Cannot hold boost

[rage385]

Hello all,

I have a 2000 A4 1.8TQM. I have an APR stage II tune. I recently changed the turbo on it (another K03). The original turbo had substantial shaft play and was leaking oil.

I post this because what I am seeing is that off the line, the car will boost up to 19-22 psi, then back down to 12 psi, then up to 16 psi, then back down to 14, then eventually even out to around 13 psi. I took a look at the vacuum lines routed to the N75, and they are correct. I also do not have any boost leaks.

I also have noticed that under high load, I will feel some surging.

Can you guys point me in the right direction as to what I should look at next?

Thank you,

John

[MetalMan]

Can you try unplugging the N75's electrical connector? You should see a steady 3-5psi at WOT.
This will test 2 things: N75, N75 to wastegate line, and boost reference line between compressor housing and N75. It's possible for the N75 lines to have a slight tear.

Can also try cleaning the MAF, maybe also do a TBA.

[rage385]

Thank you for the help. I will disconnect the N75 first, then do the TBA. The MAF is relatively new (has about 10,000 miles on it).

[rage385]

Would crossing the lines (N75 line to WG and compressor line) cause the issue?

[rage385]

Quick update. I did not have a chance to drive the car, but I was able to verify that the lines were NOT crossed. I should have time to drive the car with the N75 UNPLUGGED on monday

[czum09]

I think thats normal tapper. Might try to tighten the wastegate actuator. Surging could be spark related. How new are the plugs? What gap?

[rage385]

Plugs have about 8k on them and are gapped to .028.

[rage385]

Quick update,

Unplugged the N75, and the car would NOT boost beyond 5 psi.

Prior to unplugging the N75, I decided to scan the car.

P0140 - Oxygen Sensor B1 S2 (sensor 2). Which should not be an issue since I am an APR stage II right?
P1326 - Knock Control Cylinder 2 - would this play a factor in my surging?

I did clear the codes, so I will see if they come back.

One one more note, I decided to check my TB alginment. After running block 60 (under adjustment), I monitored the percentages at idle and at full. What I got was

Idle
G187 - 15.6%
G188 - 84.4%

Full
G187 - 87.5%
G188 - 12.5%

This is with the engine off.

Thanks

[czum09]

Yep, knock will cause surge. I'd start by making sure the o2 is turned off. Knock could be fuel related. I'd try a different source of fuel!

[rage385]

Thank you czum09. I am running an APR stage II tune, so I assume that the rear O2 is off. The knock sensor issue is an interesting one, because I replaced both knock sensors two years ago.

When you say fuel source, you are talking about the location where I buy my gas right?

[rage385]

So I spoke with APR regarding my rear o2 sensor. It turns out that the tune does NOT code out the rear o2, but rather just the part of the monitoring that will cause the catalysis issue. The fuel monitoring functionality still plays a part in the performance of the car. Maybe I should replace the rear o2 sensor and go from there. The error code has showed of before, but I always ignored it; thinking it doesn't matter. I guess it does.

[Spectre1130]

Pretty certain rear O2 does nothing more than monitor for your catalytic converter.

I would be alot more worried of the knock code. Can you check to see how many times your car threw the knocking code? Some OBD2 readers can check that.

That code can definitely affect your boost. When your car "see's" a knock its going to retard timing and maybe allow less boost through the N75 (not sure about the second one). When your car retards timing slightly you can easily get fluctuations in your boost.

Definitely check to see how many times your car is seeing a knock. If it is happening alot you need to not be driving it.

EDIT: Also with APR stage 2 I think you should be running 93 octane, 91 at least. I have an APR stage 2 on my 1.8t and bypassed my N75 running max psi and don't have an issue with knocking. If it is a single cylinder knock, you may have a clogged or bad injector.

[rage385]

Thanks Spectre,

I am using the Vagcom software, so I am uncertain as to whether it has the capability to count the amount of times a code is thrown. I can tell you, however, that this is the first time I have seen the knock code. I will try to swap injectors to see if the injector is an issue. I am running 91 octane.

Do you know which group i need to monitor on my Vagcom to see the knocking?

[Spectre1130]

I don't use Vagcom (I know I'm crazy). I use a harbor freight OBD2 reader to read my codes. I've never used Vagcom either.

But I do know if you can datalog you can look at your ignition timing. If you see the timing do a huge jump in the negative direction that is a good sign of knock.

EDIT: When you look for the jump in the negative direction, make sure you are maintaining constant throttle. You should see something like TPS being 100%, your timing being postive and then maybe a jump negative and slowly moving back to positive, all with TPS % staying the same.

[czum09]

Thank you czum09. I am running an APR stage II tune, so I assume that the rear O2 is off. The knock sensor issue is an interesting one, because I replaced both knock sensors two years ago.

When you say fuel source, you are talking about the location where I buy my gas right?

Yes, fuel source. What fuel are you running? Knock can be a pain to figure out.

[rage385]

Spectre,

Ill run some logs when I have a chance. Thanks for the insight.

Czum,

I run 91 octane (which is what the tune is for).