No boost controll after engine change?

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ian52

2000 a4 1.8tqs-yellow
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Swapped the engine in my s3 yesterday and now there isn't any boost controll. I'm sure it's all plumbed in right. I have just disconnected the n75 just now so it's running 5psi off the actuator but when it's plugged in its like it just sticks wide open letting the boost go as high as it wants. It was fine before I changed the engine. I'm thinking the map sensor or n75 could be playing up but no codes were coming up. Any ideas?
 
Is the n75 plumed the right way? And that is probably nackerd, also DV could be faulty or maybe pcv system? What happend to the old engine?
 
N75 is the wrong way round, i had this, car was boosting over 20 psi on my standard map and frowing the car into limp mode.
 
If an N75 sticks open then it will give no boost... they are open by default and pulsed closed to bring boost up... this is why they run on actuator pressure when electrically disconnected...

Sounds like its plumbed wrong.. boost in the bottom, longer outlet into the TIP, shorted (opposite to longer one) to the actuator

<tuffty/>
 
Only bit i disconnected on the n75 was where it goes into the TIP,i left the hoses connected and the hose that goes to the actuator and the pipe at the top of the engine cant get mixed up so its plummed in the same way it was before.
 
I thought the valve would open up to bleed air away from the actuator line,like you would do with a bleed valve etc and by disconnecting it its shut and not bleeding any air off the line to the actuator?
 
MAP sensor won't be giving you this issue without throwing a code and would force the ECU into limp mode as its an important sensor...

Log duty cycle and boost request vs actual...

What boost are you actually getting? do you have a boost gauge fitted?

<tuffty/>
 
Not sure if i can log stuff with vcds lite? I have a boost guage and was sitting at 20psi before but i was getting to 25 and let off. It might have went higher if i kept going. The only bit i disconnected for the n75 was where it goes into the TIP so it is 100% connected the way it was. I was thinking i might have plugged something else into it by mistake but the only plug the same nearby wouldnt reach anyway and the n75 plug branches off from the 4 pin plug on the water pipe and sits as you would expect it to.
 
the higher compression pistons in the engine 'may' be playing a part in this... the hardware has changed don't forget... subtly of course but there has been a changed... I suspect the 25 odd psi you are seeing could just be a the initial spike... the map is set to control boost based on the previous hardware config...

You may need to run an MBC in parallel for a while to cap this....

<tuffty/>
 
Is the ecu on standard map or has it been mapped?
 
Car is mapped. I could try it and see anyway. I dont want to just keep my foot burried and see it go past 30psi and see my rods bouncing down the road in my rear view mirror. It is a fair bit colder today than the last time i drove it too so that would be a couple extra psi going by previous cars too. Other than that its spot on and i reckon the head on my previous engine was on its way out for a while. The radiator hoses arent as pressurised now and it doesnt misfire for a few seconds when you start it in the morning,that would have been where my water was going then.
 
If its doing 25psi once it will do it again... 19mm rods remember...

<tuffty/>
 
Exactly,thats why i thought **** that and unplugged the n75. I looked up on the ARY engine and one place said it was the same compression as the APY and another place said it was different. If it is different i wouldnt have thought it would make that much differance to the boost level im seeing?
 
Never experienced this change in setup before so can't say for sure but higher compression would help spool potentially and I think the high boost you are seeing is a spike rather than something that would stick...

Its entirely plausible the compression increase is responsible for the spike... I also suspect though that once through it the boost will drop to what you normally see...

Either way the cheapest option would be to fit an MBC in parallel until you get you r other engine back in... be cheaper than map tweaks for this setup and back to the other...

Should be able to drop the load scale in unisettings too on ME7... that would also curb the boost a tad but is an overall setting rather than specific parts of the rev range...

What map is on it?

<tuffty/>
 
No idea what map is on it but i suspect revo because it comes in pretty brutal. Once up the revs a bit it used to drop off to maybe about 15 psi i think but it was 20 psi midrange. Im sure it was getting past 20 psi before 3k rpm so its not laggy anyway. Think ill just buy a mbc off ebay and get it running 20ps again and see how it goes. I know this engine is in theory weaker than the APY but i think if im sensible it will be fine. Famous last words.
 
ian52 said:
No idea what map is on it but i suspect revo because it comes in pretty brutal. Once up the revs a bit it used to drop off to maybe about 15 psi i think but it was 20 psi midrange. Im sure it was getting past 20 psi before 3k rpm so its not laggy anyway. Think ill just buy a mbc off ebay and get it running 20ps again and see how it goes. I know this engine is in theory weaker than the APY but i think if im sensible it will be fine. Famous last words.
Click to expand...

If its Revo then you can use an SPS box to turn down the boost or a Revo dealer should be able to do it through the revo flash tools

<tuffty/>
 
Its just a guess that its revo. I have receipts for pretty much everything else on the car but nothing for a remap. Think ill get a mbc and see how it goes. Im sure i can hear the d/v more now too for some reason.
 
ian52 said:
Its just a guess that its revo. I have receipts for pretty much everything else on the car but nothing for a remap. Think ill get a mbc and see how it goes. Im sure i can hear the d/v more now too for some reason.
Click to expand...

Will do with more boost ;P

I would recommend the Forge UNOS as this is has proven to be the most reliable MBC I have ever used... the cheaper bleed valves don't cut it tbh....

mbc_parallel.jpg


Use 6mm silicon vac pipe and cable ties

20100518_CRW_4082.jpg


<tuffty/>
 
Last edited:
Im more confused now. I was looking about on the net and found some stuff about adjusting the screw on the n75 to adjust the boost slightly so thought i would give that a try so i turned the screw in half a turn to lower the boost slightly,plugged it in and went for a drive. 10 psi so i thought great,its turned it down so its working. Turned it back 1/4 turn hoping to get somewhere nearer 20 psi but it wont budge from 10 psi now. No fault codes or anything and its holding 10psi fine but it should be 20. Anybody any idea whats going on? Im going to try the n75 out my a4 in it i think and see how it goes if its the same or not.
 
Screw??? there is no screw on an N75???

If you have inserted a screwdriver into one of the ports twisting it about I suspect you have just simply damaged the valve... there is no screw...

Do you have a link to the 'article' where you got this info from?

Should have waited for the MBC mate... looks like you need a new N75 now as well :(

<tuffty/>
 
All sorted now. It was just sticking from when i adjusted it in the way too far. When you look down on the top of the n75 there is a hole in the metal bit on the top. In that hole there is a screw maybe 5mm below flush,screw it in and it reduces boost and increases it if you screw it out. They must use it to set them from the factory i think. Dont have a link but i just searched n75 valve adjustment on google.
 
I can just see all the limp mode and bent rod threads starting now that people have read this... lol

<tuffty/>
 
Yeah i know what you mean. At least in my case im using it to tame the boost and the car is mapped at least. On loads of jap cars you can get away with just turning the boost up a bit but not these unfortunatly.
 
Talking of limp mode im surprised mine never went into it when it hit 25 psi.
 
ian52 said:
Im more confused now. I was looking about on the net and found some stuff about adjusting the screw on the n75 to adjust the boost slightly so thought i would give that a try so i turned the screw in half a turn to lower the boost slightly,plugged it in and went for a drive. 10 psi so i thought great,its turned it down so its working. Turned it back 1/4 turn hoping to get somewhere nearer 20 psi but it wont budge from 10 psi now. No fault codes or anything and its holding 10psi fine but it should be 20. Anybody any idea whats going on? Im going to try the n75 out my a4 in it i think and see how it goes if its the same or not.
Click to expand...

dont be tempted to touch the n75... most likely outcome is a dead n75 if you do
 
That's it back to how it used to be hitting 20 psi. Pulls like a train now.
 
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