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rpm drops when braking[200/1986] posted by Maarten on
Monday, 31 August 1998, at 2:00 a.m.
not very often, but every now and then the engine's RPM drops to around 400RPM when braking. It doesn't matter if I brake either hard or soft. When I release the brake the enigine sort of recovers and runs at 900RPM.
Anyone who knows what's causing the problem?
--Maarten
Re: rpm drops when braking[200/1986] posted by Paul S. on
Monday, 31 August 1998, at 10:46 a.m.
How's is your power brake booster?
How is the little white filter/inlet from the booster to the hose that goes to the intake manifold?
They could be suspect, when you hit the brake extra (unmetered) air could be entering the intake through the power brake booster.....
Or it it could be electrical...as weird as it sounds. Hitting the brake could cause something (wires) to be forced into a space already occupied by something else....don't laugh it happens....even to Volvo's!!
--Paul S. 1985 245T and 1996 855O near Seattle........Mr. Smooth...
Re: rpm drops when braking[200/1986] posted by Tom on
Monday, 31 August 1998, at 11:55 a.m.
I'm joining the club on this one.
Same problem. Only in my case, occassionally, the engine will
stall out when at 400 RPM.
Pain in the butt. Have to brake with my left foot and keep
the right one on the gass...
Re: rpm drops when braking[200/1986] posted by Don Foster on
Monday, 31 August 1998, at 2:36 p.m.
Hmmmmmmm...............
Does it do this in the driveway, not moving, at idle?
Is this definitely related to applying brakes, or could it be removing foot from gas?
When you step on the brake, it "consumes" vacuum, which means a little air flows from the booster to the intake manifold. This is sorta like a little vacuum leak. My experience with vacuum leaks is that they usually increase the RPM a little. And in fact, when I vigorously pump the brakes on my various 240s, I notice the RPM rises a bit and becomes rough.
Maarten's, Nick's, and Tom's cars all do the opposite.
I'd thinking more along the lines that when you take your foot off the gas the throttle switch closes, and the CIS motor closes. Normally, the CIS motor will begin to open when the RPM drops below setpoint. But a worn CIS motor may be stuck in the closed position until you hit the next pothole or accelerate the engine.
Paul's second theory is interesting..... A decaying wiring harness, perhaps, and when you hit the brakes several wire touch from the inertia or the engine shifting slightly.....
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