CT26 Overboost Problem (56K warning)

[Matty P]

Hi Guys,

I'm after some assistance diagnosing the cause of a problem that seems to be overboost.

The car is a stock ST185 Group A, with no boost control.

Problem started occurring after I fitted Aussie 3" dump pipe and complete the exhaust with 3" hi-flow cat and flexi (was just 2 1/2" cat back). It used to boost to about 10psi max, but is now reaching 15psi or more at times.

When it is under full load in 3rd-5th gear, it will shudder quite badly and the boost guage (Autometer) will fluctuate and I lose power. This does not occur when I am accelerating through the gears, only from coasting (especially up hill) or going back a gear (eg. roundabout).

It doesn't go into limp mode but certainly seems to be overboost.

I figured the likely culprit was a boost leak, but I checked, double-checked and triple-checked the hoses. My mechanic also checked them

So I figured it must be wastegate not opening for some reason. I removed the turbo and dump but cannot find a cause.

Wastegate looks OK in the closed position:



Wastegate actuator rod looks OK:


Using stock gasket - does not appear any evidence that it would foul the WG:


No evidence of obstruction around the WG area of the dump pipe. Aussie dumps are very well designed:


So I am back to thinking that there must be either a boost leak or an electrical issue somewhere.

Comments, opinions?

Thanks,
Matt

[ra65]

Don't even 1j's have problems if you are using to big a dump pipe? And you are using an even smaller engine....

[Gold28]

Overboosting is not uncommon with CT26´s and big exhausts and is very common on the CT20B attached to the ST205 engines.

The general concensus in the English GT4 clubs is that with very low backpressure, the wastegate doesn´t flow enough to meter the pressure correctly. That may be partially the case but I think there is probably more to it than that. Incidently their solution is to put a baffle in the tip of the exhaust to choke the flow - problem goes away but whats the point in having a nice exhaust and choking it????

Question.....When you put it into a high gear and put your foot down, does it creep up and up or does it spike then settle to a lower boost?

If it spikes and settles then it is not a wastgate problem as such but a "system" problem. There is a hysterisis or slowness/resistance in the wastegate boost control system, in other words, it takes time for the boost pressure to fill the actuator and push the rod which opens the wastegate which leaks the pressure from in front of the turbine which stops the compressor accelerating etc etc. While all this is happening, the compressor is still doing it´s job squeezing air. Hence the spiking. This may be helped with a better (or newer) actuator and clean pressure feed lines for it.



If on the other hand the boost keeps rising, then it is indeed a regulation problem and the wastegate isn´t doing it´s job. This could be a simple case of the turbo was never designed to run with such little back pressure (I find that unlikely) or there is some mechanical or aerodynamic reason why the gate isn´t flowing enough.

Make sure the gate can fully open inside the dump, that the actuator is workign correctly etc.

If it is a case of wastegate aerodynamics in the dump pipe then it´s going to be a difficult one to solve and I wish you luck and patience.

You could also try cleaning the wastegate area of the exhaust housing. The exhaust is pretty dirty on all toyota turbo cars.

Hope you get it solved soon.


Anthony

[The Real Roadrunner]

im having the same problem with an sw20 at the moment but it goes to 19-20psi.
ive dieground the wastegate ports out as big as possible but to no avail.
its off getting clipped at the moment to hopefully cure it but im not 150% confident that it will.

if yours is steelwheeled then this may be an option but if its a ceramic then you probly wont have much chance.

the cutting out you feel is the boost cut so id probly just throw an adjustable FCD on there and be done with it, 15psi aint to high just make sure its setup on a dyno by a competent workshop.

cheers
linden

[Andrew162]

I had similar issues on my (rather troublesome) RAV. The ST205 cut I got was afforded a custom dump-pipe and a full 3" exhaust all the way. I too had problems with overboosting, though I still had the stock wastegate (twin-20mm internal as Matty has shown), but I initially thought this was to do with the way the vacuum lines were routed... ie. they were screwed up from the word 'go'. I sorted the vac lines out, but it was still overboosting marginally, I gave it up to ~14-15Psi at times, no real 'spiking' though.

for clarification, the original CT20b I had was ratshit, and Precision Turbochargers told me it wasn't worth rebuilding it because of it being a ceramic unit they preferred to stuff a steel-wheel inside instead. so I got a 56mm trim comp wheel and left the turbine stock. I never gave it too much boost though, I knew what it was capable of, and from ~2300rpm onwards the boost was exceptional, and it seldom saw more than 10Psi.

[The Real Roadrunner]

Precision Turbochargers told me it wasn't worth rebuilding it because of it being a ceramic unit they preferred to stuff a steel-wheel inside instead. so I got a 56mm trim comp wheel and left the turbine stock. I never gave it too much boost though, I knew what it was capable of, and from ~2300rpm onwards the boost was exceptional, and it seldom saw more than 10Psi.

the one im having probs with had the same mods 56mm inducer with steel wheel and i cant get the boost to drop even with the actuator pushed fully open.

i think it has something to do with the wastegate ports having shite angles for the exhaust to go into.

cheers
linden

[Andrew162]

so I guess an external w/g is one solution?

[Matty P]

Thanks Guys for the input - it is greatly appreciated

I considered external wastegate, but that really seems like overkill for a mild stock setup. If I was putting on an aftermarket turbo I would, but for CT26 it seems like a waste.

Just to clarify, the boost (according to my Autometer guage at least) does not spike but seems to build slowly. The shuddering occurs even when the gauge only shows low boost (ie. less than 10psi).

One solution may be to disable the Turbo VSV. As I understand it, this would cause it to revert to the lower boost setting in all gears. The problem does not seem to occur in 2nd gear, so perhaps this might manage to keep it below boost cut?

I am also planning to get the turbo inspected to see if it needs a rebuild. It is 150,000km old, so may be a little worn out. Will also try to make sure that the wastegate is opening properly (once I work out how to do it without measured compressed air )

Thanks again guys!

[Kyoto]

Sounds like a good excuse for a GT28RS if you ask me!

Hope you get it sorted, nothing pisses me off more than malfunctioning GT-Fours.

[Matty P]

I know what you mean Tim, but a GT28RS sounds like too much $$$ and stuffing around to me

[LeeRoy]

Maybe try hooking the wastegate actuator up to a bike pump with a gauge and compare what its designed to open at and what it actually opens at. It could also be the vac lines creating a delay in the signal or pressure hence you reach 15psi or whatever but the actuator is still only sensing 10psi but after a few seconds when it all catches up to itself it opens but during that time delay youve already spiked and hit boost cut.

Delays can be caused normally by long vac lines, many t pieces, many loads such as gauges, boost controllers, blow of valves etc all on one line, and slow wastegate/actuator.

In your case all youve done is change the dump pipe which has probably increased spool time meaning the response time between actual vacuum and wastegate opening needs to be alot quicker. In the old setup the time beween 0 boost and 15psi was probably longer and the lines and gate were up to the task of sending an appropriate signal in an adequate amount of time. In your case now, this time is too slow.

Maybe try running a direct line from the turbo housing or pipe closest to the housing straight to the actuator.

[Matty P]

It's a pretty short hose on the CT26 that runs from the compressor housing to the actuator. Should not be causing a noticeable delay.

[The Real Roadrunner]

so I guess an external w/g is one solution?

not really as its probly the a/r of the turbine housing being to small or the turbine wheel not flowing enough air.

this is indicated by the fact that it builds boost slowly instead of spiking which would indicate a boost control/actuator problem.

cheers
linden

[LeeRoy]

i think it has something to do with the wastegate ports having shite angles for the exhaust to go into.

I would definately see that as being a contibuting factor.

Combine that with a less restrictive exhaust and the exhaust probably finds it alot easier to just travel out through the turbine compared with the pressure the wastegate would have recieved with a standard wastegate. This lower force on the wastegate at an rpm or gas flow where it should open probably has an effect by which it delays its opening time.

Would a solution to that be to modify the housing to create a smother flow to the wastegate or just try an actuator with a lower opening pressure?

[Matty P]

Yes - I've read about people porting their wastegates to get around the overboosting issue. Sounds like a very 'delicate' operation!

I guess an actuator with lower pressure setting might be a solution - haven't heard of that one before.