ST205 - Fuel cut logic

[RunningRich]

Hey

I've been troubleshooting an issue with my ST205 fuel cutting anywhere between 6000 and 7000 rpm.

Engine is balanced & blueprinted with a mildly ported head. 2.5" downpipe http://gtfour.supras.org.nz/pics/downpipe2.jpg and full 3" low restriction system after that.

I'm getting code 21 (O2 sensor) as soon as I drive. It is however flicking the analog voltmeter at about 1 Hz so seems to be working fine. Will check heater circuit and Vp-E1 voltage per manual tomorrow.

It is also running rich (always has, hence the name) but plugs were pretty black. Put some iridium plugs in in case that made a difference. Nup...

Have removed all temp sensors and verified they are in spec per manual, so is MAP sensor. Tried both my ECU's.

I've ripped out the Profec B and gone back to the stock boost control system. Runs around 17-18psi max. Boost response is quite strange, continuing to build almost linearly as rpm rise. Wastegate is freely moving.

Anyway the fuel cut does not seem to be a simple boost limit. Probably feeds all the engine parameters through a risk algorithm to determine a safety cut off. Does anyone have any info on what this process is?

Any other troubleshooting suggestions welcome!

[RunningRich]

Well found the O2 sensor heater circuit open circuit so that explains the code 21.

Can land genuine item for $150 from UK so will replace that and do more troubleshooting.

Apart from checking the temp sensor signals and throttle sensor at the ECU that's all I got done. Hard to troubleshoot at the ECU under the dash when a toddler is pushing in front of you to help...

[sergei]

Are you sure you are not hitting boost cut? 17-18psi is very close to the boost cut.
CT20b known to spike with free flow system.

Stock boost should be around 1 - 1.1bar. Anything above 1.2bar (~17.5psi) and you will get a boost cut.

Running rich will also be caused by ECU reacting to overboosting and knocking. On severe knock it will also feel like fuel cut (ecu dumps a lot of fuel and pulls back on ignition). Once you get above 1bar on stock ECU, it will dump fuel to save engine (and substantially pull back ignition).

[RunningRich]

Yeah by fuel cut I mean boost cut.

My car has always ran high boost (had a three inch mandrel bent system with shite mufflers before the HKS system). I guess the HKS exhaust has just allowed it to creep a touch too far.

So how to wind back the boost a tad?

Perhaps it is time to remove the downpipe and go back to a cat to add some restriction? Anyone have a suitable cat I can purchase? I loaned mine out and never got it back.

[Malcolm]

Anyone have a suitable cat I can purchase? I loaned mine out and never got it back.

Hi

I've got one here, the actual catalyst is roaming free, so it would be more suitable for gutting - not sure whether a gutted cat would be sufficiently restrictive or not? I'm selling for $50 anyway if you're interested

[matt dunn]

I would say not, even a full complete cat may not be enough.

We have struck a few cars where we have had to restrict the exhaust to lower the boost.

Other option would be some way of porting the wasteage hole or similar
which is common thing to do on Mitsi's.

[MAGN1T]

Replace your oxygen sensor, fit bigger injectors and an AFC.
Also something to monitor knock.

Steve

[sergei]

I had to fit 2 external wastegates with elaborate spacer that goes between turbo and manifold to fix boost creep on my CT20b.
Porting internal did not work.
Isolating after porting didn't work either.
Removing internal and fitting 1 single external feeding off the original ported waste gate ports (but without flap), didn't work either.

The only thing that kept the boost down was a restrictive muffler with a 2.5in system, once I removed that boost was creeping up.


I can take fresh photos what I have if you want to have a look at.
Simon (evil_si) was the man who did amazing job of fabricating this monster.

The system I have maintains original W/A intercooler and maintains twin entry system (hence two wastegates). The things need to be modified are the oil feed and oil drain (as the turbo sits lower), it is possible to maintain the A/C (as my fried replicated this setup for his ST205 and managed to work around the A/C).

I have this setup in my ST165, which has less space in engine bay.

These are old photos done by Simon during the build of the system:



This is the way (I believe) to have a free flow system and not overboost on CT20b.

[Malcolm]

just wondering, what's the lowest boost you can achieve without creep with that setup sergei? I've been looking at my downpipe and thinking that there seems to be a lot of room for improvement in terms of wastegate flow, and thinking about making a new one.

[sergei]

just wondering, what's the lowest boost you can achieve without creep with that setup sergei? I've been looking at my downpipe and thinking that there seems to be a lot of room for improvement in terms of wastegate flow, and thinking about making a new one.

The lowest I can achieve is 0.5Bar and that is because the wastegate springs are set to that. It is very stable at the lowest setting.
If I put weaker springs I believe it will go even lower (or if I apply vacuum to the top of waste gates).

I tested with vacuum on the wastegates (so they are fully open) and I achieved no boost what so ever.

One side effect of this system is that when wastegates are open the bloody thing is very loud (even though wastegates are plumbed in).

[RunningRich]

To throw something else into the mix NZ new ST205 (at least two of the Group A models anyway) had a couple of dash pots and (from memory) zener diode tapped into the MAP sensor signal wire.

I ripped it out but I suspect it was to slightly adjust the MAP sensor signal, I assume to put in place a kind of FCD.

Since it was done on two cars I can only assume this was a Toyota NZ mod.

Apart from the downpipe, exhaust and mild head porting the car is (in effect) stock from an airflow perspective.

Not really keen on further mods to fix this (540cc injectors are not big enough?!?).

Does the Power FC allow boost above 18psi?

[sergei]

To throw something else into the mix NZ new ST205 (at least two of the Group A models anyway) had a couple of dash pots and (from memory) zener diode tapped into the MAP sensor signal wire.

I ripped it out but I suspect it was to slightly adjust the MAP sensor signal, I assume to put in place a kind of FCD.

Since it was done on two cars I can only assume this was a Toyota NZ mod.

Apart from the downpipe, exhaust and mild head porting the car is (in effect) stock from an airflow perspective.

Not really keen on further mods to fix this (540cc injectors are not big enough?!?).

Does the Power FC allow boost above 18psi?

Zener diode (or even voltage divider) on a MAP sensor is a big NO-NO.
All it will do is limit voltage output of the MAP sensor thus fooling that it is getting less air than it is in reality, without supportive electronics to take over the fuelling it will lead to blown engines (cracked ring lands).

[Prymal]

mines the other one with the toyota fitted wiener diode ..

Rich , your welcome to try my PFC if you want , just to see if it makes any difference

[MAGN1T]

The zener diode mod is the poor mans FCD. It does the same job, works just as well but costs cents. It's just not appealing to the majority of customers because it doesn't have "HKS" printed on it.
It just limits the max voltage that the ECU gets from the map sensor. With one of those you need something like an AIC to go with it to give the extra fuel.

Steve