Problems after replacing master cylinder
in [Technical]
|
From: jim beam on 14 Apr 2010 00:29 On 04/13/2010 08:27 PM, Paul Hovnanian P.E. wrote: > Tegger wrote: >> >> "Paul Hovnanian P.E."<Paul(a)Hovnanian.com> wrote in >> news:4BC3EEAA.15B3869B(a)Hovnanian.com: >> >>> Air leak (intermittent) between the booster and intake manifold. You >>> lose vaccum and the brakes need a lot more pedal pressure. When the leak >>> occurs, the engine runs rough and shakes. >>> >> >> If it were an air leak, the idle would be elevated. > > Not if the ECU compensates for the leak. latitude to /decrease/ idle air is limited, and of this type, insufficient. once there is a leak, the ecu increases fuel in ratio to the air supply to maintain stoichiometry. > >> Rough-idle and shaking were characteristics of non-feedback systems that >> were unaware of loss of manifold vacuum. OP's Lexus has a feedback system. > > But a feedback system would compensate for the average error. If the > vacuum for the brake system is tapped off a part of te manifold nearer > one cylinder, it will throw that one's mixture off. and increase fuel delivery... > -- nomina rutrum rutrum
From: Tegger on 14 Apr 2010 08:30 "Paul Hovnanian P.E." <Paul(a)Hovnanian.com> wrote in news:4BC53607.C59CFD72(a)Hovnanian.com: > Tegger wrote: >> >> "Paul Hovnanian P.E." <Paul(a)Hovnanian.com> wrote in >> news:4BC3EEAA.15B3869B(a)Hovnanian.com: >> >> > Air leak (intermittent) between the booster and intake manifold. >> > You lose vaccum and the brakes need a lot more pedal pressure. When >> > the leak occurs, the engine runs rough and shakes. >> > >> >> If it were an air leak, the idle would be elevated. > > Not if the ECU compensates for the leak. The ECM/PCM compensates by correcting the mixture AND the idle speed, or it can compensate by correcting the mixture and ignoring the idle speed. OP's Lexus probably does the former. Both our cars do the latter. In any case, a feedback-system the ECM/PCM would compensate for an air leak by shortening injector pulse width until fuel mixture and/or idle speed were correct again. There would be no rough-running or shaking. > >> Rough-idle and shaking were characteristics of non-feedback systems >> that were unaware of loss of manifold vacuum. OP's Lexus has a >> feedback system. > > But a feedback system would compensate for the average error. If the > vacuum for the brake system is tapped off a part of te manifold nearer > one cylinder, it will throw that one's mixture off. > Possibly. But considering that vehicles with MAP sensors typically have only one sensor for the entire intake manifold, I'd think pressure changes at one cylinder would have minimal effect overall; certainly not enough to induce roughness and shaking. -- Tegger
From: jim on 14 Apr 2010 09:01 Tegger wrote: > The ECM/PCM compensates by correcting the mixture AND the idle speed, or it > can compensate by correcting the mixture and ignoring the idle speed. OP's > Lexus probably does the former. Both our cars do the latter. > > In any case, a feedback-system the ECM/PCM would compensate for an air leak > by shortening injector pulse width until fuel mixture and/or idle speed > were correct again. There would be no rough-running or shaking. The point was that a vacuum leak can cause a misfire if it affects one cylinder more than the rest and a regular misfire isn't going to make an engine idle smoothly. However, that doesn't have much to with the OP's stated problem. He said: "The car starts vibrating after driving for about 10 minutes on the highway. The vibration doesn't start right away." He also made it clear that the vibration is felt in brake pedal and steering wheel. and then he said: "After about 15 minutes of driving, the vibration goes away and doesn't come back during the same ride." If I read that correctly the vibration occurs between 10-15 minutes of first getting up to highway speeds and at no other time. So no point arguing about what might happen when the engine is idling. He also said: "The brake pedal sometimes works just by a little pressing, and other times I need to press it all the way down." If the booster isn't getting vacuum then the opposite would happen. The pedal would not go down as far as normal and the braking power would be reduced. -jim > > > > >> Rough-idle and shaking were characteristics of non-feedback systems > >> that were unaware of loss of manifold vacuum. OP's Lexus has a > >> feedback system. > > > > But a feedback system would compensate for the average error. If the > > vacuum for the brake system is tapped off a part of te manifold nearer > > one cylinder, it will throw that one's mixture off. > > > > Possibly. But considering that vehicles with MAP sensors typically have > only one sensor for the entire intake manifold, I'd think pressure changes > at one cylinder would have minimal effect overall; certainly not enough to > induce roughness and shaking. > > -- > Tegger
From: Tegger on 14 Apr 2010 15:34 Tegger <invalid(a)invalid.inv> wrote in news:Xns9D5A56829A77Ftegger(a)208.90.168.18: > > In any case, a feedback-system the ECM/PCM would compensate for an air > leak by shortening injector pulse width until fuel mixture and/or idle > speed were correct again. Sorry, should have read "LENGTHEN pulse width" (to compensate for the excess air). -- Tegger
From: Mark on 23 Apr 2010 11:52
Are you folks suggesting that the P0446 CEL and the problems I described earlier with the vibrating and brake master cylinder are related? Sorry, some of this is way too technical for me. Thanks ! |