I know that the N75 valve has had a great deal of attention over the years, and I have search relentlessly for a solution to my ongoing saga. Background: Fitted a standard AGU turbo to my mk2 golf with standard ECU. The car runs and sounds lovey...until you put your foot down and it massively over boosts to 20psi or so....and the ecu cuts the fuel. I changed the N75 valve because it was making a very loud ticking noise after a few seconds of idle, but that made no difference at all as the new one did the same. If I disconnect the N75 valve, the car runs perfectly but is obviously very much down on power. I took the car to have a power run, and as usual a massive over boost was witnessed and massively underpowered once the valve was disconnected. I replaced the N75 with a simple 'T' piece and made the hole to the size where I observed approx 10psi, but this meant that I either had turbo or not, (not smooth at all).. Question: Why is the N75 making a very loud ticking, i obviously have power going to it but Im not convinced that the correct signal is being fed!!! , therefore how do i fix this as whatever its doing is causing the car to over boost. Any help would be massively appreciated
The N75 is clicking as it should to regulate the pressure to the wastegate actuater with you getting mega boost, I'd say to start looking at the turbo itself the system is designed if it sees the boost un-controlable via the ECU it will and does kill the fueling so the exhaust gasses are limited to an sensible level! either something wrong with the pipe work to the turbo control, or the actual waste gate siezed shut? Thats my first guess off the info we have
Well the good news is that you've narrowed it down to "boost control" I take it you've done a full conversion? it just says "fitted a standard AGu turbo to my mk2 golf with standard ecu" STD AGU ecu would not allow 20psi for starters so 1st place to start looking would be the wastegate and its control,see if it moves freely by hand & if the hose is intact with no splits etc.. Are you sure its a standard ECU? is your fuel pump ok? (could be running lean & the ECU is shutting off boost then) MAP sensor working ok? Just a few thoughts for you.
Thanks for the responses, The N75 is clicking so loud that you can hear it over the sound of a revving engine!, so something certainly isn't right there. I have checked that the wastegate moves freely and it does, there are no leaks in any of the hoses either. Should you be able to suck the wastegate open (unsure what resistance it should have). The Engine and ECU are standard 150bhp and the car did produce 20psi before a large black cloud from the exhaust. I have changed the fuel pump too, so happy with that.., also I have checked the DTCs on vagcom and only have 00515 (Cam shaft hall sensor). What I am struggling to understand is why would it overboost and not underboost....think i'm getting confused!. What feeds the ECU for it to tell the N75 what to do? Any further help is greatly appreciated
sounds a bit like the wastegate actuator might be faulty,N75 wnats to control it but can't (hence clickng) boosts till ECU says NO! Just a thought. Std boost is about 7psi. MAP sensor tells the ECU PSI (manifold absolute pressure) so its worth checking if its working ok or swapping with another the same.
Well, if you say it moves freely, and if you just tested it with your hands, it's faulty I would say. You should see it move, if you presure it (bike pump would do the trick, connected to the hose).
Hi, I have attached a bike pump to the hose feeding the actuator and it does move as it should and then holds its pressure. I still don't understand why the valve is making such a noise (with or without the hoses attached), it always occurs about 1min after the engine has started. It definately shouldn't make such a noise. Is it possible to test the signal feeding the N75? Cheers
You were answered by the above already. The M3.8.3 engine controller as fitted to the AGU engine does have a Lader Druck Reglung (turbo pressure regulation) feature and can attempt to reduce manifold airflow due to boost via the "N75" duty valve. This solenoid can be heard is working overtime by the loud clacking sound as it is trying to regain control possibly well outside of calibrated thresholds. I would follow the suggestions above and look for a mechnical issue. I can further suggest if you suspect an issue with a W.G. hose leak or seized waste gate, you can connect a hose to the waste gate actuator and connect direct to the manifold, bypassing the N75. Run the vehicle carefully to full load and see if the turbo regulates to a default of ~ 4.5psi or 0.25bar. If you still overboost you have a turbo issue.
Really sorry to keep going on...but i’m about to burn the damd thing!!! Right, I have spent hrs today looking at all of the above. If i disconnect all 3 hoses feeding the n75 and couple the one from the turbo to the actuator with a valved t-piece i can get in the region of 8-10psi thus taking the n75 out of the equations. The waste gate isn’t seized and opens approx 8 psi (as it should) there are no leaks. If I connect the n75 (electrically ONLY...no hoses connected) it is silent for a few secs then starts to ‘clack’...a few seconds later, clacks like mad...as mentioned earlier. So i am confident that mechanically everything ios sound.....however, if I disconnect the MAF...the N75 appears to operate as it should, but obviously the car runs rubbish!, The N75 can be heard ticking once and that’s it, each time i rev...one tick!!!...so is it the MAF that’s causing the ‘ridiculously loud’ clacking
You may have a boost leak! You need to make a tester to find the leaks. I found my boost would over shoot normal levels due to turbo having to spin faster to achieve required boost. And thats with a manual boost controller.
I connected a compressor up and held the pressure at 8-10psi, I'm assuming you are talking about the 3 hoses to the n75 Cheers
