay. The cooled gases are then routed back through the EGR pipe to the EGR valve, which might or might not allow them to mix with the fresh air, and enter the combustion chambers.

The EGR cooler has a mesh-like structure inside, similar to that of the radiator at the front of the car. The fins are attached to pipes that have the engine coolant circulated through them as part of the coolant circuit, and the exhaust gases are forced to pass through the holes in the mesh, warming them up and transferring some of their heat into the coolant. So, in some way the EGR cooler is the direct opposite of the radiator, but generally works on the same principle, the only difference being the direction in which heat is transferred between coolant and gases passing through the cooler.

EGR cooling is not done continuously and at all times, only under specific circumstances. At startup the ECU is using the EGR system to recover some of the heat from exhaust that would otherwise leave the car through the tailpipe. This helps the engine to reach operating temperature faster and make combustion more perfect even while the engine is generally still very cold.

Later on, once the ideal operating temperature range is reached, the ECU uses the EGR cooling system to control the combustion temperatures and keep them in the ideal range. For this the engine periodically needs to switch cooling of the recirculated gases on and off. Generally, whenever the engine is idling or working at low loads, the ECU usually stops cooling the recirculated gases in order to avoid the combustion temperature getting too low, and turns cooling only on when the load increases.

So, the EGR cooler is switched on and off during drives.

The EGR cooler bypass valve

This is where the EGR cooler bypass valve comes into the picture. As the name says, this allows (or does not allow) the exhaust gases going through the EGR cooler to bypass the actual cooling parts and getting rerouted back into the intake without having lost most of their heat energy.

For this the EGR cooler actually has two different paths: one that leads through the previously mentioned reverse radiator / heat absorption fins, and another one that avoids the latter. How these two paths and the bypass are implemented geometrically depend heavily on the design of the EGR cooler design.

The one in the 2.5 TDI engine uses an older design, that essentially divides the cooler into a top and a bottom portion inside the cylinder that’s seen as the cooler from the outside. The top portion of the cylinder is essentially like a straight pipe where the exhaust gasses can pass through without any hindrance and at full speed, untouched; and in the bottom part they are forced through the previously mentioned heat transfer mesh that cools them down and dissipates part of their heat energy into the engine coolant.

Which path the exhaust gases are taking is determined by the EGR cooler bypass valve, which interestingly is not at the entry point of the cooler in this particular design (ie. that of the Touareg R5s), but at the exit point. This means that exhaust gases can enter both pathways from the exhaust/turbo side, but only leave and pass through one of them.

Which one this will be, is determined by the bypass valve flap position. The flap is a binary one, which in one position closes down the upper, “uncooled” route, and allows gases to pass only through the lower, cooled route; and in the other position does the opposite, and does not allow gases to flow through the actual cooler part.

The EGR cooler bypass valve vacuum actuator

The flap is moved between these two settings/positions by a relatively simple mechanism. The engine is controlling it through the N345 solenoid valve, which either allows or disallows vacuum to be applied to the vacuum pipe that’s running to the bypass valve vacuum actuator.

If vacuum is applied to said pipe, that vacuum pulls on a diaphragm inside the actuator, which has a metal rod attached to it, which in turn moves the flap in the position where it opens up the top, “uncooled” pathway and blocks the cooled one.

When vacuum is not applied to the actuator, a spring inside it and attached to the diaphragm pushes the actuator rod out, moving the valve in the position where it opens up the cooled pathway and stops exhaust gases from passing through the cooler’s top, straight, “uncooled” portion.

How does the EGR cooler fail?

The EGR cooler can fail in many ways. The most insidious way is, that the coolant lines inside the cooler crack, which allow coolant to leak into the cooler. From there the coolant might flow down into the turbo (because of gravity), or taken up by the EGR stream and carried into the intake manifold and ultimately in the combustion chambers. In the best case this corrodes the turbo or the EGR pipe and the intake, and in the worst case it can lead to the engine hydrolocking – even though chances for the latter are rather slim in the R5 design, because of the geometry of the implementation. (Video of an EGR cooler leaking coolant into the passthrough chambers can be found here).

Another common way for the EGR cooler to fail is developing a crack on its housing, which will allow exhaust fumed to leak into the engine bay and from there get into the cabin. This kind of failure is more obvious, because the exhaust smoke can be smelt and even possibly seem emanating from under the hood or through the front grille. The crack usually occurs on the turbo side, and might be a direct consequence of the coolant leaking into the cooler, which in turn can corrode it from the inside until it eats through the housing.

Sometimes the exhaust leak appears at the other end of the cooler, at the bypass valve stem. This is usually a less serious leak that’s harder to notice. The only giveaway signs are carbon/exhaust deposits on the side of the bypass valve, or in more serious cases exhaust shooting out from the valve steam. (See video here!)

These kind of failures become pretty obvious after a while, because of the loss of coolant, because of the white smoke leaving the tailpipes, and because the exhaust gases appearing at the front of the car, and visible deposits being created near in the engine bay. Also, sooner or later the ECU will flag this with a warning light or a fault code, because of the coolant reaching critically low levels or the exhaust flow not reaching expected values.

EGR cooler bypass valve actuator failure

However, there’s a less serious failure of the cooler, which goes mostly under the radar until some other parts of the car develop some serious problems – and might go undiscovered even after that. This is the failure of the EGR cooler bypass valve actuator – which is actually the focus of my post.

The bypass valve actuator usually fails as the result of the rubber diaphragm inside of the actuator housing getting punctured or torn, allowing air to pass through it. This obviously makes it impossible for the vacuum (switched through the N345 solenoid) to pull on the diaphragm and in turn pull the actuator rod, leaving the “upper hand” to the spring and the EGR cooler constantly in the “cooled” position.

This kind of failure will under no circumstances throw a fault code on its own, because the ECU has no way to verify whether the bypass valve is working or what its actual position is, and the fault will also not be detectable through coolant loss or exhaust fumes or the absence of EGR flow.

The only giveaway will be that idle will be very rough, the engine will take a lot longer to warm up, launches from standstill will put audibly more stress on the engine, and carbon deposits will collect at a faster rate than usual at the EGR valve and in the intake manifold and ports – all which are not directly observable from the outside when looking at the engine.

The only way to realize that the actuator has failed is to observe it directly visually (especially at startup) and noticing that the valve lever never moves, and that the connecting control rod always stays in the extended position, and never get pulled back (partially) into the actuator. This diagnosis can be sped up or verified by attaching a vacuum tester to the actuator and trying to pull a vacuum on it to make it move the rod. If the tester can’t develop a vacuum and the rod doesn’t move, then the actuator has definitely failed.

Fixing the EGR cooler bypass valve vacuum actuator

So, obviously the solution to this is to fix the actuator, which in turn will allow the ECU to gain back the ability to switch back and forth between the EGR cooler actually cooling the exhaust gases or leaving them pass through without their temperatures being (significantly) lowered on their way to the intake.

Interestingly enough the actuator – despite being fastened onto the EGR cooler in a way so, that it can be (and was likely meant to be) very easily replaced – is not sold separately. I found this unacceptable, especially, that the rest of the cooler showed no signs of any problems, and that a new EGR cooler was over $800 when ordered from VW, and $350 even then when ordered directly from the supposed OE supplier to VW.

It’s worth noting that Chinese EGR coolers can be had for as low as $100-150 on eBay, but I was not convinced that they do actually any cooling and aren’t just “straight piped” on both paths, or that they wouldn’t start leaking coolant (down into the turbo, or what’s even worse, into the intake) prematurely, which would obviously negate any price benefits they would otherwise have.

I will also note that the VW repair manual explicitly tells you on the component diagram to replace the actuator together with the actual cooler only, and not separately. However, even after extensive research I didn’t find the reason why this is so, and why you’d need to replace the whole cooler assembly even if it otherwise seems to be fine; nor any anecdotes or stories where something went wrong because someone has repaired their actuators. So, I just went ahead with the repair of the actuator only – albeit with keeping the warning in mind. (See more in “Concerns regarding replacing the actuator only”).

The repair process

Because the actuator is not sold separately, you mostly have two ways to repair it.

One of them is taking off the actuator off another EGR cooler – which might be a cheap Chinese clone, another genuine EGR cooler that for ex. has cracked but still has the actuator working; or you might even try to buy and adapt a generic aftermarket turbo vacuum actuator to replace the original actuator. I considered even this option and it might work with some actuators, but did not actually test the idea. (For ex. an adjustable actuator for a NISSAN Patrol GU 3.0 ZD30 GT2052v Turbo looks like a good fit – but as said, I did not test this idea in practice, and attaching the actuator rod to the valve lever is still an open question anyway).

The other solution is to repair the original actuator, by fixing the leak it has. Now, I’m not sure whether rubber can be somehow glued or patched in a way that it could withhold the pressures and temperatures present near and in the actuator, but even if it would be possible, I’d be still worried that the old, used and dried out diaphragm would likely develop just another leak at some other spot in a short time. However, there seems to be a company manufacturing and/or selling good quality replacement diaphragms for a lot of vacuum actuators – including for that used in the Touareg R5 EGR bypass valve actuator. (These can be purchased at several places, like here. The actual membrane for the R5 TDI EGR cooler bypass valve actuator is here.)

So, it seems like the solution for the actuator leak problem is in theory very simple: you just need to replace the diaphragm inside the actuator as it is. Now, this sounds simple, but is not that easy to actually pull off. This is because even though the actuator is easily dismounted from the cooler, it’s not that easy to take it apart, so you can replace the diaphragm in it.

Nonetheless it’s still doable. What you have to do is – after you’ve removed the actuator from the cooler – to bend back all the retaining metal “claws” at the bottom of the actuator’s plastic housing, so you can remove the latter, opening up the inside of the actuator. Then you have to unscrew the arm from between the two metal plates that are holding the diaphragm, then take the new diaphragm and reassemble everything in reverse order. Then bend back the “claws” to seal the housing. If everything is done correctly, the repaired actuator will be able to hold vacuum and move the control arms that flips the valve between the two positions.

Make sure that you test the actuator with a vacuum tester before you mount it back onto the cooler, because if it’s not working and unable to hold vacuum this way, then it will be not able to flip the valve when mounted back either, and the repair will be pointless. (Actually, you better test the actuators vacuum tightness before bending back all the “claws”.)

If it works, then it will most likely also work when put back on. The result – coupled with the presumed cleaning of the EGR cooler and EGR pathways – will be an improved, smoother run of the engine, less carbon deposits being created, and an improved fuel economy. This can be actually – depending on driving style and whether you’re doing it highway/city traffic – up to 1-2 liters/100 km or 5-6 more miles per gallon, which I think is a considerable saving on fuel costs, at least in the case of the R5 engine.

• When originally opening up the vacuum actuator housing pry very carefully on the retaining “claws” with a flat screwdriver! Try not to push against the vertical part of the plastic housing, because if you do, you might crack it, and then you’ll have another – now possibly unfixable – leak!
• Once you’ve slightly lifted the edges of these “claws”, you can use a side-cutting pliers, with one of the cutting edges placed under the claw and the other placed at the stem of the claw, and a rotating movement to pry the claw fairly easily back to a vertical position. Obviously make sure to not actually pinch any parts with the pliers – you only need cutting pliers because they have a raising edge which allows you to get under the claw when it’s just barely lifted from the plastic housings base.
• You’ll need a 10 mm socket wrench to losen the nut that’s holding the diaphragm and the actuator rod together. You can use another, 7 mm wrench at the stem of the rod to prevent the rod from turning.
• When putting things backs together you can use regular pliers to bend the claws back. Make sure to press them firmly onto the plastic to ensure that there will be no leaks! On the other side make sure you don’t press that hard that the plastic housing might crack.


< ... See also following post! I had to split the text in two, because it was too long :) >


556 Posts
Discussion Starter #2
< continuation of the previous post >


Concerns regarding replacing the actuator only

As noted previously, the repair manual says to replace the EGR bypass valve actuator only with the whole cooler assembly - but I did not find any information on why that would be necessary, nor any examples of where replacing the actuator on its own caused any problems for anyone.

Regardless of this I came up with a few ideas why this would not be ideal, and tried to take countermeasures against them, just in case, so I don’t cause more problems than I already have.

One reason I speculate the repair manual says to replace the whole EGR cooler assembly might be, because once the actuator stops working, the now permanently blocked passage might fill up with carbon deposits - which if the actuator will be restored into working order, might either still block that pathway, now completely hindering EGR gases to pass through the cooler, or parts of said deposits might be carried into the intake in larger chunks and cause problems there. The obvious solution for these concerns is to clean the EGR cooler when repairing the actuator on it, and checking whether gases can still pass through both (cooled and “uncooled”) pathways.

Another reason might be that the actuator usually fails because of an exhaust leak having developed at the bypass valve and spitting exhaust fumes right at the actuator. In this case replacement of the actuator is not actually a solution to the problems at hand, because the leak will still be there and because it will kill the new/repaired actuator in a short time again. However, if you don’t have carbon deposits near the valve, then this can be excluded as being the case.

The bottom line is: even though I don’t actually know why the service manual does not recommend replacing the actuator only, but it seems to me that as long as you don’t have any signs of an exhaust leak on the cooler, and you clean and check both pathways, replacement or repair of the actuator should not pose any problems, as it will only restore the normal working order of the EGR cooler as the whole. Nonetheless one should keep these considerations in mind and weigh the possible benefits and drawbacks of a full replacement of the EGR cooler vs the repair of the actuator only when contemplating what to do.

Replacing the bypass valve actuator without removing the cooler

If you are replacing the EGR bypass valve actuator with another genuine one or repairing your existing one by replacing the diaphragm, that can be theoretically done in-place, without dismounting the EGR cooler from the engine, by only removing the vacuum actuator from it. The latter is held in place by two Allen bolts and a pin at the connection point between the actuator rod and the valve lever. However, removal is very cumbersome, and you’ll most likely end up with dropping some parts between the engine parts. I also do not recommend doing it because of the concerns explained above. So, I suggest you remove the EGR cooler as a whole, which not only makes replacement of the actuator a lot easier, but also allows you to clean and check the cooler pathways; and then put it back on, after you’ve fixed everything.

Once you’ve cleaned the EGR cooler I suggest you also check whether the coolant lines do not leak into the exhaust pathways and vice versa. This can be done two ways. You can either connect a pressurized water hose to the coolant lines and see whether any water is dripping out of the ports where the exhaust is supposed to enter and exit the cooler. Or you can connect hoses with compressed air in them to said lines, and then submerge the cooler in a bucket and see whether any air bubbles are coming up from it.

Of course always make sure to blow out any remaining water from the exhaust pathways using compressed air (or with at least a leaf blower) after you’ve cleaned and tested them this way for leaks. I do not recommend you just let the cooler dry out on its own, because that will allow the minerals from the water to settle inside the cooler and possibly cause corrosion it over time.

When putting the EGR cooler back on make sure to renew the gaskets at the entry and exit ports of the EGR cooler to avoid leaks.

556 Posts
Discussion Starter #4
Top notch ! Now I want to buy an R5 just to do this repair :)

Well, the actual repair principle should be the same for any vacuum-actuated valve on any car, including turbo actuators.

Also, most of the post is not really about the repair but about the theory behind the EGR cooler and the EGR system, which again, applies to any and all cars.

The only difference is that more modern cars usually use electronic actuators instead of vacuum-based ones. So, the vacuum-related parts do not apply to those, but everything else does.

I mostly posted all this, because I could barely find any information on how the EGR cooler works (both at the theoretical level, and also internally in that particular setup), and thought what I gathered from literally days of research might prove useful for others. So, this is intended more an informational post on EGR cooling than a repair guide of the actuator, even though it was the latter why I read up on the topic and how I discovered how the actual cooler in this engine works.

1,369 Posts
Well done.
I will have to inspect the combi valve on my vr6 soon. I've been trying to solve a rich condition since I bought it in 2017. I also have had a case of disappearing coolant twice. I'll study the comi valve on the vr6 and see where that leads me.

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556 Posts
Discussion Starter #6
EGR problems and intake carbonization might be your prime suspect with any rich condition, if injectors themselves seem to work just fine.

I was surprised how much EGR problems can influence the performance and combustion properties of the engine. I would have thought that with all the sensor data the engine controller would be able compensate more for such faults and not let the engine run rich, but that's obviously not the case. At least not on R5 engines anyway.

Maybe on more modern engines the ECU can cope better with restrictions in these components.

1,369 Posts
Did some digging on the web.
Combi valve it is called for the VR6 3.2. Part number: 022131101B (160€).
This guys has some good info. I will definitely be removing and checking it to see if it has anything to do with my rich condition.

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