irregular idle when foot on brake

[niznos]

Got a '90 that seemed to have slight idle prblems for the longest time. I recently replaced my cam with the Nismo R4 cam and the idle, expectedly, got worse. I guess it wasn't that bad before the cam for me to notice, but now with the cam I've noticed the idle holds fine until my foot is on the brake. I don't think the booster is going out because the brakes are super solid. I've adjusted the idle up to about 800-900 to try to take the shudder out the the cam'd idle, but when the brake is depressed the idle moves around from 600 to 800 almost stalling. Take the foot off the brake and within a couple of seconds the idle stablizes again at the 800-900 range.

First I thought it may be a weak electrical component, as in weak batt., alt. causing a low voltage condition that is weakening my spark a low rpms. Found that I did have a weak alt. and it is replaced. No change in idle when brakes are on.

May be a vacuum problem? But why can it sustain idle without foot on brake?

Where are the super techs?

[den240]

Brake booster works by vacuum, so it could still be a vacuum problem. Maybe using a lot of vacuum to boost your brakes just lets a vacuum problem elsewhere show up more...

Brakes should only effect any electrical stuff by turning on brake lights.

[whiterps13]

i dont have cams but i have this exact problem. werd....

[DAEDALUS]

Vacuum leak in the booster probably. It only opens up when you press the brakes. Have someone press the brakes with the car running while you listen for a hissing sound.

[navysnail]

or turn the car off and see if you still have the booster when engine is off. each time you press the pedal, it should go down less and less. this is what it says in the fsm

[Ubernoober]

The brake booster should be able to hold vacuum for a while after the engine is off. If not, its a leaky booster. If you shut down the engine and hear a hiss from the booster... its a leaky booster.

With that cam change, you are making less vacuum at idle than before, so even a small leak from the booster will dramatically effect idle.

Electrically, its really only the brake lights. I cannot imagine the alternator providing enough to load the engine for just brake lights.

[niznos]

Brakes are working perfectly. Will check for other vac. leaks and keep everyone posted.

Thanks for all the input.

[niznos]

Probably half of you reading this out there has this problem: that accrodian looking piece of intake tubing just in front of the throttle body is rotten or even ripped in the folds. Nissan wanted $80+ for the stupid thing but I would recommend that everyone with this problem to replace it. Don't duct tape it, or try to silicone it up, it just won't stop the air leak. the air seeping in did not pass throught the Mass Air Flow sensor, so the ECU is not compensating for it and will give a bad idle, and likely other drivability issues depending on how bad it is.

It seems that my idle was bad, at least partly, due to that tube. I got it replaced by getting a friend to buy it at jobber price. Still $60, but it was necessary. The idle cleaned up a bit.

When I was inspecting for intake/ vacuum leaks, I also found that the AIV may have been giving some reverse flow given that where the tube hooks in had some black carbon build up. Removing the AIV today, we'll see if this helps a bit more

[niznos]

AIV is now history and with the new intake tube the idle is not perfect, but it wasn't all that great before all my recent issues. The point is, it idles like it did before I installed that Nismo cam, and that's saying a lot, since that cam is pretty lopey.

Going to replace the ancient PCV valve when it comes in, may finally get the idle issue resolved.

Thanks to all that gave me insight.

[niznos]

Okay, the problem persists.

Can the brake booster be tested with the car off and applying vacuum to the booster via the vacuum line to see if it can hold a vacuum?

[PhaneSoul]

The brake booster should be able to hold vacuum for a while after the engine is off. If not, its a leaky booster. If you shut down the engine and hear a hiss from the booster... its a leaky booster.

With that cam change, you are making less vacuum at idle than before, so even a small leak from the booster will dramatically effect idle.

Electrically, its really only the brake lights. I cannot imagine the alternator providing enough to load the engine for just brake lights.

theres ur answer

[niznos]

I've read that post, and tested as it said. The booster tested good with that test. I was thinking that an actual vacuum tester may work better at finding a really small leak and was asking if it was something I should be trying too.