Is it "normal" for engine idle speed to be affected when holding down the brake pedal while stopped?

My '84 5-sp duke runs great, stops well. I autocross it.

I notice while stopped that as long as I hold the brake pedal down, the idle is just a tad rougher. As soon as I release the pedal, the engine idles a bit smoother. The idle speed remains around 950 RPM (due to the IAC compensating for conditions), but it's obvious that the engine is working a little harder while the brake pedal is depressed.

The engine and braking system are all stock.

I realize that engine vacuum is involved here with the power brakes, but why is there a noticeable but subtle difference in idle quality when the brakes are being used?

Bad wiring and/or ground.
Weak alternator.

See my cave, watt story in charge & start section

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

quote Originally posted by theogre: Bad wiring and/or ground. Weak alternator.

I've got to admit that the charging system never occurred to me as the culprit.

Ogre, according to your site, stepping on the brakes draws about 8 amps. I assume that's for the brake lights themselves. I'm not ruling your suggestion out just yet, but if I remember correctly, turning on headlights, highbeams, heater blower on full etc does not cause the same subtle degradation of my duke's idle. And all that stuff draws a lot more than 8 amps. The engine also turns over very fast and starts up just fine. It charges at just over 14 volts when first started. I'll have to double-check now that you've brought this up, but I'm a little doubtful that electrical is the cause. Thanks though, and I'll report back tomorrow after having a look-see.

Pinch off the vacuum line off the engine to the brake booster. If it still has an idle fluctuation, it probably is electrical/alternator related. If the issue goes away, you might have a vacuum leak in the brake booster.

All the engine sensors use a return ground wire except the O2 sensor. Try unplugging it and see if you still get the same change in idle.

quote Originally posted by fieroguru: Pinch off the vacuum line off the engine to the brake booster. If it still has an idle fluctuation, it probably is electrical/alternator related. If the issue goes away, you might have a vacuum leak in the brake booster.

Yes... booster problem will cause it to.

Low pedal can cause it. Low pedal forces booster to use more vacuum.

84-86 4 cyl uses vacuum filter. Small can in line, bolted to firewall. That filter can get block w/ dirt and cuts booster performance.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

[This message has been edited by theogre (edited 04-06-2011).]

quote Originally posted by fieroguru: Pinch off the vacuum line off the engine to the brake booster. If it still has an idle fluctuation, it probably is electrical/alternator related. If the issue goes away, you might have a vacuum leak in the brake booster.

this was my thinking too - vacuum

Okay, I'm starting to get the definite impression from you guys that it isn't normal for the idle quality be affected while the brakes are being applied.

I'll see if I can try out these various suggestions during the day. Thanks guys, I'll report back.

Mine doesn't do it if I hold the brakes, but if I rapidly pump the brake pedal the idle will fluctuate a bit. I'm guessing it's from the vacuum draw.

quote Originally posted by Formula88: Mine doesn't do it if I hold the brakes, but if I rapidly pump the brake pedal the idle will fluctuate a bit. I'm guessing it's from the vacuum draw.

Yes this is normal if you pump the pedal up and down.

Had a problem pretty similar to this. If the ground for the rear tail lights its corroded it will draw alot more than it is supposed to. The ground for the rear lights is on the passenger side, under the wheel well liner, just about at the peak of the wheel well. Mine was about ready to come off and my car would almost stall when I hit the brakes.

I just put a new zip screw in it about 2 inches over from where it was and all was well.

I wasn't able to look at too much the last couple of days, but I did observe that turning on a bunch of stuff (headlights, highbeams, blower fan, etc) also affected the idle quality. Not a whole lot, but it is noticeable. So it appears that electrical draw (and/or improper grounding) is indeed having some effect here.

quote Originally posted by Riddick85: If the ground for the rear tail lights its corroded it will draw alot more than it is supposed to.

The fact that just using the brakes alone seems to have as much effect on the idle quality as everything else (electrical) turned on makes me suspect that this could be the problem (if it turns out the brake booster has no leaks).

Thanks for these sugggestions guys. It may take me a couple more days to test and report my findings, but I'll definitely post what I find.

One further question, possibly related... Although the duke/5-spd runs well (and the brakes aren't dragging), I'm getting just under 20 mpg around town. I would've expected better mileage from a healthy duke. Could this also be the result of perhaps a lot of drag from the charging system due to poor grounds, etc? I've also had ongoing problems with the alternator belt wanting to squeal (at startup) a couple weeks or so after tightening it up. Hmmm...