1UZ VVTi hits a "wall" at around 4.5krpm

[Jesus-Ninja]

Hello all,

Engine has been in the car for a while now, and I've done a couple of track-days. Everything is going very well - sounds fantastic, still handles well, and goes like stink.... sort of....

Up to 4krpm it pulls like a train, but then by 4.5k it hits a wall and won't punch any higher. The engine will definitely rev freely out of gear, but under load it hits a point where it won't rev past. The problem is more pronounced in higher gears, as if it speed is a key component. This would make sense as aerodynamic drag cubes with speed, and the air becomes increasingly difficult to punch through.

So, it's like it's got no guts above about 4.5krpm.

Engine is a 1998 VVTi in an MX5 (Miata). Manual conversion is from an S14 (200sx / 240sx).

Techstream shows no problems aside from O2 sensors, which shouldn't affect open loop.

My initial thought was around fuelling - pump is a Walbro 255lph, although I wonder maybe if the stock Mazda pipework isn't up to the flow required. It's about 6mm internal bore.

Alternatively could this be an issue with the VVTi?

• Is there any way to force it to kick in to at least prove it one way or another?
• Am I right in saying that the VVTi on the 1UZ is more complex than a simple on/off, and it is able to progressively adjust the advance depending on TPS and load?
• How does the ECU read load? I assume it reads the vacuum from somewhere?

This is the last piece of what's been an immense jigsaw, so any help is greatly appreciated.

 

[George_R]

ok so you have Techstream.
What are fuel trim values across all RPM and load range?
In particular, when warmed up, in neutral, normal idle and 2500 RPM?
The values you need are named LTFT #1 and LTFT #2

all vvti / timing issues would appear as DTCs soon or later
vvti is not an on off system, read the interwebs on the topic

I assume two things
1 overfuelling as result of incorrect fuel pressure adjustment
2 DIY MAF housing instead of factory airbox with factory MAF inserted in; DIY MAF housings are all less than optimal at best

 

[Jesus-Ninja]

Thanks for the rapid response George Here's a few picks of the engine bay I took this morning. You are correct in that I'm not using the stock airbox. Space is limited! I have used the stock MAF and housing though, in the most elegant way I could. As an aside, I've also included some shots of the breather paperwork. The catch tank is filling up very quickly on track, and what is in there is just oil (no emulsion etc) Finally, there's a shot of a plug with nothing attached. If memory serves me, this went to some kind of pressure sender which was plumbed into the vac line with the stock carbon canister - any possibility that I need this for VVTi?

 

[George_R]

I assume your MAF is mounted in the black insert. If so, I don't see apparent troubles with it as it runs enough straight pipe before it to suppress most of air turbulence.

So try to read the LTFT's as per above, because the problem doesn't seem to be apparent

on another note, chinese radiator caps do fail. Replace it with Gates RC133 before too late.

 

[litldv8]

should the pcv/catch can be plumbed in after the maf not before?

 

[Jesus-Ninja]

I assume your MAF is mounted in the black insert. If so, I don't see apparent troubles with it as it runs enough straight pipe before it to suppress most of air turbulence.

Yes, in fact the black pipe was cut straight off the original setup.

 

[Jesus-Ninja]

should the pcv/catch can be plumbed in after the maf not before?

If it's plumbed in after, doesn't it mean I'll be drawing in unmetered air? Might reduce the chances of contamination of the MAF though.

 

[George_R]

A good note on the PCV system
From factory, LH (bank1) head's vent goes to throttle body and RH head's vent connects to resonator which is installed downstream the MAF

However normally PCV system is plumbed and no air should be drawn from it.
Yet another note is that any air leak between MAF and engine becomes negligible as both RPM and engine load increase.

I prefer to know maximum possible info before trying to guess what's wrong with the engine. Using Techstream just to read DTC's is pretty much like using a microscope to batter down nails

 

[Jesus-Ninja]

So, George, are you saying that you think I should plumb the PCV system up as stock?

 

[George_R]

No
I'm saying that it's better first to gather as much data as you can, analyze it and then do something

And I'm also saying you use about 1% of Techstream currently

 

[Jesus-Ninja]

Sorry, yeah, I wasn't suggesting that the PCV plumbing had anything to do with my problems. It was more a point of interest since it was raised.

 

[deegee]

did the car go well after your conversion then become like that after a wile?
were that pcv is plumbed into the intake before the afm could contaminate it. dirty afm in Toyotas can make them do all sorts of crazy things.
I have even had one that would bearly rev past idle.

 

[Jesus-Ninja]

Fixed. Silicone elbow between the maf and the plenum was collapsing under load.

 

[cribbj]

Good to know; thanks for updating the thread with the resolution!

 

[Jesus-Ninja]

I think it's down to what appears to be a fake Apexi air filter. I'd had it previously on a Legnum twin turbo, but it was hard pipe to the turbo, and then of course all positive pressure from the turbo to plenum.

Running with no filter is immense!! Got to find a filter now though. Choice between a "real" Apexi (on the assumption that this one is a fake), or a Blitz SUS.

I've also replaced the silicone elbow with ally

Pulls like a train now

 

[George_R]

Sounds very weird. I run 82mm silicone 90deg bend and I haven't ever seen it collapsing whatever I do to the engine. I can see no way it's possible unless using way too thin silicone or clogged filter

 

[Jesus-Ninja]

I think the filter was a fake. Having done some research there appears to be a few around.

 

[Spinnetti]

What MAF housing is that? I cut up my stock airbox to do essentially the same thing, but would like to know if there is another option....

 

[Jesus-Ninja]

It's just the standard MAF from the stock LS400 setup. I've thought about making a nice aluminium one to carry the stock MAF, but it wouldn't really improve anything, I don't think.