I need some advice with 2 unrelated turbo problems on a c900

[matt]

Everyone has something to contribute and I have learned some interesting things from unlikely sources, keep your mind open, use your head and party on!

That pretty much sums it up. We're glad to have you here with us, different viewpoints and experiences are certainly a welcome addition to our little community.

[happyandy]

My first 16V turbo I got had a failed apc like that. I was used to 8V's so before I figured it out I just thought, damn! these 16V's really cook compared to the 8V's! Then the headgasket blew, fun over.

Luke, was this a stock or modded apc? and was the overboost fuel cut off bypassed?
I belive the fuel cut off switch cut off pressure point can be adjusted to a higher pressure setting, and even if it can't, I do know where to get an adjustable pressure switch that probably would work.
In my case I am not looking for sky high boost pressure. I think twenty psi would be more than enough.
I may be a newbie, but I do know that increasing turbo pressure past some point becomes ineffecient, invites cavitation, creates high heat and detonation; and its all down hill fast from there.

[DeLorean]

Well... It should be noted that there are literally hundreds of cars that have moded APC's that have not had such problems. I suspect the issue with failed APC's was probably some specific mod people were / are doing that made it not function correctly from day 1 of the modification. The mod posted on James fox's site for example when you perform the simulation actually takes the knock sensor out of the loop entirely. No matter what signal was being sent from the knock sensor, that APC will NOT reduce boost. Clearly that APC is failed and the engine would be too after a short while of running at 20+PSI on a hot day with a stock fuel system.

[Raul]

yeah the APC's aren't the issue, usually its a problem with people running way too much boost on stock, or near stock fuel injectors. Theres basically no way you can safely make over 225whp or so on the stock fuel injection, anything past there is pretty much a crapshoot as far as reliability is concerned.

Paul

You are almost right. People run too much boost on piss fuel 91 octane or lower and expect much regardless of intercooler and injectors, 91 octane with 15% of ethanol that you find in USA pumps is actually worse for power than %100 91 octane fuel. Saab factory manual calls for 91 octane minimum requirement on SPG models and they warn you not to tow anything.

[TRAILINGTHROTTLE]

91 octane with 15% ethonol! 15%ethonol?!?!, i bet that stuff will hold a lot of water per volume! you need to move somewhere where America still exists. around here i can get Sunoco 260 at the pump or Turbo Blue, come to think of it...

[99Super]

Sean,
I gotta say, blaming the failure on the APC solenoid sounds fishy to me. These systems are designed w/ a "failsafe" so that the only way for the solenoid to NOT be bleeding air back to the intake (leaving the wastegate at base boost) is for the APC to be sending power to the solenoid. Thats why the clicking stops when brakes are applied. The APC automatically goes to the base boost setting by cutting power to the solenoid and this will happen any time the APC system fails.
I'm really curious what mod in the computer would cause it to fail by sending a 12v signal to the solenoid. Never even heard dof such a thing.
Also, if you are modding the unit so much it fails in this mode, why use it at all??

john

[matt]

When things get changed around by guys who didn't build them, sometimes things go awry.

[Sean Tennis]

Sean,
I gotta say, blaming the failure on the APC solenoid sounds fishy to me. These systems are designed w/ a "failsafe" so that the only way for the solenoid to NOT be bleeding air back to the intake (leaving the wastegate at base boost) is for the APC to be sending power to the solenoid. Thats why the clicking stops when brakes are applied. The APC automatically goes to the base boost setting by cutting power to the solenoid and this will happen any time the APC system fails.
I'm really curious what mod in the computer would cause it to fail by sending a 12v signal to the solenoid. Never even heard dof such a thing.
Also, if you are modding the unit so much it fails in this mode, why use it at all??

john

Please reread my post, the solenoid doesn't fail the control unit for APC does, and it fails in the on mode not the off mode-same as you said, not in the fail safe mode.

Clicking stops when the brakes are applied?-if the APC solenoid is clicking when the car is idling there is a problem.... and yes the APC solenoid is suppose to click when the brakes are applied, unless this feature has been by passed.

Again I've only seen this type of failure in early APC units, not reprogrammed 9K- and newer.

Cheers

[happyandy]

Here is a follow up. the chattering apc solenoid is cured! I replaced the connector in the wire harness that connects to the solenoid due to a poor connection, chattering reduced, but not gone. swaped the apc for one from annother 900 that I used to have, no change. swaped in another boost pressure transducer and all chatter was gone and boost pressure was up and consistant! The bad news is that head gasket is now about to give up frothing and pressure in the coolant resivior before the engine is anywhere near warmed up Oh well, a cyl head swap was in the plans anyway