Audiboy

The mysterious boost lost I am experiencing during autoX only is becoming more apparent at other AutoX sites, and on a one time occasion, during a merge onto the freeway.

It seems the symptoms to create the problem are extreme cornering and extreme heat in the engine compartment. Cooling down the TT for about an hour will make the problem disappear until the next AutoX event - :/

I was able to obtain an N75 open circuit code also once - not repeatable. So suspecting it was an N75 valve, I obtained a local replacement, but it was a "J" code valve (like a race valve) and not a "C" valve. This may have come at as a blessing because when I install the J valve, I got no boost - I mean limp mode boost - same as hooking up the wastegate to the compressor output. I measured the J valve to be about 25ohms impedance, compared to the 35ohms of the C valve - hence the blessing. This tells me two things - resistance on the wires somewhere, or the N75 output driver of my ECU going kaputs or poor soldering around it's pins.

So if anyone has some information on which wires go to the ECU, please let me know. I plan to run new wiring to isolate that from the ECU.<ul><li><a href="https://forums.audiworld.com/tt/msgs/1603996.phtml">my previous UFO attact on boost post </a></li></ul>

TTschwing

And honestly they rarely go bad. Sounds like you're getting pull-back because of heat. The N75 harness/plug goes bad sometimes but is replaceable. Put in a new Temp sensor. Don't tell me....GIAC?

Audiboy

I didn't do any research on these valves other than I ask them if they have a replacement valve and they stock these because they were used in their big turbo projects.

I do agree with your comment that these rarely go bad. That's also what the local tuner claims - and he humors my comment about the electric field and aliens invading our boost, yet he is stumped as I am.

I checked the connector and wiring and it looks fine. I also wrapped it with tape in case it was flexing and seperating around the area where the wires get slightly exposed.

I do have a green top coolant sensor in my to do parts bin that I have yet to install I think I've been saying this for a year now - I'm slacker I think I'll do that now to eliminate that possibility.

But I've logged and logged and logged, but I see no indications of pull back to heat. It's a very on-off situation when it happens. It's really so hard to replicate on normal driving, even as aggressive as I take it on freeways or some streets. I could try a winding road I guess.

I have an MTM ECU, but (oh oh) my other ECU is being retrofitted by GIAC as we speak. Any issues you are aware of?

TTschwing

Break in one of the N75 harness lines..They're pretty fragile. Does it drop to 5psi limp when it happens? Some J's don't work so well with some 225 versions..

I'm betting the new Temp sensor may cure your lls though..

Not a GIAC fan...I've seen too many bad logs..

Audiboy

to get me some boost for my autoX last run. It worked as I got at least 10 psi. But when the thing cooled down, I spiked all the way past 2 bar - oh crap! I never thought this K04 can push that much!) I had to baby the gas pedal all the way home.
So, Before yesterday's autoX, I installed a bleeder (boost controller) valve in place of my hose rip. Sure enough, it went limp after 1 run, but I had manual overide to get me some boost and decent runs.

I searched but I couldn't find anyone documenting a break in the N75 wiring or harness. But I'm hoping that's what it is. I have some diagrams from vagcat.com, but I haven't located yet what harnesses are involved

I don't think I'll use this J valve then.

And I'm putting the temp sensor tonight

TTschwing

A bad hose from the N75 to the wategate will give you ucontrolled overboost. Get that MBC off there, it's just confusing the issue. There is a possibility that you may have a sticking wastegate...

Audiboy

problem exist. It's bandaid while I try to figure this out. This is gone when I find the root cause. I unplug the N75 before I open the boost controller. This gives me about 10-11 psi ho hum power. The only thing I would stick in there is your BM in the future for the onset control.

I didn't think about the wastegate until now - thanks! I'll look into that too! Seeing I can get some boost with a manual controller means it's not completely stuck, but it could be stuck.

JustJeff

www.alldatadiy.com $24.95

Audiboy

mediocre when it comes to electrical diagnosis. Hopefully this more updated web version is worth it.

I found the wiring anyway using ohmeters so I'm good. I'm going to open up the ECU and trace the boards.