Alpine951

I recently developed a lean idle condition. I just pulled the car out of the garage today as it was in there for about a month because I was driving my new winter ride for a while to make sure it was OK. I did a pressure test thinking maybe I had a vacuum leak while it was in the garage but didn't find anything. I don't think I sprayed around by the brake booster when I did the test or the hoses running from it, or the inline check valve. Warm idle is at 16+ and today when the car was warm sitting in a parking lot I stepped on the brake at it immediately went to 14.7 and dithered around there. when cruising it dithers around 14.7. so my question is what affect does stepping on the brake have that could affect AFR? Also what is the function of that blue check valve?

Teddy952

Does pumping the brake pedal increase the idle a little?

If pressing the brake causes the idle AFR to change there has to be a leak in the line from the manifold to brake booster or the check valve in this line or inside the brake booster itself. To test disconnect the line to the booster from the manifold and plug the port on manifold. Try to reproduce issue.

The little blue check valve is to supply vacuum to the HVAC reservoir.

jmj951

Yes, a leaky brake booster is one possibility.

The other possibility is that it's an electrical problem. A bad ground could be affecting the voltage being read by the gauge, and by stepping on the brake pedal (which obviously activates the brake lights as well) you're exposing the harness with the bad ground.

Have you tried using other electrical accessories to see if that also causes this issue?
1) Try using the headlights, fog lights, emergency flashers, and windshield wipers, and see if it has the same effect on the AFR reading as pressing the brake pedal.
2) If not, try using tape to hold the brake light sensor in the depressed position so that the brake lights never come on when you push the brake pedal. If the AFR readings don't change when you press the brake pedal now, then you know it's electrical. If the AFR readings do change even when the brake lights don't come on, then you know you have a problem with the brake booster or the vacuum lines.

Another way to test the brake booster is to hold down the pedal for five seconds, then pump it about three times, let off the brake pedal for three seconds, then start pumping again. If the pedal was hard to pump the first time, but easier to pump the second time, then you probably have a leaky booster (because after that three second delay, the booster had time to build up some vacuum again).

Humboldtgrin

Another thought... I had my venturi booster come apart. It's that "Y" piece that is by the idle air control valve, it's the brake booster line that comes off of the venturi to the brake booster. After super gluing it back together my brakes felt right again (not hard pedal like it was), and it was a vacuum leak as well. May want to look at that.

Alan 91 C2

As I understand your post, the AFR is too lean at idle (AFR 16), and stepping on the brake improves the AFR to the 14.7 to 14.9.

This is a normal response. As you step on the brake, the booster diaphragm places an air load on the engine. The air load has already passed the AFM (or MAF) So instead of going to the combustion cycle, the air load (instantenous) goes to the brake system. The lesser airflow to the combustion cylinder, with the same amount of fuel, make the idle reading richer.

The bigger issue is why is the AFR lean. air leak?

Alpine951

Thanks for the tips. no venturi as I deleted that a while back.

jmj951

I never noticed that happening on my car. Next time I take it out, I'll watch the AFR gauge and test the effect of the brakes.

At first, it sounds plausible, but the problem is that there shouldn't actually be any 'airflow' through the brake booster - it's not like it's continually sucking air and discharging it somewhere - so holding the brake pedal shouldn't have any residual effect on AFR's. There can only be a very brief effect on vacuum (if the diaphragm isn't broken), leading to the possibility of a brief impact on the AFR's.

Humboldtgrin

Maybe the brake booster has a bad diaphragm?

Alpine951

OK. same thing happens when turning on the headlights or blower fan. I cleaned up all the grounds previously except the ones under the dash. would the few remaining grounds cause this? I guess maybe I should replace the ground wire from the battery.

jmj951

If everything electrical causes this problem, then it seems like current is not able to fully return to the negative terminal of the battery, so you could be right about the negative battery cable. It's probably not just isolated to one harness. If you have any other kind of large battery cable to test with, you could install it as a 'jumper' cable, in parallel with the factory cable, to see if it makes any difference.

Alan 91 C2

i tested my car today. pumped the brakes 3 times and the AFR went from 14.9 (avg) to 16.0 for 3-5 seconds.

My normal warm AFR at 14.9 (bounces above and below 14.8-15.1)

User 52121

Hmmm.

Unplug the brake light bulbs and try it again.

mahoney944

My car does the same. I've suspected the battery cables for some time now but haven't done anything about it. If cables are confirmed maybe I'll finally commit on a new oem set

JacRyann

Measure the voltage using the + and ground wires at the wideband unit itself. Step on the brake and see how the voltage changes.