[swiftpete]

I've just had the turbo studs replaced as they were too worn to tighten the manifold before. Manifold is now tight against the turbo and it doesnt sound like a tank anymore. The only problem is now sometimes when i hit full boost the needle goes all the way into the red then the engine sort of cuts out and lurches before running ok again. Sometimes the boost needle goes into the edge of the red and stays there and when that happens the car goes fine, but i never know when its going to overboost then lurch. it happened a couple of times before i had the turbo studs replaced, but its a lot worse now.

I have a little idea it could be the wastegate not opening sometimes, leading to it overboosting, does that sound right to you lot?
If so how do i go about sorting it. And if its not that...well anyway, any ideas would be appreciated.

thanks

 

[swiftpete]

just took it out for another run, i had decided to take the ecu fuse out as had read on another forum that it can adjust iteslf and the max boost etc. Took it out for 15 mins took it for a fastish drive and its loads worse now, cant really accelerate without it kangarooing from the fuel being cut off as it gets to full boost. Really annoying..any ideas?

 

[Last_straw]

Vacuum leak from a hose ?, dodgy DI casette, I read if the DI is one the way out it can give false readings which confuse the ECU, see if you borrow a known good one to test if that is the problem, I could easliy be wrong the moderators on here are the ones with the real knowledge

 

[Marrk]

Sounds like the vacuum pipe to the actuator is leaking or the apc valve is failing. Was the actuator disturbed when the studs were done? Base boost might need to be reset. Check out BillJ's site for instructions.

http://www.saab9000.com/procedures/exhaust/baseboostadj.html

Oh, it won't be the DI unit causing overboost though...

 

[Superaero]

You are overboosting and the pressure limit switch is cutting out the fuel pump. This is a safety device to prevent you from blowing up the engine if the turbo control system doesn't function correctly.


Most likely the "vacuum" hose (actually a pressure line) from the BPC on the radiator frame to the wastegate is cracked or loose. Otherwise the BPC itself is a likely culprit.

The way Trionic (and APC before Trionic) controls boost is very elegant. The waste gate is set at a low level of boost called base boost. If the BPC valve isn't working at all so that the wastegate receives full boost pressure it will open and dump boost well below the red zone on the boost gauge. The way boost rises to such high levesl is the trionic ecu tells the BPC to bleed boost pressure between the high pressure side (turbo output) and the turbo intake so that the wastegate control line only receives low pressure. When the ecu senses detonation (knock) in a cylinder it signals the BPC to close the bled line thus exposing the wastegate line to more pressure which opens the wastgate. When the knocking stops the bleed line opens and the wastegate closes. This happens continuously and rapidly for smooth knock free boost.

Your engine is not functioning this way because the wastegate isn't opening or the bleed line is always open.

 

[swiftpete]

Well now the apc valve wiring plug has been disconnected and the car is boosting fine. You did say that it would dump boost before it reached max, but the gauge is showing it going to the edge of the red and it certainly feels like its boosting well. DOn't know how this is happening unless maybe the base boost is set too high or the valve stuck half open maybe? but it has somehow worked out that the level of boost is the right amount without the apc involved.

So do i need a new APC valve then? From the sounds of things that would help to get a bit more boost from the system, how much further into the red is it supposed to boost though when working correctly?

Is it a problem to run the car without the APC involved at all if its boosting properly?

Sorry for my constant stream of beginners questions but im new to saabs, trying to find out though!

 

[swiftpete]

Hoses are all intact by the way..

I've buggered it up..tried to clean out the apc valve, so squirted some wd40 in there. bit of gunk came out, i reconnected everything and the car was still over boosting. disconnected the electric supply to the apc and now the car only boosts to just over half way on the gauge. is that normal? i know it was supposed to do that before but with the electrics disconnected it was going to the edge of the red so i was happy, should've left it like that. dont know why i tried to fix the apc but its now slower than before. what a bummer!

I've had advice that the apc valve is not the culprit but the base boost on the turbo is too high and it should be turned down. what will that do to performance, or will the base boost make no difference to the max boost?
I don't know if its in fact the apc thats faulty.
I'm getting a bit of a headache with it really.
how can i find out which is causing me a problem?
Thanks in advance to anyone that wants to help!

 

[Superaero]

Yup, your BPC is stuck. No way you can get more than base boost with the BPC plug disconnected unless a: the BPC is stuck open (i e the boost pressure is being leaked away regardless of what the Trionis computer wants) or b: your base boost is way way way too high which the Trionic system would still try to modulate f the BPC wasn't scewed. Only sure way to check is to try a new BPC


Base boost can be checked by a dealer and should be something like .6 bar I think but don't quote me on that. The base boost cannot be checked with the BPC screwed unless the hoses are re-connected by-passing the BPC.

 

[swiftpete]

Ok, thanks for your reply, i'm not sure that the base boost is too high but i would have to get it checked properly. It is boosting to about half way down the gauge with the apc wiring disconnected and is certainly down on power, so if it is too high then i don't think its massively over. I thought the apc controlled the max boost anyhow, if the base boost was slightly too high and the apc was working ok my understanding was that it wouldn't be able to overboost.

Is that correct?
I'll be looking for a new apc unit anyway now so thanks for that. Didnt want to order one before unless i was sure i needed one, but base boost set high or not i realise that i need new one of those regardless.
I'm starting to get the hang of how it all works now but still not an expert yet!

 

[Superaero]

Base boost would be reached in the yellow zone, precise point varies from car to car as the boost gauge on the dash is not calbrated. that's one reason it has no digits to indicate boost level. the dashboard gauge is just to inform you about relative boost pressures.


The APC "tricks" the wastegate into "thinking" the actual boost is no more than base boost. Base boost is set by the spring in the waste gate and adjusted purely mechanically by turning the threaded rod actuating the wastegate. Once set the wastegate opens at that pressure.

If the APC is disconnected electrically and the air passages are "open" in the BPC/APC solenoid then the actual boost pressure reaches the wastegate and so the engine cannot develop more than base boost. If the BPC/APC solenoid is damaged or stuck partly closed then base boost may not be reaching the wastgate and so it stays closed and allows more boost. The BPC/APC solenoid bleeds boost pressure that would otherwise push down the wastegate hose back to the turbo intake until the APC or Trionic computer detects knocking in three consecutive ignition events and then it opens dumping full boost pressure into the wastgate line which then opens the wastegate.

The boost gauge reads intake pressure the wastegate receives only the amount of boost pressure permitted by the BPC/APC.

 

[swiftpete]

I'm in the process of ordering a replacement apc, WD40 seems to have sorted out the current one now though, it isn't sticking and is allowing my car to boost correctly at last! But a replacement is probably not a bad idea anyway before it goes wrong again.

Thanks for all your help, I do have more of an understanding of how it all works now.

 

[Basre]

Welll... that's a sorta right explanation.. but if it pleases you
Wastegate controls exhaust gas backpressure feeding the hot section of the Tutbo unit... (it has been documented at up to 46 PSI .. if that's of any relevance) the Actuator when fed Boost, fights the Actuator spring which spends it's time keeping the wastegate firmly shut.. Bleeding off the Exhaust gas pressure powering the Exhaust Turbine.. which powers the Compressor, is how the System controls inlet manifold pressures (boost levels)... Slow acting at best it can cause significant overshoot.. witness the all too familiar spikes in the boost levels.. even when the Bits are working at 100%.. in your case apparently the bit(s) are not working as designed. Solenoid Valve on the later Saabs is a known/proven culprit.. failing surprisingly often.. Pricey too, to add insult to injury. Not a lot of choice in yr repair avenues though.

 

[Superaero]

Huh?


Exhaust back pressure is caused by the turbine, not the other way around. The turbine is driven by the exhaust gas leaving the manifold, the turbine obstructs this flow which is what causes the turbine to turn and turn the compressor on the common shaft.

The wastegate has a preloaded spring on the valve that opens to allow exhaust gas to bypass the turbine, preventin git from developing more boost. This happens when base boost is achieved, except on the SAAB system which is modulated by the wastegate controller. The wastegate is controlled by a "vacuum" hose that receives the boost pressure from the compressor outlet, minus the pressure that the APC/BPC solenoid bleeds back to the intake side. This fools the wastegate into remaining closed even though boost pressure is higher than base boost.