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Discussion Starter · #1 ·
Hey all, I've been struggling with a bunch of problems on-and-off for a couple of months and now I'm just confused and frustrated so I figured I'd ask for some help.

First of all, engine mod list:
Apexi Power FC w/ Hand Commander - tuned ~300bhp
Z32 MAF
Blitz air filter
Benchmark coil packs
FMIC
R35 stock BOV venting to atmosphere (I know, I know, will address this further in the post)
Aftermarket turbo of some kind
Greddy Profec B Spec II boost controller
Kakimoto straight pipe exhaust
Oil catch canister

I've had my GTST for almost a year, and everything is mechanically exactly the same as it was when I bought it. For about 6 months after I got it, it was running fine. I took it off the road earlier this year to do some basic maintenance and part replacements during the lockdowns. Up until that point, it was running great for the most part.

It ended up being sat off the road for a couple of months while being worked on but when I came to drive it again, it had a bunch of problems. Some of them seem to be my fault (not replacing pipes/clips correctly, etc.), others seem to have sprung out of nowhere. I think I've sorted most of the issues now, but there's a couple of bad ones that are getting in the way of being able to drive it again.

First of all, the AAC valve doesn't seem to be working correctly. When plugged in, it makes the idle stay higher than usual, bounce up and down intermittently and does not seem to respond to the steering wheel being turned or extra load being put on the engine. Sometimes unplugging it and plugging it in again makes the idle stop hunting, but still keeps the engine idling higher than it should be (I assume).

It have taken the AAC valve off, disassembled and cleaned it thoroughly following the guides here, but it doesn't seem to have made any difference at all. I've played around with the idle control screw, but it does not seem to change the idle unless the valve unit is unplugged. The idle seems to go straight back to the same place when it's plugged in regardless of what the screw is set to.

I'm not sure how to test what's causing this, whether it's something to do with the valve unit itself or wherever the electric signal is coming from.


Second problem, engine keeps almost stalling after lifting off the throttle instead of going back down to idle position and staying there. It's most noticeable when sitting in neutral and revving; the higher I rev before letting off, the worse the near-stall is when it comes down again. This happens every time I let my foot off the pedal.

I found that covering up the BOV outlet "trumpet" stops this behaviour and I can rev the engine up to any point and it does not drop past the set idle when lifting off. Uncovering the BOV outlet again causes the idle dropping and almost-stalling after revving again. This did not happen when I got the car nor when I was driving it regularly.

The BOV that was fitted appears to be a stock R35 BOV that has had the top crushed slightly, so I bought another R35 BOV to replace it, but that one seems to behave the same as the previous one. Although I haven't driven under boost with the new BOV, it still seems to be causing the rev drops at idle when the outlet isn't covered.

Now I know that the obvious straight-forward solution is to just plumb the BOV back to the intake and make it recirculate again, but I quite like the sounds it causes and I'm confused as to why it worked venting to atmosphere for 6+ months without an issue, but is now causing trouble after not being used for a couple of months? I'm not opposed to making it recirculate again, but I would rather get it back to how I bought it without this problem, if I can.


Would appreciate any insight with these, and I'm all open to suggestions or providing more information.
 

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1997 ECR33 Skyline GTSt Type M, 40th Anniversary model
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Hey all, I've been struggling with a bunch of problems on-and-off for a couple of months and now I'm just confused and frustrated so I figured I'd ask for some help.

First of all, engine mod list:
Apexi Power FC w/ Hand Commander - tuned ~300bhp
Z32 MAF
Blitz air filter
Benchmark coil packs
FMIC
R35 stock BOV venting to atmosphere (I know, I know, will address this further in the post)
Aftermarket turbo of some kind
Greddy Profec B Spec II boost controller
Kakimoto straight pipe exhaust
Oil catch canister

I've had my GTST for almost a year, and everything is mechanically exactly the same as it was when I bought it. For about 6 months after I got it, it was running fine. I took it off the road earlier this year to do some basic maintenance and part replacements during the lockdowns. Up until that point, it was running great for the most part.

It ended up being sat off the road for a couple of months while being worked on but when I came to drive it again, it had a bunch of problems. Some of them seem to be my fault (not replacing pipes/clips correctly, etc.), others seem to have sprung out of nowhere. I think I've sorted most of the issues now, but there's a couple of bad ones that are getting in the way of being able to drive it again.

First of all, the AAC valve doesn't seem to be working correctly. When plugged in, it makes the idle stay higher than usual, bounce up and down intermittently and does not seem to respond to the steering wheel being turned or extra load being put on the engine. Sometimes unplugging it and plugging it in again makes the idle stop hunting, but still keeps the engine idling higher than it should be (I assume).

It have taken the AAC valve off, disassembled and cleaned it thoroughly following the guides here, but it doesn't seem to have made any difference at all. I've played around with the idle control screw, but it does not seem to change the idle unless the valve unit is unplugged. The idle seems to go straight back to the same place when it's plugged in regardless of what the screw is set to.

I'm not sure how to test what's causing this, whether it's something to do with the valve unit itself or wherever the electric signal is coming from.


Second problem, engine keeps almost stalling after lifting off the throttle instead of going back down to idle position and staying there. It's most noticeable when sitting in neutral and revving; the higher I rev before letting off, the worse the near-stall is when it comes down again. This happens every time I let my foot off the pedal.

I found that covering up the BOV outlet "trumpet" stops this behaviour and I can rev the engine up to any point and it does not drop past the set idle when lifting off. Uncovering the BOV outlet again causes the idle dropping and almost-stalling after revving again. This did not happen when I got the car nor when I was driving it regularly.

The BOV that was fitted appears to be a stock R35 BOV that has had the top crushed slightly, so I bought another R35 BOV to replace it, but that one seems to behave the same as the previous one. Although I haven't driven under boost with the new BOV, it still seems to be causing the rev drops at idle when the outlet isn't covered.

Now I know that the obvious straight-forward solution is to just plumb the BOV back to the intake and make it recirculate again, but I quite like the sounds it causes and I'm confused as to why it worked venting to atmosphere for 6+ months without an issue, but is now causing trouble after not being used for a couple of months? I'm not opposed to making it recirculate again, but I would rather get it back to how I bought it without this problem, if I can.


Would appreciate any insight with these, and I'm all open to suggestions or providing more information.
Hi Matt,
This is similar to issues I am having so can relate. I have changed items and all I can think is we share the same ECU and perhaps the changes are not recognised as part of the original tune? For instance, if car was tuned with a dirty MAF, cleaning it could result in confused readings. I have done so much to try and trace the issue! have a read and happy to contribute. I got rid of the HKS bov before but did not have an effect on anything.
 

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Newbie
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68 Posts
Discussion Starter · #3 · (Edited)
Okay quick update after a test run:

I've left the AAC valve unplugged and removed the vacuum pipe from the top of the (non-crushed) BOV and plugged up the pipe leading from the plenum. The BOV is only functioning on mechanical spring pressure now.

I'm no longer getting the idle drop issue when lifting off throttle, boost doesn't seem to be leaking and the engine feels a hell of a lot more smooth, consistent and powerful on acceleration and mid-range pickup.

Obviously, this means the BOV isn't dumping much air any more and this is causing some compressor surge. Not sure exactly what to do about that or what it means working better like this.

Of course it shows that I should get a hole made on the intake to plumb the BOV into, but I'm back to being stumped as to why I didn't have this problem for 6+ months but now it doesn't want to work after being taken off the road for a while. I'm pretty sure the work I did in that period shouldn't have had any effect on the BOV.

The only thing I can think of is that I still have an air leak somewhere that would cause the BOV also venting air to mess up the rev stability, but just disabling the BOV and AAC has made it run better than it has since I took it off the road with no apparent air leak symptoms. Could a faulty/unplugged AAC valve mess up the air volume specifically when the BOV dumps air?

Anyone got any info or ideas?

This is similar to issues I am having so can relate. I have changed items and all I can think is we share the same ECU and perhaps the changes are not recognised as part of the original tune? For instance, if car was tuned with a dirty MAF, cleaning it could result in confused readings.
I did consider this, but as far as I'm aware, everything is mechanically the same as when it was running perfectly on the same tune it had 6 months ago. I tried stuff like cleaning the MAF, reseating air pipes and similar stuff after I started having these problems, but apart from replacing dodgy pipe connectors, nothing has made a difference except disabling the BOV and AAC.
 
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