Intake Plenum Question - Dropping Boost - Also Good Tools To Have When Owning Skyline For $29 !

[benl1981]

Hi there,

I'm having a problem.

Just had a tune on the rb25det. Made 247rwkw at 18psi boost. Running great, power to the redline. After 1 week I have noticed boost is dropping off massively after 6000rpm.

Still hits close to the same peak but soon after the boost drops off to less than 5psi. I havent changed anything.

- Exhaust manifold looks ok (no broken studs or soot) so too does the manifold to turbo.

- Checked intercooler pipework. I have the metal piece in intake pipe so it shouldnt be sucking closed.

I got the tuner to have a quick look and he thought it may be the intake plenum gasket (between the 2 halves). It doesn't appear to be leaking at idle but might be under boost. There was a little oil around the join.

Anybody got any ideas? Have you had a similar problem?

Also, you need a ratchet spanner to take the top half of the plenum off (for the 5 bottom bolts). Crack it first with a normal spanner (otherwsie you break the ratchet). Also, wear gloves or wrap your hand in a towel when doing this. I sliced my hand open trying to crack one of the underneath bolts. That is by far the hardest part of the job. You have to disconnect the crossover pipe etc and take the throttle body off and hoses. Also, leave all hoses attached to the AAV at the back, just undo the 3 bolts holding it on.

The gasket is about $85 from nissan. Will get the part no. and add to thead. Anybody know if there are good aftermarket ones?

Hopefully the gasket will fix this problem. Can't think of any other potential problems. I thought exhaust restriction butthat wouldn't have been so sudden I think. The exhaust is 3-3.5" the whole way through with catco high flow cat.

Notice the oil residue? Any comments?

Gasket in place:

Gasket by itself:

Oil in the ports is this normal? Looks dry? Is this a low point wear it rests?

$29 Ratchet spanners. From bunnings. Good for a lot of things on the skyline. Should have had them a year or so ago!

Edited February 1, 2008 by benl1981

[The Mafia]

so... going to list your mods?

[benl1981]

Sure, but I didn't think it was necessary to the problem - I'm hitting close to 18psi and the boost drops off massively after 5500rpm to 6000rpm.. I have put up mods in rb25 turbo upgrade thread.

dyno result and mods

247.4rwkw

R33GTST

Z32 AFM

GCG High Flow - R33 exhaust housing (not vg30)

3" Batmbl bellmouth dump --> Catco Cat --> 3.5" Kakimoto with 2 mufflers

S15 injectors 450cc

Tomei Fuel Pump

Turbtech boost valve

18psi - running back to 16psi

Stock Airbox

450x300x76 FMIC (Justjap)

[r32woohoo]

Theres a thread about GCG turbo's dropping boost

It was something to do with the wastegate actuator arm

That could be it

edit: http://www.skylinesaustralia.com/forums/R3...html&hl=gcg

Edited February 1, 2008 by r32woohoo

[benl1981]

It was running fine. The actuator arm was checked prior to install. It was holding boost to start with..

Thanks anyway.

[r32woohoo]

Yeh i found the thread and its not quite the arm but i was close

[raps86]

the easiest way to tell if the gasket was leaking would b 2 get a can of carby clean and start the vehicle up and spray around all the vacumn lines and gaskets and listen 2 the revs if there is a leak the revs will climb slightly.

[benl1981]

Yeah we checked that. But I think it is only under high boost/airflow that it is actually leaking.

[robots]

So did it fix it ?

[benl1981]

I'm waiting for the new gasket from nissan..

[benl1981]

I'm just thinking that maybe I should have blocked off the BOV first to test that. I can't really hear heaps of air escaping so I was thinking it may be the standard BOV venting back into intake (would make sense). Do BOVs fail like this sometimes? It was working fine for a week - now something has happened.

Anyone used gasket goo on their old gaskets? THinking of wacking it backtogerther with a gasket goo so I can check the bov..

Wont get the new gasket till Monday!

Edited February 2, 2008 by benl1981

[Adriano]

I doubt it is the inlert gasket, especially if it doesn't leak at idle. You may be on the money with the BOV though, or could be the cat.

[benl1981]

So Adriano, do you think wack the old gasket back in with some goo? I can't see any holes in it. Anyone got any ideas from the photos?

Edited February 2, 2008 by benl1981

[benl1981]

Ok I'm waiting for the goo to dry so I can test it. I used a small smear of this all the way round.

The GOO

Seems like it bonds quite well. I know it's not really the intended use but if it works it works. If it works for a few runs then blows out Ill invest in a new gasket and washers

I have also put a 50cent coin between my BOV and the intercooler pipe (blocking it off). Will this do any harm to my turbo if I do a couple of runs at 18psi!?? I imagine the pressure build up on gear change and stalling on the compressor couldn't be good.

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Edited February 3, 2008 by benl1981

[BAMR33]

GCG High Flow - R33 exhaust housing (not vg30)

I see your problem, a restrictive rear housing.

An aftermarket exhaust manifold might help your case a bit. All the same 247kW is a pretty decent effort out of gcg hi-flow, I'd leave it at that.

Edited February 3, 2008 by BAMR33

[benl1981]

Mate. That isnt the problem. On the dyno and the first week I was really happy. Would boost to 7000rpm. Something has changed and boost drops after 5500 to 6000rpm hugely.

[Lazy-Bastard]

The easiest way to find the problem is going to be to pressure test your entire inlet tract, otherwise it could take ages to narrow it down.

Another QLD user (FineLine) has had a tester fabricated and it is a VERY handy little tool. It's simply a 6" length of 90mm alloy pipe (he has a Q45 AFM, so you'd need 76mm from memory) closed on one end with a tyre valve attached.

You simply remove your AFM, install the alloy tube in its place and slowly pressurise with a compressor up to your desired boost level (have someone watch your boost gauge!) It's amazing how many leaks you will find. Also, you aren't trying to listen for an air leak over a running engine which makes life a lot easier.

After a 2 day road trip my car started having trouble developing boost. It would build really slowly and eventually hit full boost near redline, whereas previously it would hit ~3300rpm. We pressurised the system and found 3 boost leaks in the turbo/cooler plumbing. It's impossible to find them when stationary because you can't make any boost, and all the clamps/hoses etc seemed fine from visual and manual inspection.

The owner was having idling issues the other day, he pressurised the inlet and found a hole in some flexible inlet piping and 2 leaking silicone joins. Fixed leaks/holes, fixed idle.

If you went to an exhaust shop or the like I reckon you could get one knocked up for ~$25 and it would pay for itself in about 5 minutes.

[benl1981]

Sounds like a good little idea here. Might make one up. Did you have the throttle open so you pressure the penum also?

Did you use an air compressor or just pump it up by hand?

Thanks a lot

[Lazy-Bastard]

We didn't have the throttle open, but you obviously could if need be. We used an air compressor, but wait for the compressor motor to stop before you start working so you don't have to listen over the compressor noise.

[BHDave]

The easiest way to find the problem is going to be to pressure test your entire inlet tract, otherwise it could take ages to narrow it down.

Another QLD user (FineLine) has had a tester fabricated and it is a VERY handy little tool. It's simply a 6" length of 90mm alloy pipe (he has a Q45 AFM, so you'd need 76mm from memory) closed on one end with a tyre valve attached.

You simply remove your AFM, install the alloy tube in its place and slowly pressurise with a compressor up to your desired boost level (have someone watch your boost gauge!) It's amazing how many leaks you will find. Also, you aren't trying to listen for an air leak over a running engine which makes life a lot easier.

After a 2 day road trip my car started having trouble developing boost. It would build really slowly and eventually hit full boost near redline, whereas previously it would hit ~3300rpm. We pressurised the system and found 3 boost leaks in the turbo/cooler plumbing. It's impossible to find them when stationary because you can't make any boost, and all the clamps/hoses etc seemed fine from visual and manual inspection.

The owner was having idling issues the other day, he pressurised the inlet and found a hole in some flexible inlet piping and 2 leaking silicone joins. Fixed leaks/holes, fixed idle.

If you went to an exhaust shop or the like I reckon you could get one knocked up for ~$25 and it would pay for itself in about 5 minutes.

I did something similar, but povo style. I removed one of the hoses on the plenum and pressurised the plenum with the TB closed. I sprayed soapy water everywhere and watched for bubbles. You can get a few psi even doing it this way and it was enough to tell me my IACV was leaking. (I had a problem with vacuum at idle decreasing over time to the point where the car couldn't cruise without miss firing badly)

Lazy bastards way is better as it does a better job of sealing the system.