Boost problem - Troubleshooting

[arnebret]

Hi,

My boost is only idicating 1.5bar at the stock gauge. It used to indicate te normal 1.8... What could cause this:

- Wastegate shot: wastegate not function as it should do. Only allowing a limited amount of exhaust air going to the turbo -> too little boost
- Controller hose from cyclic valve: wastegate bad. Wastegate not opening and closing the way it should -> little boost, stuck wastegate
- KLR not working properly: the KLR is damaged and cannot control the boost properly.
- Clear hose from intake manifold->KLR bad, KLR gets wrong reading from intake manifold causing in strange boost settings
- Exhaust leak or turbo gone bad: cracked exhaust causes pressure-loss or the turbo blades may be damaged.
- TPS gone bad: KLR is getting wrong reading on how far the accelerator is pressed -> wrong boost level.
- Vacuum leak after turbo causing loss of pressure along the way.

I can't really think of anything else and the self-diagnosing unit is not indicating ANY blinks. It does work, because when i disconnect the TPS it gives the right blink-code.

- As far as i know the wastegate is fine.
- Controller hose seems OK, is pretty strong stuff and is well connected with hose clamps on both ends.
- KLR functions as it still devlops boost, so KLR is doing it's job.
- The clear hose to the KLR seems to be in a bit of a damaged shape. Maybe air is leaking and the KLR gets strange intake pressure readings.
- Exhaust has just been checked up and all gaskets are replaced by a mechanic. I checked myself, did a great job, absolutly no leaks. Checked turbo inlet-blades, fine. No damage that could cause low pressure.
- Had some trouble with TPS, causing in strange idle speeds. The bad one is still one, ordered new one.
- Checked ALL hose clamps, all seems fine, air leak seems to be absent.

So, TPS and maybe the clear KLR-hose. Will have a closer look at the little klr-hose and tps needs replacement and will get it asap.

Any other ideas???

I really want my boost, it's a turbo.. RIGHT?

[arnebret]

Hmm, i replaced TPS still getting about 1.6 indicated. Although, a couple of days ago i was in 4th gear under full boost (about 110mph) and the gauge was suddenly blasting all the way to almost 2 bar. This was under extreme load and the turbo was already howling away for a couple of seconds. I have no idea why it doesn't boost all the way up under normal conditions. It seems the acceleration of the car is fine.

[Lawrence Coppari]

Did you check the rubber hose pieces where boost tubes connect to intercooler? I have seen them crack and leak boost.

See if the valve that lets pressurized air back into compressor intake is ok. Should not be able to blow through it unless there is vacuum applied to nipple at top.

[arnebret]

I never really know how the bypass-valve in a 951 functions. But if i'm right the little hose will create a vacuum at the top nipple of the valve. This causes the valve to open and to let the compressed air that bounces back to lead back into the intake side of the turbo?

I also never understand why they didn't use an open-end valve that will sneeze air back into the engine compartment? For instance, all high-powered turbo cars around (think about the world rally cars) use the open ones. You can always hear it sneeze or squeek. Some 951 users did installed open valves, is there any advantage/disadvantage.

[arnebret]

Just checked my valve. Took it out and blew at the side opening of it (not the little nipple, and not the bottom one that goes into the intake again). Air was only coming out of the little nipple. When i closed the nipple with my finger air wasn't going anywhere. When i pressed the valve open from the bottom hole all air went through the bottom hole. Is this right?

I also checked the two connecting pieces and couldn't really find a hole or something. I did notice they are pretty old and show some cracks on the outside, but it doesn't seem this can let any air pass.

The strange thing is: i can get the max pressure, but so far only in 4th under full load...

[arnebret]

I'm very sorry guys, this is really becoming a turbo topic, but the 944T forum is just too empty.

I just looked some stuff up from the manual (see below).

... maybe the 1.52 bar is actually completly right and all is just fine.. How will i know my engine type? I always tought it was M44.51...

[Lawrence Coppari]

I think your valve has a broken diaphram. The little nipple shouldn't allow any air to move without moving the valve. The nipple opens the valve against a spring when it senses vacuum. So if the nipple is getting leakage from the positive pressure under boost, your whole control system is messed up because it is getting postive pressure when there should be vacuum. Put a piece of hose on the nipple and draw a vacuum on it. It should hold vacuum and open the valve and keep it open until you let loose the vacuum. It is not connected to anything; it's a dead end. It's sole purpose is to move the valve.

[arnebret]

Isn't it connected via a little plastic hose to the fuel damper or something?? My nipple (no.. ) is connected with some sort of little hose on the top.

[Lawrence Coppari]

Sure, a line goes from the little nipple over towards the fuel rail. There should be a 'Y' connector in the vicinity of the fuel rail that goes to the cycling valve (I think) and to the damper (or whatever that thing is) on the fuel rail. But the valve with the diaphram is and must be a dead end. It should not permit continuous flow of air. Because if it does, it is changing the pressure (amount of vacuum) in the other parts of the vacuum system by virtue of the 'Y'. This would mess up turbo control. If your diaphram leaks, you are pressurizing the vacuum line with compressed air due to boost. This messes up the control of the turbocharger.

A cracked 'Y' is another cause of boost control problems. The 'Y' hardens with age and cracks or breaks. The plastic line also gets very brittle with age so careful when you bend it.

[robm-951]

Isn't it connected via a little plastic hose to the fuel damper or something?? My nipple (no.. ) is connected with some sort of little hose on the top.

This is correct. The line that goes to the top of the CBV ties into the "Y" that also feeds the fuel dampner (on the front of fuel rail) this line then goes to the banjo bolt between the intake tubes for the 3rd and 4th cylinders. This line will provide a boost/vacuum signal to the CBV. Under enough vacuum (off throttle) the CBV opens and allows the the turbo outlet air to recycle to the inlet of the turbo to help keep it spooled up. When under boost, the the boost pressure in this line hold the CBV closed along with the internal spring pressure. Bottom line is that of you can blow through the little nipple on you CBV, then it's toast. These do fail regularly and are about 30~40 bucks for an original or you can upgrade to an aftermarket one for 140 here. These are rebuildable and should be more durable. If you get an original one, make sure it's the newer 993 part number that has the metal diaphram.

Later,
Rob

[arnebret]

Is the 'new' blow-off valve 99311033750 or is there even a newer stock version available?

[nize]

has this ever been resolved?