Diagnosing Common Problems
The GT-Four is blessed with the legendary Toyota reliability but it is not
Inevitably, as with any high power 4WD car, the driver will be faced
with a problem sooner or later.
In these pages we hope to help solve some of the more common ones
To narrow down the range of options please choose your car model below
This is a common problem on the GT4, and is generally caused by movement in the rear differential.
It is caused by a failure/breakdown of the stock differential mounting which has a large rubber bush in it. Over time this rubber slowly fails until it allows the differential to move creating the characteristic clunk
There is an excellent write-up on diff cushion replacement here on gtfours.co.uk
If you are suffering from a light clunk when braking in reverse do not worry. This is actually not a problem as such but is a feature of the brakes used on the GT-Four
Th calipers allow the brake pads to move slightly when wheel rotation direction changes. So you get a light clunk the first time you brake in reverse and a light clunk first time you brake going forwards.
Othen than a quick visual inspection to make sure that the brake pads are correctly seated in the calipers and all of the brake pad retaining pins are in place there is nothing more to do.
This could be something very simple, or very complicated!
- The first and most simple thing to check is your tyre pressures. Ensure that
the pressures comply to those in your owners manual or printed on the sticker in
the glovebox. It is best to check your pressure before the car has been driven
far so that the air in the tyres does not have chance to warm up and expand.
- Bushings on front suspension arms. TRD manufacture stronger rubber bushings
which might be better than the standard Toyota ones if your car is used in a
harsh climate where rubber deteriorates fast.
- Front suspension mountings on top of front shock absorbers, access under bonnet/hood.
The rubber on these can expand over time. The rubber should be flatish and sit
below the level of the mounting rim. If the rubber is bulging over the rim, you should
change the mountings. Toyota mountings are OK for street use, although TRD
make a stiffer component.
- Steering rack - check for oil leakage. A gasket replacement kit is available
from Toyota. You can also adjust the effort needed to turn the steering wheel by
fiddling with a control on the rack; if this adjustment is out of alignment, then
you may be experiencing excess feedback from the road.
- Power steering pump - check for fluid leakage. Again, a gasket replacement kit
- Rear axle carrier - check for play.
- Check chassis alignment on jig
There are many reasons why this might happen from simple to expensive!
If it happens on all gears then the problem is very likely to be clutch related as it is rare to experience gearbox related problems in all gears
The first thing to check is clutch pedal adjustment to make sure that the clutch is dis-engaging correctly
If this is in order then then it is possible that the clutch pedal bracket in the drivers footwell has snapped. This is seemingly quite a common problem on ST185 variant models and has been occasionally reported on other models to.
To inspect you need to get into the drivers footwell "upside down" so that your head is down by the pedals. This can be a painful experience and requires a certain amount of wriggling.
When you get there check the bracket that the edals are attached to. You are looking for a crack in either end of the support which allows the braket to move and hence prevents full clutch dis-engagement
If this fails then read through the clutch related issues section below and see if anything there seems amiss
If the selection problem is related to only one or two gears then it's likely to be one of two things :-
The linkages could be worn. This is best diagnosed from inside the car. With the engine off put the car in any gear and try to "wiggle" the gearstick. If it moves in any direction by more than about 1/2" then there is play in the linkages which could be causing the problem
The play problem can be solved quite simply although it is a fiddly job. The linkage cables are attached to the gearbox selectors with 2 rubber bushes which, over time, degrade and become loose. The solution is to replace these bushes, either with OEM Toyota parts or aftermarket replacements.
Shown below are a set of stock (worn) Toyota bushes alongside a set of aftermarket replacements
If the bushes do not seem worn or replacing them does not help the problem and you have exhausted all simple clutch maintenance problems then it is quite possible that the problem lies within the gearbox itself, likely a worn synchromesh. Tackling this sort of problem goes beyond the scope of simple home maintenance!
Problems with the clutch can cause a number of different issues with the car from difficult
gear selection to slippage
Unfortunately most problems will require inspection of the clutch assembley. This in turn requires
removal of the gear/transfer box from the engine. Not a task for the fainthearted but it is possible
for the enthusiast DIY fan
Some common symptoms
- Clutch grabs/judders
- Engine Mount Failed - Mount can be replaced in situ
- Clutch disc damaged/worn - Clutch should be replaced
- Clutch disc glazed - Clutch should be removed and re-faced
- Clutch disk contaminated - Gearbox or crank seal failed. GB must be removed
- Spongy Pedal
- Clutch system needs bleeding
- Unlikely but master or slave cylinder damaged
- Noisy Clutch
- Release Bearing damaged/dry
- Uncommon but possible GB input shaft bearing failure
- A reguler rhythmic rattle could indicate an aftermarket clutch - OS Giken units are famous for it!
- Clutch slipping
- Clutch pedal out of adjustment - See Adjust Clutch Pedal
- Clutch worn out
- Clutch contaminated - replace clutch and check GB and crankshaft oil seals
- Does not disengage/Hard to get all gears
- Clutch pedal out of adjustment - See Adjust Clutch Pedal
- Clutch spline needs grease
- Clutch plate worn/damaged
- Flywheel damaged/distorted
Firstly how do you define poor?
Well a well setup car should return 25+mpg with sensible driving. If you have
removed your lead boots and still cannot get near this you may have a problem!
There are a number of potential reasons for this:-
For ST165 and ST185 models any leak in the vacuum system will cause overfueling.
If it is a small leak it may not be obvious!
However, a common cause is a failing O2 sensor. These do degrade with age and mileage.
It is one of the common causes for poor fuel consumption
It is possible to test the sensor if you have a voltmeter
- Run the car up to normal operating temperature. Leave it running
- Open up the cover on the diagnostic port (On the LH suspension turret under the bonnet)
- The reverse of the cover should have the pinout of the diagnostic connector
- Locate the OX and E1 pins
- Connect the ground of the voltmeter to the E1 terminal and the positive to the OX pin
- On IDLE the voltmeter should swing about 0.5V (indicating a 14.7:1 AFR). If it reads less than 0.4V
for any significant period od time then the sensor is probably on it's way out
It is difficult to make an exact diagnosis of this due to the fact that there
are so many cars out there running modified boost levels. The good news is that reduced
boost is not always a sign of a blown turbo! Turbo failure generally tends to be either
catastrophic resulting in no boost or non boost effecting (blown seals etc)
Some common tricks and tips:-
Number 1 most important test. CHECK FOR ERROR CODES (follow THIS LINK. The ECU has a number of strategies for dealing
with engine problems and boost reduction is one of them.
If you are running an EBC try switching it off. This might allow you to establish the
base actuator assuming you haven't got boost creep problems
If you are running a relief valve or bleed valve remove and inspect them. Bleed valves
can becone blocked and relief valves can become sticky. In both cases a visual inspection
and good clean might solve the problem
Other Things you can try to diagnose actuator problems:-
Boost creep occurs because fundamentally the turbo wastegate is too small
This means that even when the wastegate is fully open there is still to much
exhaust flow through the turbine which in turn produces more boost
The normal symptoms are a boost curve which rises in line with rpm. It is quite common
to see boost creep sufferers who have 0.6 bar boost at 3000 rpm and 1.5+ bar boost at 7000rpm
even when running wastegate actuator pressure only. This is a sure sign of boost creep
So what causes it? I have a stock turbo so why am I suffering?
It is almost always caused by improvements in the engine breathing systems.
That means free flowing induction system in conjunction with a free flowing de-cat exhaust
The exhaust is the main culprit. As the exhaust system is improved exhaust gases are able to
exit the exhaust system easier. This also means that they can flow through the turbo that
much faster which leads to boost creep
So how do I fix it?
It depends ;)
There are a few avenues of attack. Some good, some bad
- Turbo wastegate porting
This is where you remove your turbo and try to open out the wastegate ports
The problem with this is an inherent design problem in the CT2X series turbos. The twin
entry design means that one of the runners must have a small port. It is possible to
open it out more than stock but ultimately results have been mixed. In some cases the
problem has been solved but in others it has only been eased.
The problem is there's no real way of knowing if wastegate porting will solve your problem. One
thing for sure is that it involves a great deal of work to find out!
- Intake restriction
Opinions vary on this type of setup. Essentially it limits the amount of air which can
flow through the turbo. In some respects this is a very good thing. When the engine demands
little airflow, like at low to medium revs the restrictor has little effect. This means that
you can get good boost at low-mid rpm - good for torque :)
As airflow demands increase the restrictor comes into effect more and more. So as revs rise
the restrictor will choke airflow more and more reducing achievable boost pressure
Sounds ideal. However, there is a little but here
By choking the inlet airflow the restrictor is making the turbo work harder to produce boost. This
will lead to slightly increased outlet charge temps - the turbo is working harder and will be
slightly less efficient as revs rise. So you compromise high boost high rev performance!
It also stresses the turbo. You have created a mismatch between what the turbine side is trying to flow vs what
the compressor side can flow. This will stress the CHRA and the turbo shaft.
- Exhaust restrictions
This is a more common approach to solving boost creep. By introducing some more back pressure in the exhause
you slow the overall exhaust gas flow out of the engine. This (hopefully) allows the wastegate to
control boost pressure again.
This can also be done simply. All it takes is to replace the downpipe/flexi gasket with one with a smaller
center bore. This can be made from simple sheel steel
The upside of this mod is that the turbo is not as stressed as with an inlet restrictor
The downside is that the restrictor has a negative effect throughout the rev range and will make the
engine slightly less efficient at all rpm
One additional upside is that often a hi flow CAT can be used to impose this exhaust restriction. This
might have the desirable side effect of making a non legal de-cated car legal again :)
- Combined Intake and exhaust restriction
This potentially has the benefits of both without too many side effects. A small exhaust restriction
will not have a great impact on the overall VE of the engine and a small inlet restriction will not
stress the turbo exccessivly. The combined effect might combine to produce controllable boost with few downsides
- External Wastegate
This is the absolute best solution but at a cost. External wastegates are always bigger than any
CT2X internal wastegate and should allow complete boost control of even the most outrageous hybrid
The problem is that it is difficult to fit an external wastegate to a stock exhaust manifold.
It should also have a beneficial effect on power as a significant amount more exhaust gas can bypass
the known restrictive CT2X turbine. This should have a beneficial impact on engine VE at high rpm - a