RB25 S2 Idles but doesn't want to rev

[Trilbyte]

I'm a bit stuck with this one fellas, I recently decided to undertake a turbo conversion on my GTS-4, ended up pulling the motor to replace all the seals and do an mls head gasket and arp head studs while I was at it.

Everything has gone according to plan, no oil or coolant leaks, compression checked out, and decided to give it a test before I book it in for a tune

As it stands right now, the turbo only has oil lines plumbed so that I don't put boost into my untuned engine, and I'm using the factory GTS-T crossover pipe with a maf and poddy as a temporary intake.

The car starts with relative ease and idles amazingly, however as the title suggests it really doesn't want to rev up, runs very smooth but almost feels choked out and unresponsive to the throttle, almost like it's running lean or something?

I've triple checked my vacuum lines and timing with a timing light, injectors haven't been touched as of yet to avoid flooding the engine and worked perfectly beforehand, new plugs and coilpacks, I've even unbolted the cat just to see if it spontaneously collapsed, no dice

The car currently has a nistune r32 gtst ecu installed so I have no way to check for fault codes.

At this point I'm a bit stumped. Is this normal behaviour for a car with a turbo bolted onto the exhaust going nowhere? I'd imagine the turbo wouldn't cause this much restriction in the exhaust without the boost going through the intake. Any ideas? If necessary I can link a video of how it responds to full throttle while in neutral.

 

[Kinkstaah]

Um. Was the ECU the same as the one previously there? I know R33's needed R32 GTST ECU's or other tomfoolery to run Nistune.

This is such a wild setup. Most people would plumb in the turbo then not drive the car (i.e tow it to a tuner).

What's actually changed since it last ran?

[Trilbyte]

Haha yea it's a bit of a weird setup at the moment, just wanted to make sure I sorted out any headaches before dyno day.

At the moment I've changed the exhaust manifold, turbo and downpipe, 460lph fuel pump and rising rate reg as well as the previously mentioned headstuds and gasket. The nistune was already mapped for the car as it was and drove around for a few months no worries.

The plan is to run flex fuel hence the big pump, which I've wired direct to the battery through a relay to avoid voltage issues.

[Kinkstaah]

Right, so is the fuel pressure the same as it was previously?
If not, well, it's not going to work well.

I can understand the theory I assume you are going for, which is:

"Run it N/A, and if I have the original AFM, and same amount of fuel and intake air as previously, the fact there's a turbo in my exhaust should still make it run"

Which it... might?

Provided you don't actually change how much air or fuel is going into the car.. or getting out.
But if you have changed that, all bets are off.

What is the purpose to even have the car run this way? To get it to a tuner?

And if the goal is to limp it to a tuner, you're still going to what - plumb up a whole intercooler system and new intake in the parking lot out front before your tune......?

[Trilbyte]

No intention of driving the car the way it's set up now, just wanting to make sure I haven't made any mistakes along the way so that the tune can go smoothly, I don't want to waste anyone's time with stupid issues. And of course I intend to get it towed to the dyno in a completed state 😅

I have the base fuel pressure with the vacuum disconnected set at about 40psi, which I believe is at least relatively close to stock fuel pressure, maybe I'll have a fiddle with the reg and see what happens. For all I know I could be wasting my time with this and I should just plumb it in and see what the tuner reckons. I just find it a bit strange that it seems to be under so much load just free revving, but maybe that's just a result of the turbo slowing down the exhaust gasses and making a restriction in the flow, with no boost to compensate.

[Duncan]

Just checking, is there still and AFM and then leak free intake piping to the plenum? that would kill it if it not.

An unloaded turbo in the exhaust won't cause an issue, it you are worried just disconnect the wastegate so it is open

[Trilbyte]

Still got the afm on the intake, clamps are shut tight, only loose hose is the one that goes from the j pipe towards the IACV, since it's next to impossible to find a factory hose and the barbs are different sizes (I'm still using clamps on this hose to try and help it seal on the iacv side)

I've ordered parts to make up the hotside of the intercooler pipes, I'll plumb it in and see what happens in a few days I suppose

The turbo's internally gated, can I just unscrew the tension rod to let the gate open?

[GTSBoy]

7 minutes ago, Trilbyte said:

The turbo's internally gated, can I just unscrew the tension rod to let the gate open?

Don't worry about. Just don't try to drive hard enough to make boost and you'll be fine.

[MBS206]

Connect all the plumbing up properly.

The actual worst part about having the turbo unable to actually build boost, is you can and likely would over soon the turbo if you drove the car (free rev while stationary isnt likely to manage it).

 

If you do have an air leak as Duncan suggested, you'll have a bad time as the AFM will be saying different to reality.

 

As you have had the engine out, I'd go over all electrical connections again. If you have a consult cable, plug it in, and see what the ECU thinks is going on.

Did you touch the timing belt? If so, double check your timing on the gears is correct AND put a timing light on it while it's idling and see what ignition setting you have.

Is the AFM you've installed the one that was in the car previously?

You said you replaced seals, what seals exactly? What have you had apart?

 

From memory when I had my RB25DET with factory throttle, there was two plugs the TPS plug on the loom could go to. One would leave the car running like shit, the other made it work, I think one plug on the TPS area was for something else (an option). It's too many years since I saw that part of the loom to remember properly though.

 

Is the car idling smooth (as smooth as an RB can)? Does it smell fuelled up?

 

I have a feeling either timing in timing belt is off, or a sensor isn't right.