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Brain Transplant to fix Low Boost? Posted by TonyJ [Email]  (#407)  [Profile/Gallery]
(more from TonyJ) on Sun, 9 Nov 2003 19:41:48
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Re: Here's a thought...(Posted Oct23 by George)
George, thanks for your response, which described a case of low boost in a 97 being caused by an evap purge control valve problem. We have eliminated just about everything else (see following paragraph) but the brain and perhaps the evap purge valve. In your case, besides the OBDC readout, was there any engine warning light indication? The 95 does not have OBD, but an ISAT scan didn't reveal anything and there are no warning light indications. What was wrong with your Evap Purge Valve and how did you fix it? With everything off, our evap purge valve acts as a one-way check valve towards the intake manifold; that seems correct, but what other checks can we do? The Saab shop manual does not list this problem as a source of low boost.
Other Saab Gurus: Are we down to the brain?
My son's recently-acquired 95 Aero (93K) has low boost (mid yellow, wich is only slightly above base boost) and noticebly slower acceleration than our 94 Aero. Base boost is within tolerance, the wastegate and control rod work freely, there is >2 turns of preload, and the diaphram is intact. The plugs and air cleaner were changed, althought hey looked almost new. The car runs well on 93 octane premium and in fact gets excellent MPG (>500 miles/tank on the highway). Pulling the top hose off the BPC or energizing the boost-increase BPC coil create untrolled boost and lots of power well into the red until abrupt fuel cut off, so the wastegate is closing, the turbo is working, and there are no major leaks in the intercooler or plumbing. We swapped the BPC, hooter, and DI with the 94. In this state, the only difference is that the boost momentarily goes to mid red, then immediately drops back to mid yellow. There seems to be slightly more boost in 5th gear. Meanwhile, the 94 performs properly using the BPC, hooter, and DI from the 95. We verified that the reverse and brake signals are arriving properly at the brain. Vehicle speed is also getting there, as evidenced by the upshift indicator working. We unplugged the cruise control and measured about 9 volts on the signal that goes to the brain. We pulled it up to 12Volts via a 1K resistor to ensure that computer wasn't thinking the cruise was engaged. We checked out the IAC valve electrically and mechanically. We swapped the MAP sensor from the 94. The brain could not be swapped because the connectors have different indexing tabs. We measured the voltage at both BPC coils while driving the car. The duty cycle must be very small, because both sides dropped by only about 0.5 volts below battery voltage (as averaged in the DVM) as the transistors in the brain switched on, but both the increase and decrease boost voltages otherwise behaved as expected and neither drive transistor appears to be shorted or open. These experiments were interspersed with adaptation runs. Is there anything else other than the brain or the Evap Purge valve as in George's case that could be the culprit? Any advice would be greatly appreciated before we do a brain transplant, which is the next and last step in the Saab shop manual.
TonyJ 94 Aero, 93 CSET AndyJ 95 Aero, 92 S
posted by 24.154.21...
, Sun, 9 Nov 2003 19:41:48 <-- Viewing This Message, Sun, 9 Nov 2003 19:54:39
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