pumping brakes increases idle

pappentl · Post by **pappentl** » Sun Mar 15, 2009 6:32 pm

If the car ('81, stock 2.

is sitting at a light and I pump the brakes, I find the idle will climb 3-400 rpm and then settle back down. Is this normal? If not, what should I check? TIA.

...tom

grumpsjr · Post by **grumpsjr** » Sun Mar 15, 2009 7:04 pm

Peter Florance told me years ago that this is a symptom of the idle mixture being rich. I believe you can adjust this via the AFM. You might start there.

Brian

On Mar 15, 2009, at 16:32, "pappentl" <pappentl@aol.com (pappentl@aol.com)> wrote:

If the car ('81, stock 2. is sitting at a light and I pump the brakes, I find the idle will climb 3-400 rpm and then settle back down. Is this normal? If not, what should I check? TIA.

...tom

...tom
1981 528i, no. 6 in a series...

Post generated using Mail2Forum (http://www.mail2forum.com)

Mike W. · Post by **Mike W.** » Sun Mar 15, 2009 8:43 pm

It's normal. By pumping the brakes, which are vacuum assist, more air is introduced into the system. At least on 528i's, with the oxygen sensor constantly correcting the mixture, it then injects more fuel to compensate for more air. So if the idle goes up, it means everything is working correctly.

1st 5er · Post by **1st 5er** » Tue Mar 17, 2009 10:59 pm

Interesting.
So, they been like this since new?

Mike W. · Post by **Mike W.** » Thu Mar 19, 2009 9:37 pm

swatterssr wrote:Interesting.
So, they been like this since new?

Yep, pretty much any vacuum boosted car is like this. E28's aren't due to having a hydroboost system running off the power steering pump, but that's the exception, not the rule.

1st 5er · Post by **1st 5er** » Thu Mar 19, 2009 11:36 pm

I just tried the 'pump the brakes when stopped at idle' with no variation in idle speed, so, do I have a problem with my 79 528i?
I am still trying to solve my rough idle after engine reaches operating temp issue, which results in very poor acceleration at low rpm's, any gear.

Mike W. · Post by **Mike W.** » Fri Mar 20, 2009 12:17 pm

swatterssr wrote:I just tried the 'pump the brakes when stopped at idle' with no variation in idle speed, so, do I have a problem with my 79 528i?

You may not be pumping fast enough. Regardless, if the brakes work ok, you're ok.

I am still trying to solve my rough idle after engine reaches operating temp issue, which results in very poor acceleration at low rpm's, any gear.

Bad AFM is the first thing that comes to mind, what does the O2 sensor say when that is happening? That is the best single troubleshooting tool on the car. Use it!

1st 5er · Post by **1st 5er** » Fri Mar 20, 2009 1:23 pm

Mike W. wrote:Regardless, if the brakes work ok, you're ok.

Bad AFM is the first thing that comes to mind, what does the O2 sensor say when that is happening? That is the best single troubleshooting tool on the car. Use it!

Brakes are fine.
I have taken apart the AFM and adjusted the swiper arm, no change.
I have installed a new O2 sensor, no change.

I have several E28's (85 528e, 85 535i, 87 535i, and the M5), are the AFM's interchangeable with the E12, if so which one? Real OEM says no.

Mike W. · Post by **Mike W.** » Fri Mar 20, 2009 2:43 pm

I have taken apart the AFM and adjusted the swiper arm, no change.
I have installed a new O2 sensor, no change.

But what does the O2 say when you're driving around? It may look goofy, but you've got to wire it up out of the hood, in the window, so you can watch it while you're driving. That will give you a much better idea of what's going on.

1st 5er · Post by **1st 5er** » Fri Mar 20, 2009 3:52 pm

Mike W. wrote:
I have taken apart the AFM and adjusted the swiper arm, no change.
I have installed a new O2 sensor, no change.
But what does the O2 say when you're driving around? It may look goofy, but you've got to wire it up out of the hood, in the window, so you can watch it while you're driving. That will give you a much better idea of what's going on.

Not sure how to "wire it up out of the hood".
How about some simple instructions? Mine is a single wire.
Tech FAQ does not cover this.

Don Daynes · Post by **Don Daynes** » Sat Mar 21, 2009 1:23 pm

What Mike is suggesting you do is to monitor the output of the O2 sensor with a voltmeter. What you should see is a voltage that goes up and down. The ECU interprets this signal and shortens or extends the pulse duration to the fuel injectors. That is to say, that when the O2 sensor senses that there is a rich mixture, the ECU shortens the pulse to the injectors; at the next refresh when the O2 sensor senses a lean mixture, the ECU increases the pulse. This causes the mixture to oscillate back and forth between lean and rich.

The ECU has limited ability to effect the mixture. It cannot compensate for many problems, like vacuum leaks or fuel pressure that is too high. When I attempted to monitor the O2 sensor with my DVM, I found it difficult to read the voltage on the display. (Perhaps my meter’s refresh rate is slower than most?) I found the Split Second air/fuel ratio meter much easier to use. Incorporating the meter into the pod on the dash made it available anytime for troubleshooting, or if I just wanted a light show. http://www.splitsec.com/

1st 5er · Post by **1st 5er** » Sat Mar 21, 2009 3:53 pm

Connected positive to O2 wire and negative to ground.

Results:
At idle it fluctuated between 0.78 and 0.18 volts.
When I removed the oil filler cap on the valve cover it would drop drastically.

Driving fluctuated between 0.78 and -0.04 volts.
The higher numbers while accelerating, mid numbers while cruising, and lower numbers in a no load condition such as coasting to stop, between gear shifts, etc.

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