Erratic boost, Real cause found

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Erratic boost, Real cause found

Postby ChrisAudi80 » Wed Dec 24, 2014 5:32 am

Hi guys, I know this is the SEM subforum, but not sure where else to put it.

First up for my set up:
-Stock AEB, unchipped at 185k kms
-MAP-ECU3 piggyback. I know, some people look down on piggys. :hide:
-630cc Dekas with 3bar FPR
-E85
-2.5" DP and exhaust, no cat

I have some weird boost spiking happening when pressing hard in 3rd to 5th gear. I am currently running intercept mode on MAF signal and timing. Not controlling boost at this time with the piggyback.

If you will look at the screen shot of one of my logs you will see:

WB AFR in white
MAP in blue
MAF voltage signal in teal
MAF output to ECU in yellow
TPS in light brown
RPM in red

As you can see, MAP is very erratic. Up and down like a saw tooth. Correspondingly, the MAF signal goes crazy. I see a peak boost of 13psi in this log. Since I am stock boost level, it should not go over 8. Luckily, I have plenty of fuel.

After searching around, it could be that the N75 is on its way out. However, it ohms out at 30Ohm, which is in the acceptable range of 25-35.
I am assuming this is the original N75 C valve. Some say that it could be because of overheating. I do have a N75H valve here. I know that it is not recommended to use this in a stock unchipped set up, but I am controlling fuel with the piggy. Would it be an idea to swap the H valve and see what happens?

Any ideas what could be the issue?
Attachments
Log shot MAP ECU.jpg
Log shot MAP ECU.jpg (188.24 KiB) Viewed 3002 times
Last edited by ChrisAudi80 on Sat May 09, 2015 5:08 am, edited 3 times in total.
96 S6 auto
95 80Q AEB VEMS
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Re: N75 valve borked? Advice requested

Postby ChrisAudi80 » Wed Dec 24, 2014 5:37 am

D'oh, Christmas eve... guess it will take while to get a response.
96 S6 auto
95 80Q AEB VEMS
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Re: N75 valve borked? Advice requested

Postby bradyzq » Mon Jan 05, 2015 10:31 am

This might be interesting, since the AEB does not regulate the N75 duty cycle according to boost, because it has no MAP sensor. It uses the MAF. So, if you scaled the MAF down for your injectors and E85 with your piggyback, the ECU will ask for more duty cycle from the N75 in order to achieve the target airflow.

It doesn't explain the oscillations, but does explain why the boost is higher.
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Re: N75 valve borked? Advice requested

Postby ChrisAudi80 » Mon Jan 05, 2015 9:35 pm

bradyzq wrote:This might be interesting, since the AEB does not regulate the N75 duty cycle according to boost, because it has no MAP sensor. It uses the MAF. So, if you scaled the MAF down for your injectors and E85 with your piggyback, the ECU will ask for more duty cycle from the N75 in order to achieve the target airflow.

It doesn't explain the oscillations, but does explain why the boost is higher.


Hmm, interesting.

Its one of those chicken and egg situations. Is the MAF signal oscillating because of the increased "suction" from the turbo, or is it because of what you said?

It behaves well if gently going to WOT.
Someone else suggested the wastegate diaphragm or the pivot sleeve could be worn out.
96 S6 auto
95 80Q AEB VEMS
ChrisAudi80
 
Posts: 736
Joined: Mar 2, 2013
Location: Bangkok, Thailand

Re: N75 valve borked? Advice requested

Postby ChrisAudi80 » Sat Jan 17, 2015 7:14 am

I've tried to make the MAF signal reduction less to see if this has any effect. So far, inconclusive.
It did richen up the AFR in boost.

Could be that it is hardware related. Bad, WGFV and/or diaphragm. Will make some more logs to see what is happening.
No fault codes from stock ECU, except limit adaption errors, but that is likely caused by the ECU not being able to trim the overly rich fuel levels sometimes.

Could a bad MAF element cause this?

On a side note, did a long distance drive last week. Got 9km per l. :D And that was with a loaded car and climbing steep hills.
Not bad for E85.
96 S6 auto
95 80Q AEB VEMS
ChrisAudi80
 
Posts: 736
Joined: Mar 2, 2013
Location: Bangkok, Thailand

Re: Erratic boost, Cause found

Postby ChrisAudi80 » Thu Jan 22, 2015 4:58 am

Alright, after looking around some more and working it through, it seems that I have found the cause for the overboost peaks.

Most likely culprit is PCV system leak. In my case, from the plastic S hose connection to the block. There is oil spray everywhere.
This makes sense as I had something similar a few years ago on the S6. The PRV was not connected to the hose, thereby overboosting and fuelcut.

As always, when having a problem, it usually is something simple. Back to basics.
96 S6 auto
95 80Q AEB VEMS
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Posts: 736
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Re: Erratic boost, Cause found

Postby ChrisAudi80 » Sat May 09, 2015 5:07 am

Actually, the issue persisted until I unplugged the narrow band O2. The stock ECU was fighting the enrichment I had programmed.

I'm a dumbass.
96 S6 auto
95 80Q AEB VEMS
ChrisAudi80
 
Posts: 736
Joined: Mar 2, 2013
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