car will not boost

i have a 91 st185 with the third gen swap. i have had it for a couple months now and it ran fine just up until the other day. it will not build boost. it has vac. -20 and builds up to 0 and stops. an ideas? car runs fine other than that

Sounds like your turbo I'm sorry to say, most likely the ceramic turbine wheel is no more.

If it can build up to 0 I'd say it has a massive boost or exhaust leak. If the turbo wheel was gone I doubt it would get to 0.

underscore":2bjdffv0 said:

If it can build up to 0 I'd say it has a massive boost or exhaust leak. If the turbo wheel was gone I doubt it would get to 0.

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+1 to that. Blown wheels hardly get close to 0. I'd check the piping, couplers and vacuum lines. Run a boost leak test, it's cheap and very easy to do. Does the turbo burn oil? Do you see oil in the intake? Smoke out of the exhaust? Also check the exhaust manifold. Do you hear a high pitch whistle? It could be a crack in the exhaust manifold or a blown gasket.

it has been lowing a little bit of smoke out but only for a little while then it would go away..i will check vac lines and all that tomorrow and let you know. could it possibly be a stuck wastegate?

jmoz519":3o2o1bq7 said:

could it possibly be a stuck wastegate?

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Just like no turbine wheel, an open wastegate will not let you get past 0psi
The good money is on a massive boost leak right in front of the turbo,
I cant think of anything else that could stop u going past 0psi as a leak at the throttle body/intercooler side (which is the next largest [volume] air leak spot) would still allow you to achieve upto 3psi as you reach the upper limits of the rev band, and the hissing sound would be a dead giveaway.
It may be as simple as the coupling hose has gotten old, stiff and brittle so even though you tighten the clamp it wont fully seal.
Get some new coupling hoses if yours are in bad condition, if they are soft, flexible and without cracks and cuts then i would just recommend you remove your intercooler, check there isn't a flood of oil inside the turbo compressor while the intercooler is off, then just put it back on.

The act of removing the intercooler is beneficial in 2 ways,
First it allows you to remove, clean and inspect the rubber coupling hoses for degradation,
Second it will allow you to put the hoses back on with a confirmed seating so you know 100% that its not your problem anymore.
Just remember to tighten everything with a socket set and not a screw driver, most worm gear clamps have the Phillips head screw as an option but you can only apply limited torque before you start to strip the head.

When the throttle is wide open it would be close to 0psi anyway wouldn't it? I know if I stamp on the throttle at low rpm, no boost the boost gauge comes out of vacuum..

You would still build boost if the wastegate was stuck open. The wastegate simply doesn't flow enough.

Edit: I'll just add, that it can't be the ECU as a turbo is a mechanical thing and if the car is revving through to red line then there is nothing the ECU can do to prevent it from building boost - the exhaust flow has to do SOMETHING.

I would expect a car that is leaking at least 17psi of boost pressure (remember, no boost pressure = no wastegate) to atmosphere would be making one hell of a racket. Hence why I suggested the turbine wheel. If that was gone, no boost, no noise, and car would drive normal except for no boost.

We checked it out a little today. The waste gate rod. Moves freely. There was a small amount of pil in the intake pipe from the wta into the turbo. The compressor wheel looks a little dinged up but has no shaft play and spins freely. We started to tighten the manifold to block bolts because they were loose.

When you say the wastegate rod moves freely is that the good type of freely where you apply a small amount of compressed air and it works as it should or the bad type of freely were the wastegate actuator is dead and just the idle exhaust pressure opens the wastegate flap.

One of the turbo to manifold bolts was missing the nut. Taken everything off and tightening it up. Hopefully fixes the problem

Even if it was leaking that wouldn't cause it to not boost. You'd notice a lot of carbon around it too if it was leaking. You should take the dump pipe off and look at the turbine.

I'm interested if you have sorted the issue yet?

Thought I should leave this here in case the OP comes back to check on it:

viewtopic.php?t=46361

My guess is the shaft that connects the two wheels together has snapped. The would both spin but independent of each other.

Like people have said...its probably your turbo. Check it over for shaft play on both ends.

i had kinda the same issue with my st185 mine was the turbo ... but was caused by the catalitic converter being cloged
.. any car will make 0 vac when throttle plate is open
if you dont make any pressure at all
i am 100% that its your turbo

example

if it was boost leak.. you would still make atleast 2psi unless turbo was compleatly disconnected
exhaust leak.. same as above a small lean in the exhaust can lower your back pressure but exhaust still
has to go some ware throu the turbo in that case it will create some kind of boost

some one said broken shaft <<< has happend to me so i would check shaft play again pulling in and out

heres a simple test if you havent got it fixed yet

start the car look into turbo see if its spinning...