Rotorious

iTrader: (7)

OK all you FD geniuses out there, explain this one to me! I'm really curious as to how/why these two aspects are related.

A few days ago, out of nowhere, my car suddenly started feeling different while driving stop-and-go style on the parking lot they call a highway that takes me to/from work. Every time I'd start moving a little bit, I'd have to come back to a stop shortly after, and each time I slowed down, the engine RPMs dropped sharply to around 500 for just a second and then rose back up to normal. The engine hesitated to a varying extent each time. Sometimes it felt like it came close to dying. It didn't take me long to realize that this was happening each time I applied the brakes at low speeds.

Well, this is happening regularly now, and it is driving me crazy. I never had any idle issues up until now, and then this issue appears out of nowhere. For whatever reason, applying the brakes consistently causes the RPMs to drop sharply, and it gets that stumbling feeling each time.

How the heck can using my brakes effect the idle so much, so suddenly?

Thanks in advance for any efforts to help me!

- Jim

DaveW

Sounds like your brake booster check valve (should be in the vacuum line to the brake booster to build up and maintain vacuum for the vacuum brake assist to use) is not working properly. Each time you apply the brakes, some air is being sucked out of the intake manifold, reducing the idle speed. You could also have a leak in the diaphragm in the vacuum booster, which could also cause this problem. Have you noticed an occasional increase in pedal effort needed to brake?

There was a recall to replace the original separate check valve with one that is in the above hose. Was this recall done on your car?

Dave

mad_7tist

how is the alternator output? is it manual or auto?
with the car off pump up the brakes until it is a good high hard pedal
with pressure still applied to the pedal start the car
if the booster is ok the pedal should depress slightly and then stop

Rotorious

iTrader: (7)

Sorry for the late response. I forgot I posted this for awhile.

In response to your questions:

All the recalls have been performed on the car.

I have not noticed any difference in braking power.

The alternator is fine as far as I know, though I don't doubt it could have issues without me knowing it.

The car is manual.

I performed the test suggested by mad_7tist, and it worked as described.

Some additional information:

I read another thread regarding a failed brake booster check valve, and it was stated that a sign of failure is hearing a "whooshing" sound coming from beneath the hood directly in front of the driver. I do hear this noise when the window is open. It consistently whooshes under acceleration, getting louder as RPMs go up.

All signs seem to suggest the valve is bad, but I don't understand why I am not noticing any difference in braking power.

What do any of you think?

Thanks for any help you can offer.

- Jim

Rotorious

iTrader: (7)

This is my first and last "bump" of this thread in a desperate attempt to get whatever thoughts you knowledgable types will spare me!

I know it hurts some of you to know an FD is in pain, even if it is relatively minor, so help if you can!

Thanks in advance,

- Jim

ObliqueFD

iTrader: (32)

I have the same problem. I have heard it is the dashpot, throttle position sensor, idle speed control, or possibly a weak engine. I haven't figured mine out yet, but I'm leaning toward the dashpot. Check out the factory service manual, section F-133 Deceleration Control System.

White94RX

I've got the same exact problem. I've replaced both the hoses on the brake booster. I'm definately leaning towards the low compression theory. How many miles are on your motor? I've got 85k on my reman, and I definately think it's the culprit. Runs like a champ off idle, but if I make a panic stop, the engine will stall. we all know compression numbers will be lower the slower the engine spins, which is why off idle, while it's spinning faster, it runs fine.

Rotorious

iTrader: (7)

Sorry, this was a double post and I'm too dumb to figure out how to delete this, so just ignore this one!

Rotorious

iTrader: (7)

It's at just a bit under 39k miles, completely stock, and rarely beat on. I really hope you are wrong about the compression being low. If compression is the issue, wouldn't the problem have appeared gradually over time rather than jumping straight from perfectly fine to noticeably problematic? I really ain't sure, but that is what my logic is suggesting to me.

Also, if that whooshing sound I hear under acceleration is not due to the check valve being bad, am I correct in guessing there is a boost leak somewhere?

Thanks for your input!

- Jim

White94RX

well, sounds like you might be onto something. low compression from the housings wearing out I would think would only happen on high mileage motors, like mine at 85k. but if you're hearing a wooshing sound, very likely is a boost leak. and a boost leak, could very likely also be a vacuum leak, and could cause rough idle problems.

yeah, my idle problems came on kinda gradually, and it's kinda leveled off as being pretty bad, but I've got it figured out how to keep it from doing it. but it will definately stall if i panic stop.