James_PowerMad

Hi Guys, Newbie here...

I've always been a tad power hungry when it comes to my cars, but I'm fairly sure that my MY98 is underpowered / not behaving as it should.

As an engineer, I take a keen interest in knowing how my car's turbo bits work, and have recently been wondering whether my boost pressure is anywhere near what it should be.

I have not yet found a pressure gauge to hook up to the system, so have been making other observations.

I first guessed that the signal to the valve that stops the wastegate opening (I think you call it the boost control valve) would tell me whether the boost was reaching its target, by being turned off by the ECU - hence allowing turbo pressure to open the WGate.

I observed that the ECU used a surprisingly fast PWM modulation to control this valve (considering it is a mechanical device), but when driven hard the level never drops very low (in terms of duty cycle). This made me suspect that the turbo may not be producing enough pressure to require limiting by the ECU.

I also observed that (when disconnected) the WG actuator required a surprising (to me) approx 1 bar of pressure to make it start to move (very large syringe, halving of volume). I would have expected it to actuate at a quite low pressure, and the excess then being allowed by the ECU opening the boost control valve for more of the time.

Anyways... Lots of speculation about PWM ratios etc... I really wanted to know whether the WG was opening much of the time (that would indicate that the boost was being controlled). I tried several arangements of wires and microswitches on springs, but in the end I just used a bit of v.thin wire and two bits of Blutack!

The WG actuator doesn't move a peep when driven hard (not for long enough to melt the Blutack luckily).

Is my turbo not producing enough boost? Is it likely to be screwed after only 50k miles (I cant vouch for prev owners, but I am very particular about warm up/down)?

Does anyone have any ideas?

James

PS. I guess I really need to take another Scoob for a test drive from a Dealer to compare the power (this is the only one I've ever driven in). Plus, must find a cheap pressure gauge!

john banks

Wastegate spring pressure in our cars is about 8 PSI. The PWM controlled solenoid acts to bleed off air from the wastegate diaphragm back into the intake, reducing wastegate creep before you target boost. However, it is rarely/never continually open AFAIK.

The peak boost is controlled by the size of the restrictor in the line off the turbo nipple before the T-piece on its way to the wastegate diaphragm. The held boost appears to be controlled by closed loop feedback using the MAP sensor and the ECU controlling the PWM to produce consistent held boost figures.

Suggest you get a boost gauge or run your car on a Select Monitor at the dealers. A lot of boost gauges seem to under-read by up to 1 PSI. Typical held values on a UK car are 13 PSI.

You could also try 30-70 on your speedo. Start in 2nd from about 25mph and time as you go past 31 then after a change to 3rd gear 73mph (to correct for speedo error). If you get around 5.5 seconds you are on the ball for a UK car.

If your wastegate was not actually opening you would at some point around 3000-4000rpm probably see around 25+ PSI of boost and get a sudden fuel cut where you will feel like your head is going through the windscreen.

James_PowerMad

John,

Thanks for the reply... Just a couple more Q's - please!

On my car, there is only (what looks like) a plain tube between the turbo output nipple and the plastic T, which splits off to the WG actuator, and the solenoid. Is the restrictor part of the T, or do I have a bit missing? :-)

Select Monitor... I guess you mean that it can read back what the ECU knows about the recent boost behaviour of the engine?? If so, is it possible (or likely) for the car's own pressure sensor to drift out of cal etc without the ECU knowing?

Lastly, the possible reason for the WG never opening (that I initially had in my mind) was for some fault in the system (tired turbo / pressure leak) causing an inability for the system to produce enough pressure. Hence producing less than the 8 PSI that you mention to actuate the WG (or some higher pressure that is still too little - given the behaviour of the solenoid). Is this scenario possible / probable in anyone's experience?

Thanks

James

john banks

Restrictor is near the T-piece in that first curve tube off the turbo nipple. Feel along it with your fingers after unplugging from the T-piece.

Select Monitor will only tell you error codes or current boost behaviour.

Last thought seems possible, but time that 30-70 run - it would be noticeably down if you were running 8 PSI. If you had no restrictor it would run about 8 PSI. Or a damaged solenoid valve might do it.

A Scooby should give you a pretty good punch in the back

I would buy a boost gauge - they are about £30 for a cheapo one which may read out (usually under) by 1PSI but gives you an idea.

[Edited by john banks - 12/31/2001 2:59:46 PM]