[GTLee]

Hey folks,

I recently bought my TDI and have been doing massive amounts of research (of which this site has greatly contributed). One of the things I read was that I could use a VCDS to recalibrate the EGR so as to never have worry about future black sticky tar build-up.

However, I read in part 2 of the TDI buyer's guide on this site that "Many people suggest turning down the EGR with a VCDS but the ECU will adapt and the result is lower fuel economy."

This is the first time I am hearing of this. What kind of lower fuel economy are we talking about? 1-2 MPG? 5-10 MPG?

What is the ECU adapting to that results in the lower MPGs?

What does the TDI community have to say?

Regards,
Lee

 

[chittychittybangbang]

This was found years ago and was put into the FAQ of another site....but has since been discredited as giving lower fuel economy. I'm not sure how the ECU is adapting because I don't know how to read the maps, few people do. From what I've heard, it's only a few mpg. This became popular because it was thought it would help with intake manifold clogging.

To turn down the EGR you have to physically block or restrict it. This will throw a CEL-MIL, engine tuning is required to remove the CEL.

 

[GTLee]

What if I plugged the vacuum line to the EGR valve and left everything else intact? Would the ECU still set off a CEL-MIL?

 

[chittychittybangbang]

Yes because the EGR valve also changes how the MAF behaves. The EGR gasses displace intake air.

 

[Seatman]

Glad I left my car alone and didn't do the egr thing. I don't mind taking the manifold off once in a while for a clean though because I like to tinker anyway.

 

[GTLee]

Yes because the EGR valve also changes how the MAF behaves. The EGR gasses displace intake air.

But the EGR is way downstream of the MAF. There's even a turbo in between them. How would it change it's behavior?

Was is confusing me is that I don't see what the feedback mechanism to the ECU is regarding the EGR. How would the ECU know that the EGR is malfunctioning? What's the sensor that blabs to the ECU that somethings up?

 

[chittychittybangbang]

To the best of my understanding, the ECU knows if EGR is malfunctioning in part, off MAF readings. I could be wrong but this is how I think it works.

Let's say the EGR at idle is 50%. This is about normal. I don't know how much EGR gas is as a percent of intake air so let's just say 20% as an example. If the car is telling the EGR to give 20% intake air by working the valve at 50%, the MAF should also go down by 20%. If it doesn't it knows something is out of bounds. You car has no oxygen sensor to test the exhaust air.

 

[larrysmiths1]

To the best of my understanding, the ECU knows if EGR is malfunctioning in part, off MAF readings. I could be wrong but this is how I think it works.

Let's say the EGR at idle is 50%. This is about normal. I don't know how much EGR gas is as a percent of intake air so let's just say 20% as an example. If the car is telling the EGR to give 20% intake air by working the valve at 50%, the MAF should also go down by 20%. If it doesn't it knows something is out of bounds. You car has no oxygen sensor to test the exhaust air.

This is correct. I had done a few of these adaption changes in conjunction with intake cleaning to try to help the customers save or prolong some future expense. In keeping with the old axiom "no good deed goes unpunished" several did notice loss of fuel economy so I set them to original spec and have discontinued changing them. The MAF readings did change a good bit (15 - 20 %) after the reduction of EGR duty cycle.
On the subject of intake cleaning, I have always preferred to clean them manually with and old table knife to get most of the stuff out of the throat and ports. I also use a piece of 1/4 " cable 36" long folded in half. After a few uses the ends of the cable begin to unravel and it is similar to a couple of flexible wire brushes to get all the other stuff out. It also is not as bad for your lawn as with the pressure washer method. The video of the redneck method is pretty funny