Excess Back Pressure From Turbos

[4drlvr]

Hi Guys,

For some time now ive been having this problem where when i rev the car the rpm drops to about 500 hunts and comes back up to the normal idling rpm. It happens even when im driving and have to come to a sudden stop, espically if ive been driving on the freeway it sometimes even turns off and then back on by itself as i come to a stop, very rarely does it turn off and not come back on.

Now all this happens only when its at operating temp and doesnt do it when the car is cold???

The mechanic suggests getting the pfc djetro or the haltech which doesnt read of the afm's is the best way to go but the way i see it is that by putting the pfc djetro on we just bypassing the problem of backpressure.

To give u the background of my car its got an rb26dett fully forged with standard turbos and hks cams that havent been dialled in and stock ecu. The car has done about 15000ks after we put the rb26 in it. The mechanic seems to say that the stock plump back bov system isnt here on this car since we would have to customise the shit out of the piping since its a gtst, so he says its an atmospheric bov and the afms are getting the wrong reading from the back pressure on the turbos. Initially when the engine got built i dont remember it having this issue. I blew bottom end bearin when the oil pump packed itself and then did a full rebuild on the bottom end again.

Ever since the 2nd rebuild i seem to this idle issue. The only thing i can remember that is different between the 2 rebuild is the pod filter, i had some cone shape pod filters which i changed to the hks ones.

So far we've changed bov's, ive tried closing the existing bov, checked for vacuum leaks, afm.

Also when i rev the car from the engine bay and listen to the pod when i let go of the trottle i get a chhhhhhh........ noise from the pods, is that what the back pressure is???

I apologise if i havnt been clear but i just need an opinion from you exprets out there on what to do to fix this the proper way.

Anythin i can check or need to do to sort this issue out????

HELP!!!!!!!!!!!!!

Edited December 27, 2008 by 4drlvr

[Green_s13]

do you have a standard bov that is venting to the atmosphere? if so i would say that is your problem

[4drlvr]

Ive got an aftermarket bov which is connected to the cooler piping. I do remember the mechanic sayin that it doesnt have the standard bov. Does that help Green_s13

[116.hks]

Hey mate. Me wifes 33 does the same thing. If you come of throttle to quick under boost, it wants to stall or drop in idle. It's only since i've put a atmosphere type BOV on. The valve stays open a bit longer and upsets the afm. When you are revving the motor and realising it and you hear that "chhhhhh", it's just the pressure realising out the front housing and not out the valve, like it should. Make sure that the vaccum hose to your valve isn't crimped somewhere, hasn't got a hole in it and check to make sure that valve isn't leaking. The standard valves for a gtr a hiden in the inner guard, not sure its pass or drivers, so if you have custom intercooler piping then you wont have a standard valve. Hope this helps mate.

[JMW]

Sounds like air reversion from the compressor housing. As you back off the pressure pulses back and forth from the compressor housing past the AFM and confuses the hell out of it mainly when running higher boost.

Have heard a good solution to this is to move the AFM to the pressure side of the turbo (boost does not bother AFM's as they only measure air flow).

I know Ken Nomura does this on nearly all his URAS drift cars and also remember Adam Trewella doing it on his Ceffy a few years back.

Hope this might help a little to solve whats called "FC Disease"

[hugwase]

Get stock gtr bovs/plumb back hose on it, surely the stock gtr piping would come close to fitting your cooler

[BeEnAr]

I had this problem on the GTR and it turned out to be the AFMs. They needed the points re-soldered. Although yours does soung like it's caused by the BOV sticking open. Wind the screw in a bit to help it close off throttle and put a tiny bit of oil in the vent to give it some lube. I had to do the oil bit on my GFB stealth on the rally car all the time

[PHA71L]

what dose it do if you free rev it to about 3000rpm? dose it cut then come back then cut then come back?

[bigslick]

If it happens while the car is stationary and you just give it a quick rev, or just driving normally and then put the clutch in when you come to a stop, and only happens when the car is warm, then it sounds like the oxygen sensor.

It would be a good idea to have a plumb-back BOV too. Less confusing for the AFM's.

[4drlvr]

116.hks - i do have custom cooler piping running to a GTR Cooler, ive looked at the bov which is on the drivers side of the inner guard and ive got a little black hose which is connected to the bov and the other that is blocked off, i remember the mechanic sayin that it was incase i was running 2 bovs. Do i follow those 2 hoses back to where they came from and check for blocks or leaks????? or is it some other hose that i need to look for and where should i be looking.

Jamesrb25 - how do i move the afm's to the pressure side of the turbo, which side is that and whats involved in doing it??

Dirt Diggler - il try the lube thing with the bov tomorrow and let you know how it goes.

PHA71L - yup thats exactly wat it does, any ideas??

Bigslick - ive put the car on the scanner and it came up with no errors so how else would i check the oxy sensor

Im really sorry guys i havnt really played around in the engine bay alot and im sorry if i sound like an idiot but im happy to try out what u tell me just a lil more detail pleeaaaaseeeeee, dnt mean to b a pain just wana get it sorted.

But thanks heaps for ur replies, n keep em comin

[JMW]

It involves a little fabrication work on the pipes, but basically you can move them where ever aslong as the direction of air flow stays the same. With the twin AFM's of a GTR it makes it a little harder to position them, but even just moving them away from the compressor housing a litlle reduces the strength of the pulsing air.

On RB25's i've seen them mounted just after the intercooler in the RH side under the front guard, just running extended wiring.

Or even on an off chance it could be your AAC valve playing up or even a coil pack on its way out too!

[R31Nismoid]

Stock ECU with bigger cams + aftermarket BOV

Thats your problem.

Put the stock BOV's back on, get a aftermartket ECU, and get it tuned.

Although, why would you build a motor and then put stock turbos back on is a bit of a mystery, because if a turbo fails its likely to destroy your forged build.

As for the crap about not being able to put GTR BOV's on, thats a load of shit. He is just being lazy. There is always a way.

Even using a single one, there is no problem there and i used a single for over 350rwkw/24psi.

I think the best thing you can do is take the car elsewhere.

[PHA71L]

my bet mate is the tps switch!!!!!

i did the same thing on mine after the rebuild either bumped it or it moved a little bit.....

there is two little bolts on it you just need to adjust it back.... there is a real way to do it with a mulity meter how ever you speel it lol

but i had a close look on mine and you could see were the washer had made a mark on it so i just put it back to there and BANG PERFECT!!!!!

ps this was after about 4 hours of trying to set up the throttle bodys and throttle links because thats what i thought was causing the problem haha

all the best let me know how you go!

[PHA71L]

also mine would only do it once it was warmed up!

you could drive it for a while befor it would start doing it again....

so its a good bet you have the same problem mate

[4drlvr]

Jamesrb25 - Thanks il look into moving the afms as you mentioned and will clean the aac valve.

R31Nismoid - i dnt have and never had the stock bov in the car, since the engine was put in, ive always had the same bov. With regards to why ive still got the standard turbos and ecu, i wanted to runin the engine before i started modifying the other stuff, although i will keep the thought of the stock turbos stuffin up the engine incase i blow blow

PHA71L - Il def check the tps tomorrow morning and see if i can make out any marks or anything

Thanks all once again and will keep you's posted on how i go, in the meantime if there is something else to check please lemme know

Thanks All

[4drlvr]

Ok Guys I cleaned the BOV that was on the car and put a little oil in for some lube for the spring, the car now does the hunting and drop in rpm only once now and no where as bad as the drop in rpm as before, but its still hunting

As for the TPS, PHA71L i cant seem to see any marks of it being moved at all but will check it out with a multimeter n see.

I also noticed something else on both turbos of the car, there is a a piece of rubber that has been cable tied to it, im pressumin to block it off, anyone know what im talkin bout or do i need to get a pic???

Another hose on the drivers side towards the back of the engine, the one next to the hose that goes to the brake booster is also blocked. The hose that im talkin about is closer to the firewall than the hose that goes to the brake booster.

Anyone know what that is?? or if any of these blockings could cause this issue??

Im still gona keep checkin the other stuff that u guys told me but just thought id let u know what ive done n achieved so far.

Keep the idea coming pleaseeee

[PHA71L]

maybe air in the water system need bleeding? i know in charades it can cause hunting no idea in a gtr tho....

[doo doo]

Ok Guys I cleaned the BOV that was on the car and put a little oil in for some lube for the spring, the car now does the hunting and drop in rpm only once now and no where as bad as the drop in rpm as before, but its still hunting

As for the TPS, PHA71L i cant seem to see any marks of it being moved at all but will check it out with a multimeter n see.

I also noticed something else on both turbos of the car, there is a a piece of rubber that has been cable tied to it, im pressumin to block it off, anyone know what im talkin bout or do i need to get a pic???

Another hose on the drivers side towards the back of the engine, the one next to the hose that goes to the brake booster is also blocked. The hose that im talkin about is closer to the firewall than the hose that goes to the brake booster.

Anyone know what that is?? or if any of these blockings could cause this issue??

Im still gona keep checkin the other stuff that u guys told me but just thought id let u know what ive done n achieved so far.

Keep the idea coming pleaseeee

Think you're talking about the clutch booster line? Is there a black plastic 90 degree piece (one way valve) coming out of your clutch master cylinder that's not hooked up to a line from under the plenum? The lines for the brake booster and clutch booster both come off the same attachment from the vacuum box under the inlet manifold.

[jeffworld2]

go around and re-block the million vacum hoses you have blocked. most likely one is open somewhere which isnt helping you diagnose this problem. tighten the bov if you can.

the real answer is your ecu is not tuned for your bigger cams, and never will be until you buy a new one.

no amount of moving afm's and adjusting bov's is going to fix that, the bigger cams want to idle a bit different.