boost questions

[s_morrison57]

Hi
I'm a new P car owner , picked up my 87 930 last Sept. and with work and winter have only got to put about 500 miles on it. I've never owned any car with a turbo and have nothing to compare to mine but it seemed like the boost gauge would go to .8 and then I heard/felt a muffled popping from the exhaust as the car was still accelerating from about 5500 RPM, I assumed this to be the pop off/waste gate working , next time out the boost gauge would hardly move under the same type of conditions, get to .4-.5 BAR and I would shift , have been using 62-6300 as my red line, that was in the beginning, now the car has none of the muffled popping and the boost gauge pins itself way past .8 BAR and I'm getting close to .8 @ 5000 RPM.
Is this a bad boost gauge? Am I misinterpreting the muffled popping sound, my concern is over boosting the engine, is it possible for the pop off to fail and stay closed? I did notice that I could back off on the throttle and keep the boast @ .8 while still accelerating, do you guys accelerate through the waste gate popping or what?? The car had a turbo replacement @41000 miles and has 60K on it now, no details as to turbo size etc... when it was changed. What does your car do under under hard acceleration, Thanks for the feedback

[Benny771]

Sorta new myself, I'm sure more will help you but in the mean tI'me.....

I had a issue with my waste-gate diaphragm. It was torn from age/use so the boost was erratic and MAX at tI'mes.
There is a over-boost valve that releases around 1.2 that stops the engine from destruction (if the previous owner has not unplugged it).


Good-luck.

[mark houghton]

It sounds as if one minute you're overboosting, the next right on, and the next underboosting. Normal operation would have a stock 930 (assuming yours still has the stock 3LDZ turbo, intercooler, and exhaust) reaching full .8bar boost at around 5000 rpm or maybe a little before....at which point the wastegate will open and you'll get a roar of unmuffled exhaust shooting out the wastgate exit pipe. The sound is not to be confused with a "popping" noise, nor will you feel any hesitation or rough running associated with it. The only thing you should feel is your cheeks being plastered against your ears as the G forces increase.

Overboost could be a bad diaphram in the WG, not allowing the valve to open completely. Low boost could be a sticking WG valve that isn't fully closed and wasting boost when it should be trying to build it, or it could be an air leak in the feed pipe or intercooler O rings, allowing that boost to escape rather than making it to the intake manifold. Low boost associated with an air leak will also result in the engine stumbling when trying to reach wide open throttle higher rpms.

The stock boost gauges are not all that accurate but are generally close enough when operating within the stock engine setup. It's when you're trying to higher boost levels that a more accurate gauge should be used when operating on the ragged edge of things.

The WG can be checked by pressurizing it to .8bar while the engine is running (pressurize through the hose connected to the intercooler - don't forget to plug the nipple on the intercooler). You should hear it open with exhaust exiting the pipe.

[Miguel Antonett]

I would check - firast - the WG connection to the intercooler to be sure the WG is opening.

If the connection is OK w/o leaks, there is a simple test: put a piece of waded paper towel inside the WG exhaust pipe - sort of loose - and have someone keeping an eye on the back of the car watching for the paper towel to be thrown out when the WG opens. It should happened when the gage starts showing boost. If it does fly away at least you knoe the WG works.

[mark houghton]

Originally Posted by Miguel Antonett

I would check - firast - the WG connection to the intercooler to be sure the WG is opening.

If the connection is OK w/o leaks, there is a simple test: put a piece of waded paper towel inside the WG exhaust pipe - sort of loose - and have someone keeping an eye on the back of the car watching for the paper towel to be thrown out when the WG opens. It should happened when the gage starts showing boost. If it does fly away at least you knoe the WG works.

I was tempted to mention the old "potato in the pipe trick", but a wad of paper should work as well!!!

[p930t]

Regarding the original boost gauge, it is not that accurate and may be showing the wrong value.
It works by measuring resistance from the pressure transmitter mounted on the plenum chamber. (easy access)
The reference values from the factory are:
Bar 0.00 0.25 0.50 0.75 1.00
Ohm 10 45 75 104 132

I put the values into a spreadsheet to convert to a more detailed scale:
Bar 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
Ohm 10 24 38 51 63 75 87 98 110 121 132

Then I purchased a cheap variable resistor 0-200ohm 2%deviation.
Soldered two wires to it . Connected one to ground and one to the connector that i disconnected from the pressure transmitter in the engine bay.
While sitting inside the car i read the pressure shown on the gauge with ign.on. Starting on 0.00Bar. Then increase the variable resistance to read the next 0.1bar level. For each 0.1 bar i turned off the ign. and measured the Ohm on the variable resistor with a multimeter (The ign must be OFF).

This takes 20 min. if you do it 3 times , ( from 0.00 to 1.0 bar down again to 0.00 and upp to 1.0 ) insert into a spreadsheet and you will see if the gauge is in acordance to the specification. If not it may be the transmitter.

Regarding your overboost problem. I wold take off the waste gate and take it apart. Easy job. If the diaphragm is old and/or ruptured... buy a new one, and a new spring as well. The spring will loose its tension over the years.

Happy hunting

[Miguel Antonett]

If you decide to attack the WG dissasemblig, as mentioned above, BE CAREFUll while taking it apart; the spring load might create a violent snap when unbolting it...

[HelmetHead]

Originally Posted by Miguel Antonett

If you decide to attack the WG dissasemblig, as mentioned above, BE CAREFUll while taking it apart; the spring load might create a violent snap when unbolting it...

LOL.... I must have had my webcam on that day.

I highly suggest putting a C-Clamp on it before unbolting it!!!

[beepbeep]

Be careful of "pegged boost gauge". That means that wastegate isn't doing what it should and that overboost protection switch is disconnected.

Next thing you might hear is sound of cash register at parts store, while looking for new pistons.