Boost pressure varying?

[GT4-Addict]

Hey guys, im just new here. I own a ST185 GT-four celica (1990) and its just started having a weird problem. i drive fine for a couple of hours on the open road, or maybe an hour round the town, and suddenly i go from boostin 7Psi (im just running stock boost) down to about 4 or 5 psi.

It also happens if i drive the car, it warms up to normal operating temperature, i stop, turn it off, go do something for 10 min, and then come back and start it up and drive off.

I think it is heatsoak personally, whether the intercooler is buggered or something similar could that be the problem?

I have also just noticed a tiny amount of oil around the top of the turbo, where the plastic/flexyish connector thing joins it to the intercooler, could this mean its blown a seal? and could this contribute to my original problem?

I came here because i thought it would be best to get advice from people who live, breath and dream toyota, so any help would be Much Much appreciated!!

[AJz]

First thing that comes to mind. Has it got an aftermarket boost controller ?

Possibly boost leak ? I had a feeling they ran a little higher that 7 once warmed up but its been a while since i had mine.

They do suffer from a bad case of heatsoak

Something that could be causing it is a dodgy vsv. It keeps you from raping it when its cold and has the boost at about 5psi. Once its warmed up it will then raise the boost to about 8 - 10 i think. If this was bung it would explain your problem.

You can bypass it and this would tell you if it is your vsv.

If you understand this picture below, i would have a go at this, and see if it fixes your problem.

Baring in mind you will need a joiner or new hose to change it back to stock. The car will run at the higher boost, even at start up.

[GT4-Addict]

There isnt any boost controller, it's pretty much stock under the hood.

Is there anyway to test for a boost leak?

I'm going to try disabling the vsv, I think it will be that or heatsoak. I forgot to mention that if when this happens I drive really easy for 10min normal boost comes back for a while.

If it IS heatsoak, what could I do to try and fix it?u

[sergei]

VSV can only raise the boost. If you disable the VSV it will permanently lower the boost.
With boost lower than waste gate spring pressure, there are only few faults cause this.
Boost leak (loose clamps on intercooler for example, stuck BOV if you have one, etc). (loud hissing/turbo noise).
Faulty waste gate actuator (burned through valve, or broken diaphragm)
Semi-seized turbo.
Substantial exhaust leak. (sounds like tractor)
Engine does not produce enough gas flow to spin the turbo (collapsed catalytic converter, or something completely catastrophic). you would notice this as the car would not perform at all.

[GT4-Addict]

The thing that confuses me is that it runs fine most of the time. If it was a boost leak etc wouldn't it always only run 4-5 psi?

[barryogen]

sounds like something is warming up and coming loose... as in, something that is connected fine when cold, expands when warmed, and opens something..

rubber hose somewhere that holds boost?

[GT4-Addict]

i never even thought of that. might be time to go check and if needed replace any pipes and hoseclamps i can find

[AJz]

yea i was under the impression it had an aftermarket controller sorry.

as sergei said. listen in the engine bay for hiss and check all the hoses you can see

[cedwards]

exhaust manifold warp/leak?? varies with temperature loosing exhaust pressure when hot? Should hear tractor type noise when problem is present. also may see soot around leak (if not concealed by heat shield)

[GT4-Addict]

It just sounds normal. I have just repaired a hole in the flex joint of the exhaust and it sounds quiet again. To me it's looking more and more like heat soak

[bbq1988]

im not a mechanic but when it gets heat soaked wont it sill boost 7-8psi?
Its just heating the air up, still the same pressure?

I would look for leaking joiners or a leaking I/C
Yay for Heat expansion!

[AJz]

has it got a bov ?

[GT4-Addict]

Yup its got a racepro big blaster i think