Failed MAP sensor?

[spikemoto]

So I have an MTM stage 1+ chip with a 3 bar MAP on my AAN powered 1990 coupe quattro. Lately it has been cutting out hard at certain boost levels in each gear (15-17psi in lower gears and 12-15psi in higher gears). It seems to only cut out for a brief second and the tach drops to 0 and springs back up instantly. I decided to log the incident using VAGCOM to see what I could find.

Where the RPM (in red) and TPS (in blue) drops significantly is where it cuts out.
The most significant piece of information from the log is the manifold pressure or "mbar" on the graph which is flatlined.. obvious MAP failure? I've checked all of the lines and they all look good.

[UrS4boy]

Could be MAP sensor but is more likely to be boost leak or bad MAF sensor connection.

Here is some MAP info:

http://forums.quattroworld.com/s4s6/msgs/20417.phtml

Here is some MAF info:

http://forums.quattroworld.com/s4s6/msgs/20442.phtml

[prj]

So I have an MTM stage 1+ chip with a 3 bar MAP on my AAN powered 1990 coupe quattro. Lately it has been cutting out hard at certain boost levels in each gear (15-17psi in lower gears and 12-15psi in higher gears). It seems to only cut out for a brief second and the tach drops to 0 and springs back up instantly. I decided to log the incident using VAGCOM to see what I could find.

Where the RPM (in red) and TPS (in blue) drops significantly is where it cuts out.
The most significant piece of information from the log is the manifold pressure or "mbar" on the graph which is flatlined.. obvious MAP failure? I've checked all of the lines and they all look good.

1. Where it cuts out your ECU probably resets, check all the grounds to ECU, pull codes and make sure there are no trigger errors with a running engine (Hall sensor, crankshaft position sensor, engine speed sensor).
2. The pressure in mbar is absolute pressure from F96 sender, not the signal from MAP sensor.

Even if the MAP did fail, it could not possibly cause your issue, as it is only used for boost control, and it'd just revert to mechanical boost with a bad sensor.
To me it sounds like a bad trigger signal.

[spikemoto]

1. Where it cuts out your ECU probably resets, check all the grounds to ECU, pull codes and make sure there are no trigger errors with a running engine (Hall sensor, crankshaft position sensor, engine speed sensor).
2. The pressure in mbar is absolute pressure from F96 sender, not the signal from MAP sensor.

Even if the MAP did fail, it could not possibly cause your issue, as it is only used for boost control, and it'd just revert to mechanical boost with a bad sensor.
To me it sounds like a bad trigger signal.

Good to know, looks like I need to track down some bad grounds or ecu connections. The only code I have been getting is "idle speed adaption limit exceeded" which I would think indicates a vacuum leak. Also I broke the AAN Hall sensor so I am running a 7A dizzy for Hall signal. I would not be surprised if that is causing my issues but the real puzzler is that it only happens at higher boost or idle...

[UrS4boy]

... Also I broke the AAN Hall sensor so I am running a 7A dizzy for Hall signal. ...

I assume, therefore, that you are running the 7A intake cam in order to drive the 7A dizzy/Hall sender. This is very similar to the very early AANs (to mid-Feb 92) that used a "tuna can" Hall sender. Provided you lined everything up properly, that should work (how did you modify the harness? open it up and pull back the G40 connector?)

http://forums.quattroworld.com/s4s6/msgs/22580.phtml

[spikemoto]

Yes, I am running 7A cams. All I did was make an extension harness to go from the front to the rear by the dizzy everything has been working just fine for about 3k miles unless the plastic gear on the dizzy spun slightly... but even then it only cuts out at certain boost levels. I can rev the car to redline as long as I don't exceed ~15psi

[UrS4boy]

...the dizzy spun slightly... but even then it only cuts out at certain boost levels. I can rev the car to redline as long as I don't exceed ~15psi

Then we have to ask (again?), have you checked for boost leaks. Not sure how you are plumbed downstream of the turbo but with a normal AAN, the most likely culprits are the turbo to cross-over pipe hose and the Michelin man hose (cross-over to intercooler). If you used an UrS6 plastic cross-over pipe or the UrS6 intercooler with plastic head tanks, they are also some of the usual suspects for a boost leak.

My thoughts on boost testing (others disagree and includ the PCV system) : http://forums.quattroworld.com/s4s6/msgs/29530.phtml

Some of the usual suspects, AAN style:

[spikemoto]

I am running a front mount, custom piping and silicone hoses. I have tested the intake tract from turbo to throttlebody hose and found no leaks. I am wondering about the PCV system though, any common leaks there?

[UrS4boy]

Positive Crankcase Ventilation(PCV) System with revised diagrams, PNs and Links

I am running a front mount, custom piping and silicone hoses. I have tested the intake tract from turbo to throttlebody hose and found no leaks. I am wondering about the PCV system though, any common leaks there?

There are a couple of places where the PCV system could cause a problem. The first would be the bleeder valve (PCV valve) - if fails open under boost, then boost could leak out any of the PCV hoses or the pressure regulating valve, as explained here:

http://forums.quattroworld.com/s4s6/msgs/22506.phtml

[prj]

I am running a front mount, custom piping and silicone hoses. I have tested the intake tract from turbo to throttlebody hose and found no leaks. I am wondering about the PCV system though, any common leaks there?

How did you test?

[spikemoto]

I plugged the pipe right before the throttle body hose and applied pressure via a regulator to the turbo side of the intercooler. Maybe I should come up with a way to test more of the components...

[UrS4boy]

I plugged the pipe right before the throttle body hose and applied pressure via a regulator to the turbo side of the intercooler. Maybe I should come up with a way to test more of the components...

Ummm..., yup. You are probably missing a few things, e.g. turbo to intercooler hose/pipe, etc. Use this AAN diagram as a guide and test at least from the turbo outlet to the throttle body:

[UrS4boy]

Found this diagram. Proably better. Suggest testing from turbo coldside outlet i.e. through hose 12 (which might be the issue) and onward through the intercooler, the 895805853 cross member (item 24), items 15, 25, and 26 to and through the item 16 pipe to just before the throttle body. Watch out for the ISV and BPV connections, you might want to block them off at least for the first test.

[spikemoto]

What is overboost on a stock AAN? I put a stock ECU in and it's still cutting around 15psi

[prj]

I plugged the pipe right before the throttle body hose and applied pressure via a regulator to the turbo side of the intercooler. Maybe I should come up with a way to test more of the components...

Yea, here is your problem.
What you should do is exactly as follows, nothing more and nothing less. This is assuming everything is plumbed in:
1. Remove the MAF.
2. Plug the MAF hole in the MAF-turbo hose with a 80mm dummy.
3. Disconnect the MAF-turbo hose from the PCV (front of rocker cover).
4. Plug the resulting hole in MAF-turbo hose with a 20mm dummy.
5. Remove the vacuum line from the Bypass valve.
6. Feed air into the system through the vacuum line (insert a tyre nipple there).
7. Spray everything with soapy water. Use a fluid that bubbles well.

Alternatively, if you can make yourself a 80mm or 20mm dummy with a hole in the middle, you can insert a tyre nipple there, then you don't have to touch the bypass valve vacuum feed.

Do not block off or disconnect anything else. The system should hold 0.5 bar like this for a long time. If it doesn't, you are leaking.