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'98 540i Idle Surge

341832
Posts: 4
Submitted by 341832 on January 1, 2008 - 6:52pm.

540i with ~115k miles. Occasionally the engine will miss a beat at idle, resulting in a 200 - 300 rpm rev, and then settle down. Not consistent, but enough to be annoying. This does not happen with the A/C on and the idle up at 600 - 700 rpm, only at the low 500 rpm idle. Also, it seems that any pressure at all on the throttle will stop the surging; even if it isn't enough to change the rpm, just taking up 'slack' in the dead band. No codes. Camshaft position sensor and idle control valve were replaced about a year ago to address another issue.

Thanks!


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329012
Posts: 42
January 1, 2008 - 8:21pm

My 540i began having a rough idle the first time it was started each day. May have been similar to what you discribe ???. I used 93 octane and injector cleaner for a couple of tanks of fuel and it's OK now.


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341832
Posts: 4
January 7, 2008 - 8:05pm

Thanks. That was my first thought, too. I ran some Redline through a tank, and have been running 93 the whole time. Didn't seem to help (but it did help smooth out the early morning warm up!) It only does the surge dance once warmed up and the idle goes down to the 500 rpm mark. Odd.


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rcutclif
rcutclif's picture
Posts: 2
January 7, 2008 - 10:33pm

My 325i (1993) does this as well, and I haven't yet fixed it (I live in Colorado and its cold here now;) but I'm pretty sure the problem lies in a dirty idle solenoid. I don't know where it would be on the 540i, but I would guess the underneath the intake manifold somewhere. This little thing is controlled by the engine computer to control the idle speed (imagine a small separate throttle that is only used when the main throttle plate is closed <-This may be simplified, but I think it should help understanding). If this is dirty and sticky, the idle will stutter and surge, and when you hold the main throttle open just enough, the problem is bypassed. Also, as you noticed, a/c usually raises the idle speed a tad and might just give the big engine enough inertia to keep spinning smoothly. Fuel injector cleaner might clear this up a little, because it would be cleaning the solenoid a little.

When this weather clears up, I'm gonna start by pulling the solenoid and giving it a good hand-cleaning. Hope this helps!

--

RAwB


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382330
Posts: 3
February 21, 2008 - 10:57pm

The idle speed control valve is a round cylinder attached to the throttle body. Use a stethescope (get at auto store for about $8) and listen to it while the car is idling. It should be buzzing. Also watch the rpm as you switch on and off the A/C. The rpm should increase about 200-300 rpm. If that is working then you speed control valve is probably good. You probably have an air leak in the intake manifold or other part of the system. Look for cracks in the boot connecting the Mass air flow sensor and the throttle. It may be a gasket on the throttle body or intake. the back of the manifold has a cover and sometimes that fails. I have also heard that a dirty throttle body can cause it. Also check any vacuum lines. There is probably one in front of the water pump that has a rubber cap. Some times they go bad.


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