I was wondering if boost gauges were built with a reduced orifice to dampen intake pulses. I'm monitoring boost with a regular gauge for diagnostic purposes and the needle is all over the place, sometimes +/- 5PSI. Is this just normal intake pulses, or is the bypass valve on the intake side of the turbo opening prematurely?

Thanks, Mark

hmm...how have you got this guage connected..??

the standard way to connect a boost guage is to connect it to the inlet manafold.

when connected to the main manaofld you should get a STEADY reading (smootly increaing, smoothly decreasing, steady whatever) not pulsed (definately NOT)..

I very much doubt you have what you call a bypass valve o nteh inlet side of the turbo, normally you have a waste gate on the exhaust side of the turbo which opens progresively at a preset presure (ish.manual gates start opening over a range .. of course if you have dawes devices etc connected this can alter this behaviour)..

you will have some of blow off valve on the inlet side designed to stop the compressor stage stalling, but again this is normally presure controlled and should recirculate the air - unless you have fitted a true blow off, which is not exactly a great idea on a volvo with fuel injection...

so in short you shouldnt see pulses under normal situations, and it would scare the hell out of me if i did! Obviously you will swing from vacume to boost under accelaeration engine loading etc, but it should be a smooooooth transistion.

Of course IF you had the take off well into one branch of the manafold i suppose you might see presure fluctuations in that branch with the opening and closing of the inlet valves for that branch...

Hi,

Thanks for your response. I installed a tee in the large (approx. 3/8") vacuum line at the drivers side end of the intake manifold. The car is a '99 with the ME7 engine management system. What I call the bypass valve you call a blow off valve, but in any case it redirects compressed air directly back into the compressor inlet. I'm concerned that the valve is fluttering or in some other way misbehaving and opening when it's not supposed to.
I'm probably going to invest in a hand vacuum pump and see if the diaphraghm is intact. I hope it's OK, because it's nearly impossible to remove without taking the turbocharger out of the car. I'll do a compression test as well, just because, but I'd be surprised to find a problem at this stage of the game.

Thanks,

Mark