ST205, loss of power in ~3000rpm range

[sergei]

My ST205 has developed a new problem:
quiet often when cold or slightly warm, and occasionally when hot it losses power between 2500-3500rpm. It feels exactly like traction control kicking in on a chaser/supra on the wet.
While this is happening it generates boost fine (boost goes up, while car goes no where), and it is sort of intermittent (similar to missfire but not quiet, like runs out of fuel).

Here the things I have checked out so far:
1) Spark plugs are new (needed new set anyway)
2) Leads are fine.
3) Cap is fine and dry (no cracks or build up).
4) Rotor is fine and dry (no cracks or build up).
5) Tested with different coil with same result

I suspect fuel delivery problem - feels like fuel pump is on its way out.
Initially I suspected ignition system, but after doing the basics I doubt it. Symptoms aren't as rhythmic as it would be with ignition system (and are way more intermittent than one would expect if ignition was at fault).

Fuel is fresh, 98, there is 20L of it in there.

No error codes.

Things left to check:

Fuel pump relay
Igniter (one would think that would bring an error code).
ECU
Replace fuel filter (it is due).
Replace fuel pump (OEM if under $300 or Walbro).

MAP sensor and MAP sensor line is fine because the factory boost gauge is working off it and I clearly see plenty of boost when the fault is happening.

This fault is happening under medium to high load on low RPM. Once it is past 4000rpm it goes fine (perhaps fuel pump relay is at fault?).

[KinLoud]

Failing knock sensor perhaps...
They can develop a crack which resonates at particular rpms.
OR
something is loose on the engine causing extra resonance

Maybe connect another knock sensor but just cable tie it to a radiator hose or something that is isolated from vibration or some variation of this theme.

Ken

[sergei]

I pulled apart fuel pump relay (the $&#$% plastic got very brittle and case is muntered now), and cleaned the contacts, took car to work today and no fault yet.

Will see how much is new relay, buy it, or if it is two expensive will override the whole twospeed system and put the fuel pump full time full speed.

[XS1V]

I should have a factory relay for it if you want one

Brad

[RunningRich]

My ST165 used to do this in really bad/wet weather. Did it at the Manukau stage of the rally one year cause it was so feking wet. Also drove a ST185 doing it once. They would boost but without any power, felt like you were dragging an elephant with handbrake on.

My understanding was it was a knock sensor fault causing the ECU to retard ignition to a ridiculous point. In the case of my ST165 it never hung around once dried out.

Unplug the knock sensor (using multi-jointed tiny arms if available!) and Spray well with CRC CO Contact Cleaner. Ensure sensor is tight and the wire isn't damaged. A common MA71 Supra repair is to replace the shielded knock sensor wires in the loom but I've never heard that on any 3S.

One thing to be aware of is that my ST165 ran very very lean (mixture meter) when on boost and it was playing up in this manner. ST205 may be different but care is required.

[Prymal]

I have the same issuies in the wet with my 205 atm .. Might seek a thought or two rich , if your local to my shop and drop in .. Its about time we cought up again!

[sergei]

That is odd that it goes lean with knock sensor fault.
One would think that it would retard ignition and dump fuel if there is anything coming from knock sensor.

I am still yet to get this problem again. When I have time I will do an experiment:
I will hose the area with water and see if it causes the symptom to come up.

[Perky]

Hi Sergei

Did you ever get to the bottom of this?

My ST206, with 3SGE, has had exactly the same symptoms the whole time I've had it. I've seen similar problems described here before, but never a solution.

When I got it, several years ago, I changed plugs, leads and distibutor cap (rotor's fine). At some point the O2 sensor produced an error code, so I replaced that too. Nothing solved the problem.

It had a new fuel filter when I got it. I have suspected the fuel pump but I've never investigated it, or even thought about its relay.

Exactly the same revs are affected in my car, which makes me doubt that it's a freak resonance, but who knows.

Because it's so rev-specific, I've suspected the TPS. It measures okay closed and at at the specified opening. Do you know if it's actually a potentiometer?

Like yours, the problem comes and goes. I've never been able to pin down a common factor to its occurance. I don't think it's dampness. I seem to notice it most at the beginning of a journey, when it's cold, but I think that's just because most journeys are short. It can certainly happen when it's hot.

I have a tentative impression is that it's worse when the tank is nearly empty and better when it's full, but I'm not really sure. I had thought that if the fuel pump was the problem, it would not be so rev-specific. I'm not 100% confident that the fuel cap makes a good seal; could that affect pump performance?

What is the two-speed system? Something that switches around 3700 revs would be a prime suspect.

[sergei]

My problem went away.
I have suspicion that runningrich was right and that was knock sensor connection, to be exact, I have suspicion that it was wet (since I washed the body from underneath with water blaster right before symptom appeared).

While I was mucking around with plugs and what not, it dried up and "fixed" itself.
The car is garaged for months at a time so I cannot confirm 100% it went away. If it appears again I will try to diagnose it further.

As for TPS, yes it is a potentiometer, with ignition ON it should linearly go up to ~5V from 0V when you open throttle.

As far as I am aware of the fuel pump operation it switches on full during moderate to high load, and switches to half speed on cruising and idling. Really easy to diagnose - bypass relay with the jumper (or short the fuel pump resistor).

If the dampness is a factor it would be around a connector, most sensitive sensors to the dampness are the ones that are not fed by 5V (not TPS, not WTS or any other). It would be CAS and Knock sensors as they are very low voltage signals.
To crudely test the knock sensor (if your getting problem frequently enough to be able to action), just disconnect it entirely (it will bring engine light on, and it will go in limp mode). If the miss goes away it means that knock sensor plug was affected by dampness.You could put some grease inside of the connector to fix it semi-permanently.