HELP ! NO DTC but engine cuts boost over 0.8+ Bar ???

[dodgydave]

Guys,

I'm pulling my hair out!!

I've just fixed one problem and after finally driving happy for a week the next one arises. Now, I don't know what it is and the car is undrivable.

When I accelarate softly or full throttle and the boost rises up to approx. +0.8 Bar the ecu cuts power. Then the boost jerks around and the car jumps like a Kangoroo!!

I already changed the MAF, N75, DV's (<-Brand new Forge) & MAP coz off my previous problem. Now, I tried changing N75 and MAP again just on the offchance the new parts broke after a week but it seems this problem comes from somewhere else. The weird thing is that I have no DTCs.

Could it be the EGT sensors ?? How difficult is it to change them myself on a LHD RS4 ??
How can I test them ? VAG Block ? etc

Please any help is really appreciated because I need the car! ;-(


Cheers,
Dave

[S2tuner]

Dave, I had offered you some help earlier on but you have always seemed to ignore me....

Anyway, EGT sensors will always trigger fault codes, so you have to look elsewhere IMHO...

Mihnea

[dodgydave]

??? I don't remember ignoring you ? Appologies if I came across that way.

Back to the problem, what else could it be ?

What about knock sensors ?

I'll try to do some logging tonight. Any tips what VAG-COM blocks I should include ?

Cheers,
Dave

[Golich]

Given the symptoms, I doubt its an EGT. However, my previous S4 had problematic EGTs that didn't always register a DTC. Eventually one did, but the other didn't show itself until AMD were in the middle of a remap and it was producing an odd output.

Block 112 is the EGTs.

Having just removed an EGT last night on my RS4. The right hand side one is a real pain.(if looking into the engine bay from the front of the car) I took 3 hours trying various combinations of spanners and sockets. I couldn't even get an open ended 17mm spanner onto the nut, so I cut the EGT wire with a junior hacksaw because I couldn't get a squeeze on any one of the three wire/cable cutters I have. In the end I managed to get a small cheap ring spanner onto the EGT nut after removing the driveshaft splash guard and bending the spanner.

Even then I could only get 5mm of movement. Thankfully, the nut wasn't too tight. Now I've still got to put the new one on. So I'm going to have to cut a slot in the ring spanner so I can get the ring over the wire and hope that I can apply enough torque to the nut. If not, I'll either have to grind down an open ended spanner and probably reduce its length. I might add I had to do all this because the water and oil pipes into the turbo are in the way and I don't have any suitably clean containers to drain into which I was sure I wouldn't have do. If both of them were out of the way it would have made things easier but still tricky as the heat shield is in the way.

Do a search on Audiworld.com for EGT in the A4/S4 forum. Unfortunately, they have not put this into the technical section - yet. But the post is very good with loads of pics. That lad was able to get his off without cutting. PS If you have to cut, I'd disconnect the multiplug from the EGT unit incase its possible to blow anything.

PPS VAGPARTS are £91 per EGT V's Audi's £161!

[dodgydave]

Okay,

I did some more testing and the EGTs seem fine. They always show 935 C but I think that's because the temp is below that and I can't really build up temp because I can't build up any boost.

Also, my initial description was incorrect as the boost pressure where I get the problem varies. If i slowely open the throttle I can build up more boost and hold it as long as I don't floor it.
The feeling is the same as I use to get on my old mechanical turbo cars when the wastegate was broken and the boost obvershoots. In one hour of testing I got one DTC that said something like "Maximum boost pressure excceded'. I can't remeber the number as I cleared the memory whilst driving. So, the DTC would tie in with my theory.

I was thinking that the waste gate on one turbos proberly failed and therefor the ECU still has some control over the pressure, which is still enough to stear clear of DTCs most of the time.

The only thing I don't understand is that when I slowely accelerate and then floor it one out of ten times I would get +0.41 and the boost would stay there even in the higher revs. When I then take the foot off the throttle and floor it again I get the big jerking again. The jerking is so severe that I fear I'll damage the transmission soon.


Anyone any ideas ? Anyone had a broken wastegate on one of the turbos before ?

Can test the wastegates on the turbos without taking the engine out ?


Cheers,
Dave

[smurfbus]

You better do a pressuretest to be sure no leaks in the lines or hoses.

[Turbo Joe]

Sorry bro, sounds like your EGT to me. I have exactly the same problem at the moment where my EGTs are reading 945 degrees. I wished it wasn't but the symptoms were the same as when one of them went the first time round. I just changed the driver's side so I reckon it's the passenger side this time. Problem is with this modified "F" sensor replacing the "D", if you had your car remapped with original "D"s fitted, you will notice a reduction in torque when you replace them with the "F"s. Audi have deliberately done this to reduce claims for blown turbos under warranty. A sneaky method I must say. I am currently looking to change one of mine and the harrowing stories of how difficult it is to change on the pass side are coming through thick and fast. If you need more info, check the thread about them in the S4 forum where I go into much more detail.

[dodgydave]

Okay,

I've spent all weekend trying to locate the problem. I did a pressure test and the system is airtight.
Then I just pumped air into the wastegates from both turbos and they are operating as expected. They open and close syncron and there is no leakage either. They're fully open at around +0.5 Bar, which must be right. As I remember correctly the max boost pressure without N75 is around +0.4 Bar, which means by that pressure the wastegates are nearly fully open.

Now, I started get the following DTC:

16622 - Manifold Pressure / Boost Sensor (G31): Signal too High

P0238 - 35-10 - - - Intermittent

When I pull the plug on the MAP (G31) I get the same powercut/jerking problem but just much earlier, almost imediately when I try to accelerate. I have a new MAP and an old one and have the same problem with both. So, I reckon it must be either the wiring or the ECU ??

In the AUDI Workshop manuals states that there should be around 5 volt between pin 1 and 3, which I have. But I also measured the other pins and here are the results in volts:

Pin 1+3 = 5.01
Pin 1+4= 5.64
Pin 3+4 = 0.63

As you can see the 1+4 seems to equal (1+3)+(3+4). Does that seem right ???

Anyway, it seems that all the wires are connected to the ECU as they all carry different signals. It would be handy to know how the Sensor works to see if the voltages make sense. Anyone ???

The workshop manual continues and suggests to attach the cable to a VAG 1598/31 tester and messure the voltages whilst driving on socket 108/98 on the testbox. Now I don't know what the box does and how I could test this without, again anyone ??

Could it be the ECU ?

So, please any help is appreciated!!!


Cheers,
Dave

[Turbo Joe]

I still say it's your EGT sensor. Vag-com never showed up mine as faulty either and I was getting a constant 945*. I know you don't wanna hear it but the car's behavior is classic signs of an EGT gone defective. Stop running from the EGT possibility. At least change the oldest one and see what happens after that. Defective EGTs can throw up all sorts of fault codes. When I went to AMD 5 came up on my car that didn't show themselves on vag-com but I know the EGT passenger side is the route of all of them.

[dodgydave]

Update,

I'm still pulling my hair out and at this rate I'm probably bold before I got my baby back in working order.

So after checking with a lot of RS4 experts I got the same answer. Everybody thinks including me that it is impossible that the ETGs trigger the DTC I get occasional and that is must have to do with the G31, which I've already swapped.
Now, I checked all the wires from the G31 to the ECU and they're fine.
But when I attempt to do the voltage test in the vag book (look attached file) I fail to get the require voltages. I always get 5.64 Volt between Pin 1 and 4 instead of 1.9-3.0 Volt.
Unless the VAG test box does something fancy I think here might be a problem that points to a faulty ECU ????

Anybody any thoughts on that ??

Cheers,
Dave

[RS4_STEVO]

Hi Dave

I know where you are coming from regarding rippin your hair out! It ain't funny and very frustrating, but you will get it fixed in the end, i'm sure!

Are ECU's interchangable for testing only?

One thing for sure it ain't a boost problem causing a boost fault, I would say it's definate electrical, sensor causing the boost fault!

I'm going to take mine to AMD or JZ Machtech, specialist I feel in this field!

[S2tuner]

Dave, shoot me a call and come and see us today if you can and we'll figure it out...

Thanks,

Mihnea

[dodgydave]

Hi Dave

I know where you are coming from regarding rippin your hair out! It ain't funny and very frustrating, but you will get it fixed in the end, i'm sure!

Are ECU's interchangable for testing only?

One thing for sure it ain't a boost problem causing a boost fault, I would say it's definate electrical, sensor causing the boost fault!

I'm going to take mine to AMD or JZ Machtech, specialist I feel in this field!

Yes,

You can swap ECUs for testing but you need to adapt it to your car otherwise the immobilizer will kick in.
Nothing that can't be fixed with a quick VAG-COM session.

Dave

[dodgydave]

AHHHHH !!!!!

After another week of testing up and down and getting G31 errors and sometimes mixture too lean I did the fuel filter, which was pretty clogged up. Still no luck. ;-( Then I messured the fuel pump flow and pressure - also hunky dory.

Today, I decided to run any VAG-log I can possibly get and the second block I meassured I saw the obvious cause off my problems. The MAF wouldn't go over 124 g/s.

AHHHRG!!!! I never took the MAF in concideration because I just changed it a week before I got the problem. Now, everything is clear.

Btw: I'm Bold now !!! *lol*

See what Audi got to say to this ?

Dave out!

[S2tuner]

Dave, disconnect the MAF and see if your mixture is still lean... Gee it was so late when you came over that I didn't even think about disconnecting it, mainly because you said it was new....

Mihnea