"bypassing" EGR?

n3qik · **Posted:** Sun Jan 08, 2006 11:05 pm

greiswig wrote:

n3qik wrote:

There is two ways to put the CRD in "off road" mode.

One is to block off the pipe from the exhaust going into the EGR vavle. This will only cause one code, P0401.

(SNIP)

I just wonder what the long-term effect of doing this might be. On my old ODBII vehicle, throwing certain codes caused things to go into "limp home" mode, which made things run far from optimally. Richer mixture, ignoring input from other sensors, etc. It would be nice to have some confidence that throwing this particular code caused no other issues other than...throwing the code itself.

I went 5 days with it blocked. Drove very well with no problems.

Ranger1 · **Joined:** Mon Oct 17, 2005 7:40 pm **Posts:** 1137

I ran across a link in the TDI forum, posted back in 2004, where someone with a manual on the Bosch EDC-16 system described some of its operation. I wish I had seen this earlier. This is the same electronic engine control system that our CRD uses. The one feedback loop they describe that makes it extremely difficult to decrease EGR flow is an O2 sensor... the ECM (EDC-16) is adjusting fuel trim, and monitoring EGR flow by measuring oxygen content in the exhaust. They thought this measuring was occuring somewhere in the millisecond range - maybe hundreds of times per second. If that's true, and we have an O2 sensor feedback loop, it looks like EGR is here to stay on our CRD's.

I looked for a sensor in the CRD exhaust pipe, starting from the turbo exit all the way down to the muffler. I don't see anything. The only place I haven't looked at yet is the exhaust manifold, hidden by the heat shield for the turbo and other accessories. Has anyone seen any O2 sensors on their CRD exhaust system?

Link is here:

http://forums.tdiclub.com/showthread.ph ... tuning+box

oldnavy · **Posted:** Fri Jan 13, 2006 9:48 am

No O2 sensors on diesels period. Not on Dodge, VW, MB, or Jeep.

greiswig · **Posted:** Fri Jan 13, 2006 1:36 pm

Ranger1 wrote:

I ran across a link in the TDI forum, posted back in 2004, where someone with a manual on the Bosch EDC-16 system described some of its operation. I wish I had seen this earlier. This is the same electronic engine control system that our CRD uses. The one feedback loop they describe that makes it extremely difficult to decrease EGR flow is an O2 sensor... the ECM (EDC-16) is adjusting fuel trim, and monitoring EGR flow by measuring oxygen content in the exhaust. They thought this measuring was occuring somewhere in the millisecond range - maybe hundreds of times per second. If that's true, and we have an O2 sensor feedback loop, it looks like EGR is here to stay on our CRD's.

I looked for a sensor in the CRD exhaust pipe, starting from the turbo exit all the way down to the muffler. I don't see anything. The only place I haven't looked at yet is the exhaust manifold, hidden by the heat shield for the turbo and other accessories. Has anyone seen any O2 sensors on their CRD exhaust system?

Link is here:

http://forums.tdiclub.com/showthread.ph ... tuning+box

Thanks as usual, Ranger1. Good information to have. I'm puzzled by one thing: in looking through the service manual, it is pretty clear that there is no O2 sensor on the CRD. How could it use the same ECM but not use an O2 sensor...did they just disable that part of the code?

The other thing is that I'm not sure what if any overall benefit there would be from just disabling the EGR, since I thought it only injected exhaust at low engine loads. Is that false?

oldnavy · **Posted:** Fri Jan 13, 2006 2:15 pm

In the TDI and CRD the EGR only functions under certian conditions at low rpm's. I don't remember the rpm point for the TDI, but I seem to remember under 1300 or 1500 rpm's was only time the EGR functioned. It was one of the reasons why people that drove the TDI real frugal to get best mileage from the car were the people first impacted along with non synthetic oil users and in some cases maybe non diesel oil requardless of synthetic or non synthetic oil. The excess oil fumes from some oils along with the soot clogged up some cars before 30,000 miles, while some cars that used a diesel synthetic oil and were almost always heavy throttled on a daily basis seldomed had an intake needing cleaned prior to 100k mile mark. Also it seemed type of fuel used also played a big part, I seem to remember the guys using B100 always seem to not have any thing but oily intakes.

I remember there being on average three basic average mile points for the A4 cars, 30k miles, 65k miles and 90k miles when they developed clogged intakes bad enough to cause driveability problems. The problem was also partly the fault of the desighn of the intake on the 99 through 03 non PD TDI engine, it had a very sharp bend in the intake were it also narrowed down also at that same point. The A3/B4 model VW cars that had different head & intake design seldom ever had a problem till way past 100k miles or more.

My take on the soot in intake problem is it will basically disappear after we switch to ULSD fuel later this year.

Ranger1 · **Joined:** Mon Oct 17, 2005 7:40 pm **Posts:** 1137

This is what I find puzzling.. its an excerpt from the link I posted. It shows the 04 PD TDI's using an O2 sensor... I'm concerned with the grime factor being put back into the engine...soot isn't the only thing coming out the exhaust. It also raises exhaust temperatures by recycling the heat. That coolant chamber on the egr valve doesn't come close to reducing the exhaust temperature to 150 or 175 fresh air is.

"recent discussion of the new PD O2 sensor and how to do an "off-road" EGR modification got me thinking. I pulled out my Bosch Yellow Book on Electronic Diesel Control (EDC) to see if it had anything about the use of the oxygen sensor. Sure enough, it does! I bought this quite a while back, and I guess I just glossed over the O2 sensor section. I'll try to sum up what it says about the O2 sensor since I don't think it's been addressed here at Fred's yet. This is all technical stuff... don't read on if you find this stuff boring.

First of all, the use of the oxygen sensor allows a 10-20% fleet reduction in NOx emissions, and is only available with EDC16. EDC16 is the new ECU architecture, and is used on the 2.0L engines. All the european 1.9L PD engines use the older EDC15, which is similar to what was used with the older distributor-pump engines. This confirms why EDC16 and the O2 sensor were included in the north american PD100 engine... gotta meet those NOx levels to sneak into EPA Tier II... but it still wasn't enough to meet CARB requirements.

The main purpose of the O2 sensor is to allow closed-loop Lambda-based EGR control. Lambda is the symbol commonly used to refer to air-fuel ratio. By measuring the amount of uncombusted oxygen in the exhaust the system can compute what the air-fuel ratio was during combustion.

The system is basically a dual-loop controller. This means that there is a fast loop and a slow loop. The fast loop is the same as in older engines... the MAF is used to measure fresh air flow into the engine, and the EGR valve is used to modulate it to a desired setpoint given the engine operating conditions. This is a fast loop... it measures MAF and recalculates the command to the EGR valve many, many times a second... a period probably on the order of milliseconds."

Ranger1 · **Joined:** Mon Oct 17, 2005 7:40 pm **Posts:** 1137

"
Thanks as usual, Ranger1. Good information to have. I'm puzzled by one thing: in looking through the service manual, it is pretty clear that there is no O2 sensor on the CRD. How could it use the same ECM but not use an O2 sensor...did they just disable that part of the code?"

Good question, first question that occured to me when I read this post from TDI club. I don't know - I suppose they could have tied the O2 input to a fixed reference voltage and leave it at that. Or maybe Bosch provides software hooks to use or not use the O2 feature. If we don't have O2, thats good news as far as I'm concerned.

"bypassing" EGR?

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